Earth terminal

Dropdown slide rails for circuit breaker withdrawal 16 SF6 circuit breaker chamber 17 Auxiliary wiring harness multi-pin plug and socket 18 Integral skid underbase 19 Voltage transformer 20 Earth terminal 21 Fuses 22 Split clamp connectors 23 Earth fault current transformer 24 Incoming adaptor and half coupler. 

Earthing is the connection of the exposed conductive parts of an electrical installation to the main protective earthing terminal of the installation. 

Circuit protective conductor (CPC) - a protective conductor connecting exposed conductive parts of equipment to the main earthing terminal of the installation. This is the green and yellow insulated conductor in twin and earth cable. 

This is the type of supply more often found when the installation is fed from overhead cables. The supply authorities do not provide an earth terminal and the installation s circuit protective conductors (CPCs) must be connected to earth via an earth electrode provided by the consumer. lET Regulation 542.2.3 lists the type of earth rod, earth plate or earth tapes recognized by BS 7671. An effective earth connection is sometimes difficult to obtain and in most cases a residual current device (RCD) is provided when this type of supply is used. The arrangement is shown in Fig. 3.36. 

Where PVC insulated and sheathed cables are concealed in walls, floors or partitions, they must be provided with a box incorporating an earth terminal at each outlet position. PVC cables do not react chemically with plaster, as do some cables, and consequently PVC cables may be buried under plaster. Further protection by channel or conduit is only necessary if mechanical protection from nails or screws is required or to protect them from the plasterer s trowel. However, lET Regulation 522.6.101 now tells us that where PVC cables are to be embedded in a wall or partition at a depth of less than 50 mm they should be run along one of the permitted routes shown in Fig. 3.45. Figure 3.44 shows a typical PVC installation. To identify the most probable cable routes, lET Regulation 522.6.201 tells us that outside a zone formed by a 150mm border all around a wall edge, cables can only be run horizontally or vertically to a point or accessory if they are contained in a substantial earthed enclosure, such as a conduit, which can withstand nail penetration, as shown in Fig. 3.45. 

Earth electrodes connected to the earth terminal of an electric fence controller must be separate from the earthing system of any other circuit and should be situated outside the resistance area of any electrode used for protective earthing. The electric fence controller and the fence wire must be installed so that they do not come into contact with any power, telephone or radio systems, including poles. Agricultural and horticultural installations should be tested and inspected in accordance with Part 6 of the Wiring Regulations every three years. 

Where earthed electrical equipment may come into contact with the metalwork of other services, they too must be effectively connected to the main earthing terminal of the installation (lET Regulations 411.3.1.2 and 544). 

A d.c. test using an ohmmeter continuity tester is suitable where the protective conductors are of copper or aluminium up to 35 mm. The test is made with the supply disconnected, measuring from the consumer s main protective earthing terminal to the far end of each CPC, as shown in Fig. 4.32. The resistance of the... 

A protective conductor connecting exposed conductive parts of equipment to the main earthing terminal. 

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