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How Does An RCD Work?

RCD's work on the current balance principle, using Kirschoff's First or Node Law. That is the algebraic sum of all the electric currents meeting at any point in a circuit must be zero, otherwise the circuit will be broken.

In a RCD the incoming supply, phase and neutral, passes through an iron core or toroid, which acts as the primary winding of a current transformer. A secondary winding around the toroid is connected to the trip mechanism.

Under normal conditions the phase and neutral currents are equal and opposite so no flux is induced in the toroid and hence no current flows in the secondary winding.

If an insulation fault occurs and current flows to earth, the phase and neutral currents will no longer be balanced. A flux will be induced in the toroid and a current will flow in the secondary winding which will activate the trip mechanism and cut the incoming supply.

The trip mechanism operates when the residual current is between 50% and 100% of the rated tripping current, as required by the relevant British Standards.

Fire Protection

Many electrical fires are caused by earth leakage current due to faulty wiring. In fact, leakage current of 1A or less can start a fire. Yet normal protection devices will not detect such low current levels. However, RCD's will trip out and greatly reduce the risk of an electrical fire.

Personal Protection

The safety of an electrical installation is reduced, and the risk of a person receiving an electric shock is increased by substandard, incompetently wired or unearthed equipment. Whilst fuses (rewireable/HRC) and MCB's provide protection against short circuits and current surges, they do not protect against current flowing through the human body.

A RCD (available in many different models) will detect small currents and greatly reduce the risk of death due to electrocution by cutting the flow virtually instantaneously.

The IEC Publication 479 gives details of the effect of current passing through the human body and relates current flow to time. Essentially the longer the current flow and the larger the current, then the grater the probability of heart fibrillation (asphyxiation as the muscles surrounding the heart pulse at different rates to the heart) and even death.

A RCD with a trip range of 30mA greatly reduces the possibility of heart fibrillation and death.

Wiring Regulations

All products comply with the 16th Edition IEE wiring regulations.


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