16th Edition (reference only) – NOW superseded by the 17th Edition IEE Regulations.

chapter 5

chapter 6

  5.1 - The earthing principle 5.6 - Protective multiple earthing (PME)
  5.2 - Earthing Systems 5.7 - Earthed concentric wiring
  5.3 - Earth fault loop impedance 5.8 - Other protection methods
5.4 - Protective conductors 5.9 - Residual current devices (RCDs)
5.5 - Earth electrodes

5.10 - Combined functional and protective

5.9.1 - Why do we need residual current devices?

{5.3} has stressed that the standard method of protection is to make sure that an earth fault results in a fault current high enough to operate the protective device quickly so that fatal shock is prevented. However, there are cases where the impedance of the earth-fault loop, or the impedance of the fault itself, are too high to enable enough fault current to flow. In such a case, either:

1. - current will continue to flow to earth, perhaps generating enough heat to start a fire, or

2. - metalwork which is open to touch may be at a high potential relative to earth, resulting in severe shock danger.

Either or both of these possibilities can be removed by the installation of a residual current device (RCD).

In recent years there has been an enormous increase in the use of initials for residual current devices of all kinds. The following list, which is not exhaustive, may be helpful to readers:

RCD residual current device
RCCD residual current operated circuit breaker
SRCD socket outlet incorporating an RCD
PRCD portable RCD, usually an RCD incorporated into a plug
RCBO an RCCD which includes overcurrent protection
SRCBO a socket outlet incorporating an RCBO

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Extracted from The Electricians Guide Fifth Edition
by John Whitfield

Published by EPA Press Click Here to order your Copy.

Click here for list of abbreviations