CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 17 – Protection
Protection Unit Aims By the end of the unit participants should be able to: List basic principles of shock protection, circuit overload and short-circuit protection Syllabus Reference: 2.3.07
Protection against Electric Shock (Chapter 41) The definition of „electric shock‟ is: “A dangerous physiological effect resulting from the passing of an electric through a human body or livestock” (from BS 7671 „Definitions‟). Chapter 41 (Protection against Electric Shock) of BS 7671 details the means of providing appropriate protection. Two types of protection is specified as follows: Basic Protection Fault Protection. Basic Protection: “Protection against electric shock under fault-free conditions” (from BS 7671 „Definitions‟). Basic Protection is provided to prevent a person or livestock touching parts that can normally be expected to be live under normal circumstances, e.g. live terminals, conductors, bus bars, etc. The example right shows a person coming in to contact with the line terminal of a socket outlet; the current returning via the person through earth back to the supply. This was previously referred to as “Protection against direct contact”. Fault Protection: “Protection against electric shock under single-fault conditions” (from BS 7671 „Definitions‟). Fault Protection is provided to prevent danger to a person or livestock touching conductive parts, whether forming part of electrical equipment (exposed conductive parts) or not (extraneous conductive parts), that would not normally be expected to be live but have become live due to a fault, e.g. gas pipes, water pipes, equipment enclosures, metal bath tub, structural steelwork, etc. The example below shows a person coming in to contact with a radiator that has become live due to it coming into contact with a line conductor.
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 17 – Protection
Protective Measure: Automatic Disconnection of Supply (411) Automatic disconnection of supply is a protective measure in which: (i)
basic protection is provided by basic insulation of live parts or by barriers or enclosures, in accordance with Section 416, and
(ii)
fault protection is provided by protective earthing, protective equipotential bonding and automatic disconnection in case of a fault, in accordance with Regulations 411.3 to 411.6.
Basic Protection (416) Basic protection shall be provided by one of the following means: Basic insulation of live parts (Regulation 416.1) Barriers and enclosures (Regulation 416.2) Obstacles (Regulation 417.2) Placing out of reach (Regulation 417.3) Basic insulation: Live parts shall be completely covered with insulation which can only be removed by destruction. For equipment, the insulation shall comply with the relevant standard for such electrical equipment (Regulation 416.1). Barriers and enclosures: Live parts shall be inside enclosures or behind barriers providing at least the degree of protection IPXXB or IP2X except that, where larger openings occur during the replacement of parts, such as certain lampholders or fuses, or larger openings are necessary to allow the proper functioning of equipment according to the relevant requirements for the equipment: (i)
suitable precautions shall be taken to prevent persons or livestock from unintentionally touching live parts, and
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 17 – Protection (ii)
it shall be ensured, as far as practical, that persons will be aware that live parts can be touched through the opening and should not be touched intentionally, and
(iii)
the opening shall be as small as is consistent with the requirements for proper functioning and for replacement of a part. (Regulation 416.2.1)
See also Regulations 416.2.2 to 416.2.5. Obstacles: This method can only be used where the installation is controlled or supervised by skilled persons as defined in Regulation 410.3.5. Obstacles shall prevent: (i)
unintentional bodily approach to live parts, and
(ii)
unintentional contact with live parts during the operation of live equipment in normal service. (Regulation 417.2.1)
Placing out of reach: Again, this method can only be used where the installation is controlled or supervised by skilled persons as defined in Regulation 410.3.5. Note:
Protection by placing out of reach is intended only to prevent unintentional contact with live parts.
A bare or insulated overhead line for distribution between buildings and structures shall be installed to the standard required by the Electricity Safety, Quality and Continuity Regulations 2002. (Regulation 417.3) See also Regulations 417.3.1 to 417.3.3. Fault Protection (411.3) Protective Earthing (Regulation 411.3.1.1) (i)
Exposed-conductive-parts shall be connected to a protective conductor. Exposedconductive-parts are defined as “conductive part of equipment which can be touched and which is not normally live, but which can become live when basic insulation fails” (from BS 7671 „Definitions‟).
(ii)
Simultaneously exposed-conductive-parts shall be connected to the same earthing system individually, in groups or collectively.
(iii)
Conductors for protective earthing shall comply with Chapter 54 which relates to, amongst other things, the size of protective earthing conductors.
(iv) A circuit protective conductor (cpc) shall be run to and terminated at each point in wiring and at each accessory except a lampholder having no exposed-conductiveparts and suspended from such a point (e.g. pendant lampholder). Protective Equipotential Bonding (Regulation 411.3.1.2) In each installation main protective bonding conductors complying with Chapter 54 shall connect to the main earthing terminal extraneous-conductive-parts. Extraneousconductive-parts are defined as “a conductive part liable to introduce a potential, generally Earth potential, and not forming part of the electrical installation” (from BS 7671 „Definitions‟) and includes the following:
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 17 – Protection (i)
Water installation pipes
(ii)
Gas installation pipes
(iii)
Other installation pipework and ducting
(iv) Central heating and air conditioning systems (v)
Exposed metallic structural parts of the building.
Connection of a lightning protection system to the protective equipotential bonding shall be made in accordance with BS EN 62305. Where an installation serves more than one building the above requirement shall be applied to each building. To comply with the requirements of these Regulations it is also necessary to apply equipotential bonding to any metallic sheath of a telecommunications cable. However, the consent of the owner or operator of the cable shall be obtained. Automatic Disconnection in the event of a Fault (Regulation 411.3.2) A protective device shall automatically interrupt the supply to the line conductor of a circuit or equipment in the event of a fault of negligible impedance between the line conductor and an exposed-conductive-part or a protective conductor in the circuit or equipment within the disconnection time required by Regulation 411.3.2.2, 411.3.2.3 or 411.3.2.4. The maximum disconnection time stated in Table 41.1 shall be applied to final circuits not exceeding 32 amperes (Regulation 411.3.2.2).
TABLE 41.1 Maximum disconnection times System
50V < UO ≤ 120V seconds
120V < UO ≤ 230V seconds
230V < UO ≤ 400V seconds
UO > 400V seconds
a.c.
d.c.
a.c.
d.c.
a.c.
d.c.
a.c.
d.c.
TN
0.8
NOTE 1
0.4
5
0.2
0.4
0.1
0.1
TT
0.3
NOTE 1
0.2
0.4
0.07
0.2
0.04
0.1
In a TN system, a disconnection time not exceeding 5 seconds is permitted for a distribution circuit and for a circuit not covered by Regulation 411.3.2.2 (Regulation 411.3.2.3). In a TT system, a disconnection time not exceeding 1 second is permitted for a distribution circuit and for a circuit not covered by Regulation 411.3.2.2 (Regulation 411.3.2.3). Requirements for Basic Protection and Fault Protection Regulation 414.2) Basic protection and fault protection is deemed to be provided where: (i)
the nominal voltage cannot exceed the upper limit of Voltage Band I (50 volts a.c. or 120 volts d.c.), and
(ii)
the supply is from one of the sources listed in Regulation 414.3, and
(iii)
the conditions of Regulation 414.4 are fulfilled.
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 17 – Protection This will generally be achieved by using either Separated Extra-Low Voltage (SELV) or Protective Extra-Low Voltage (PELV). SELV is defined as, “an extra-low voltage which is electrically separated from earth and from other systems in such a way that a single-fault cannot give rise to the risk of electric shock” and PELV is defined as, “an extra-low voltage system which is not electrically separated from Earth, but which otherwise satisfies all the requirements of for SELV”. Extra-low voltage is defined as not normally exceeding 50V a.c. or 120V ripple-free d.c., whether between conductors or to earth. Most SELV systems are fed from a safety isolating transformer with no electrical connection between the primary and secondary sides also with no connection between the secondary live conductors and earth. The diagram on the following page shows a typical arrangement:
Other SELV sources include motor-generators whose windings provide appropriate electrical separation, batteries and certain electronic sources. Additional Protection (411.3.3) In a.c. systems, additional protection by means of an RCD in accordance with Regulation 415.1 (residual operating current (I∆n) not exceeding 30 mA and an operating time not exceeding 40 ms at a residual current of 5 I∆n.) shall be provided for: (i)
socket-outlets with a rated current not exceeding 20 amperes that are for use by ordinary persons and are intended for general use, or
(ii)
mobile equipment with a current rating not exceeding 32 amperes for use outdoors.
Overload Current An overload is defined as, “An overcurrent occurring in a circuit which is electrically sound” (from BS 7671 „Definitions‟). An overload is a situation that occurs in a circuit which is still electrically sound. It is generally caused by trying to take more power from a circuit than it is designed for; this results in a larger than normal current flowing in the circuit. If the load is reduced then the circuit can continue to function without any need for repair.
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 â&#x20AC;&#x201C; Principles of Electrotechnology
Unit 17 â&#x20AC;&#x201C; Protection A typical example of this is a radial circuit which has been extended and the load has increased to exceed that originally intended (see right). Each item connected to the circuit and the circuit supplying the equipment is healthy, and so is the circuit supplying the outlet, but more current is being drawn through the cable than was originally intended. Every cable has some resistance and the result of drawing more current through the cable results in more heat being produced in the cable. This rise in temperature will, over a period of time, result in the insulation becoming less effective and eventually breaking down. In the case of severe overload the insulation becomes so hot it begins to melt and may even catch fire. This is obviously a serious fire risk and steps must be taken to stop this happening (see diagram right).
Protection Devices The usual way of protecting each cable is by installing a device in each circuit that will automatically disconnect it from the supply when an overload occurs. There are several different protection devices that can be used, one of which is the semi-enclosed rewireable fuse to BS3036 (see right). However, these devices are not the most effective and their use requires special consideration to be made for cable sizing.
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 17 – Protection Overloads are sometimes difficult for a device to detect, as they can build up over a period of time as different pieces of equipment are switched on. For example, a 6A lighting circuit is usually designed for up to 13 100 watt lamps. If each 100 watt lamp is replaced by a 150 watt and they were all switched on one after the other until they were all on, the circuit would eventually be carrying one and half times the load it was designed for. It would depend on the protection device as to whether the circuit would be automatically disconnected under these conditions.
Short-Circuit Current “An overcurrent resulting from a fault of negligible impedance between live conductors having a difference in potential under normal operating conditions.” (from BS 7671 „Definitions‟). „Live conductors‟ means all those carrying current under normal conditions, which includes the neutral conductor. If a short-circuit is two conductors touching then it can be assumed that the resistance or impedance of that connection would be so low that it can be ignored. So a short circuit can occur between: Conductors connected to different phases for example, brown phase to black phase or brown phase to grey phase, Any phase and neutral. Let us consider for a moment the implications of this. If the fault has negligible impedance then the only restriction to the amount of current that will flow in the circuit is that of all the conductors. As conductors are of a low resistance then this total value will itself be low and the current that flows can be very high.
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 17 – Protection
Referring to the diagram above it can be seen that the impedance of the total circuit under these conditions is made up of: The supply transformer winding (0.01Ω) The supply cable, both phase and neutral (0.01Ω + 0.01Ω) The cables up to the short circuit (0.15Ω + 0.15Ω) This gives a total resistance of: 0.01 + 0.01 + 0.01 + 0.15 + 0.15 = 0.33Ω If the system is operating at 230V then the current flowing under these fault conditions would be 230 volts divided by 0.33 ohms. 230V. 0.33 Ω
=
697 amperes
Considering that this circuit may be protected by a 30A fuse or circuit breaker this is a large current to flow in a domestic installation. Not only is the current flow going to be high, it also tends to occur very quickly, so the protective device has to be able to cope with a very different set of conditions from those of an overload. In this case the temperature builds up very rapidly, within fractions of a second, and the device must sense this current and disconnect it from the supply before any damage is done to the cable or equipment. The risk of fire under these conditions is considerable. Now, if a fault should develop at the intake position of an installation the limiting impedance is only that of the supply conductors.
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 17 – Protection Taking the example in the diagram above, the external impedance to the installation would be: 0.01 + 0.01 + 0.01 = 0.03Ω This means that on a 230V supply the fault current would be: 230V. 0.03 Ω
=
7666.6 amperes or 7.6 kA
This value is known as the „prospective short circuit current‟ (PSCC). Protection equipment fitted at any point needs to be capable of breaking the prospective short circuit current, which may occur at that point, without damage to the equipment.
Protection Index of Protection (BS EN 60529: 1992 [2004]) IP code for ingress protection are given in BS EN 60529: 1992 (2004] which is a standard used to form the basic requirements of electrical equipment standards. The degree of protection provided by an enclosure is indicated generally by two numbers and followed by an optional letter and/or optional supplementary letter e.g. IP44 B H. The first numeral indicates the level of protection against ingress of solid foreign bodies and protection against access to hazardous parts inside an enclosure .The second numeral refers to the level of protection against ingress of water. If not required the numeral can be replace by X or XX. The additional letter indicates the degree of protection of persons against access to hazardous parts. It is only used if the protection against access to hazardous parts is higher than that indicated by the 1st character, or if only the protection against access to hazardous parts, and not general ingress is indicated, the 1st character numeral being replaced by X. The letter B - is protection against access with a test finger of minimum thickness 12mm diameter test finger and 80mm long, e.g. IPXXB. See the following page for the complete range of IP ratings:
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 17 – Protection
IP Ratings
For more information see pages 145 to 150 of „Electrical Installations Level 2 2330 Technical th Certificate‟ – revised for the 17 Edition IEE Wiring Regulations (ISBN 978 0 435401 09 2).
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