How Does an RCD Work....?

RCDs (Residual Current Devices)

RCD is a generic term which embraces the various forms such as RCCB, RCBO, etc.
Residual Current Devices are now firmly established around the world as a primary means of providing protection against electrocution and fires caused by electrical faults. Less than one quarter of an amp (250mA) leaking from a faulty installation can generate sufficient heat to start a fire (the heating effect is proportional to the current squared), or if leaking through a human body for only 200mS can cause heart fibrillation and subsequent death.

An RCD protects by constantly monitoring the current flowing in the live and neutral wires supplying a circuit or an individual item of equipment. Under normal circumstances, the current flowing in the two wires is equal. When an earth leakage occurs due to a fault in the circuit or an accident with the equipment, an imbalance occurs and this is detected by the RCD, which automatically cuts off the power before injury or damage can result.

To be effective, the RCD must operate very quickly at a low earth leakage current. Those designed to protect human life are engineered to trip out with an earth leakage current of 30mA within 200mS and at a higher earth current of 150mA, they will trip in less than 40mS. These limits are well inside the safety zone, within which electrocution or fire would not be expected to occur.

The medical aspect of an electric shock

The effects of electricity on the human body have been widely studied and researched by the IEC and its findings have been published in several reports (IEC publication 479-1, 1984 and 479-2, 1987). The reports contain a number of useful graphs, showing the effects of both AC and DC currents and also the influence of frequency, and are recommended for further reading. From the reports and other sources, it may be observed that an electrical shock, whilst not always sufficiently serious to cause death, can still have a long term adverse effect on a person’s health.
Much of the data generated refers to adult people in good health at the time of the shock, but if the victim is a child or a person in poor health, the effects can be more serious and the need for RCD protection is even greater.

The degree of risk depends not only on current, but also on time – the higher the current or the longer the time of shock, the greater the danger. In considering a 230V 50Hz AC supply, the following effects of current are typically observed.

How Does an RCD Work....?

RCDs (Residual Current Devices)

The medical aspect of an electric shock

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