16 Electric Wiring: Domestic not conform to the IEE Regulations regarding ‘installation circuit arrangement’. The requirement is that circuits which need to be separately controlled, for example lighting and power, remain energized in the event of the failure of any other circuit of the installation. Hence, an earth fault on, say, a socket-outlet circuit would cause the whole of the installation to be cut off if protected by one 30 mA RCD. One preferred arrangement is to protect the whole installation by a 100 mA RCD and, using a ‘split-load’ consumer unit, protect the socket-outlet circuits with a 30 mA RCD (Figure 2.2). Meter kwh
Consumer unit Bonding to gas and water, etc. Cut-out
Earthing conductor
Main earthing terminal
Earth clamp to armouring Service cable
FIGURE 2.2
TN-S system.
Alternatively, combined RCD/CB devices (RCBOs) may be used to protect each circuit individually (Figure 2.3). In many domestic situations, ‘off-peak’ electricity is used, as this can be a means of reducing electricity bills. Energy is consumed out of normal hours, for example 11.00 pm to 7.00 am, and the tariff (the charge per unit of energy used) is a lot less. This arrangement lends itself to the use of storage heaters and water heating, and the supply intake equipment will incorporate