What is an RCD and how many do I need

RCD stands for residual current device. Most people call it a safety switch. It is the single most important electrical safety device in a home, and plenty of older houses around the Illawarra still do not have enough of them, or any at all.

What an RCD does

Electricity flows out to an appliance on the active conductor and back on the neutral. In a healthy circuit those two currents match. If some current is leaking to earth, perhaps through a damaged cable, a wet fitting or a person touching something live, the currents no longer match.

An RCD measures that difference continuously. When the imbalance reaches a small threshold, typically 30 milliamps for household circuits, it disconnects the power in a fraction of a second. That speed is what makes the difference between a fright and a serious injury.

How it differs from a circuit breaker

A circuit breaker protects cables from overload and short circuits. It does not detect leakage to earth, so it will not trip if a person is receiving a shock at normal current levels. An RCD protects people. Most homes need both. Read ceramic fuses vs circuit breakers for more on overload protection.

Types you will see

• RCD: a safety switch protecting one or several circuits
• RCBO: an RCD and circuit breaker combined, protecting a single circuit
• Portable RCD: a plug-in unit, useful for power tools, but not a substitute for protection at the board

How many do I need

There is no single number for every house, but there are good principles.

Every final circuit should be protected

Current wiring rules in AS/NZS 3000 require RCD protection for lighting and power circuits in new installations and for many alterations. Older homes may have been compliant when built but fall short of today's standard.

Do not put everything on one

If one RCD protects all circuits, a single fault cuts power to the whole house. The rules limit how many circuits can share one RCD for this reason. Spreading lighting and power circuits across at least two RCDs means that when one trips, you still have light to see by.

RCBOs give the best separation

With an RCBO on each circuit, a fault on the laundry power only affects the laundry. It costs more space and components than a couple of shared RCDs but makes tripping much easier to diagnose and far less disruptive.

Test them regularly

Press the test button on each RCD every few months. It should trip immediately. Reset it afterwards. If it does not trip, call an electrician. Electricians use dedicated test equipment to confirm trip times, which is part of what we do on every safety switch job.

If you rent, it is worth asking your landlord or agent whether the property has safety switches. Many tenants never check, and it is a simple question to raise at the start of a lease.

When to add more

• Your home has fuses and no safety switches
• One RCD covers every circuit
• You are adding a pool, spa, sauna or outdoor power
• You are renovating or renting the property out
• You have young children or older relatives living with you

Adding RCDs usually happens at the switchboard. If the board is old or full, it may make sense to upgrade the board at the same time. Our switchboard services page covers that.

Next step

Not sure what your home has? Text a photo of your switchboard with the door open to Kalani on 0424 400 346, or call. We will tell you what protection is there and what would make your home safer.