CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 13 – Electrotechnical Systems
Electrotechnical Systems Unit Aims By the end of the unit participants should be able to: Identify basic types of electrotechnical systems and features a) power sources b) wiring/cable systems c) controls d) electrical components e) supports/fixings Syllabus Reference: 2.4.05
Electrotechnical Systems Power Sources Normally, the Regional Electricity Company (REC) will run a three phase supply cable along the road and a single- or three-phase service cable is taken from this to feed an individual premises. This looks similar to the diagram below.
Here can be seen a three-phase, four-wire supply. The fourth wire is the neutral (now coloured blue rather than black) and the other three are the brown, black and grey lines (previously red, yellow and blue phases) usually labelled for the supply as L 1, L2 and L3.
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 13 – Electrotechnical Systems Between each of the three lines to the neutral there is a nominal voltage of 230V. This is what is referred to as „230V single-phase supply‟ and is most common in domestic installations. The other type of supply, although less common is the „400V single-phase supply‟. Here there are two of the lines that come into the property and the voltage that appears across those two points is 400V. The final type of supply is a „400V three-phase supply‟. Here all the conductors come into the installation and the full range of single phase and three-phase supplies is available. This type is most commonly used in businesses, farms, industry etc. The nominal voltages therefore in the UK are: Single-phase Single-phase Three-phase
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230V 400V 400V
Legislation allows the supply voltage to vary from the nominal value. This variation is currently +10% and -6% on the nominal supply voltage. This gives the voltage ranges shown in the table below: Nominal Voltage 230V 400V
Minimum Voltage 216.2V 376V
Maximum Voltage 253V 440V
A three-phase supply is typical around the world, and although the voltages may vary between countries, the basic principles remain. Now, at a higher level of study we will consider this in more detail but at present you need to know that a three-phase supply provides options that aren‟t open to us when we are simply dealing with a single-phase supply. Wiring/Cable Systems In any electrical system the means of delivery of electrical energy to a load will vary. Different types of cable are used as well as different ways in which these cables are supported. The name attached to these are called wiring systems and they fall into general areas. Cable Tray: Cable tray is commonly used to support a number of cables where individual clipping is difficult or not economical. Tray is used either above ceilings or in industrial or commercial settings. It has a range of sizes from 50mm to 1000mm. Ladder Rack: This is similar to cable tray but with greater strength and is used in industrial settings to deliver large numbers of sub-main cables. Basket Tray: Again, this is similar to cable tray but is more likely to be used in commercial premises for large amounts of smaller cables. Steel Conduit and Trunking: Generally used to deliver single core cables in industrial, commercial and health settings. They are strong and able to resist high mechanical stresses.
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 13 – Electrotechnical Systems Plastic Conduit and Trunking: As with steel conduit and trunking single core cables are generally installed in schools and commercial premises. It is less robust than steel conduit but easier to install. Clipped direct: We can clip mineral insulated (MI), steel wired armoured (SWA), crosslinked polyethylene (XLPE), polyvinylchloride/polyvinylchloride (PVC/PVC) cables direct to a surface using appropriate clips. The setting defines the nature of the clips and how the cables are run. Controls Many controls are required in electrotechnical installations for various purposes. These can be classified into three main categories: Safety Isolation Functional Safety Switching: this is provided to enable electrotechnical circuits to be disconnected when a situation could potentially become dangerous. Examples include circuit breakers (CBs), overload trips, residual current devices (RCDs) and stop-buttons. Isolation Switching: When work is to be carried out on an electrotechnical circuit it must be fully isolated. To this end, there must be a straight-forward means of isolating all circuits. Isolation is defined as “a function intended to cut off for reasons of safety the supply from all, or a discrete section, of the installation by separating the installation or section from every source of electrical energy” (BS 7671 Definitions). This means that the device used for isolation must break all live conductors including the neutral. Functional Switching: Functional switching is defined as “an operation intended to switch ‘on’ and ‘off’ or vary the supply of electrical energy to all or part of an installation for normal operating purposes” (BS 7671 Definitions). It includes switches, push buttons and contactors. All electrical installations and circuits must contain a means of carrying out all three types of switching listed above. Each device my apply to the whole installation, e.g. the main switch in a consumer will provide isolation for the whole of a domestic installation, or each circuit or device may have its own means of switching, e.g. separate isolator switch for a lathe. Some devices my combine more than one function. For example, a switch-fuse can be used for safety switching and also for isolation provided that it can be locked-of and the contact separation is sufficient. Electrical Components All electrotechnical installations will comprise of many components which have already been discussed including wiring systems, enclosures, controls, accessories and current using equipment.
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CGLI 2330 Certificate in Electrotechnical Technology Level 2 Inst Tech: Unit 202 – Principles of Electrotechnology
Unit 13 – Electrotechnical Systems Supports and Fixings To support the electrotechnical system it will be necessary to provide various supports and fixings. This will depend on the items being fixed, i.e. size, weight, etc., the environment the system is being installed in and the nature of the building structure containing the system, i.e. brick, block, concrete, stud partition, etc. Conclusion All of the items mentioned will be dealt with in greater detail in other parts of the course.
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