What is an RCD and what does it do?

A residual current device (RCD) is a protective device used to disconnect the electrical supply automatically when an imbalance (the residual current) is detected between Line and Neutral conductors. The device normally operates when the residual current reaches a preset limit (IΔn).

RCD operation

RCDs are primarily intended to give protection against the risk of injurious electric shocks and to provide an additional protective measure for insulation faults and fire hazards due to a persistent earth fault current.

An RCD does not provide overcurrent protection, which is usually provided by a fuse or a circuit‑breaker. Combined RCDs and circuit-breakers are available and are called RCBOs.

RCDs are defined by three main characteristics:

  • the rating in amps
  • the rated residual operating current of the protective device in amps, known as the IΔn (pronounced ‘I delta N’), and
  • the instantaneous trip that occurs or an intentional time delay to permit discrimination*. Such devices are called ‘S’ or Special/Selective.

Are RCDs reliable?

We’ve found that fixed RCDs are about 97% reliable. This improves if they are tested regularly. If you have fixed RCD protection, it will reduce the risk of electric shock to you and your family. It can also protect your home against the risk of fire caused by faulty wiring or appliances.

Remember – Although RCD protection reduces the risk of death or injury from electric shock it does not reduce the need to be careful. Have your wiring checked at least once every 10 years to ensure the safety of you, your family and your home.

Don’t forget to test – You should test all fixed and socket RCDs about every three months. Manufacturers recommend that portable RCDs are tested every time you use them.

Beware – If you hold the test button in for a long time and the RCD does not switch off the electricity supply, then get advice from a registered electrician.

The UK standard for safety – Since July 2008 virtually all circuits in new or rewired homes have been required to include an RCD under the latest edition of BS 7671.