Part 2
1.1 Practical beneďŹ ts
P2
1.2 Environmental beneďŹ ts
P2
2.1 The photovoltaic effect
P3
2.2 Photovoltaic modules
P4
2.3 Inverters
P6
2.4 Connections
P7
2.5 Battery chargers
P7
2.6 Off grid or grid connected
P8
3.1 How to ensure safety during normal operation?
P10
3.2 Protection against overvoltage: Surge protection
P13
3.3 How to ensure safety during maintenance or emergency
P15
3.4 How to ensure safety during all the life cycle of the installation
P17
4.1 Common characteristics of PV architectures
P18
4.2 Architectures for installations connected to the grid
P19
4.3 Sizing
P21
4.4 Installation type
P22
4.5 Electrical equipments selection
P23
5.1 Types of monitoring systems
P31
5.2 Monitoring systems
P31
5.3 Sensors
P33
5.4 Security of the installation
P33
Schneider Electric - Electrical installation guide 2015
This technology enables to produce electricity directly from the sun light, which is a source of renewable energy. There are two ways for this: Solar Thermal Energy is captured through an exchange between a circulating fluid exposed to the sun and a load circuit (accumulation tank or heat pump). Solar photovoltaic Energy is produced using the principle of the photovoltaic cell discovered by Edmond Becquerel in 1839. It is particularly beneficial to use solar radiation reaching the Earth since: This radiation remains stable (to within 10%) on average from one year to the next; At ground level, it supplies an average of 1000 Wh/m² per day although this depends on the following principal criteria: The latitude The angle of the surface and the direction faced The degree of pollution The time of year The thickness of the cloud layer The time of day The shade The global horizontal irradiation, which is the amount of energy received yearly on a plane varies from 700 kWh/m² per year in the North of Europe to 2500 kWh/m² per year in African desert areas.
Using solar energy allows to reduce the consumption of “fossil” fuels which are the likely cause of global warming and atmospheric pollution. It contributes to sustainable development and is also in accordance with the policies of the European Union, which passed a decree in March 2007 setting the following targets to be met by 2020: Reduction of greenhouse emissions by 20% Reduction of energy consumption by 20% 20% renewable energy as a proportion of total energy consumption
Schneider Electric - Electrical installation guide 2015
This is the ability to transform solar energy into electricity and is achieved by using photovoltaic (PV) cells. A PV cell (see ) is capable of generating voltage of between 0.5 V and 2 V depending on the materials used and a current directly dependent on the surface area (5 or 6 inch cells). Its characteristics are shown in a current/voltage graph as shown in .
Amperes
4 2 0 0,0
0,2
0,4
0,6
0,8
Volts
The photovoltaic effect is dependent on two physical values (see )– irradiance and temperature: As irradiance E (Wm²) increases, so do the current and power produced by the cell As the temperature (T°) of the cell increases, the output voltage decreases significantly, the current increases only slightly, so overall the output power decreases. In order to compare the performance of different cells, the standard has set out Standard Test Conditions (STC) for irradiance of 1000 W/m² at 25°C.
Current
Current 1000 Wm2
MPP
800 Wm2 600 Wm2
75 °C MPP 50 °C
400 Wm2
25 °C 200 Wm2 0 °C Voltage An increase in solar radiation increases the power generated by the cell
Voltage An increase in temperature decreases the power generated by the cell
MPP : Maximum Power Point
To make it easier to use energy generated by photovoltaic cells, manufacturers offer serial and/or parallel combinations grouped into panels or modules.
Schneider Electric - Electrical installation guide 2015
These combinations of cells (see ) enable the voltage and current to be increased. To optimise the characteristics of the modules, these are made up of cells with similar electrical characteristics. Each module providing a voltage of several tens of volts is classified by its power level measured in Watt peak (Wp). This relates to the power produced by a surface area of one m² exposed to irradiation of 1000 W/m² at 25°C. However, identical modules may produce different levels of power. Currently, the IEC standard specifies a power variation of ±3%. Modules with typical power of 160 Wp include all modules with power of between 155 Wp (160 -3%) and 165 Wp (160 +3%). It is therefore necessary to compare their efficiency which is calculated by dividing their power (W/m²) by 1000 W/m². For example, for a module of 160 Wp with a surface area of 1.338m² (*), the peak power is 160/1.338 which gives 120 Wp/m². Therefore the efficiency of this module is: 120/1000 = 12%. Nota: Manufacturers may have different production tolerance limits according to local standards or habits (example: JISC8918 specifies ±10%), so it is recommended to always check product catalogues for actual tolerance values. (see table in Figure P5 as an example)
156 x 156 mm 60 24 V 250
255
260
0/+5
0/+5
0/+5
30.1 V
30.2 V
30.4 V
8.3 A
8.4 A
8.6 A
8.9 A
9.0 A
9.1 A
37.2
37.4
37.5
1 000 V CC Isc = +0,065%/°C Voc = -0,34%/°C Pmax = -0,43 % /°C Under Standard Test Conditions (STC) : irrandiance of 1000 W/m², spectrum AM 1,5 and cells temperature of 25°C
However when photovoltaic cells are connected in series, a destructive phenomenon known as the “hot spot” may occur if one of the cells is partially shaded. This cell will operate as a receiver and the current passing through it may destroy it. To avoid this risk, manufacturers include bypass diodes which bypass damaged cells. Bypass diodes are usually fitted in the junction box behind the module and enable 18 to 22 cells to be shunted depending on the manufacturer. These modules are then connected in series to achieve the level of voltage required, forming chains of modules or “strings”. Then the strings are arranged in parallel to achieve the required level of power, thus forming a PV array. Since there are increasing numbers of PV module manufacturers throughout the world, it is important to consider the various options carefully when choosing equipment. Installers should also: Ensure the compatibility of the electrical characteristics with the rest of the installation (inverter input voltage). Ensure that they are compliant with the standards. Select suppliers likely to be in business in the long-term to ensure that faulty modules can be replaced as these must be identical to those already installed. This final point is important as installers are responsible for the warranty granted to their clients.
(*) The dimensions of these modules (L x W x D) in mm are: 1237 x 1082 x 38. Schneider Electric - Electrical installation guide 2015
Different technologies are currently being used to manufacture photovoltaic generators. These are divided into two categories - crystalline modules and thin film modules.
There are two main categories of crystalline silicon modules – mono-crystalline modules and multi-crystalline modules. Mono-crystalline modules are currently best in terms of performance, with efficiency of 16 – 18%. They are also more expensive. The efficiency of multi-crystalline modules is between 12 and 14%. They are more commonly used, especially in the residential and service sectors. These modules have a service life of more than 20 years. They lose some of their power over time (< 1% per year) but continue to produce electricity. Depending on the look required, bi-glass modules are available with two plates of glass which make the module semi-transparent, or Tedlar or Teflon glass modules which are less expensive but completely opaque.
Extensive research is currently being carried out on thin film modules and current efficiency levels of 6 to 8% should increase in coming years. They are cheap and suitable for large areas provided that the surface is not a valuable part of the facility. This category of thin film modules includes a number of technologies of which there are 3 main types: a-Si – thin film or amorphous silicon CdTe (cadmium telluride) CIS (copper indium selenide) It should be noted that at present we do not yet have 20 years’ experience of this type of technology and thus still do not know how these modules will age. In their technical specifications, reputable manufacturers indicate initial and stabilised values. The table in
provides a comparative overview of all these technologies.
STC module efficiency Maximum
20.4 %
16 %
10 %
14.4 %
15.5 %
Average
16 %
15 %
6%
11 %
11 %
Relative cost ($/Wp)
0.8 to 1
0.8 to 1
0.75
0.65
0.85
Temperature coefficient at the power peak (%/°C)
-0.3 / -0.5
-0.3 / -0.5
-0.2
-0.2
-0.3
Schneider Electric - Electrical installation guide 2015
These devices which convert direct current into alternating current are special inverters for photovoltaic power supply (see ). Different types of photovoltaic inverters or “PV inverters” are available. They fulfil three main functions: Inverter function: Converts direct current into alternating current in the form required (sinusoidal, square, etc.) MPPT function: Calculates the operating point on the photovoltaic surface or array which produces the most power in terms of voltage and current - also known as the Maximum Power Point Tracker (see ). Automatic disconnection from the network function: Automatically commands the inverter to switch off and the system to disconnect from the network in the absence of voltage on the electrical network. This protects the inverter and any maintenance staff who may be working on the network. Therefore, in the event of a network failure, the inverter no longer supplies energy to the network and energy produced by the photovoltaic modules is wasted. “Grid interactive” systems are nevertheless available which function in back-up mode. Batteries need to be installed for these systems as well as an additional control panel to ensure that the network is disconnected before supplying their own energy. Different models Some “multi-MPPT” inverters have a double (or triple, quadruple, etc.) MPPT function. This function enables PV supply to be optimised when the array includes strings facing in different directions. There is however a risk of total loss of supply if one inverter is faulty. Nevertheless, it is possible to install one less powerful inverter per string, which is a more expensive solution but increases the overall reliability of the system. “Multi-string inverters” are also available. These inverters are not necessarily multiMPPT as described above. The name simply indicates that several strings can be connected to the inverter and that they are paralleled inside the inverter.
6,00
180,00 160,00
5,00 140,00 4,00
120,00 100,00
3,00
2,00
Maximum power point
80,00 60,00 40,00
1,00 20,00 0,00 0 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 Vmpp Voc : Module’s short circuit current Voc : Module’s open wire voltage
0,00
P (V) (A)
In order to compare the various appliances, a level of efficiency has been determined based on different operating points, simulating the average daily performance of an inverter. This “European efficiency” is calculated using the following formula: 0.03 x ( 5%) + 0.06 x ( 10%) + 0.13 x ( 20%) + 0.1 x ( 30%) + 0.48 x ( 50%) + 0.2 x ( 100%)
Schneider Electric - Electrical installation guide 2015
Ingress protection and temperature parameters are important when choosing an inverter. Almost all manufacturers of inverters offer IP65 inverters which can be installed outdoors. However, this does not mean that they should be installed in full sunlight as most inverters operate in degraded mode in temperatures over 40°C (50°C for Xantrex inverters manufactured by Schneider Electric) and thus output power is reduced. Installing inverters outdoors in full sunlight also incurs the risk of premature aging of some of the inverter’s components such as the chemical condensers. This considerably reduces the inverter’s service life from 10 years to as few as 5 years!
Photovoltaic installations require special cables and connectors. Since modules are installed outdoors they are subjected to climatic constraints associated with high voltages caused by the installation of modules in series. Besides being ingress protected, the equipment used must also be resistant to UV rays and ozone. It must furthermore display a high level of mechanical resistance and a high level of resistance to extreme variations in temperature.
The voltage drop between the PV array and the inverter must be calculated and this must not exceed 3% for nominal current (UTE recommendation: 1%). The DC cables used should be double-insulated single wire cables and since these are not standardised, cables indicated by the manufacturer as being specifically for PV should be used.
In general, photovoltaic modules are supplied with two cables equipped with one male and one female connector. Using these cables, it is possible to connect two modules installed side by side, thus creating a series without any difficulties. The male connector connects to the female connector of the following module and so on until the required level of direct current is attained. These special connectors including the Multi-Contact MC3 or MC4 with locking systems offer protection if touched while they are disconnected. This protection is necessary since as soon as a photovoltaic module is exposed to irradiation, it supplies voltage. If the cables connecting the modules are handled (to alter or extend them) they must either first be disconnected or the DC isolator for the DC circuit must be activated at the input to the connection box. It is also possible to use different connectors available on the market. These should be chosen carefully for their quality, contact and male-female mating to avoid any poor contact which may lead to overheating and destruction
In remote locations, batteries need to be charged to supply energy after sunset. There are two types of chargers: Current chargers – the voltage of the PV array must be the same as the charge voltage of the battery and is regulated in terms of current. MPPT chargers – these chargers operate at the maximum power point. They manage the charge of the battery, limit the current and voltage, and control floating. This type of charger is more expensive than the type mentioned above but allows an optimal number of PV modules to be installed and reduces the overall cost of the installation.
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Historically, these were the first places in which photovoltaic systems were used, supplying telecommunication relay stations or remote settlements which were difficult to access and could not be connected to the network. They remain one of the only means of supplying electricity to 2 billion people who currently do not have access to it. In order to size these installations correctly, it is first necessary to identify the load curve required and the number of days where the installation will not be exposed to sunlight in order to identify how much energy needs to be stored in the batteries. This information is used to determine the size and type of batteries required. Then, the surface area of the photovoltaic sensors must be calculated to ensure that the batteries can be recharged in the worst case scenario (shortest day of the year).
This method entails over-sizing the system to ensure continuity once or twice a year. As a result, this type of installation is very expensive! It should be noted that according to the EPIA (European Photovoltaic Industry Association) this type of installation will account for 20% of the photovoltaic market in 2012 and 40% in 2030.
Storage is crucial to this type of installation. Several types of batteries are available: Lead batteries These batteries operate in cycles (charge/discharge). Open batteries are recommended to prevent inflating which may occur due to excessively rapid charging and large emissions of hydrogen. Their purchase price is certainly their main advantage although they have short service lives. This is influenced by the depth of discharging but they last no more than 2 or 3 years at a discharging rate of 50% and above. Furthermore, deep discharging may “kill” the battery. Therefore, when operating such equipment at a remote site, the batteries should be changed on a regular basis to maintain their charging performance. Ni-Cd or Nickel Cadmium batteries These batteries have the advantage of being much less sensitive to extreme temperature conditions and deep charging or discharging. They have a much longer service life (5 to 8 years) but are more expensive to purchase. However, the cost of the Wh stored over the service life of the installation is lower than that of lead batteries. Li-ion batteries These are the batteries of the future for these types of operations. They are insensitive to deep discharging and have a service life of up to 20 years. At present, they are prohibitively expensive but prices are set to fall by 2012 with the start of mass production. They will therefore become the most economic variety for this type of usage.
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Owners of power generation systems connected to the grid have 2 options: Sell all the power they produce (option known as “total sale”). For this option, a separate connection must be established to the network, apart from the connection for consumption. This also requires an administrative declaration. Use the power they produce locally as required and only sell the excess (option known as “sale of excess”) which has two benefits: The difference in the rates payable by the producer (purchase) and the consumer (sale) It is not necessary to establish a new connection which may be expensive and requires an administrative declaration. Since different rates are charged, a profitability analysis should be carried out to choose the best option.
The following points are important to note with regard to installations connected to the network: In contrast to independent installations, no correlation is required between consumption for the building and output. For the “total sale” option, the two elements are completely independent. For the “sale of excess” option, the network will compensate when production does not cover consumption. The network must be present in order to supply and sell energy. Furthermore, energy distributors require automatic disconnection systems to be in place in case of incidents on the network. When activated, these stop supply and therefore sales. Reconnection occurs automatically when the network returns to its nominal operating conditions. As a general rule, no provision is made for local storage using batteries or other means. This is true for mainland France where there is a high quality network with the capacity to absorb all the energy produced. However, the system does have one fault. If the network fails, owners of installations who are also generally consumers are left with a power generation facility which they cannot use (see previous point). In countries or towns with frequent network incidents, systems are being developed which include batteries.
Schneider Electric - Electrical installation guide 2015
IEC standard 60364 Part 7-712 sets out rules for ensuring that solar photovoltaic power systems are safe and supplies a number of the definitions used in these pages
Two particular characteristics of PV generators are their DC voltage levels and the fact they cannot be shut off as long as PV modules are exposed to the sun. The short-circuit current produced by the PV module is too low to trigger the power supply’s automatic disconnect. The most frequently used protective measures do not therefore apply to PV systems. However, as PV modules are installed outdoors they are exposed to the elements. And since they can be installed on roofs, critical attention should be paid to the risk of fire and the protection of fire fighters and emergency services staff.
IEC 60364-7-712 stipulates that PV systems whose maximum UOC MAX is higher than 120V DC should use « double or reinforced insulation » as a protection against electric shock. Switchgear, such as fuses or circuit-breakers on the DC side, do not afford protection against electric shock as there is no automatic disconnect of the power supply. Overcurrent protection, when used, protects PV cells against reverse current and cables against overload.
Generally speaking there are three situations that can lead to abnormally high temperatures and the risk of fire in a PV system: insulation fault, a reverse current in a PV module, and overloading cables or equipment.
Double or reinforced insulation is a protective measure against electric shock but it does not exclude all risk of insulation fault. (The assumption here is that the likelihood of an insulation fault and of someone touching an energised part of the installation at the same is very low. Insulation faults in themselves do happen more frequently, however.) DC insulation fault could be more dangerous as arc has less chance to extinguish by itself as it does in AC.
It is impossible to earth one pole. AC protection can be used to detect insulation faults. An overcurrent protective device (which also detects insulation faults) should be used to trip the grounded conductor in the event of a fault, if the PV cell technology (e.g. thin films of amorphous silicon) requires one of the conductors to be directly grounded. An insulation monitoring device should be used if the PV cell technology requires one of the conductors to be resistance-grounded. An insulation monitoring device should also be used when PV cell technology does not require either conductor to be earthed. Insulation monitoring device shall be selected taking into consideration both U OC MAX and the capacitance between poles and earth causes leakage current. In addition cables and inverter capacitance should be also considered. An Insulation monitoring device able to handle capacitance up to 500 F is suitable for PV system.
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The literature provided by manufacturers of photovoltaic modules yield the following figures:
1 MW
8000 m2
1 nF / m2
8 µF
100 kW
800 m2
5 nF / m2
4 µF
100 kW
800 m2
50 nF / m2
40 µF
Some measurements made in European plants are giving the following figures: Maximum power developed with a single inverter
Surface necessary to develop such a Power
Lowest capacitance measurement
Highest capacitance measurement
Maximum measured capacitance by m²
Frameless glassglass module with aluminium frame on an assembly stand (open air)
Plant 1: 1 MW
8000 m²
Sunny afternoon: 5 µF
Rainy morning: 10 µF
1,25 nF / m²
Plant 2: 750 kW
5000 m²
Sunny afternoon: 2 µF
Rainy morning: 4 µF
0,8 nF / m²
In-roof glassglass module with aluminium frame
Plant 1: 100 kW
800 m²
Sunny afternoon: 2 µF
Rainy morning: 4 µF
5 nF / m²
Plant 2: 50 kW
400 m²
Sunny afternoon: 0,5 µF
Rainy morning: 1 µF
2,5 nF / m²
Thin-film PV module on flexible substrate
Plant 1: 100 kW
800 m²
Sunny afternoon: 30 µF
Rainy morning: 50 µF
62,5 nF / m²
Plant 2: 50 kW
400 m²
Sunny afternoon: 15 µF
Rainy morning: 25 µF
62,5 nF / m²
Inverter
= ~
A short circuit in a PV module, faulty wiring, or a related fault may cause reverse current in PV strings. This occurs if the open-circuit voltage of one string is significantly different from the open voltage of parallel strings connected to the same inverter. The current flows from the healthy strings to the faulty one instead of flowing to the inverter and supplying power to the AC network. Reverse current can lead to dangerous temperature rises and fires in the PV module. PV module withstand capability should therefore be tested in accordance with IEC 61730-2 standard and the PV module manufacturer shall provide the maximum reverse current value (IRM)
String overcurrent protection is to be used if the total number of strings that could feed one faulty string is high enough to supply a dangerous reverse current: 1.35 IRM < (Ns -1) I SC MAX where: IRM is the maximum reverse current characteristic of PV cells defined in IEC 61730 Ns is the total number of strings
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As in any installation, there should be protection against thermal effect of overcurrent causing any danger. Short-circuit current depends on solar irradiance, but it may be lower than the trip value of overcurrent protection. Although this is not an issue for cables as the current is within current-carrying capacity, the inverter will detect a voltage drop and stop producing power. It is therefore recommended that the maximum trip current should be significantly lower than I .
Where string overcurrent protection is required, each PV string shall be protected with an overcurrent protection device. The nominal overcurrent protection (Fuse or Circuit breaker) rating of the string overcurrent protection device shall be greater than 1.25 times the string short circuit current I stc_string.
The nominal rated trip current (ITRIP) of overcurrent protection devices for PV arrays (Fuses or Circuit breaker) shall be greater than 1.25 times the array short-circuit current Isc stc_array The selection of overcurrent protection rating shall be done in order to avoid unexpected trip in normal operation taking into account temperature. A protection rating higher than 1.4 times the protected string or array short-circuit current I sc_stc is usually recommended.
Circuit breakers or fuses can be used to provide overcurrent protection. Fuses, usually on the fuse holder or directly connected to bars or cables, do not provide a load-break switch function. So when fuses are used, load-break switches should also be used to disconnect fuses from the inverter in order to allow cartridge replacement. So an array box with fuses on fuse holders as string protection, for example, should also incorporate a main switch. Circuit breakers offer fine-tuned adjustment and greater accuracy than fuses in order to allow the use of cables, especially for sub-array cables, that are smaller than fuses
PV systems are either insulated from the earth or one pole is earthed through an overcurrent protection. In both set-ups, therefore, there can be a ground fault in which current leaks to the ground. If this fault is not cleared, it may spread to the healthy pole and give rise to a hazardous situation where fire could break out. Even though double insulation makes such an eventuality unlikely, it deserves full attention.
Inverter
= ~
The fault level could be low (e.g. two insulation faults or a low short-circuit capability of the generator in weak sunlight) and below the tripping value of overcurrent protection (circuit breaker or fuses). However, a DC arc fault does not spend itself, even when the current is low. It could be a serious hazard, particularly for PV modules on buildings. Schneider Electric - Electrical installation guide 2015
Circuit breakers and switches used in PV systems are designed to break the rated current or fault current with all poles at open-circuit maximum voltage (UOC MAX). To break the current when UOC MAX is equal to 1000V, for instance, four poles in series (two poles in series for each polarity) are required. In double ground fault situations, the circuit breaker or switches must break the current at full voltage with only two poles in series. Such switchgear is not designed for that purpose and could sustain irremediable damage if used to break the current in a double ground fault situation. The ideal solution is prevent double ground faults arising. Insulation monitoring devices or overcurrent protection in grounded systems detect the first fault. However, although the insulation fault monitoring system usually stops the inverter, the fault is still present. Staff must locate and clear it without delay. In large generators with subarrays protected by circuit breakers, it is highly advisable to disconnect each array when that first fault has been detected but not cleared within the next few hours.
Enclosures on the DC side shall provide double insulation.
The thermal behaviour of switchgear and enclosures warrants careful monitoring. PV generator boxes and array boxes are usually installed outdoors and exposed to the elements. In the event of high ambient temperatures, high IP levels could reduce air flow and thermal power dissipation. In addition, the way switchgear devices achieve high voltage operation – i.e. through the use of poles in series – increases their temperature. Special attention should therefore be paid to the temperature of switchgear inside outdoor enclosures on the DC side. Cable protection should comply with requirements of IEC 60364. Part 7-712 of the standard stipulates that all enclosures on the DC side should meet the requirements of IEC 61439. This standard covers low voltage switchgear and control gear assemblies and sets out requirements that guarantee the risk of temperature rises has been factored into the safe design of DC boxes (generator and array boxes).
In addition to the standard criteria for selecting enclosures in PV systems with UOC MAX of 1000V, some equipment may show IEC 60947-1 Pollution Degree 2 rather than Pollution Degree 3.
Overvoltage may occur in electrical installations for various reasons. This may be caused by: The distribution network as a result of lightning or any work carried out Lightning bolts (nearby/on buildings and PV installations, or on lightning conductors) Variations in the electrical field due to lightning. Like all outdoor structures, photovoltaic installations are exposed to the risk of lightning which varies from region to region. Preventive and arrest systems and devices should be in place.
The first safeguard to put in place is a medium (conductor) that ensures equipotential bonding between all the conductive parts of a PV installation. The aim is to bond all grounded conductors and metal parts and so create equal potential at all points in the installed system.
SPD are particularly important to protect sensitive electrical equipments like AC/DC Inverter, monitoring devices and PV modules but also other sensitive equipments powered by the 230VAC electrical distribution network. The following method of risk assessment is based on the evaluation of the critical length L crit and its comparison with L the cumulative length of the d.c. lines.
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Surge Protection Device is required if L
Lcrit
Lcrit depends on the type of PV installation, and is calculated according to the following table:
L is the sum of: distances between the inverter(s) and the junction box(es), taking into account that the lengths of cable located in the same conduit are counted only once, and distances between the junction box and the connection points of the photovoltaic modules forming the string, taking into account that the lengths of cable located in the same conduit are counted only once. Ng: arc lightning density(nb of stike/km²/year)
Array box
Generator Box
AC Box
Main LV switch board
= SPD 1
LDC
~
SPD 2
LAC
SPD 3
SPD 4
SPD Protection Location
PV Modules or Array box
Inverter DC side
LDC Criteria
<10m
>10m
Type of SPD
No need
SPD 1 Type 2 *
Inverter AC side
Main board
LAC
SPD 2 Type 2*
Ligthning rod
<10m
>10m
Yes
No
No need
SPD 3 Type 2
SPD 4 Type 1
SPD 4 Type 2 if Ng>2,5 & overhead line
The number and location of SPD on the DC side depends on the length of the cables between the solar panels and inverter (if this length is>10 metres a second SDP is necessary and located in the box close to the solar panel, the first one is located in the inverter area. To be efficient, SPD connections cables to L+, L- network and also between earth terminal block of SPD and ground busbar must be as short as possible (d1+d2<50cm)
Schneider Electric - Electrical installation guide 2015
Generator
N L
iPRD-DC
1
Generator
Conversion
iPRD-DC
1
N L
iPRD-DC
1
Conversion
To ensure staff safety during maintenance and emergencies disconnect devices should be appropriately located and enclosures installation should be failsafe.
The switch disconnectors on the AC side and DC side of the inverter shall be installed for inverter service and maintenance. As many switch disconnectors should be installed as are needed to allow operation on the PV generator, particularly to replace fuses in the array boxes and generator junction boxes. For PV systems inside buildings, a remotely-controlled switch disconnector should be mounted as closely as possible to the PV modules or to the point of entry of DC cables in the event of an emergency.
Array box
Generator Box
AC Box
= ~
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Main LV switch board
Enclosures for different PV generator boxes and switch boards on the DC side need to ensure double isolation, equipment protection against such outdoor hazards as temperatures, the rain, vandalism, and shock. Enclosure and their auxiliary equipment must ensure temperature and humidity control to allow equipment to operate smoothly. It is, however, difďŹ cult to propose a generic solution. Each installation needs to be analysed in order to optimize the sizing of its enclosures and auxiliary equipment.
Cold
Heat
Thermostat O Fans IP55
Heating resistance
Thermostat F
Humidity
Hygrostat
Heating resistance
Thermal risks and heating / cooling solution shall be studied
Schneider Electric - Electrical installation guide 2015
IEC60364-6 requires initial and periodic verifications of electrical installations. Specificities of photovoltaic installation (outdoor, high DC voltage, unsupervised installation) make periodic checking very important. If usually the efficiency of all the system is checked in order to ensure the maximum production, we recommend to perform periodic maintenance of equipment. PV system operating conditions involve various environmental stresses: wide temperature variations, humidity, and electrical stresses. In order to ensure performances of equipment during all the life cycle of installation particular attention shall be paid to the following: Enclosure integrity (Double isolation IP level) Switchgears operating condition and integrity to evaluate if any overheating has occurred to examine switchgears for the presence of dust, moisture… Visual check of electrical connections Functional test of equipment and auxiliaries Insulation monitoring device test Insulation resistance test
Schneider Electric - Electrical installation guide 2015
A PV array is made up of a number of modules in series or parallel, corresponding to the input characteristics of the inverter. However, since these modules are interconnected, the array is very sensitive to shade or differences in terms of the direction faced. By following a few simple cabling rules, supply can be optimised and any operating problems may be avoided.
If, when installing a PV array on a roof, panels need to face in different directions, it is essential to assemble at least one string per direction and ensure each string is facing in just one direction to ensure optimised supply. Each string must be connected to a specific inverter (or to inputs of a multi-MPPT inverter - see Section 3). If this instruction is not observed, the array will not be damaged but supply will be reduced, thus increasing the time needed for a return on investment.
Besides the risk of destruction of shaded modules within a PV array due to the “hot spot phenomenon” as described in Paragraph 2.2 for which manufacturers have devised solutions, research conducted by the Institut National des Energies Solaires (INES – France’s national institute for solar energy) suggests that shading of 10% of the surface area of a string may cause more than a 30% reduction in output! It is therefore important to eliminate direct shading. However, in many cases this is difficult (trees, chimney, neighbouring wall, pylon, etc.). If a PV array includes several strings: If possible, shaded modules should be included in a single string Otherwise, a technology should be chosen which responds better to diffuse light than direct light
When connecting components, the first precaution to take is to avoid loops in the cabling within strings. Even though direct lightning strikes on arrays are relatively rare, currents induced by lightning are much more common and these currents are particularly destructive where there are large areas of looping. shows how to improve an array including a large loop.
Charge
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Charge
Where photovoltaic installations are connected to the grid and energy is sold, it is necessary to optimise efficiency and reduce installation costs. With this in mind, a relatively high DC operating voltage of between 200 and 500 V is often used for residential applications, with up to 1000 V being used for applications requiring a higher level of power. All the modules in a PV array should be identical (same brand and same type) and selected to supply the same level of power. For example, the modules should all be 180 W, even though there exists other power levels in the same PV modules product range (170 W, 180 W and 190 W). In practice, the protection units (DC and AC units) should be positioned close to the inverters for ease of maintenance.
This is the simplest configuration (see ). It is used for small PV arrays with peak power of up to 3 kWp depending on the modules deployed. In most cases, it is used for residential PV operations.
PV
DC unit
Inverter
AC unit
=
Meters kWh
kWh
~
Modules are connected in series, supplying direct current of between 200 and 500 VDC in this instance. Optimal efficiency is obtained from the inverter within this voltage range. A single DC line is fed through to the inverter. The PV array can be isolated from the inverter by means of a load break switch near the inverter.
This configuration (see ), mainly deployed on buildings or in small PV power plants on the ground, is used for PV installations of up to thirty strings in parallel with power output of some 100 kWp. This limit is imposed for technological and financial reasons. If exceeded, the required width of the main DC cable would be impractical. Direct current can be determined based on the number of modules in series per string and in this instance is between 300 and 600 VDC. By paralleling identical strings, the power required for the installation can be attained. The strings are paralleled in a PV array box. This box includes the safety devices required for paralleling the strings and appliances used to measure the strings’ current. A single DC cable connects these boxes to the inverter. The PV array can be isolated from the inverter by means a load break switch near the inverter.
PV
PV array box
Inverter DC enclosure
Meters AC enclosure
Supply point kWh
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Network kWh
As a variation on this diagram, several single-phase inverters can be installed in a three-phase arrangement (see ).
PV
PV array box DC enclosure
AC enclosure
Inverter
When power levels exceed 50 or 100 kW, photovoltaic arrays are split into subgroups (see ) to make it easier to connect the various components. Strings are paralleled on two levels. Strings in each subgroup are paralleled in subgroup PV array boxes. These boxes are fitted with safety devices, the necessary measuring equipment and monitoring devices. The outputs of these boxes are paralleled in a PV array box near the inverter. This box is also fitted with the required safety devices as well as the measuring and monitoring equipment necessary for paralleling the subgroups. The array can be isolated from the inverter using a load block switch which may or may not be fitted in the PV array box. The array’s direct current is approximately 1000 VDC.
PV
PV array box
DC enclosure
AC enclosure Inverter
Schneider Electric - Electrical installation guide 2015
It is absolutely essential to take account of location (geographic location, latitude, altitude, shade, etc.) and installation factors (direction faced, angle, etc.). Firstly, the approximate power output may be calculated based on the available surface area: 10 m² = 1 kWp 7140 m² (=football ground) = 700 kWp The PV array should always be arranged around the inverter. The calculations involved should compare the characteristics of the modules and those of the inverter with a view to identifying the optimal configuration. String composition: NB: Number of modules x Voc (at t° min) < inverter Vmax The no load voltage of the string (Voc x number of modules in series) at the minimum temperature of the installation location must be lower than the inverter’s maximum input voltage. => This must be strictly observed. Otherwise the inverter may be destroyed. Apart from the aforementioned rule for preventing destruction of the inverter Number of modules x Voc (at t° min) < inverter Vmax – two other limits must be observed: Number of modules x Vmpp (at t° max) > inverter Vmin The operating voltage (Vm x number of modules in series at all temperatures at the installation location) should fall within the inverter’s MPPT voltage range. Otherwise, the inverter will stall and energy supply will cease. Isc strings < inverter I max The total Isc current for strings in parallel must be lower than the maximum input current for the inverter. Otherwise, the inverter limits the supply of energy delivered to the network.
In Europe, the power level of the inverter must be between 0.8 and 1 times the power of the array: 0.8 < Pinverter / Parray < 1 Below this (under 0.8 Parray), the inverter limits power significantly. The energy sold to the network will thus be inferior to that which the panels are capable of supplying and therefore it will take longer to secure a return on investment. Above this (over Parray), the inverter is too large for the power level of the array. Again, it will take longer to secure a return on investment. Single-phase or three-phase A decision should be made over these two options in consultation with the local energy distributor based on the devices available in manufacturers’ product ranges, often within the following limits: Inverter Pn < 10 kW => single phase inverter 10 kW < Pn < 100 kW => either three-phase inverter(s) or single-phase inverters split between the three phases and neutral. The management of unbalances between phases needs to be checked in this instance. Pn > 100 kW => three-phase inverter(s) Configuration software Manufacturers of inverters help design offices and installers to size strings for residential and service sector installations based on the equipment available by supplying sizing software.
Schneider Electric - Electrical installation guide 2015
The installation type is a factor which should not be neglected since, in countries including France, the purchase price for power supplied is dependent on this. Along with shading, it should be taken into account when choosing a module. There are three installation types â&#x20AC;&#x201C; building integrated, partially integrated and ground-based: Building Integrated PhotoVoltaic (BIPV) This installation type fulďŹ ls a dual role (energy supply and roof waterprooďŹ ng, shading, etc.). Partially integrated This is the simplest assembly to install and, most importantly, does not alter the water resistance of a roof. However, its major drawback is that, in France, operators cannot charge the highest rate for it. This installation type is most commonly used in Germany and Switzerland. Ground-based This installation type is used for power supply plants covering large areas (photovoltaic farms). Again, in France it is not eligible for the highest purchase price.
Schneider Electric - Electrical installation guide 2015
Typically, a 5kW grid-connected single-phase inverter. with UOC MAX 600V. One or two strings – Isctc < 25A, IAC < 32A. In this design there is no string protection. A PV main switch is necessary. When the inverter is indoors, an additional remotecontrolled switch at the DC cable entry point is recommended for emergencies services.
E1
E2
Inverter with or without galvanic isolation
E3
= Q1
Q2
Q3
~
SPD 2 Outdoor
Isolation Switching (Making & breaking rated current) Control Over-current protection
SPD 3
Indoor
•
• (d)
(a)
• (d)
• DC21B
• (d) DC21B
(a)
• (d)
• (b)
• (d)
(e)
• (d)
(c)
• (f)
Protection against Insulation fault
(h)
Outdoor Double insulation
(h) RCD type B or A SI
• type 2
• type 1 or 2
Indoor Double insulation
Standard AC requirement + grid code requirement Inverter relevant parameters
Energy
a PV array main switch could be included in the inverter. This solution makes inverter service or replacement more difficult. b Remote switching for emergency services located as closely as possible to the PV modules or to the point of entry of DC cables in the building. c No protection is required when the number of str ing does not exceed 2. d Service and emergency switching e Inverter shall include a protection for anti-islanding (in accordance with VDE 0126 for example) f Overload and short-circuit protection B curve recommended. g This SPD could be unnecessary if there is another SPD in the A C installation at a distance of less than 10 metres. h If the inverter provides no galvanic separation a RCD protection is necessary on AC side. IEC 60364-712 specifies RCD type B Some local regulations require RCD type A SI
Schneider Electric - Electrical installation guide 2015
Typically, 10kW to 36kW grid-connected inverters, UOC MAX probably higher than 600V (i.e. 800V or 1000V), Isctc < 125A, Iac < 63A. In this range of power, inverters usually have between 2 and 4 maximum power point tracking (MPPT) inputs, so the number of strings in the same DC sub-network is equal to one or two. There is no need for string protection. A PV main switch for each MPPT input is necessary. When an inverter is indoors, additional remote-controlled switches at DC cable entry point are recommended for emergencies services.
E1 SPD 11
21
Q11
Q21
Q12
Q22
SPD 12
Inverter with or without galvanic isolation
E2 SPD
E3
= Q3
~
SPD 22
SPD 3 Indoor
Indoor / Outdoor
Isolation Switching (Making & breaking rated current) Control Over-current protection
•
• (d)
(a)
• (d)
• DC21B
• (d) DC21B
(a)
• (d)
• (b)
• (d)
(e)
• (d)
(c)
• (f)
Protection against Insulation fault
(h)
Outdoor IP5x Double insulation
(h) RCD type B or A SI
• type 2
• type 1 or 2
Indoor IP5x Double insulation
Standard AC requirement + grid code requirement Energy
a PV array main switch could be included in the inverter. This solution makes inverter service or replacement difficult. b Remote switching for emergency services located as closely as possible to the PV modules or to the point of entry of DC cables in the building. c No protection is required when the number of str ing does not exceed 2. d Service and emergency switching e Inverter shall include a protection for anti-islanding (in accordance with VDE 0126 for example) f Overload and short-circuit protection (B curve recommended). g If there is no SPD in the inverter or if the distance between DC box and inverter exceeds 10m a SPD is necessary in this box. h If the inverter provides no galvanic separation a RCD protection is necessary on AC side. IEC 60364-712 specifies RCD type B Some local regulations require RCD type A SI If the inverter provides at least simple separation - Without functional earthing: insulation monitoring is necessary, it's usually done by the inverter in this range of power. - With functional earthing: the earthing shall be done with a DC MCB breaker (C60PV 4P series 2 – 10A) or a fuse.
Schneider Electric - Electrical installation guide 2015
Typically, 30kW to 60kW grid-connected inverters. UOC max is generally higher than 600V (up to 1000V), Isctc does not exceed 200A, I AC does not exceed 100A. This design has more than 2 strings. Reverse current protection is therefore necessary. A main PV switch is required. When an inverter is inside, additional remote-controlled switch at DC cable entry point is recommended for emergencies.
Q0
E1 E2
Inverter with or without galvanic isolation
E3
=
Q0 Q1
Q3
Q2
Q0 SPD 1
~
SPD 2
Q0
SPD 3
Indoor Outdoor
Isolation Switching (Making & breaking rated current) Control Over-current protection
•
• (d)
(a)
• (d)
• DC21B
• (d) DC21B
(a)
• (d)
• (b)
• (d)
(e)
• (d)
•
(c)
Protection against Insulation fault
• (f) (h)
Outdoor IP5x Double insulation
(h) RCD type B or A SI
• type 2
• type 1 or 2
Indoor IP5x Double insulation
Standard AC requirement + grid code requirement P,Q, PF, Energy
a PV array main switch could be included in the inverter. This solution makes inverter service or replacement difficult. b Remote switching for emergency services located as closely as possible to the PV modules or to the point of entry of DC cables in the building. The main switch in array box can be equipped with tripping coil and motor mechanism for remote reclosing for that purpose. d Service and emergency switching e Inverter shall include a protection for anti-islanding (in accordance with VDE 0126 for example) f Overload and short-circuit protection (B curve recommended). g If there is no SPD in the inverter or if the distance between DC box and inverter exceeds 10m a SPD is necessary in this box. h If the inverter provides no galvanic separation a RCD protection is necessary on AC side. IEC 60364-712 specifies RCD type B Some local regulations require RCD type A SI If the inverter provides at least simple separation - Without functional earthing: insulation monitoring is necessary - With functional earthing: the earthing shall be done with a DC MCB breaker (C60PV 4P series 2 – 10A) or a fuse.
Schneider Electric - Electrical installation guide 2015
Typically, 6x5 to 20x5kW grid-connected inverters. The design used for residential building can be duplicated as often as necessary. In that case, the DC system is very simple and the AC system is very similar to usual AC systems.
E1
Indoor
=
E3
Q1
~
SPD 1
Q3
Outdoor
E1
=
Q1 SPD 1
E1
~
Q3
~
Q3
~
Q3
~
Q3
~
Q3
=
Q1 SPD 1
E1
=
Q1 SPD 1
E1
Q4
SPD 3
=
Q1 SPD 1
E1
=
Q1 SPD 1
See 5kW design
(h)
• (d)
• type 2
• type 1 or 2
Outdoor IP5x Double insulation
Standard AC requirement + grid code requirement Energy
P,Q, PF, Energy, unbalance
h If the inverter provides no galvanic separation a RCD protection is necessary on AC side. IEC 60364-712 specifies RCD type B Some local regulations require RCD type A SI
Schneider Electric - Electrical installation guide 2015
Typically, 60kW to 100kW grid-connected inverters with 2 arrays. Array cable protection is not necessary for 2 or 3 arrays. The Isctc array 200A, Isctc 400A, and Imax AC 200A. A PV main switch is required close to the inverter. Remotely operated switches in array boxes allow disconnects to be located close to the PV modules in the event of emergencies.
E1 Q0
E2 Q1
Q0
Inverter with or without galvanic isolation
SPD 1
= Q3
Q2
Outdoor
~
SPD 2
Q0
E3
SPD 3
Q1
Q0
SPD 1
Indoor
Outdoor
Isolation Switching (Making & breaking rated current) Control Over-current protection
•
• (d)
(a)
• (d)
• DC21B
• (d) DC21B
(a)
• (d)
• (b)
• (d)
(e)
• (d)
•
(c)
Protection against Insulation fault
• (f) (h)
Outdoor IP5x Double insulation
(h) RCD type B or A SI
• type 2
• type 1 or 2
Indoor IP5x Double insulation
Standard AC requirement + grid code requirement P,Q, PF, Energy
a PV array main switch could be included in the inverter. This solution makes inverter service or replacement difficult. b If switching for emergency services is required, the main switch in array box can be equipped with tripping coil and motor mechanism for remote reclosing. c No protection is required when the number of arr ays <= 3 (No cable sizing benefit) d Service and emergency switching e Inverter shall include a protection for anti-islanding (in accordance with VDE 0126 for example) f Overload and short-circuit protection. g If there is no SPD in the inverter or if the distance between DC box and inverter exceeds 10m a SPD is necessary in this box. h If the inverter provides no galvanic separation a RCD protection is necessary on AC side. IEC 60364-712 specifies RCD type B Some local regulations require RCD type A SI If the inverter provides at least simple separation - Without functional earthing: insulation monitoring is necessary - With functional earthing: the earthing shall be done with a DC MCB breaker (C60PV 4P series 2 – 10A) or a fuse.
Schneider Electric - Electrical installation guide 2015
Typically, 150kW to 500kW single inverter. This design is very similar to the previous one except that it has more arrays, which requires array cable protection. Istc <= 400A, IAC <= 600A.
Q0
Q0
SPD 0
= Q2
Outdoor Q0
Isolation
•
Switching (Making & breaking rated current) Control Over-current protection
Q21 SPD 1
Q3
~
SPD 3
Q2
Q1
Q0
Inverter without galvanic isolation
Q2
Q1
Indoor
SPD 0
•
•
• (a)
• (d)
• DC22A
• DC22A
• (a)
• (d)
• (a)
• (d)
• (h)
• (h)
• (b) •
• (c )
Protection against Insulation fault
• (f)
(g)
• type 2
• type 1 or 2
Outdoor IP5x Double insulation
Indoor Double insulation
Standard AC requirement + grid code requirement
Energy
P,Q, PF, Energy, Alarm, THD, individual harmonics
a PV array main switch could be included in the inverter. This solution makes inverter service or replacement more difficult. b If switching for emergency services is required, the main switch in array box can be equipped with tripping coil and motor mechanism for remote reclosing. c Array cable protection is recommended to prevent cable overszing. To ensure fast trip of protections 6 to 8 arrays are recommended. e Inverter shall include a protection for anti-islanding (in accordance with VDE 0126 for example) f Overload and short-circuit protection. g If there is no SPD in the inverter or if the distance between DC box and inverter exceeds 10m a SPD is necessary in this box. h Galvanic insulation is provided by LV/MV transformer, PV system without functional earthing: insulation monitoring is necessary: IMD - IM20 and accessory IMD-IM20-1700 PV system With functional earthing: the earthing shall be done with a DC MCB breaker (C60PV 4P series 2 – 10A) or a fuse.
Schneider Electric - Electrical installation guide 2015
Typically 10x20 to 20x30kW grid connected inverters Uoc max <=1000V one or two string per inverter. IAC max 50A for one inverter.
E1
SPD 11
Indoor
=
Q11 Q12
~
=
SPD 12
~
= ~
=
E3 Q5
Q5
Q5
Q6
~
= ~
= ~
= E1
SPD 11
Q11
~
=
Q12
~
SPD 12
Q5
Q5
SPD 3
Q5
Q5
Q5
See 10 to 36kW design • type 2
• type 1 or 2
Outdoor IP5x Double insulation
Standard AC requirement + grid code requirement Energy
Schneider Electric - Electrical installation guide 2015
P,Q, PF, Energy, Alarm,
Typically 500kW -630kW inverters with LV/MV transformers and MV substation.
Q0
Q2
Q1
Q0
SPD 0
= Q3 b
Q0
SPD 1
Isolation
•
Switching (Making & breaking rated current) Control Over-current protection
Q5
~ SPD 3
SPD 2
Q2
Q1
Q0
Q4
SPD 0
2nd inverter similar scheme
Indoor
•
• (a)
See next page
•
• DC22A
• (a)
See next page
•
• (b) •
• (c )
Protection against Insulation fault
• (h) (g)
• type 2
Outdoor IP5x Double insulation
Indoor Double insulation
Energy
See next page
•
See next page
• (f)
See next page (g)
• (h) • type 1 or 2
P,Q, PF, Energy, Alarm, Power quality
a PV array main switch is usually included in the inverter panel. b If switching for emergency services is required, the main switch in array box can be equipped with tripping coil and motor mechanism for remote reclosing. c Array cable protection is recommended to prevent cable overszing. To ensure fast trip of protections 6 to 8 arrays are recommended. f Overload and short-circuit protection. g If there is no SPD in the inverter or if the between DC box and inverter >10m a SPD is necessary in this box. h Galvanic insulation is provided by LV/MV transformer, PV system without functional earthing: insulation monitoring is necessary: IMD - IM20 and accessory IMD-IM20-1700 PV system With functional earthing: the earthing shall be done with a DC MCB breaker (C60PV 4P series 2 – 10A) or a fuse.
Schneider Electric - Electrical installation guide 2015
Since the profitability of photovoltaic installations depends mainly on operational uptime, it is essential to ensure that they are permanently up and running. The best way of ensuring this is to install a monitoring system covering key equipments of the installation. This system should notify all faults immediately, be capable of detecting drifts in output, and possibly control equipment remotely.
Several types of monitoring systems are available for installations, depending mainly of the size of the installation. Systems for Residential up to commercial, 1 to 1000 kWp, are able to monitor the inverters – status, measurements and alarms - and key electrical values related to the output of the installation. These systems are based on a data -logger, mostly equipped with a RS232/485 serial port to communicate with the inverters, using Modbus or a proprietary protocol. Data acquisition is based on low speed polling rate, every 10 minutes in average. Data may be stored locally in the data-logger, for free, but for a short period of time, or pushed to an external server which store the data over the years and deliver a front end, providing an annual service fee. In that case, the communication with the distant server can be or via GPRS, or via Ethernet The data-logger can also be equipped with auxiliary inputs, such as analogue inputs to monitor temperature or irradiance sensors, digital input to monitor the status of an equipment and/or pulse input to connect with an energy meter equipped with digital output. Systems for large commercial up to Utility scale power plant, from 500 kWp upwards, are able to monitor the complete installation, from the string input to the point of connection to the grid. These systems are based on a SCADA (Supervision Control And Data Acquisition) system, which enable multi site monitoring, DC & AC measurements, remote control of motorized equipments, smart alarming, generation of reports, performance indication and other capabilities such as in-depth analysis. These systems also include other equipment to run the site more efficiently, such as weather station ( temperatures, wind rain gauge ), irradiance sensors, a plant controller - device which communicate with the grid operator, to adapt the production of the site to the grid variation ( Voltage, Power Factor ) – and specific meters such as revenue grade meters, close the popint of connection These scada systems can be local and/or remote, with redundancy capabilities and high performance for data processing. This type of installation is mostly served by a Service contract for Operations & Maintenance and in many cases, with performance objectives which can be production, performance ratio or availability.
Schneider Electric - Electrical installation guide 2013
These systems may be autonomous or include remote monitoring, accessible from the "cloud".
Large screen display
SMS
Files
GSM GPRS
SMTP
FTP
Data logger
Sensors
=
=
~ Inverter 1
=
~ Inverter 2
~ Inverter 3
Once the data is collected locally, the system sends output data and alerts as soon as they are generated to a remote monitoring system capable of managing stand-by periods for maintenance work. This enables the installation to be monitored closely, which is essential where operators of photovoltaic installations are not necessarily the site occupants.
Remote
Site 2 Site 1 Large screen display and control in a control room
On-site server
Router
TCP/IP Ethernet network
SCADA RS485 - TCP/IP
Sensors
=
~ Equipment status, measurements Inverter 1
Ethernet TCP/IP network
=
=
~
~
Inverter 2
Inverter 3
Schneider Electric - Electrical installation guide 2013
Operation & Maintenance Remote monitoring system
Sensors provide data to the monitoring systems and include: A sensor for measuring instantaneous luminous flux such as a pyranometer (heat flow sensor used to measure the quantity of solar energy in natural light (W/m²), see ). This is the standard reference for the installation. It may be used to identify shifts over time and is recommended to all suppliers wishing to conduct comparative analyses and compile statistics for their installations. A temperature sensor – this is an important factor for photovoltaic power supply (see Paragraph 2.1). This sensor either serves as an external probe or is attached to the back of a module. A kilowatt hour meter When selling power, only the kilowatt hour meter operated by the energy distributor purchasing the electricity may be used as a reference. The other meters fitted within an installation (in the inverter or next to the official meter) are only indicators with their own specific levels of accuracy. Variations of more than 10% may occur between the values given by an installation’s devices and that given by the official meter. However, these variations are not only due to different levels of accuracy. They are also caused by energy lost in the cables and safety devices downstream from the inverter. It is therefore important to use cables of minimal length and clearly identify: The location where the installation will be connected to the network The locations where the energy distributor’s meters will be connected
Since modules are expensive and in some cases openly accessible, sites need to be monitored by security cameras. NB – although this type of surveillance is authorised for private sites, filming of public highways is prohibited.
Schneider Electric - Electrical installation guide 2013
Schneider Electric - Electrical installation guide 2013
1.1 General
Q2
1.2 Distribution boards components
Q2
1.3 Protection of people
Q4
1.4 Circuits
Q6
1.5 Protection against overvoltages and lightning
Q7
2.1 ClassiďŹ cation of zones
Q8
2.2 Equipotential bonding
Q11
2.3 Requirements prescribed for each zone
Q11
Schneider Electric - Electrical installation guide 2015
Most countries have national regulations and-or standards governing the rules to be strictly observed in the design and realization of electrical installations for residential and similar premises. The relevant international standard is the publication IEC 60364.
The vast majority of power distribution utilities connect the low voltage neutral point of their MV/LV distribution transformers to earth. The protection of persons against electric shock therefore depends, in such case, on the principle discussed in chapter F. The measures required depend on whether the TT, TN or IT scheme of earthing is adopted. RCDs are essential for TT and IT earthed installations. For TN installations, high speed overcurrent devices or RCDs may provide protection against direct contact of the electrical circuits. To extend the protection to flexible leads beyond the fixed socket outlets and to ensure protection against fires of electrical origin RCDs shall be installed.
(see
)
Distribution boards (generally only one in residential premises) usually include the meter(s) and in some cases (notably where the supply utilities impose a TT earthing system and/or tariff conditions which limit the maximum permitted current consumption) an incoming supply differential circuit-breaker which includes an overcurrent trip. This circuit-breaker is freely accessible to the consumer.
Enclosure
Service connection Distribution board Lightning protection
Incoming-supply circuit breaker Combi surge arrester
Overcurrent protection and isolation Protection against direct and indirect contact, and protection against fire Remote control
MCB phase and neutral
Differential MCB
Differential load switch
Remote control switch TL 16 A
Energy management Programmable thermostat THP
Load shedding switch CDSt
Programmable time switch IHP
Contactors, off-peak or manual control CT
Schneider Electric - Electrical installation guide 2015
On installations which are TN earthed, the supply utilities usually protect the installation simply by means of sealed fuse cut-outs immediately upstream of the meter(s) (see ). The consumer has no access to these fuses.
Meter
Fuse …
or …
Circuit breaker depending on earthing system
(see
Distribution board
)
The consumer is allowed to operate this CB if necessary (e.g to reclose it if the current consumption has exceeded the authorized limit; to open it in case of emergency or for isolation purposes). The rated residual current of the incoming circuit-breaker in the earth leakage protection shall be 300 mA. If the installation is TT, the earth electrode resistance shall be less than 50 V 166 . In practice, the earth electrode resistance of a new installation 300 mA R ( ).. shall be less than 80 2 R
(see
)
This board comprises: A control panel for mounting (where appropriate) the incoming supply circuitbreaker and other control auxiliaries, as required A distribution panel for housing 1, 2 or 3 rows (of 24 Acti 9 units) or similar MCBs or fuse units, etc. Installation accessories for fixing conductors, and rails for mounting MCBs, fuses bases, etc, neutral busbar and earthing bar, and so on Service cable ducts or conduits, surface mounted or in cable chases embedded in the wall to facilitate future modifications to the installation, it is recommended to keep all relevant documents (photos, diagrams, characteristics, etc.) in a suitable location close to the distribution board. The board should be installed at a height such that the operating handles, indicating dials (of meters) etc., are between 1 metre and 1.80 metres from the floor (1.30 metres in situations where handicapped or elderly people are concerned).
The installation of lightning arresters at the service position of a LV installation is strongly recommended for installations which include sensitive (e.g electronic) equipment. These devices must automatically disconnect themselves from the installation in case of failure or be protected by a MCB. In the case of residential installations, the use of a 300 mA differential incoming supply circuit-breaker type S (i.e slightly timedelayed) will provide effective earth leakage protection, while, at the same time, will not trip unnecessarily each time a lightning arrester discharges the current (of an overvoltage-surge) to earth.
In the case where the resistance to earth exceeds 80 , one or several 30 mA RCDs should be used in place of the earth leakage protection of the incoming supply circuit-breaker.
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On TT earthed systems, the protection of persons is ensured by the following measures: Protection against indirect contact hazards by RCDs (see ) of medium sensitivity (300 mA) at the origin of the installation (incorporated in the incoming supply circuit-breaker or, on the incoming feed to the distribution board). This measure is associated with a consumer installed earth electrode to which must be connected the protective earth conductor (PE) from the exposed conductive parts of all class insulated appliances and equipment, as well as those from the earthing pins of all socket outlets When the CB at the origin of an installation has no RCD protection, the protection of persons shall be ensured by class II level of insulation on all circuits upstream of the first RCDs. In the case where the distribution board is metallic, care shall be taken that all live parts are double insulated (supplementary clearances or insulation, use of covers, etc.) and wiring reliably fixed Obligatory protection by 30 mA sensitive RCDs of socket outlet circuits, and circuits feeding bathroom, laundry rooms, and so on (for details of this latter obligation, refer to clause 3 of this chapter)
300 mA
30 mA
Diverse circuits
Socket-outlets circuit
30 mA
Bathroom and/or shower room
In this case: An insulation fault to earth could result in a shutdown of the entire installation Where a lightning arrester is installed, its operation (i.e. discharging a voltage surge to earth) could appear to an RCD as an earth fault, with a consequent shutdown of the installation Incoming supply circuit-breaker with 300 mA differential and High sensitivity 30 mA RCD (for example differential circuit-breaker 1P + N type Declic Vigi) on the circuits supplying socket outlets High sensitivity 30 mA RCD (for example differential load switch type ID’clic) on circuits to bathrooms, shower rooms, laundry rooms, etc. (lighting, heating, socket outlets)
This type of CB affords protection against fault to earth, but by virtue of a short time delay, provides a measure of discrimination with downstream instantaneous RCDs. Tripping of the incoming supply CB and its consequences (on deep freezers, for example) is thereby made less probable in the event of lightning, or other causes of voltage surges. The discharge of voltage surge current to earth, through the surge arrester, will leave the type S circuit-breaker unaffected.
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(see ) Incoming supply circuit-breaker with 300 mA differential type S and High sensitivity 30 mA RCD (for example differential circuit-breaker 1P + N type Declic Vigi) on the circuits supplying washing machines and dish-washing machine High sensitivity 30 mA RCD (for example differential load switch type IDâ&#x20AC;&#x2122;clic) on circuits to bathrooms, shower rooms, laundry rooms, etc. (lighting, heating, socket outlets)
300 mA - type S
30 mA
30 mA
30 mA
1 Diverse High-risk location Socketcircuits (laundry room) outlet circuit
Bathroom and/or shower room
2
5
300 mA
3
4
30 mA
30 mA
30 mA
Bathroom and/or shower room Socket-outlet circuit Diverse circuits
High-risk circuit (dish-washing machine)
In this case the protection of persons must be ensured by: Class II level of insulation up to the downstream terminals of the RCDs All outgoing circuits from the distribution board must be protected by 30 mA or 300 mA RCDs according to the type of circuit concer ned as discussed in chapter F. Where a voltage surge arrester is installed upstream of the distribution board (to protect sensitive electronic equipment such as microprocessors, videocassette recorders, TV sets, electronic cash registers, etc.) it is imperative that the device automatically disconnects itself from the installation following a rare (but always possible) failure. Some devices employ replaceable fusing elements; the recommended method however as shown in , is to use a circuit-breaker. Figure P7 refers: 1. Incoming-supply circuit-breaker without differential protection 2. Automatic disconnection device (if a lightning arrester is installed) 3. 30 mA RCD (for example differential circuit-breaker 1P + N type Declic Vigi) on each circuit supplying one or more socket-outlets 4. 30 mA RCD (for example differential load swith type IDâ&#x20AC;&#x2122;clic) on circuits to bathrooms and shower rooms (lighting, heating and socket-outlets) or a 30 mA differential circuit-breaker per circuit 5. 300 mA RCD (for example differential load swith) on all the other circuits
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National standards commonly recommend the subdivision of circuits according to the number of utilization categories in the installation concerned (see ): At least 1 circuit for lighting. Each circuit supplying a maximum of 8 lighting points At least 1 circuit for socket-outlets rated 10/16 A, each circuit supplying a maximum of 8 sockets. These sockets may be single or double units (a double unit is made up of two 10/16 A sockets mounted on a common base in an embedded box, identical to that of a single unit 1 circuit for each appliance such as water heater, washing machine, dish-washing machine, cooker, refrigerator, etc. Recommended numbers of 10/16 A (or similar) socket-outlets and ďŹ xed lighting points, according to the use for which the various rooms of a dwelling are intended, are indicated in
Socketoutlets
Lighting
Heating
Washing Cooking machine apparatus
Living room 1 5 Bedroom, lounge, 1 3 bureau, dining room Kitchen 2 4 (1) Bathroom, shower room 2 1 or 2 Entrance hall, box room 1 1 WC, storage space 1 Laundry room 1 (1) Of which 2 above the working surface and 1 for a specialized circuit: in addition an independent socket-outlet of 16 A or 20 A for a cooker and a junction box or socket-outlet for a 32 A specialized circuit
IEC and most national standards require that each circuit includes a protective conductor. This practice is strongly recommended where class I insulated appliances and equipment are installed, which is the general case. The protective conductors must connect the earthing-pin contact in each socketoutlet, and the earthing terminal in class I equipment, to the main earthing terminal at the origin of the installation. Furthermore, 10/16 A (or similarly sized) socket-outlets must be provided with shuttered contact oriďŹ ces. (see
)
The c.s.a. of conductors and the rated current of the associated protective device depend on the current magnitude of the circuit, the ambient temperature, the kind of installation, and the inďŹ&#x201A;uence of neighbouring circuits (refer to chapter G) Moreover, the conductors for the phase wires, the neutral and the protective conductors of a given circuit must all be of equal c.s.a. (assuming the same material for the conductors concerned, i.e. all copper or all aluminium).
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indicates the c.s.a. required for commonly-used appliances Protective devices 1 phase + N in 2 x 9 mm spaces comply with requirements for isolation, and for marking of circuit current rating and conductor sizes.
Fixed lighting
1.5 mm2 (2.5 mm2)
2,300 W
Circuit-breaker Fuse
16 A 10 A
10/16 A
2.5 mm2 (4 mm2)
4,600 W
Circuit-breaker Fuse
25 A 20 A
2.5 mm2 (4 mm2)
4,600 W
Circuit-breaker Fuse
25 A 20 A
Dish-washing machine
2.5 mm2 (4 mm2)
4,600 W
Circuit-breaker Fuse
25 A 20 A
Clothes-washing machine
2.5 mm2 (4 mm2)
4,600 W
Circuit-breaker Fuse
25 A 20 A
Cooker or hotplate (1)
6 mm2 (10 mm2)
7,300 W
Circuit-breaker Fuse
40 A 32 A
Electric space heater
1.5 mm2 (2.5 mm2)
2,300 W
Circuit-breaker Fuse
16 A 10 A
Individual-load circuits Water heater
(1) In a 230/400 V 3-phase circuit, the c. s. a. is 4 mm 2 for copper or 6 mm 2 for aluminium, and protection is provided by a 32 A circuit-breaker or by 25 A fuses.
The choice of surge arrester is described in chapter J
Three principal rules must be respected: 1 - It is imperative that the three lengths of cable used for the installation of the surge arrester each be less than 50 cm i.e.: the live conductors connected to the isolating switch from the isolating switch to the surge arrester from the surge arrester to the main distribution board (MDB) earth bar (not to be confused with the main protective-earth (PE) conductor or the main earth terminal for the installation.The MDB earth bar must evidently be located in the same cabinet as the surge arrester. 2 - It is necessary to use an isolating switch of a type recommended by the manufacturer of the surge arrester. 3 - In the interest of a good continuity of supply it is recommended that the circuit-breaker be of the time-delayed or selective type.
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Bathrooms and showers rooms are areas of high risk, because of the very low resistance of the human body when wet or immersed in water. Precaution to be taken are therefore correspondingly rigorous, and the regulations are more severe than those for most other locations. The relevant standard is IEC 60364-7-701. Precautions to observe are based on three aspects: The deďŹ nition of zones, numbered 0,1, 2, 3 in which the placement (or exclusion) of any electrical device is strictly limited or forbidden and, where permitted, the electrical and mechanical protection is prescribed The establishment of an equipotential bond between all exposed and extraneous metal parts in the zones concerned The strict adherence to the requirements prescribed for each particular zones, as tabled in clause 3
Sub-clause 701.32 of IEC 60364-7-701 deďŹ nes the zones 0, 1, 2, 3 as shown in the following diagrams (see below to opposite and next pages):
Zone 1*
Zone 1* Zone 2
Zone 3
Zone 0
Zone 2
Zone 3
Zone 0 0.60 m
2.40 m
2.40 m 0.60 m
Zone 1
Zone 2
Zone 3
2.25 m Zone 1
Zone 0
0.60 m
2.40 m
(*) Zone 1 is above the bath as shown in the vertical cross-section
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Zone 0 Zone 1
Zone 2
Zone 0 Zone 1
Zone 3
Zone 2
Zone 3
2.40 m
0.60 m
2.40 m 0.60 m
Zone 2
Zone 1
Zone 3
2.25 m Zone 1 Zone 0
0.60 m
2.40 m
Fixed shower head (1)
Fixed shower head (1)
0.60 m Zone 1 0.60 m Zone 2
0.60 m Zone 1 0.60 m Zone 2 Zone 3
Zone 1
2.40 m
Zone 2
2.40 m
Zone 3
Zone 3
2.25 m
(1) When the shower head is at the end of a flexible tube, the vertical central axis of a zone passes through the fixed end of the flexible tube
0.60 m Prefabricated shower cabinet 0.60 m
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Classes of external influences AD 3 BB 2 BC 3
Classes of external influences AD 3 BB 2 BC 3
Zone 3 Dressing cubicles (zone 2)
AD 3 BB 3 BC 3 AD 7 BB 3 BC 3
AD 3 WC BB 2 BC 3
Shower cabinets (zone 1)
Classes of external influences h < 1.10m AD 5 1.10m < h < 2.25m AD 3 BB 3 BC 3 AD 7 BB 3 BC 3
Classes of external influences h < 1.10m AD 5 1.10m < h < 2.25m AD 3 BB 3 BC 3
Dressing cubicles
Zone 2
Zone 1
WC
Classes of external influences
Classes of external influences
AD 3 BB 2 BC 3
h < 1.10m AD 5 1.10m < h < 2.25m AD 3 BB 3 BC 3
AD 3 BB 2 BC 3
h < 1.10m AD 5 1.10m < h < 2.25m AD 3 BB 3 BC 3
Dressing room Zone 2
Zone 2
Zone 1
AD 7 BB 3 BC 3
: Classes of external inï¬&#x201A;uences (see Fig.Q46).
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(see
)
To the earth electrode
Metallic pipes h 2m
Water-drainage piping
Socket-outlet
Gaz Radiator
Lighting
Metal bath
Equipotential conductors for a bathroom
Metal door-frame
The table of clause 3 describes the application of the principles mentioned in the foregoing text and in other similar or related cases
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below summarizes the main requirements prescribed in many national and international standards. : Section in brackets refer to sections of IEC60364-7 (example: "section 701" means IEC60364-7-701)
Domestic dwellings and other habitations
Bathrooms or shower rooms (section 701) Zone 0
TT or TN-S systems 20 Differential protection 300 mA if the earth electrode resistance is 80 ohms instantaneous or short time delay (type S) 30 mA if the earth electrode resistance is 500 ohms surge arrester at the origin of the installation if supply is from overhead line with bare conductors, and if the keraunic level > 25 a protective earth (PE) conductor on all circuits Supplementary equipotential bonding in zones 0, 1, 2 and 3 SELV 12 V only 27
Zone 1
SELV 12 V
25
Zone 2
SELV 12 V or 30 mA RCD
24
Zone 3
Swimming baths (section 702) Zone 0
Switch operating handles Protection by and similar devices on 30 mA RCDs distribution panels, to be mounted between 1 metre and 1.80 metre above the ďŹ&#x201A;oor
Class II limited to strict minimum Class II limited to strict minimum Class II limited to strict minimum
21
Supplementary equipotential bonding in zones 0, 1, and 2 SELV 12 V
28
Special aplliances Water heater Special appliances Water heater Class II luminaires Only socket-outlets protected by : 30 mA RCD or Electrical separation or SELV 50 V
Class II limited to strict minimum Class II limited to strict minimum
Zone 1
25
Zone 2
22 (indoor) 24
Saunas (section 703) Work sites (section 704) Agricultural and horticultural establishments (section 705) Restricted conductive locations (section 706)
24
Class II
44
Mechanically protected
Conventional voltage limit U L reduced to 25 V Conventional voltage limit U L reduced to 25 V Protection against ďŹ re risks by 500 mA RCDs
Special appliances
Special appliances
Special appliances
Only socket-outlets protected by : 30 mA RCD or electrical separation or SELV 50 V
35
2x
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Adapted to temperature Protection by 30 mA RCDs Protection by 30 mA RCDs
Protection of: Portable tools by: SELV or Electrical separation Hand-held lamps By SELV Fixed equipement by SELV Electrical separation 30 mA RCDs Special supplementary equipotential bonding
Fountains (section 702)
Protection by 30 mA RCDs and equipotential bonding of all exposed and extraneous conductive parts
Caravan park (section 708)
55
Marinas and pleasure craft (section 709)
The cable length for connection to pleasure craft must not exceeded 25 m
Medical locations Group 2 : Operating theatres and similar (section 710)
IT medical system equipotential grouding, limited to one operating theatre and not exceeding 10 kVA
Medical locations TT or TNS Group 1 : Hospitalization and similar (section 710) Exhibitions, shows and TT or TN-S systems stands (section 711) Balneotherapy Individual: see section 701 (cure-centre baths) (volumes 0 and 1) Collective: see section 702 (volumes 0 and 1) Motor-fuel ďŹ lling Explosion risks in security zones stations Motor vehicules Protection by RCDs or by electrical separation External lighting installations (section 714) Mobile or transportable The use of TN-C system is not units (section 717) permitted inside any unit
Flexible cable of 25 metres length
Only magnetic protection for the primary of LV/LV transformer. Monitoring of secondary loads and transformer temperature
Socket-outlets shall be placed at a height of 0.80 m to 1.50 m from the ground. Protection of circuits by 30 mA RCDs (one per 6 socket-outlets) Protection of circuits by 30 mA RCDs (one per 6 socket-outlets) Protection of circuits by thermal-magnetic protection only. One to three per circuit.
Protection by 30 mA RCDs
4x
Protection by 30 mA RCDs
Limited to the necessary minimum
23
Protection by 30 mA RCDs 30 mA RCDs must be used for all socket-outlets supplying equipment outside the unit
Schneider Electric - Electrical installation guide 2015
Schneider Electric - Electrical installation guide 2015
3.1 Equipotential bonding inside and outside buildings 3.2 Improving equipotential conditions 3.3 Separating cables 3.4 Raised ďŹ&#x201A;oors 3.5 Cable running 3.6 Busway 3.7 Implementation of shielded cables 3.8 Communication networks 3.9 Implementation of surge arresters 3.10 Cabinet cabling (Fig. R20) 3.11 Standards
R5 R5 R7 R7 R8 R11 R11 R13 R15 R15 R18
3.12 Electrostatic discharge protection
R18
4.1 General 4.2 Common-mode impedance coupling
R20 R21
4.3 Capacitive coupling
R22
4.4 Inductive coupling
R23
4.5 Radiated coupling
R24
5.1 Signal classes (see Fig. R37) 5.2 Wiring recommendations
R26 R26
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The system earthing arrangement must be properly selected to ensure the safety of life and property. The behaviour of the different systems with respect to EMC considerations must be taken into account. below presents a summary of their main characteristics. European standards (see EN 50174-2, EN 50310 and HD 60364-4-444) recommend the TN-S eathing system which causes the fewest EMC problems for installations comprising information-technology equipment (including telecom equipment).
Safety of persons
Safety of property
Availability of energy EMC performance
Correct Use of residual current devices (circuit breaker) is mandatory Correct Medium ground-fault current (< about ten amps) Correct Correct - Risk of overvoltages - Equipotential problems - Need to manage devices with high leakage currents
Correct Continuity of the PE conductor must be ensured throughout the installation
Poor High ground-fault current (about 1 kA)
Correct Excellent - Good equipotential situation - Need to manage devices with high leakage currents - High ground-fault currents (transient disturbances)
Correct Low ground-fault current for ďŹ rst fault (< about ten mA), but high for second fault Excellent Poor (to be avoided) - Risk of overvoltages - Common-mode ďŹ lters and surge arrestors must handle the phaseto-phase voltages - RCDs subject to nuisance tripping if common-mode capacitors are present - Equivalent to TN system for second fault
Poor High ground-fault current (about 1 kA)
Correct Poor (not recommended) - Neutral and PE are combined - 50/60 Hz and harmonics currents circulate in the earthing and grounding structures - High ground-fault currents (transient disturbances)
When an installation includes high-power equipment (motors, air-conditioning, lifts, power electronics, etc.), it is advised to install one or more transformers speciďŹ cally for these systems. Electrical distribution must be organised in a star system and all outgoing circuits must exit the main low-voltage switchboard (MLVS). Electronic systems (control/monitoring, regulation, measurement instruments, etc.) must be supplied by a dedicated transformer in a TN-S system. below illustrates these recommendations.
Transformer
Disturbing Sensitive devices devices
Disturbing Sensitive devices devices
Not recommended
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Preferable
Disturbing devices
Sensitive devices
Excellent
This section deals with the earthing and equipotential bonding of information-technology devices and other similar devices requiring interconnections for signalling purposes.
Earthing networks are designed to fulfil a number of functions. They can be independent or operate together to provide one or more of the following: Safety of persons with respect to electrical hazards Protection of equipment with respect to electrical hazards A reference value for reliable, high-quality signals Satisfactory EMC performance The system earthing arrangement is generally designed and installed in view of obtaining a low impedance capable of diverting fault currents and HF currents away from electronic devices and systems. There are different types of system earthing arrangements and some require that specific conditions be met. These conditions are not always met in typical installations. The recommendations presented in this section are intended for such installations. For professional and industrial installations, a common bonding network (CBN) may be useful to ensure better EMC performance with respect to the following points: Digital systems and new technologies Compliance with the EMC essential requirements of Directive 2004/108/EC (emission and immunity) The wide number of electrical applications A high level of system safety and security, as well as reliability and/or availability For residential premises, however, where the use of electrical devices is limited, an isolated bonding network (IBN) or, even better, a mesh IBN may be a solution. It is now recognised that independent, dedicated earth electrodes, each serving a separate earthing network, are a solution that is not acceptable in terms of EMC, but also represent a serious safety hazard. In certain countries, the national building codes forbid such systems. Use of a separate “clean” earthing network for electronics and a “dirty” earthing network for energy is not recommended in view of obtaining correct EMC, even when a single electrode is used (see and ). In the event of a lightning strike, a fault current or HF disturbances as well as transient currents will flow in the installation. Consequently, transient voltages will be created and result in failures or damage to the installation. If installation and maintenance are carried out properly, this approach may be dependable (at power frequencies), but it is generally not suitable for EMC purposes and is not recommended for general use.
lightning rods "Clean" earthing network
Electrical earthing network
Separate earth electrodes
lightning rods "Clean" earthing network
Electrical earthing network
Single earth electrode
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The recommended configuration for the earthing network and electrodes is two or three dimensional (see ). This approach is advised for general use, both in terms of safety and EMC. This recommendation does not exclude other special configurations that, when correctly maintained, are also suitable.
Equipotential bonding required for multi-level buildings lightning rods "Electrical" and "communication" earthing as needed
Multiple interconnected earth electrodes
In a typical installation for a multi-level building, each level should have its own earthing network (generally a mesh) and all the networks must be both interconnected and connected to the earth electrode. At least two connections are required (built in redundancy) to ensure that, if one conductor breaks, no section of the earthing network is isolated. Practically speaking, more than two connections are made to obtain better symmetry in current flow, thus reducing differences in voltage and the overall impedance between the various levels in the building. The many parallel paths have different resonance frequencies. If one path has a high impedance, it is most probably shunted by another path with a different resonance frequency. On the whole, over a wide frequency spectrum (dozens of Hz and MHz), a large number of paths results in a low-impedance system (see ). Each room in the building should have earthing-network conductors for equipotential bonding of devices and systems, cableways, trunking systems and structures. This system can be reinforced by connecting metal pipes, gutters, supports, frames, etc. In certain special cases, such as control rooms or computers installed on false floors, ground reference plane or earthing strips in areas for electronic systems can be used to improve earthing of sensitive devices and protection interconnection cables.
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The fundamental goals of earthing and bonding are the following: Safety By limiting the touch voltage and the return path of fault currents EMC By avoiding differences in potential and providing a screening effect. Stray currents are inevitably propagated in an earthing network. It is impossible to eliminate all the sources of disturbances for a site. Earth loops are also inevitable. When a magnetic field affects a site, e.g. the field created by lightning, differences in potential are created in the loops formed by the various conductors and the currents flowing in the earthing system. Consequently, the earthing network is directly affected by any counter-measures taken outside the building. As long as the currents flow in the earthing system and not in the electronic circuits, they do no damage. However, when earthing networks are not equipotential, e.g. when they are star connected to the earth electrode, the HF stray currents will flow wherever they can, including in control wires. Equipment can be disturbed, damaged or even destroyed. The only inexpensive means to divide the currents in an earthing system and maintain satisfactory equipotential characteristics is to interconnect the earthing networks. This contributes to better equipotential bonding within the earthing system, but does not remove the need for protective conductors. To meet legal requirements in terms of the safety of persons, sufficiently sized and identified protective conductors must remain in place between each piece of equipment and the earthing terminal. What is more, with the possible exception of a building with a steel structure, a large number of conductors for the ligthning rods or the lightningprotection network must be directly connected to the earth electrode. The fundamental difference between a protective conductor (PE) and a ligthning rod down-conductor is that the first conducts internal currents to the neutral of the MV/LV transformer whereas the second carries external current (from outside the installation) to the earth electrode. In a building, it is advised to connect an earthing network to all accessible conducting structures, namely metal beams and door frames, pipes, etc. It is generally sufficient to connect metal trunking, cable trays and lintels, pipes, ventilation ducts, etc. at as many points as possible. In places where there is a large amount of equipment and the size of the mesh in the bonding network is greater than four metres, an equipotential conductor should be added. The size and type of conductor are not of critical importance. It is imperative to interconnect the earthing networks of buildings that have shared cable connections. Interconnection of the earthing networks must take place via a number of conductors and all the internal metal structures of the buildings or linking the buildings (on the condition that they are not interrupted). In a given building, the various earthing networks (electronics, computing, telecom, etc.) must be interconnected to form a single equipotential bonding network. This earthing-network must be as meshed as possible. If the earthing network is equipotential, the differences in potential between communicating devices will be low and a large number of EMC problems disappear. Differences in potential are also reduced in the event of insulation faults or lightning strikes. If equipotential conditions between buildings cannot be achieved or if the distance between buildings is greater than ten metres, it is highly recommended to use optical fibre for communication links and galvanic insulators for measurement and communication systems. These measures are mandatory if the electrical supply system uses the IT or TN-C system.
Even though the ideal bonding network would be made of sheet metal or a fine mesh, experience has shown that for most disturbances, a three-metre mesh size is sufficient to create a mesh bonding network. Examples of different bonding networks are shown in next page. The minimum recommended structure comprises a conductor (e.g. copper cable or strip) surrounding the room.
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Mesh BN
IBN PE
Mesh BN
Mesh IBN
Local mesh
Local mesh IBN
Trunk Tree structure IBN
Star (IBN) CBN
BN: Bonding network CBN: Common bonding network IBN: Isolated bonding network
The length of connections between a structural element and the bonding network does not exceed 50 centimetres and an additional connection should be installed in parallel at a certain distance from the first. The inductance of the connection between the earthing bar of the electrical enclosure for a set of equipment and the bonding network (see below) should be less than one µHenry (0.5 µH, if possible). For example, it is possible to use a single 50 cm conductor or two parallel conductors one meter long, installed at a minimum distance from one another (at least 50 cm) to reduce the mutual inductance between the two conductors. Where possible, connection to the bonding network should be at an intersection to divide the HF currents by four without lengthening the connection. The profile of the bonding conductors is not important, but a flat profile is preferable. The conductor should also be as short as possible.
The purpose of a parallel earthing conductor is to reduce the common-mode current flowing in the conductors that also carry the differential-mode signal (the commonmode impedance and the surface area of the loop are reduced). The parallel earthing conductor must be designed to handle high currents when it is used for protection against lightning or for the return of high fault currents. When cable shielding is used as a parallel earthing conductor, it cannot handle such high currents and the solution is to run the cable along metal structural elements or cableways which then act as other parallel earthing conductors for the entire cable. Another possibility is to run the shielded cable next to a large parallel earthing conductor with both the shielded cable and the parallel earthing conductor connected at each end to the local earthing terminal of the equipment or the device. For very long distances, additional connections to the network are advised for the parallel earthing conductor, at irregular distances between the devices. These additional connections form a shorter return path for the disturbing currents flowing through the parallel earthing conductor. For U-shaped trays, shielding and tubes, the additional connections should be external to maintain the separation with the interior (“screening” effect).
Bonding conductors may be metal strips, flat braids or round conductors. For highfrequency systems, metal strips and flat braids are preferable (skin effect) because a round conductor has a higher impedance than a flat conductor with the same cross section. Where possible, the length to width ratio should not exceed 5. Schneider Electric - Electrical installation guide 2015
The physical separation of high and low-current cables is very important for EMC, particularly if low-current cables are not shielded or the shielding is not connected to the exposed conductive parts (ECPs). The sensitivity of electronic equipment is in large part determined by the accompanying cable system. If there is no separation (different types of cables in separate cableways, minimum distance between high and low-current cables, types of cableways, etc.), electromagnetic coupling is at its maximum. Under these conditions, electronic equipment is sensitive to EMC disturbances flowing in the affected cables. Use of busbar trunking systems such as Canalis or busbar ducts for high power ratings is strongly advised. The levels of radiated magnetic fields using these types of trunking systems is 10 to 20 times lower than standard cables or conductors. The recommendations in the “Cable running” and “Wiring recommendations” sections should be taken into account.
The inclusion of the floors in the mesh contributes to equipotentiality of the area and consequently to the distribution and dilution of disturbing LF currents. The screening effect of a raised floor is directly related to its equipotentiality. If the contact between the floor tiles is poor (rubber antistatic joints, for example) or if the contact between the support brackets is faulty (pollution, corrosion, dust, etc. or if there are no support brackets), it is necessary to add an equipotential mesh. In this case, it is sufficient to ensure effective electrical connections between the metal pedestals. Small spring clips are available on the market to connect the metal pedestals to the equipotential mesh. Ideally, each pedestal should be connected, but it is often sufficient to connect every other pedestals in each direction. A mesh 1.5 to 2 metres in size is suitable in most cases. The recommended cross-sectional area of the copper is 10 mm2 or more. In general, a flat braid is used. To reduce the effects of corrosion, it is advised to use tin-plated copper (see ). Perforated floor tiles act like normal floor tiles when they have a cellular steel structure. Preventive maintenance is required for the floor tiles approximately every five years (depending on the type of tile plate and the environment, including humidity, dust and corrosion). Rubber or polymer antistatic joints must be maintained, similar to the bearing surfaces of the floor tiles (cleaning with a suitable product).
Raised floor
Spring clips
Metal pedestals 10 mm2
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Selection of materials and their shape depends on the following criteria: Severity of the EM environment along cableways (proximity of sources of conducted or radiated EM disturbances) Authorised level of conducted and radiated emissions Type of cables (shielded?, twisted?, optical fibre?) EMI withstand capacity of the equipment connected to the wiring system Other environmental constraints (chemical, mechanical, climatic, fire, etc.) Future extensions planned for the wiring system Non-metal cableways are suitable in the following cases: A continuous, low-level EM environment A wiring system with a low emission level Situations where metal cableways should be avoided (chemical environment) Systems using optical fibres For metal cableways, it is the shape (flat, U-shape, tube, etc.) rather than the crosssectional area that determines the characteristic impedance. Closed shapes are better than open shapes because they reduce common-mode coupling. Cableways often have slots for cable straps. The smaller the better. The types of slots causing the fewest problems are those cut parallel and at some distance from the cables. Slots cut perpendicular to the cables are not recommended (see ).
Lower
OK
Better
In certain cases, a poor cableway in EMI terms may be suitable if the EM environment is low, if shielded cables or optical fibres are employed, or separate cableways are used for the different types of cables (power, data processing, etc.). It is a good idea to reserve space inside the cableway for a given quantity of additional cables. The height of the cables must be lower than the partitions of the cableway as shown below. Covers also improve the EMC performance of cableways. In U-shaped cableways, the magnetic field decreases in the two corners. That explains why deep cableways are preferable (see ).
NO!
YES!
Area protected against external EM field
Different types of cables (power and low-level cables) should not be installed in the same bundle or in the same cableway. Cableways should never be filled to more than half capacity.
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It is recommended to electromagnetically separate groups from one another, either using shielding or by installing the cables in different cableways. The quality of the shielding determines the distance between groups. If there is no shielding, sufďŹ cient distances must be maintained (see ). The distance between power and control cables must be at least 5 times the radius of the larger power cable.
Forbidden
Correct
Ideal
Power cables Auxiliary circuits (relay contacts) Control (digital) Measurements (analogue)
Note: All metal parts must be electrically interconnected
Metal building components can be used for EMC purposes. Steel beams (L, H, U or T shaped) often form an uninterrupted earthed structure with large transversal sections and surfaces with numerous intermediate earthing connections. Cables should if possible be run along such beams. Inside corners are better than the outside surfaces (see ).
Recommended Acceptable Not recommended
Both ends of metal cableways must always be connected to local earth network. For very long cableways, additional connections to the earthing system are recommended between connected devices. Where possible, the distance between these earthing connections should be irregular (for symmetrical wiring systems) to avoid resonance at identical frequencies. All connections to the earthing system should be short. Metal and non-metal cableways are available. Metal solutions offer better EMC characteristics. A cableway (cable trays, conduits, cable brackets, etc.) must offer a continuous, conducting metal structure from beginning to end. An aluminium cableway has a lower DC resistance than a steel cableway of the same size, but the transfer impedance (Zt) of steel drops at a lower frequency, particularly when the steel has a high relative permeability Âľr. Care must be taken when different types of metal are used because direct electrical connection is not authorised in certain cases to avoid corrosion. That could be a disadvantage in terms of EMC. When devices connected to the wiring system using unshielded cables are not affected by low-frequency disturbances, the EMC of non-metal cableways can be improved by adding a parallel earthing conductor (PEC) inside the cableway. Both ends must be connected to the local earthing system. Connections should be made to a metal part with low impedance (e.g. a large metal panel of the device case). The PEC should be designed to handle high fault and common-mode currents.
Schneider Electric - Electrical installation guide 2015
When a metal cableway is made up of a number of short sections, care is required to ensure continuity by correctly bonding the different parts. The parts should preferably be welded along all edges. Riveted, bolted or screwed connections are authorised as long as the contact surfaces conduct current (no paint or insulating coatings) and are protected against corrosion. Tightening torques must be observed to ensure correct pressure for the electrical contact between two parts. When a particular shape of cableway is selected, it should be used for the entire length. All interconnections must have a low impedance. A single wire connection between two parts of the cableway produces a high local impedance that cancels its EMC performance. Starting at a few MHz, a ten-centimetre connection between two parts of the cableway reduces the attenuation factor by more than a factor of ten (see ).
NO!
NOT RECOMMENDED
YES!
Each time modiďŹ cations or extensions are made, it is very important to make sure they are carried out according to EMC rules (e.g. never replace a metal cableway by a plastic version!). Covers for metal cableways must meet the same requirements as those applying to the cableways themselves. A cover should have a large number of contacts along the entire length. If that is not possible, it must be connected to the cableway at least at the two ends using short connections (e.g. braided or meshed connections). When cableways must be interrupted to pass through a wall (e.g. ďŹ rewalls), lowimpedance connections must be used between the two parts (see ).
Mediocre
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OK
Better
According to the WHO (World Health Organisation), exposure to electromagnetic fields can be a health hazard starting at levels as low as 0.2 micro-Teslas and could represent a long-term risk of cancer. Some countries have created standards that stipulate limits (e.g. 0.2 µT at 1 metre in Sweden). All electrical conductors generate magnetic fields proportional to the distance between them. The design of busbar trunking with tightly spaced conductors in a metal enclosure helps to considerably reduce radiated electromagnetic fields. The electromagnetic field characteristics of busbar trunking are welldefined and measurements show that they are far below potentially dangerous levels (see ). In specific cases, where particularly low values are required (computer rooms, medical rooms, some offices), it is important to minimize the magnetic induction generated by power cables. Magnetic induction is: proportional to the current proportional to the distance between the conductors inversely proportional to the square of the distance with respect to the busbar. Busbar with a steel casing provides a good screening effect compared to power cables: magnetic field reduced from 2 to 30 times, depending on the Canalis model. This is particularly low because of the short distance between the bars and the additional attenuation provided by the steel casing.
1000
100
10
1.0
0.1 100
1000
10000
Distance from the center of the Busbar (mm)
KTA10 (1000 A) KTA16 (1600 A) KTA20 (2000 A) KTA40 (4000 A)
When the decision is made to use shielded cables, it is also necessary to determine how the shielding will be bonded (type of earthing, connector, cable entry, etc.), otherwise the benefits are considerably reduced. To be effective, the shielding should be bonded over 360°. below show different ways of earthing the cable shielding. For computer equipment and digital links, the shielding should be connected at each end of the cable. Connection of the shielding is very important for EMC and the following points should be noted.
Schneider Electric - Electrical installation guide 2015
If the shielded cable connects equipment located in the same equipotential bonding area, the shielding must be connected to the exposed conductive parts (ECP) at both ends. If the connected equipment is not in the same equipotential bonding area, there are a number of possibilities. Connection of only one end to the ECPs is dangerous. If an insulation fault occurs, the voltage in the shielding can be fatal for an operator or destroy equipment. In addition, at high frequencies, the shielding is not effective. Connection of both ends to the ECPs can be dangerous if an insulation fault occurs. A high current flows in the shielding and can damage it. To limit this problem, a parallel earthing conductor (PEC) must be run next to the shielded cable. The size of the PEC depends on the short-circuit current in the given part of the installation. It is clear that if the installation has a well meshed earthing network, this problem does not arise.
All bonding connections must be made to bare metal Not acceptable
Acceptable
Collar, clamp, etc.
Bonding bar connected to the chassis
Bonding wire (“pigtail”)
Poorly connected shielding = reduced effectiveness
Correct
Collar, clamp, etc.
Equipotential metal panel
Ideal
Cable gland = circumferential contact to equipotential metal panel
The following figure shows how to prepare the screen when EMC clamps are used.
Screen Cable insulated coat
Screen Cable insulated coat
Screen Cable insulated coat
Added insulated coat
Schneider Electric - Electrical installation guide 2015
EMC clamp width
It is highly recommended to follow the European Standards EN 50173 series to perform Information Technology cablings. To ensure a reliable data transmission, the quality of the whole link shall be homogeneous. That means the category of the different cables shall be the same, the connecting interfaces shall be adapted to the cables. Cables and connections of different categories may be mixed within a channel however the resultant performance will be determined by the category of the lowest performing component. The shield continuity of the whole link (patch cords, Terminal Outlets, horizontal cable) shall be ensured and controlled by tests. The Terminal Outlets (TO) could be used to earth the screen terminations in the cabinet. The choice of these TO is very important. Communication networks are mostly extensive. They interconnect equipment located in different areas where the feeding power supplies could have different earthing systems.
Equipment 1 Patch cord
Equipment 2 TO
Main link
TO
Patch cord
Dirturbing currents
Equipment 1 Patch cord
Equipment 2 TO
Main link
TO
Patch cord
Dirturbing currents
If these different areas are not well equipotential, harsh transient currents could appear (lightning, main power fault, etc.) causing high voltage potential differences between interconnected equipment. Communication interfaces (board, module, etc.) could be disturbed or damaged by this common mode over voltages. The use of TN-S earthing system and well equipotential installation minimize this issue. In any case, the use of Surge Protective Device (SPD) installed in Common Mode and/or Differential Mode is recommended. Schneider Electric - Electrical installation guide 2015
If the different areas/zones are not equipotential, if the power supply earthing system is TN-C or IT, or if there is a doubt and the previous 2 points, optical fiber links are highly recommended. To avoid electrical safety issue, the optical fiber link should not have any metallic parts.
AC and mostly DC Coils (relay, contactor, actuator, etc.) are very disturbing sources.
x
x
Arcing voltage
Arcing voltage
Coil Relay
Un
Un
Coil Relay
TVS
To minimize these High Frequency disturbances the following solutions could be implemented. (In grey, the preferred choice).
R-C network
Y
Y
2 to 3 . Un
1 to 2 times the standard time
Metal Oxide Varistor
Y
Y
< 3 . Un
1.1 to 1.5 times the standard time
Transient Voltage Suppression Diode Bidirectional
Y
Y
< 2. Un
1.1 to 1.5 times the standard time
Transient Voltage Suppression Diode Directional
N
Y
Un + 0.7 V
3 to 10 times the standard time
Free wheeling diode
N
Y
Un + 0.7 V
3 to 10 times the standard time
Resistor
Y
Y
< 4 . Un
1.5 to 2.5 times the standard time
To be efficient, the TVS shall be installed closely to the coil.
Schneider Electric - Electrical installation guide 2015
Refer to chapter J - paragraph 4 "Installation of SPDs"
Each cabinet must be equipped with an earthing bar or a ground reference metal sheet. All shielded cables and external protection circuits must be connected to this point. Anyone of the cabinet metal sheets or the DIN rail can be used as the ground reference. Plastic cabinets are not recommended. In this case, the DIN rail must be used as ground reference.
Potential Reference Plate
Each cabinet, cubicle or enclosure shall be ďŹ tted, as a minimum, with an earthing bar and a reference metallic plate or grid (grounding plate). All the metallic parts (frames, panels, roof, door, etc.) shall be interconnected together with adapted features. The use of speciďŹ c washer is recommended. Some examples of preferred ones are shown below:
Schneider Electric - Electrical installation guide 2015
PE EMC Painted sheet metal Ensure metal-to-metal contact
paint
paint
L < 10 cm
All the cables shall be laid on the grounded/earthed metallic structures. All EMC components (e.g. EMI ďŹ lter, EMC clamps) shall be ďŹ xed directly on the metallic plates without any insulating coating (e.g. free of paint or varnish).
Schneider Electric - Electrical installation guide 2015
Screened cables coming or going out from the cubicle shall be bonded to the earthing bar or grounding plate if these cables are coming from long distance and/or from non equipotential zones. The goal is to divert the disturbing currents at the cabinet entrance and not inside the cabinet. Non metallic cabinet are not recommended for EMC purposes. To protect electronics equipment against low frequency magnetic ďŹ eld, it is recommended to use (galvanized) steel cabinets. Non magnetic metals (e.g. aluminum, stainless steel) are more efďŹ cient for high frequencies environment. Power and low level apparatus shall be physically separated and cables segregation and distances between power and sensitive cables shall also be respected as shown on the ďŹ gures below.
Power
To power components
Mains
LOW level
Actuators
Schneider Electric - Electrical installation guide 2015
Probes Detectors
LOW level
Power
Metal cable trough
It is absolutely essential to specify the standards and recommendations that must be taken into account for installations. Below are several documents that may be used: EN 50174-1
Information technology - Cabling installation. Part 1: Specification and quality assurance
EN 50174-2
Information technology - Cabling installation. Part 2: Installation planning and practices inside buildings
EN 50310
Application of equipotential bonding and earthing in buildings with information technology equipment.
EN 50173
Information Technology - Generic cabling systems
HD 60364-4-444 Low-voltage electrical installations Part 4-444: Protection for safety - Protection against voltage disturbances and electromagnetic disturbances
Normally, the use of specific tools or packages is required to handle or carry electronics boards or components (CPU, memory, analog, PCMCIA modules, etc.) which are sensitive to Electrostatic discharge (ESD). Our products comply with standard ESD tests but ESD conditions are in some cases over the specs. ESD threat could cause semiconductors aging and failures. Without any care, the semiconductor devices could be damaged or burned without users noticing.
The use of specific anti ESD wrist strap is highly recommended. This wrist strap shall be installed inside each cabinet and correctly connected to the earthed cabinet metallic frame.
Schneider Electric - Electrical installation guide 2015
Provide a procedure which depicts the good conditions of use. An example is shown below.
Static is produced by the contact and separation of materials: Shoes and ďŹ&#x201A;oors, clothes and the human body, parts being moved on or from surfaces. The generated charge will reside on the body until it is discharged - the familiar "zap" that all of us have experienced. It's the "zap" that does the damage. If we can prevent any static charge from building up on the body, then there is essentially nothing to be discharged. A properly grounded wrist strap effectively prevents any static charge from building up. Any static charge that would tend to be created is instantly "drained" by the wirst strap. The wrist strap maintains the potential equilibrium that is accomplished the hard way with the "zap".
Schneider Electric - Electrical installation guide 2015
An EM interference phenomenon may be summed up in
below.
Source
Coupling
Victim
Origin of emitted disturbances
Means by which disturbances are transmitted
Equipment likely to be disturbed
Example:
Radiated waves
Walkie-talkie
TV set
The different sources of disturbances are: Radio-frequency emissions Wireless communication systems (radio, TV, CB, radio telephones, remote controls) Radar Electrical equipment High-power industrial equipment (induction furnaces, welding machines, stator control systems) Office equipment (computers and electronic circuits, photocopy machines, large monitors) Discharge lamps (neon, fluorescent, flash, etc.) Electromechanical components (relays, contactors, solenoids, current interruption devices) Power systems Power transmission and distribution systems Electrical transportation systems Lightning Electrostatic discharges (ESD) Electromagnetic nuclear pulses (EMNP) The potential victims are: Radio and television receivers, radar, wireless communication systems Analogue systems (sensors, measurement acquisition, amplifiers, monitors) Digital systems (computers, computer communications, peripheral equipment) The different types of coupling are: Common-mode impedance (galvanic) coupling Capacitive coupling Inductive coupling Radiated coupling (cable to cable, field to cable, antenna to antenna)
Schneider Electric - Electrical installation guide 2015
Two or more devices are interconnected by the power supply and communication cables (see ). When external currents (lightning, fault currents, disturbances) ďŹ&#x201A;ow via these common-mode impedances, an undesirable voltage appears between points A and B . This stray voltage can disturb low-level or fast electronic circuits. All cables, including the protective conductors, have an impedance, particularly at high frequencies.
Device 1
Device 2
Z sign.
Stray overvoltage
2 ECPs
Signal line
ECPs
1
Z1
Z2
The exposed conductive parts (ECP) of devices 1 and 2 are connected to a common earthing terminal via connections with impedances Z1 and Z2. The stray overvoltage ďŹ&#x201A;ows to the earth via Z1. The potential of device 1 increases to Z1 1. The difference in potential with device 2 (initial potential = 0) results in the appearance of current 2. I2 Z1 Z1 I 1 Zsign Z2 I 2 I 1 Zsign Z2 Current 2, present on the signal line, disturbs device 2.
(see
)
Devices linked by a common reference conductor (e.g. PEN, PE) affected by fast or intense (di/dt) current variations (fault current, lightning strike, short-circuit, load changes, chopping circuits, harmonic currents, power factor correction capacitor banks, etc.) A common return path for a number of electrical sources
Disturbed cable Device 1
Device 2 Signal cable Disturbing current Difference in potential ZMC
Schneider Electric - Electrical installation guide 2015
Fault currents
Lightning strike
(see
)
If they cannot be eliminated, common-mode impedances must at least be as low as possible. To reduce the effects of common-mode impedances, it is necessary to: Reduce impedances: Mesh the common references, Use short cables or flat braids which, for equal sizes, have a lower impedance than round cables, Install functional equipotential bonding between devices. Reduce the level of the disturbing currents by adding common-mode filtering and differential-mode inductors
Device 1
Z sign.
Stray overvoltage
Device 2
2
Z sup. Z1
PEC
1
Z2
If the impedance of the parallel earthing conductor PEC (Z sup) is very low compared to Z sign, most of the disturbing current flows via the PEC, i.e. not via the signal line as in the previous case. The difference in potential between devices 1 and 2 becomes very low and the disturbance acceptable.
U Vsource
The level of disturbance depends on the voltage variations (dv/dt) and the value of the coupling capacitance between the disturber and the victim.
t
Vvictim
t
Capacitive coupling increases with: The frequency The proximity of the disturber to the victim and the length of the parallel cables The height of the cables with respect to a ground referencing plane The input impedance of the victim circuit (circuits with a high input impedance are more vulnerable) The insulation of the victim cable ( r of the cable insulation), particularly for tightly coupled pairs shows the results of capacitive coupling (cross-talk) between two cables.
(see
opposite page)
Nearby cables subjected to rapid voltage variations (dv/dt) Start-up of fluorescent lamps High-voltage switch-mode power supplies (photocopy machines, etc.) Coupling capacitance between the primary and secondary windings of transformers Cross-talk between cables
Schneider Electric - Electrical installation guide 2015
Differential mode
Common mode Source
Vs v
DM
Victim
Vs
v
CM
CM
DM
Source
Victim
Vs DM: Source of the disturbing voltage (differential mode) v DM: Disturbing current on victim side (differential mode) Vs CM: Source of the disturbing voltage (common mode) v CM: Disturbing current on victim side (common mode) Metal shielding
(see
C
Source
Victim
)
Limit the length of parallel runs of disturbers and victims to the strict minimum Increase the distance between the disturber and the victim For two-wire connections, run the two wires as close together as possible Position a PEC bonded at both ends and between the disturber and the victim Use two or four-wire cables rather than individual conductors Use symmetrical transmission systems on correctly implemented, symmetrical wiring systems Shield the disturbing cables, the victim cables or both (the shielding must be bonded) Reduce the dv/dt of the disturber by increasing the signal rise time where possible
The disturber and the victim are coupled by a magnetic ďŹ eld. The level of disturbance depends on the current variations (di/dt) and the mutual coupling inductance. Inductive coupling increases with: The frequency The proximity of the disturber to the victim and the length of the parallel cables, The height of the cables with respect to a ground referencing plane, The load impedance of the disturbing circuit. (see
next page)
Nearby cables subjected to rapid current variations (di/dt) Short-circuits Fault currents Lightning strikes Stator control systems Welding machines Inductors
Schneider Electric - Electrical installation guide 2015
Disturbing cable
Disturbing cable
H
H
Victim loop
Victim pair i
i
Victim loop
Differential mode
Common mode
Limit the length of parallel runs of disturbers and victims to the strict minimum Increase the distance between the disturber and the victim For two-wire connections, run the two wires as close together as possible Use multi-core or touching single-core cables, preferably in a triangular layout Position a PEC bonded at both ends and between the disturber and the victim Use symmetrical transmission systems on correctly implemented, symmetrical wiring systems Shield the disturbing cables, the victim cables or both (the shielding must be bonded) Reduce the dv/dt of the disturber by increasing the signal rise time where possible (series-connected resistors or PTC resistors on the disturbing cable, ferrite rings on the disturbing and/or victim cable)
The disturber and the victim are coupled by a medium (e.g. air). The level of disturbance depends on the power of the radiating source and the effectiveness of the emitting and receiving antenna. An electromagnetic field comprises both an electrical field and a magnetic field. The two fields are correlated. It is possible to analyse separately the electrical and magnetic components. The electrical field (E field) and the magnetic field (H field) are coupled in wiring systems via the wires and loops (see ).
E field
H field i
V
Field-to-cable coupling
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Field-to-loop coupling
When a cable is subjected to a variable electrical field, a current is generated in the cable. This phenomenon is called field-to-cable coupling. Similarly, when a variable magnetic field flows through a loop, it creates a counter electromotive force that produces a voltage between the two ends of the loop. This phenomenon is called field-to-loop coupling. (see
)
Radio-transmission equipment (walkie-talkies, radio and TV transmitters, mobile services) Radar Automobile ignition systems Arc-welding machines Induction furnaces Power switching systems Electrostatic discharges (ESD) Lighting
E field
EM field
Signal cable Device 1
Device 2
i Device h
h
Area of the earth loop
Ground reference plane
Example of field-to-cable coupling
Example of field-to-loop coupling
To minimise the effects of radiated coupling, the measures below are required. Reduce the antenna effect of the victim by reducing the height (h) of the cable with respect to the ground referencing plane Place the cable in an uninterrupted, bonded metal cableway (tube, trunking, cable tray) Use shielded cables that are correctly installed and bonded Add PECs Place filters or ferrite rings on the victim cable Reduce the surface of the victim loop by reducing the height (h) and the length of the cable. Use the solutions for field-to-cable coupling. Use the Faraday cage principle. Radiated coupling can be eliminated using the Faraday cage principle. A possible solution is a shielded cable with both ends of the shielding connected to the metal case of the device. The exposed conductive parts must be bonded to enhance effectiveness at high frequencies. Radiated coupling decreases with the distance and when symmetrical transmission links are used.
Schneider Electric - Electrical installation guide 2015
(see
Unshielded cables of different groups
)
1 - Power connections (supply + PE)
Shielded cables of different groups
2 - Relay connections Device
e
h
NO!
Ground reference plane
YES!
4 - Analogue link (sensor)
3 - Digital link (bus)
Risk of cross-talk in common mode if e < 3 h
Sensitive cable
Sensitive cable
Disturbing cable
Disturbing cable
30 cm NO!
Cross incompatible cables at right angles
Four classes of internal signals are: Class 1 Mains power lines, power circuits with a high di/dt, switch-mode converters, powerregulation control devices. This class is not very sensitive, but disturbs the other classes (particularly in common mode). Class 2 Relay contacts. This class is not very sensitive, but disturbs the other classes (switching, arcs when contacts open).
1m YES!
Class 3 Digital circuits (HF switching). This class is sensitive to pulses, but also disturbs the following class.
NO!
Class 4 Analogue input/output circuits (low-level measurements, active sensor supply circuits). This class is sensitive.
YES!
It is a good idea to use conductors with a specific colour for each class to facilitate identification and separate the classes. This is useful during design and troubleshooting. Standard cable
Two distinct pairs
Poorly implemented ribbon cable
Correctly implemented ribbon cable
(see
above)
Digital connection Analogue pair Bonding wires
(see and ) In general, a 10 cm separation between cables laid flat on sheet metal is sufficient (for both common and differential modes). If there is enough space, a distance of 30 cm is preferable. If cables must be crossed, this should be done at right angles to avoid cross-talk (even if they touch). There are no distance requirements if the cables are separated by a metal partition that is equipotential with respect to the ECPs. However, the height of the partition must be greater than the diameter of the cables.
Schneider Electric - Electrical installation guide 2015
(see ) If it is necessary to use a cable to carry the signals of different groups, internal shielding is necessary to limit cross-talk (differential mode). The shielding, preferably braided, must be bonded at each end for groups 1, 2 and 3. (see ) The overshielding acts as a HF protection (common and differential modes) if it is bonded at each end using a circumferential connector, a collar or a clampere However, a simple bonding wire is not sufďŹ cient.
NO! Shielded pair Electronic control device
Sensor Unshielded cable for stator control Electromechanical device
YES! Bonded using a clamp Shielded pair + overshielding Electronic control device
NO! Power + analogue
Digital + relay contacts
YES! Power + relay contacts
Digital + analogue
Sensor Shielded cable for stator control Electromechanical device
(see ) Except where necessary for groups 1 and 2 (differential mode). If a single connector is used for both analogue and digital signals, the two groups must be separated by at least one set of contacts connected to 0 V used as a barrier. (see ) For group 4, these connections are not advised for lines with very low voltage and frequency levels (risk of creating signal noise, by magnetic induction, at the transmission frequencies).
Shielding Power connections
Digital connections
Relay I/O connections
Analogue connections NO!
YES! Electronic system
NO!
Electronic system
YES!
Wires not equipotentially bonded
Digital connections Analogue connections
Equipotential sheet metal panel
Schneider Electric - Electrical installation guide 2015
Equipotential sheet metal panel
(see ) This is particularly important for low-level sensors. Even for relay signals with a common, the active conductors should be accompanied by at least one common conductor per bundle. For analogue and digital signals, twisted pairs are a minimum requirement. A twisted pair (differential mode) guarantees that the two wires remain together along their entire length.
NO!
YES!
Area of loop too large
PCB with relay contact I/Os
PCB with relay contact I/Os
+ Power supply
+ Power supply
But they should be made of twisted pairs to ensure compliance with the previous section. (see ) For example: Covers, metal trunking, structure, etc. In order to take advantage of the dependable, inexpensive and signiďŹ cant reduction effect (common mode) and anticross-talk effect (differential mode).
NO!
NO!
YES!
Chassis 1
Chassis 1
Chassis 2
Chassis 2
Chassis 3
Chassis 3
YES! Metal tray
Power supply Power or disturbing cables
I/O interface
Power supply
I/O interface
All metal parts (frame, structure, enclosures, etc.) are equipotential
Relay cables Measurement or sensitive cables
(see
)
Schneider Electric - Electrical installation guide 2015
Schneider Electric - Electrical installation guide 2015