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A Guide to Using Residual Current Devices

1. What is an RCD?
2. In what circumstances can electrocution occur?
3. How does the RCD work?
4. How should an RCD be installed?
5. What should the tripping current be?
6. What can RCDs provide protection for?
7. What do RCDs NOT provide protection for?
8. What to do if the RCD trips?
9. Testing
10. Why do RCDS respond faster to high fault currents than lower ones?
11. What kind of RCDs are there?

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1. What is an RCD?

   A Residual Current Device, is an electrical safety device which breaks a faulty circuit. When installed correctly, an RCD can detect faults in the electrical system and is designed to disconnect quickly enough to prevent potential injury or death from electrocution by switching off the supply almost instantly.

2. In what circumstances can electrocution occur?

   RCDs provide protection against electrical currents flowing to earth. This can occur when a person accidentally touches a live part or metalwork of an electrical system, for example by running over a cable with a lawnmower. They would then suffer an electric shock from the current flowing through their body to earth. Or, if the flow was through wiring or electrical appliances this could cause a fire hazard.

3. How does the RCD work?

   When an electrical system is working properly the phase and neutral currents are equal and opposite, however, when a fault occurs and current flows to earth, the phase and neutral currents will no longer be balanced, and the RCD will react to this and cut the incoming supply almost simultaneously.

4. How should an RCD be installed?

   Ideally the RCD should be built into the main switchboard so that the electrical supply is permanently protected. Alternatively an RCD can be built into an electrical socket outlet, or a plug in RCD adaptor can be used.

   However, be wary of using the same RCD to protect the entire electrical system as a fault in one area could result in a complete loss of power. For example, using the same RCD as the one connected to the lighting circuit could cause a dangerous blackout should the RCD trip.

5. What should the tripping current be?

   To be effective enough to prevent accidents an RCD should have a tripping current of 10mA or 30mA. An RCD with a trip range of 30mA greatly reduces the risk of heart fibrillation and death. Higher tripping currents are used to protect against fire, in which case the RCD would have an operating current of 100mA or 300mA .

6. What can RCDs provide protection for?

   RCDs can be used for both direct and direct contact with an electrical system. It provides protection against substandard wiring and unearthed equipment Whilst short circuits and current surges are covered by fuses and MCBs, only an RCD will prevent an electric shock.

7. What do RCDs NOT provide protection for?

   Faults which do not come from an external leakage, overloads, or short circuits between phase and neutral or phase to phase. For this, over current protection in the form of a fuse or circuit breaker must be employed.

8. What to do if the RCD trips?

   Do not use the electrical system again until it has been checked as the RCD is designed to trip when there is a fault. It is possible that the RCD itself is faulty, in which case both should be tested to eliminate risks.

9. Testing

   Do not assume that the RCD will always be working correctly. RCDs have a test button which should be checked each time the electrical system is used to ensure it is functioning properly.

10. Why do RCDS respond faster to high fault currents than lower ones?

    RCDs are designed to do this to avoid nuisance tripping which can be caused by minor occurrences such earth leakage, voltage spikes and surges.

11. What kind of RCDs are there?

    There are four types of RCD:

    1. RCCB, Residual Current Circuit Breaker;
    2. RCBO, Residual Current Breaker with Over current protection;
    3. SRCD, Socket outlet fitted with an RCD function; and
    4. PRCD, Portable plug or adaptor with RCD function.

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