A residual current device (RCD) should not be used in an installation in such a way that its operation would be liable to cause danger or inconvenience.
An example of such inappropriate use of an RCD is shown in the diagram in Figure 1. As can be seen from the diagram, an earth fault in any of the circuits supplied from the distribution board, or in equipment supplied by the circuit, would cause the RCD to operate. This would result in a loss of supply to all of the circuits, possibly causing inconvenience or even danger, for example the risk of a person falling down stairs, having been plunged unexpectedly into darkness.
Regulation 314-01-01 of BS 7671 requires, amongst other things, that:
‘Every installation shall be divided into circuits as necessary to avoid danger and minimise inconvenience in the event of a fault.’
Furthermore, Regulation 314-01-02 requires that:
‘A separate circuit shall be provided for each part of an installation which needs to be separately controlled for compliance with the Regulations or otherwise to prevent danger, so that such circuits remain energised in the event of failure of any other circuit of the installation, and due account shall be taken of the consequences of the operation of any single protective device’.
Most installations include socket-outlets rated at 32 A or less which may reasonably be expected to supply portable equipment for use outdoors. Such socket-outlets are required by Regulation 471-16-01 to be provided with supplementary protection against direct contact by means of an RCD having a rated residual operating current (IDn) not exceeding 30 mA and an operating time not exceeding 40 ms when type-tested at a residual current of 5 times IDn. The electrical installation designer may be tempted to meet this requirement by employing an arrangement such as that shown in Figure 1.
However, to do so would result in a non-compliance with the requirements of Regulations 314-01-01 and 314-01-03, already mentioned, due to the possibility of danger or inconvenience being caused in the event of the RCD operating.
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(Article originally published in Connections 157)