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.
What does RCD stand for?
RCD stands for residual current device. The Conduction Company uses the term the way the New Zealand wiring rules do: a device that switches a circuit off when current leaking to earth goes above a set level (AS/NZS 3000, clause 2.6.2.1). Residual current is the difference between the current going out on a circuit and the current coming back. You will also hear it called a safety switch or an RCCB.
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.
Spa pools carry their own RCD rules, which The Conduction Company explains in its guide to spa pool electrical requirements. The Conduction Company's spa pool wiring service fits that protection on every spa supply.
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.
What circuits need RCD protection?
In a New Zealand home, three kinds of circuit need a 30 mA RCD: circuits supplying power points, circuits supplying lighting points, and circuits supplying directly connected hand-held equipment, such as a hard-wired hair dryer (AS/NZS 3000, clause 2.6.3.3.1). The Conduction Company checks all three when it works on a switchboard.
That is New Zealand's rule, and it is narrower than Australia's. The Australian clause of the same standard requires RCDs on all final subcircuits in a home (clause 2.6.3.2.2). Much of the advice online is Australian, which is why you will often read that every circuit needs an RCD.
Is an RCD a legal requirement in an existing house?
In The Conduction Company's reading of the wiring rules, only when work on the house brings the rule in. The RCD requirement in AS/NZS 3000 applies to complete new circuits, and the standard deals with additions and alterations in a separate clause (clause 2.6.3.3.1, note 2, and clause 2.6.3.3.4).
Three kinds of work bring RCDs in. A new circuit for power points or lighting needs one. Power points added to an existing circuit need RCD protection. And replacing all of the circuit protection on a switchboard brings every circuit on that board under the rule. Replacing an existing power point that has no RCD does not, on its own. So adding two power points to an old circuit means RCD protection for the new power points, which the standard allows at the start of the added wiring, though it prefers the switchboard. That rule does not by itself call for a whole new switchboard.
When is an RCD not required?
Under AS/NZS 3000, an RCD is not required when replacing a socket outlet that is not RCD protected, or when adding lighting points to an existing lighting circuit. Added socket outlets are different and do need RCD protection. The Conduction Company checks which exception applies before deciding an RCD is not needed.
Lighting points cover more than light fittings. The standard also counts combination fan, light and heater units, smoke alarms, exhaust fans and ceiling sweep fans as lighting points, so adding an extractor fan to an existing lighting circuit does not by itself call for a new RCD. Bathrooms and other wet areas have their own rules, which can still require one. A socket for a fixed cooking appliance, like an oven or hotplate, is also exempt, provided it is unlikely to be accessed for general purposes and is clearly marked for that use. Two things bring the full requirement back. If all of the circuit protection on a switchboard is replaced, every circuit it supplies needs RCD protection, and a full rewire or a brand new subcircuit needs a 30mA RCD on the circuits supplying power points and lighting in a home.
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.
An RCD that keeps tripping with everything unplugged needs RCD fault finding, which The Conduction Company does across Auckland.
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.
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.
How long should a 30mA RCD take to trip?
No more than 300 milliseconds when 30 mA leaks to earth. That limit comes from AS/NZS 3190, the RCD standard, and it is the figure The Conduction Company tests to. The more current leaks, the faster the RCD must trip: for a 30 mA RCD the standard sets 150 milliseconds at twice that leakage and 40 milliseconds at five times it (Table 5).
The New Zealand wiring rules, AS/NZS 3000, require household RCDs to comply with AS/NZS 3190 or its sister standards (clause 2.6.2.2.1). In New Zealand, testing a newly installed RCD must also include its trip time, measured with test equipment rather than the test button (clause 8.3.10, as changed for New Zealand by the Electricity (Safety) Regulations). So when The Conduction Company fits an RCD, it measures how fast it trips before signing the work off.
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. The Conduction Company sets out what changed in New Zealand's electrical regulations for 2026, including the new rules on choosing and testing RCDs.
What does an RCD protect against, and does it protect against overload?
An RCD protects people against electric shock from current leaking to earth. It does not protect against overload. The Conduction Company explains the split the way AS/NZS 3000 sets it out: an RCD switches off when earth leakage passes a set value (clause 2.6.2.1), and protection against overcurrent is a separate requirement (clause 2.5.1.1), met by a circuit breaker. An RCBO does both jobs in one device. If the power goes off when too many appliances run at once, a circuit breaker or an RCBO has tripped, and The Conduction Company's guide to why a circuit breaker keeps tripping covers the causes.
How many times can a breaker trip before it goes bad?
There is no fixed number. When The Conduction Company checked the standards and the makers' own documents, the answer depended on how much current the breaker breaks each time. Switching on and off is what a breaker is built for. The international standard for household circuit breakers, IEC 60898-1, tests one for 4,000 on-off cycles at full rated current, and Schneider Electric rates the Resi9 breaker it sells in New Zealand for 10,000.
A trip on a real fault is harder on it. Schneider Electric says its breakers are designed to withstand 12 overloads at six times their rating, and that a breaker which has cleared a short circuit at its full rated breaking capacity should be replaced. That is why The Conduction Company's advice is to reset a tripped breaker once, and if it trips again, leave it off and have the circuit tested.
The Conduction Company also checked whether an RCD can take more trips than a breaker. It is not built to: the standards set a lower minimum for RCDs and RCBOs, 2,000 operating cycles against 4,000 for a breaker, and Schneider's own ratings put its RCDs above, level with, or below its breakers, depending on the range.
Do RCDs wear out, and can an RCD be faulty?
Yes, an RCD can fail, and there is no fixed lifespan in years. As The Conduction Company found above, the standards set RCDs a minimum number of operating cycles, not an age. A faulty RCD usually shows itself in one of two ways: it does not switch off when you press the test button, or it trips when there is no fault on the circuit, which electricians call nuisance tripping. Either way, have a registered electrician test it, because an RCD that will not trip gives no protection.
Does a circuit breaker or RCBO protect against arc faults?
Not reliably. A standard circuit breaker protects against overload and short circuit, and an RCBO adds earth-leakage (RCD) protection, but neither is designed to detect an arc fault. AS/NZS 3000 defines an Arc Fault Detection Device (AFDD) as one that disconnects a circuit when it detects an arc fault. The Conduction Company can advise whether AFDD protection suits your switchboard.
A test that trips an RCBO by creating a direct short circuit proves nothing about arc fault protection, because an RCBO is built to clear exactly that kind of fault every time. The arc that causes fires is usually a loose connection or a damaged flex lead, which draws normal current and leaks nothing to earth, so neither a breaker nor an RCD sees it. That is what an AFDD exists to catch.
RCD vs circuit breaker: what is the difference?
- RCD detects earth leakage, meaning an imbalance between the current flowing out and the current coming back.
- Circuit breaker detects overloads and short circuits.
- RCCB is another term for an RCD.
- RCBO is an RCD and a circuit breaker combined in one device.
- GFCI is the US name for an RCD. You will see it on imported appliances and overseas guides.
An RCD does not protect your appliances from a power surge, The Conduction Company explains. In AS/NZS 3000 these are two different devices: an RCD cuts the circuit when earth leakage passes a set value, while a surge protection device limits voltage spikes from lightning and switching. The Conduction Company fits whole-house surge protection in Auckland switchboards.
RCD vs RCBO: what is the difference?
As The Conduction Company puts it to clients, an RCD protects people from electric shock, and an RCBO protects people and the wiring. An RCBO is the two devices built into one, so it also switches off on overload and short circuit.
A standalone RCD sits in front of a group of circuits, each with its own circuit breaker. In a home, AS/NZS 3000 allows no more than three circuits behind any one RCD (clause 2.6.2.4). When that RCD trips, every circuit in the group goes off with it, which is why one fault can take out the lights and the power together.
Sharing also affects nuisance tripping. Electrical equipment can leak a small amount of current to earth in normal use, and on one RCD the leakage from every circuit behind it adds together. AS/NZS 3000 clause 2.6.2.1 requires RCDs to be chosen and circuits split so normal leakage is unlikely to cause unnecessary tripping. An RCBO only sees the leakage on its own circuit, and when it trips, only that circuit goes off.
The Conduction Company recommends RCBOs when a switchboard is upgraded, so a fault switches off one circuit rather than a group. It also fits an RCBO on the dedicated circuits it runs for an EV charger installation or underfloor heating.
The Conduction Company sets an RCD, an RCBO and an MCB side by side in its guide to RCD, RCBO or MCB.
What's dangerous about an old switchboard with rewireable fuses and no RCD?
An older switchboard like this carries two risks. Rewireable fuses are an old form of protection that AS/NZS 3000 no longer allows in new work, and periodic electrical inspections specifically flag them. With no RCD fitted, nothing disconnects fast enough if someone touches a live part, which is a shock risk. The Conduction Company checks both during a switchboard inspection.
A rewireable fuse uses a thin wire that melts to break the circuit. It does not cut off an overload as closely as a modern circuit breaker, and it can be rewired with the wrong size of fuse wire, so a cable can run hotter than it should without the fuse blowing. A periodic assessment of an older switchboard also checks whether its equipment is likely to contain asbestos, which is another reason to have one looked at by a registered electrician.
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.
Do I need new RCD protection when adding outlets or lighting?
It depends on what is added. Under AS/NZS 3000, socket outlets added to an existing circuit need RCD protection, and so does any brand new power or lighting circuit. Lighting points added to an existing lighting circuit do not. Where an added outlet's circuit has no RCD, The Conduction Company fits one.
The standard prefers the new RCD at the switchboard where the circuit starts, but allows it where the new wiring begins, with its location then recorded at the switchboard. Sometimes the simpler answer is to connect the new outlet to a circuit that already has RCD protection. Before adding anything, The Conduction Company checks the switchboard has room for the extra protection and that the existing circuits pass the AS/NZS 3000 tests. A quick photo of your switchboard is often enough for us to tell in advance whether it already has RCD protection.
How to test your RCD
As good practice, test your RCD every 6 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 6-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.
What is RCD testing, and is it a legal requirement?
RCD testing checks that an RCD switches off, and how fast. There are two kinds. The test button, which anyone can press, confirms the RCD operates, but it does not measure the trip time. An electrician's test uses test equipment to measure how quickly the RCD trips.
In New Zealand, the electrician's test is required on every newly installed RCD before the work is signed off, and it must include the trip time (AS/NZS 3000, clause 8.3.10, as changed for New Zealand by the Electricity (Safety) Regulations). The Conduction Company's advice to press the test button every 6 months at home is recommended practice, not a legal requirement.
How many RCDs do I need?
At least two in a home with more than one circuit, with no more than three circuits on any one RCD (AS/NZS 3000, clause 2.6.2.4). The Conduction Company counts the circuits that need RCD protection and divides by three, so a board with seven of them needs at least three RCDs. Where a home has more than one lighting circuit, the lighting is shared between the RCDs, so one trip does not put every light out. On an upgrade, The Conduction Company recommends an RCBO on each circuit, which gives every circuit its own protection.
How do I know if my switchboard has enough RCD protection?
The clearest sign is too few RCDs for the circuits on the board: AS/NZS 3000 allows no more than three circuits behind any one RCD in a home. Where the enclosure is sound, The Conduction Company fits extra RCBOs in place of the old breakers, extends cabling, and fits a new main switch instead of replacing the whole board.
If an RCD trips, every circuit behind it loses power, so the standard requires at least two RCDs in a home with more than one circuit, with lighting circuits shared between them where there is more than one. Missing pole fillers, which leave access to live parts inside the board, are another thing worth checking for. Pricing for this kind of upgrade assumes the existing circuits pass the tests set out in AS/NZS 3000:2018. Occasionally the newly fitted protection trips because of an existing wiring fault that only shows up once the upgrade is done. The Conduction Company discusses the fix and any extra cost with you before going ahead if that happens.
RCDs are one of several things The Conduction Company checks on an old board, and its guide covers the other signs a switchboard needs upgrading.
RCD safety switch installation in Auckland: what does it cost?
The Conduction Company installs RCD safety switches in Auckland homes, either as a single RCD added to an existing board or as RCBOs on every circuit.
We quote on facts, not guesses
Before we give you a price, we carry out a proper assessment of your property and how you'll use your switchboard.
That way you know exactly what you're buying and the total cost, with no surprises.
Clear expectations from the outset, start to finish.
The estimates below are for standard installation of RCD protection on your existing switchboard. The Conduction Company confirms final pricing after a property assessment.
These figures come from Auckland switchboard jobs The Conduction Company has sold. The Conduction Company adds a single RCD to an existing board from about $175 including GST. Changing a typical home's switchboard to RCBOs usually costs around $1,000 to $1,250 including GST, plus certification. Where the enclosure itself has to go, The Conduction Company replaces the whole board as a switchboard upgrade.
In short
A residual current device switches a circuit off when current leaks to earth, which is what protects a person from a serious shock. It is not an overload device; that is the circuit breaker's job, and an RCBO does both. In a New Zealand home, the wiring rules call for 30 mA RCDs on new circuits supplying power points, lighting points and directly connected hand-held equipment, with no more than three circuits on any one RCD. In The Conduction Company's reading of AS/NZS 3000, an existing house comes under that rule through work: a new circuit, added power points, or replacing all of a board's protection.
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.
What is the difference between an RCD and an RCBO?
An RCD cuts the power when current leaks to earth, which protects people from electric shock. An RCBO does that and also works as a circuit breaker against overload and short circuit. One RCD can cover up to three circuits in a home and switches them all off when it trips. The Conduction Company recommends RCBOs when a switchboard is upgraded, so each circuit has its own protection.
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 6 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.
Should an RCD be on or off?
On. The circuits behind an RCD only have power while it is switched on. If it will not stay on, leave it off and call The Conduction Company or another registered electrician to find the fault.
How do I know if I have an RCD?
Look on your switchboard for a device with a test button, usually marked T or TEST. Many also show their rating, such as 30 mA, on the front. A power point with an RCD built in has its own test button. If you are not sure, a photo of your switchboard is often enough for The Conduction Company to tell you.
