Trade & Electrical Guides
June 8, 2026

Residual Current Devices (RCDs): How They Work & Why They Trip

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A residual current device (RCD) is a safety switch that cuts the power in a fraction of a second when it detects electricity leaking to earth, for example through a person. It is one of the most important safety devices in your home. In New Zealand, an RCD with a maximum rated residual current of 30mA is required on household circuits that supply your power points and your lighting, under the New Zealand Wiring Rules (AS/NZS 3000).

We are The Conduction Company, registered electricians and electrical inspectors in Auckland. Below we explain what an RCD does, why yours might keep tripping, and when the rules require one, in plain language.

What does an RCD do?

A residual current device (RCD) is a safety switch that constantly compares the current flowing into a circuit with the current flowing back out. If the two no longer match, which can happen when current leaks to earth through a person or a fault, the RCD switches the power off in a fraction of a second. This is what protects you from a serious electric shock, and it is why RCDs are fitted in homes across Auckland and New Zealand.

A RCD is an electrical protective device, usually found within your switchboard. Sometimes you’ll find them built in to a socket outlet or as part of an extension lead.

When are RCDs required in New Zealand?

In New Zealand, the Wiring Rules (AS/NZS 3000) require additional protection by an RCD with a maximum rated residual current of 30mA on the final subcircuits that supply socket-outlets and lighting points in homes. In practice that means the power points and lights in a New Zealand house should be protected by 30mA RCDs, installed at the switchboard the circuit runs from. The Conduction Company are registered electricians and electrical inspectors in Auckland, and confirming this protection is part of a proper switchboard inspection.

There are some exceptions to the rule, but in general it is always a good idea to have them installed. Schools, childcare centres, and patient areas within medical facilities are required to have specific RCDs installed for a greater level of protection.

Why is my RCD tripping?

There are many different causes for a tripping RCD. The most likely cause of tripping is a faulty appliance. Another common cause is outdoor electrical fittings (sensors, lights, outdoor outlets, and so on). These fittings can become waterlogged, which causes current leakage as water comes into contact with live parts within the fitting.

If your residual current device tripped after water-blasting or heavy rain, this is likely the cause. The RCD trips to protect you from electric shock.

The RCD itself can also malfunction and start ‘nuisance’ tripping. If you are unsure what fault is occurring, give us a call or an email. Our team will diagnose the issue and get your electricity restored quickly.

What can I do to fix it?

If your RCD tripped after turning on or plugging in an appliance:

  • Disconnect the appliance, then reset the RCD to see if it trips again.

If the RCD tripped randomly:

  • Leave the RCD off, then test all appliances that are plugged in.
  • Unplug every device that won’t turn on, then switch the RCD back on.

If the RCD doesn’t trip:

  • Start plugging your appliances in one by one. If it trips again, you have identified the faulty appliance.

If the RCD trips immediately:

  • Contact a qualified electrician to investigate further.
Office building with electrical wiring - Commercial electrical services in Auckland
How does an RCD prevent electric shock?

An RCD detects electric shock by monitoring the balance between the live and neutral conductors. If a current imbalance occurs, such as when electricity flows through your body to the earth, the RCD detects this and rapidly disconnects the circuit, preventing harm. This happens fast enough to protect a person from a serious or fatal electric shock.

Is an RCD the same as a circuit breaker?

An RCD is not the same as a circuit breaker. A circuit breaker protects the wiring from overload and short circuit, and will not necessarily trip if current leaks through a person. An RCD is built to detect that leakage to earth and disconnect fast enough to protect a person from a serious shock. Many modern switchboards use combined RCBO devices that do both jobs in one. The Conduction Company can check what is fitted in your Auckland home and whether it meets the current Wiring Rules.

Why do both the lights and power turn off when the RCD trips?

In New Zealand, it’s common for electricians to install a single RCD to protect multiple circuits, usually 2 power circuits and 1 lighting circuit. If an RCD trips on a power circuit, it may also affect lighting circuits. For greater protection, install one RCD per circuit.

How to test your RCD

As good practice, test your RCD every 3 months. Press the test button marked "T" or "TEST" on the switchboard, check the power switches off straight away, then switch it back on. A 3-month check is recommended advice to make sure your safety switch still works, not a legal requirement. If the RCD does not trip when you press the test button, it may have failed, and you should have it checked by a registered electrician.

If you are uncertain about the level of protection needed for your installation, we offer RCD testing and installation services in Auckland to make sure your system meets current safety standards. A worn or outdated switchboard is often the place to start, so it can be worth considering a switchboard upgrade if yours is ageing.

Frequently asked questions

What does an RCD (residual current device) do?

An RCD is a safety switch that turns the power off in a fraction of a second when it detects electricity leaking to earth, for example through a person who has touched a live part. It is designed to protect people from a serious or fatal electric shock.

Are RCDs required in New Zealand homes?

Yes. The New Zealand Wiring Rules (AS/NZS 3000) require additional protection by an RCD with a maximum rated residual current of 30mA on the household circuits that supply socket-outlets and lighting points. The RCD is installed at the switchboard the circuit runs from.

Why does my RCD keep tripping?

An RCD trips because it has detected current leaking to earth somewhere on the circuit. The usual causes are a faulty appliance, moisture getting into wiring or fittings, or a worn or damaged cable. The fastest way to find the cause is to unplug appliances on the affected circuit one at a time. If it keeps tripping with everything unplugged, the fault is in the wiring and you should call a registered electrician.

Is an RCD the same as a circuit breaker?

No. A circuit breaker protects the wiring from overload and short circuit. An RCD detects electricity leaking to earth and disconnects fast enough to protect a person from a shock. A combined RCBO does both jobs in one device.

Can I test my own RCD?

Yes. Every RCD has a test button marked "T" or "TEST". Pressing it should switch the power off straight away, which confirms the RCD is working. If pressing the test button does nothing, the RCD may have failed and you should have it checked by a registered electrician.

How often should I test my RCD?

As good practice, test your RCD every 3 months. Press the test button marked "T" or "TEST" and check the power switches off straight away, then switch it back on. This is recommended advice to keep your safety switch working, not a legal requirement. If the RCD does not trip when you press the test button, have it checked by a registered electrician.

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