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Knowledgebase Article

What is an RCD?


You may have heard the term "RCD" or maybe "Earth Leakage Relay" or even "Safety Switch" from an electrician and wondered exactly what it was. Well wonder no more, today we will explain to you exactly what it is and why it is important to have on your house.

It has been legislation for some time now that RCDs be installed on all rental properties, schools and public buildings. Many houses (particularly new ones) now have RCDs installed, but data shows that around 30% of houses still don't have one.

RCD stands for "Residual Current Device". We know - it doesn't make a whole lot of sense to us either. An RCD is a device that detects if there is a small amount of current flowing to earth.

To make it simple, in an electrical circuit current should flow from the "active" wire to the thing needing power (e.g. toaster, light, fridge etc) and then flow back out through the "neutral" wire. You can think of electricity like water flowing through a pipe, out of your tap and then back down the drain. But what if the sink started to overflow? In this case not all of the water that came out of the tap went back down the drain. Instead you would have water flow onto the floor - not a good thing. The same can happen with electricity - if not all of the electricity that comes from the active wire goes back down the neutral wire you have a problem on your hands.

Usually when this happens it either means you have a bare wire somewhere touching the earth (a potential fire hazard) or you have a person who is being electrocuted.

An RCD is designed to detect when this happens and then trip the circuit. RCDs used in houses are designed to trip within 40 milliseconds if they detect any more than 30 milliamps of current difference. This is very important, because tests have shown that is all it takes to electrocute somebody. It's scary to think that it only takes 0.03 amps of current to electrocute you when the outlet on your wall can typically supply 10 amps of current all the time.

One RCD can be used to protect your entire house, or you can also have seperate RCDs on each circuit. The second option is preferred on newer houses because you can isolate the fault easier. On older houses it may be much easier to just retrofit one RCD instead of changing all the circuits.

RCDs can also be combined with circuit breakers, and are called combination RCD/MCBs or RCBOs. A circuit breaker performs a similar but slightly different safety function (it is there to cut the power supply in the event of a short circuit).

It is important to protect your house with an RCD. Merlin Discount Electrical has a number of affordable RCDs for sale that comply with Australian standards, making sure to trip within 40 milliseconds if more than 30 milliamps is "leaked". For only a few dollars you can help protect your loved ones from electrocution.