Handbook of Electrical Installation Practice - BeKnowledge

Substations and Control Rooms 63
Distribution boards and switch-fuses
After testing all insulation, phasing and earth continuity ensure that the ratings and
types of all fuses are suitable for the circuits before they are energised. Make sure
that the fuses are the right type for the fuse carriers provided, and that the ratings
are correct for the size of the cable or for the load supplied, whichever is the smaller.
This will also confirm that the circuit wiring is correct for the load to be supplied.
HBC fuse carriers must not be fitted with wire elements because the enclosure is
not suitable.
In distribution boards wiring disposition should be examined and all wires dressed
clear of live metal work.
Terminal connections on incoming and outgoing cables must be tested for tightness,
correct stripping of insulation and clearances. Where a number of neutrals
enter a distribution board they should be sensibly disposed along the neutral bar in
the same order as that of the live conductors.
Earthing arrangements
The earthing of l.v networks in the UK is largely determined by the l.v. supplies.
However, if the incoming supplies are at 11kV and the transformers are in the
ownership of the user, the l.v. 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. For 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 crosssectional
area shall be the same size as the phase conductor up to 16mm2 .When
the phase conductor is above 16mm2 then the protective conductor may remain at
16mm2 until the phase conductor is 35mm2 , 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