Handbook of Electrical Installation Practice - BeKnowledge

114 Handbook of Electrical Installation Practice
Fig. 5.7 Calculation of prospective fault current.
If the Electricity Distributor advises on the fault level at the incoming terminals
the prospective line/earth fault current can be determined at any point on the
site.
The values of I f and the voltage drop to the sub-mains positions should be noted
as subsequent calculations and material selection involve these values.
The fault level provides the information necessary to select the protective device
at the main intake position. The breaking capacity rating has already been ascertained
relative to the prospective fault current and it is now necessary to decide the
speed of operation of the main device.
BS 7671 (the IEE Regulations) stipulates that the maximum disconnection times
under fault conditions are 5 seconds for fixed equipment and 0.4 seconds where the
circuit feeds socket-outlets. However, this latter requirement does not apply to
reduced voltage circuits described in Regulation Group 471-15. Similarly BS 7375:
1996: Distribution of electricity on construction and building sites, advises that protection
against earth fault may be obtained in two ways:
(1) By providing a low impedance path to enable the overcurrent device protecting
the installation to operate in a short space of time;
(2) By adopting a TT system with residual current device (rcd) and associated
installation earth electrode.
When the earth path is via cable armour as in (1) above and Fig. 5.7, extreme
care is required in protective device selection. The armour impedance may result in
insufficient current flowing to allow the device to trip within the required time, and
changes to specifications do occur; such things as cable routes, equipment positions,
etc. could be revised, and it may be necessary to install a cable with high conductivity
armour, use an additional parallel protective conductor, or provide an rcd as
in (2) above.
The types of load can vary enormously, so the area distribution unit needs to be
versatile enough to incorporate a variety of final circuit protection devices. The
largest single load is probably the local tower crane where the motors may total
some 80kVA.Various factors need to be considered such as motor starting or dampness,
and because of these a final circuit device providing protection against earth
leakage in addition to overcurrent is recommended, particularly if the main site protection
includes earth leakage protection. Careful selection of rated tripping current