How is a Residual Current Device or RCD different to a Ground Fault Circuit Interrupter or GFCI?

Answer 1

Nowadays terms Residual Current Device or RCD and Ground Fault Circuit Interrupter or GFCI refer to the same type of electrical safety device.

If there is no fault anywhere in a circuit supplying single-phase alternating power to a unit such as an electrical appliance, machine or other equipment, the current flowing to the unit at any instant in the "hot" or "live" wire should exactly match the current flowing away from the unit in the neutral wire. Similarly, there should be no current flowing in the unit's safety "ground" or "earth" wire.

It is a basic fact of electrical engineering design that all current flowing to an electrical appliance, machine or other equipment from the power generation station via its supply circuit's "hot" or "live" wire should only return to the power station via that same circuit's "neutral" wire.

So, as a result of that basic fact, if any current isflowing in the ground wire, it must be caused by a fault condition and the supply of current to the circuit needs to be stopped urgently. Many years before today's electronic RCDs or GFCIs were designed, much simpler electro-mechanical relays called Earth Leakage Circuit Breakers (ELCBs) were invented so that, if any such ground current exceeding just a few milliamps was detected, they would "trip" - meaning "operate" - to break the current supply to the circuits for which they were installed to protect.

An Earth Leakage Circuit Breaker was the first name given to what is now called a Ground Fault Circuit Interrupter (GFCI) . The original type of ELCB or GFCI was designed only to detect a current flowing in the safety "ground" or "earth" wire.

So the original type of ELCB or GFCI did not check for any difference in current flowing in the live and neutral wires, a difference that would be an indication of another type of serious fault condition. Such a fault can be serious because, even if no current can be detected flowing in the ground wire, the "missing current" may actually be flowing to ground via someone's body!

After miniature electronic circuits were invented the invention of RCDs became possible: an RCD constantly checks for small differences in the currents flowing in the live compared to the neutral wire and for any current flowing in the earth wire. If any such "fault current" is detected, the device is designed to act very fast to shut off the supply of power to the circuit and the unit(s) it is protecting, because the "missing current" may actually be flowing to ground via someone's body!

That is the reason why, when RCDs were invented, most manufacturers of GFCIs adopted the same technology because it offers so much more protection to users than the original GFCI could ever give.

In the US and Canada such devices are still commonly known as "GFCIs" or "GFIs" even though they have the additional "residual current detector" functionality, whilst in Europe and elsewhere the more accurate name of "Residual Current Detector" or RCD has been widely adopted for general use instead of using the name of the originally much simpler GFCI device.

For more information on all these topics see the answers to the Related Questions and the Related Link shown below.

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As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

Before you do any work yourself,

on electrical circuits, equipment or appliances,

always use a test meter to ensure the circuit is, in fact, de-energized.

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REFER THIS WORK TO QUALIFIED PROFESSIONALS.