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March 5, 2026

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

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RCDs: Residual Current Device Electrical Safety

Many homeowners wonder how to rewire a house safely and why professional electricians are essential for this work. In New Zealand, installing a residual current device (RCD) is a crucial step in ensuring electrical safety in homes and commercial properties. Understanding how a residual current device works in New Zealand can help prevent electric shocks and property damage.

What do RCD’s do? 

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. RCDs are essential for preventing electric shocks and property damage by automatically disconnecting the circuit when it detects current leakage.

Why is my RCD tripping? 

There are many different causes for tripping RCD. The most likely cause of  tripping is a faulty appliance. Another common cause of  tripping is outdoor electrical fittings (sensors, lights, outdoor outlets etc). These fittings can become waterlogged,  which will cause 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.

If your RCD has started tripping since you have done some water-blasting, or after heavy rain, then this is the likely cause. In such cases, the residual current device will trip 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
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Office building with electrical wiring - Commercial electrical services in Auckland
How do RCDs (Residual Current Devices) work?

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.

What causes a RCD to trip? 

A RCD will trip when it detects a current imbalance in the live conductors of a circuit. 
A current imbalance typically occurs when one of the live conductors comes into contact with earth.

The connection to earth can be through conductive building materials, or worse – through you! (electric shock).
When this happens, some of the normal circuit current will flow through earth. 

A RCD detects this fault and automatically disconnects power to the circuit, preventing property damage and physical harm.

How does a RCD prevent electric shock? 

If you come into contact with a live electrical conductor, current will flow from the point of connection - through your body - to the earth (electric shock).

As long as the shock current value is larger than 30mA (milliamps), the RCD is designed to detect it and automatically disconnect the supply, preventing further shock. 

Why 30mA? because any shock current above 50mA can cause cardiac arrest (heart attack) in an adult human. 

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.

Can an RCD trip with no earth conductor? 

Yes – even if your circuit cables don’t have an earth conductor, current can flow to earth through any conductive materials that the live conductors come into contact with. 

Is an RCD the same as a circuit breaker? 

No – a residual current device (RCD) and a circuit breaker each have their own protective function. A RCD detects current imbalances, whereas a circuit breaker detects current overloads & short circuits. Both devices work by disconnecting the circuit when their respective faults are detected. 

Will an RCD trip on overload? 

No – an RCD (or RCCB) will not trip if there is an overload on the active conductors, it only detects current imbalances.
For this reason they must be combined with MCB’s or alternative overload protection. 

RCBOs are devices which combine both MCB (overload) and RCD (residual current) protection into one. 

Is an RCD a legal requirement? 

Yes – in NZ, RCD’s are legally required to be installed on residential installations where new electrical work or additions have been carried out. There are a few exceptions to this rule, butin general it’s 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.
RCD’s are not typically required in commercial and industrial settings, however, it is good practice to have them installed on circuits containing general-purpose outlets used by staff or the public.

If you are uncertain about the level of protection needed for your installation, we offer RCD testing and installation services in Auckland to ensure your system meets safety standards.

Testing and Using the RCD Switch

Regular testing of the residual current device switch is essential. Press the test button every six months to ensure it trips correctly and disconnects the circuit. This simple step keeps your home and business safe from unexpected electrical faults.

How do I test if my RCD is working?

Test your RCD by pressing the test button.It should trip, disconnecting the circuit. Regular testing every 6 month sensures your residual current device is functioning correctly, keeping your home and business safe. Our electrical maintenance services ensure your systems are always running smoothly, including RCD functionality, to prevent any unexpected issues.

A residual current device in New Zealand is a critical safety feature for every home and workplace. Understanding how RCDs work, testing the RCD switch regularly, and knowing why your residual current device tripped can prevent electric shocks and property damage. If you’re unsure about your RCD’s operation or installation, professional electricians in Auckland can provide testing, maintenance, and expert advice to ensure your system meets modern safety standards.

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