Earthing of low voltage networks
The earthing of low voltage networks in the UK is largely determined by the Low Voltage Supplies. However, if the incoming supplies are at 11kV and the transformers are in the ownership of the user, the LV supplies may be earthed in a less conventional way using a high impedance. This arrangement is not allowed for public supplies.
However, it is a useful system when it is more important to maintain supplies than it is to clear the first earth fault.
EXAMPLE: An emergency lighting scheme for the evacuation of personnel from a hazardous area could use a high impedance system if it were considered less dangerous to maintain supplies after a first earth fault than to disconnect the light completely. The Channel Tunnel could be such a case.
Even in these circumstances the original earth fault should be corrected as quickly as possible.
The more conventional earthing arrangements are:
- TN-C where the earth and neutral are combined (PEN) and
- TN-S where they are separated (5 wire) or
- TN-C- S.
The latter is very common as it allows the single-phase loads to be supplied by phase and neutral with a completely separate earth system connecting together all the exposed conductive parts before connecting them to the PEN conductor via a main earthing terminal which is also connected to the neutral terminal.
For protective conductors of the same material as the phase conductor the cross-sectional area shall be the same size as the phase conductor up to 16 mm2. IMPORTANT: When the phase conductor is above 16 mm2 then the protective conductor may remain at 16 mm2 until the phase conductor is 35 mm2, after which the protective conductor should be half the size of the phase conductor.
For conductors which are not of the same material the cross-sectional area shall be adjusted in the ratios of the factor k from Table 43A in BS 7671. The k factor takes into account the resistivity, temperature coefficient and heat capacity of the conductor materials and of the initial and final temperatures.
Lastly there is the TT system which uses mother earth as part of the earth return.
The neutral and the earthed parts are only connected together via an electrode system back to the source earth (and neutral). To check that conventional systems are satisfactory, i.e. that the protection operates on the occurrence of an earth fault, it is necessary to calculate the earth fault loop impedance (Zs) and ensure that the fault current through it will cause the protection to operate.
This is quite a tedious process, involving as it does the calculation of the impedances afforded not only by the earth return but also by:
- The phase conductor
- Supply transformer
- Supply network
- Any neutral impedance.
This information must be requested early. The Electricity Distributor should be able to give the fault level or the equivalent impedance of the supply network and the manufacturer can provide the appropriate impedances for the transformer.
However, time will be required to obtain the answers so enquiries should be made at the commencement of the project.
The substation will house the circuit breakers of fuses for the main cable connections to the sub-distribution boards and motor control centres. These protective devices must discriminate with those further down the line nearer the ultimate loads. A system study must therefore establish the correct ratings of the substation equipment to discriminate with the distribution network.
Earthing of equipment should be electrically complete and confirmed mechanically sound and tight.
Earthing conductors (previously termed earth leads) must be checked for compliance with the IEE Regulations, i.e. they must not be aluminium and they must be not less than 25 mm2 for copper and 50 mm2 for steel, unless they are protected against corrosion. These conductors are for connection to the earth electrodes.
The protective conductors previously known as earth continuity conductors must also comply with BS 7671 (the IEE Regulations) and in general for phase conductors of less than 16 mm2; this means the protective conductors must be the same size as the phase conductors. When the phase conductor is above 16 mm2 then the protective conductor remains at 16 mm2 until the phase conductor is 35 mm2, after which the protective conductor should be half the cross-sectional area of the phase conductor.
Another important point to bring out is that the earthing conductor to the earth electrode must be clearly and permanently labelled ‘SAFETY ELECTRICAL CONNECTION – DO NOT REMOVE’ and this should be placed at the connection of conductor to the electrode.
Fuse ratings should also be checked in relation to other fuse ratings in the supply circuit or against the settings of protective relays to assure correct sequence of operation and discrimination. Circuit charts for distribution boards should be completed and designation labels fitted to ensure safe operation of switches and isolators.
All tests should be carried out as required in BS 7671, Part 7, and an Electrical Installation Certificate given by the contractor to the person ordering the work.
Many installations now incorporate rcds and fault current operated protective devices. These also must be tested using appropriate test equipment, full details of which can be found in BS 7671 or for more elaborate apparatus in BS 7430 and Guidance Notes which are published separately and amplify the requirements in the British Standard.
The nominal voltages at present are:
- 230V + 10% and -6%
- 400V + 10% and -6%
Reference: Handbook of electrical installation practice fourth edition – Eur Ing GEOFFREY STOKES
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