DIY How To

RCD Principles

It is recommended to use an RCD when using portable hand operated 240v electrical equipment. Ideal uses in the garden are lawnmowers, hedge trimmers, leaf shredders etc. In the home it is wise to use one with power drills, electric tile cutters etc.

RCD Principles

When a Load is connected to the circuit supplied through an RCD the incoming supply, phase and neutral, passes through an iron core or toroid, which acts as the primary winding of a current transformer.

Diagram of how an RCD works

A secondary winding around the toroid is used as a sensing coil to detect any out of balance between the current flowing through the phase and neutral conductors.

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 sensing coil.

If a 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 sensing coil which if of sufficient magnitude and duration will cause the actuator to function and trip the RCD thereby disconnecting the supply.

A test circuit is also incorporated whereby a connection is made from the load phase to supply neutral via a Test Coil and Test Resistor and activated by a Test Button.

This test circuit is designed to pass a current well in excess of the related tripping current of the RCD in question.

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