Video- How Does An RCD Work?

by Editor on November 3, 2011

An RCD device (Residual Current Device) can also be known as-

an RCBO (Residual Current Circiut Breaker) has both RCD and MCB combined for leakage and overload protection

an ELCB (Earth Leakage Circuit breaker) Old UK Terminology

a GFCI / GFCB (Ground Fault Circuit Interrupter/Breaker) in the USA and Canada

The RCD device is designed to cut off the supply within 40 mS (milli seconds) when an earth fault current (residual) is detected. Where a 30 mA (milli Amp) tripping RCD is used on a 240 volt supply, this is fast enough to prevent a fatal electric shock.

Typical scenarios for an earth fault to occur would be when the electrical wiring in an installation or to an appliance is damaged, becomes loose or gets wet. This could cause the live parts of the circuit to leak across or connect to earth (residual) and create a danger of electrocution should anyone come into contact with it.

Please Note: ‘Nuisance Tripping’ is usually happening for a reason and although it may be annoying you should find out the reason and rectify the fault. Don’t just blame the RCD device!

{ 1 comment… read it below or add one }

JimS November 6, 2012 at 23:11

Big ‘S’ is the SI unit of conductance, little ‘s’ is the SI unit of time (second).

Residual is not another term for ‘earth’, it is the ‘left over’ current when the currents in the Live and Neutral wires are compared. In a proper functioning circuit the current flow is that through the load and obviously the Live and Neutral current is the same. If they aren’t the same then there must be another path for a current to flow. Usually that will be someone touching the live wire or an insulation breakdown from Live or Neutral to Earth. That current can only flow, of course, if there is a supply side connection to Earth. Conventionally Neutral is ‘Earthy’ so touching the Neutral is unlikely to trip the RCD as the available voltage difference is too small to produce a large enough difference (residual) current. A direct Neutral/Earth contact will invariably trip the RCD.

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