Earthing conductor

Heidelberg, E., Generation of Igniting Brush Discharges by Charged Layers on Earthed Conductors, 1967 Static Electrification Conference, Institute of Physics, London. 

Figure 6.46 Terminal block for protective earthing conductors.

Figure 6.48 Terminal block for three protective earthing conductors, cage clamp technique.

L1, L2, L3 phase conductors 1/1, 1/2, 1/3 voltages phase to neutral C1, C2, C3 capacitances phase to earth PE protective earthed conductor RF earth fault resistance UN, PE voltage between neutral N and PE in case of an earth fault... 

L1, L2, L3 phase conductors N neutral PE protective earthed conductor ... 

With method B, the PE and UL conductors are at identical PE potential, i.e. the protection method is reduced solely to earth fault monitoring. An insulated auxiliary conductor is used as a monitoring conductor to ensure a protective earthed conductor correctly connected all over the total system length. 

The earth loop impedance is the total impedance seen by the source of voltage in a faulted circuit which involves the earthing conductors. Eigure 13.10 shows the situation for a three-phase cable supplying a load such as a motor or static load. 

The overhead earthing conductor will divert some of the L-G fault current from entering the ground at the foot of the pole. The extent of diversion will be in proportion to the impedance of the overhead line compared with that of the earth resistance path back to the source. The calculations required for determining the fault current and its diverted amounts are shown in Appendix H by way of an example, and Figure 13.12. 

Table H.la shows the value of // for different values of Rep. It also shows the division of current between the overhead line earthing conductor and the footings of the pole.

Let the overhead earthing conductor divert some of the fault current. 

The device may be grounded for safety reasons (safety class I, as shown in Figure 10.35, maximum resistance in the protective earth conductor between power plug and chassis 0.2 Q), or double insulated (safety class II). 

Here Ze is the impedance of the supply side of the earth fault loop. Tfie actual value will depend upon many factors the type of supply, the ground conditions, the distance from the transformer, etc. The value can be obtained from the area electricity companies, but typical values are 0.35for TN-C-S (protective multiple earthing, PME) supplies and 0.8n forTN-S (cable sheath earth) supplies. Also in the above formula, R is the resistance of the line conductor and R2 is the resistance of the earth conductor. The complete earth fault loop path is shown in Fig. 3.32. 

Twenty-eight years ago the United Kingdom agreed to adopt the European colour code for flexible cords, that is, brown for live or phase conductor, blue for the neutral conductor and green combined with yellow for earth conductors. However, no similar harmonization was proposed for non-flexible cables used for fixed wiring. These were to remain as red for live or phase conductor, black for the neutral conductor and green combined with yellow for earth conductors. 

Single-phase supplies red line conductors, black neutral conductors, and green combined with yellow for earth conductors. 

The advantages of a PVC conduit system are that it may be installed much more quickly than steel conduit and is non-corrosive, but it does not have the mechanical strength of steel conduit. Since PVC conduit is an insulator it cannot be used as the CPC and a separate earth conductor must be run to every outlet. It is not suitable for installations subjected to temperatures below 25°C or above 60°C. Where luminaires are suspended from PVC conduit boxes, precautions must be taken to ensure that the lamp does not raise the box temperature or that the mass of the luminaire supported by each box does not exceed the maximum recommended by the manufacturer (lET Regulations 522.1 and 522.2). PVC conduit also expands much more than metal conduit and so long runs require an expansion coupling to allow for conduit movement and to help prevent distortion during temperature changes. 

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