Start here — it is probably not faulty
The phrase "nuisance tripping" has done a lot of damage, because it frames the RCD as the problem.
An RCD trips because current is returning somewhere other than the neutral conductor. Somewhere on that circuit, electricity is finding a path to earth. That is the definition of the fault the device exists to catch, and in the overwhelming majority of cases it is doing its job correctly.
The right question is not "why does it keep tripping". It is "where is the current going".
Answer that and the tripping stops as a side effect.
The four causes
1. A genuine earth fault in one appliance. The classic. A heating element that has broken down internally, a motor with degraded winding insulation, a lead with damaged insulation touching earthed metal, water inside an appliance.
Signature: tripping correlates with one appliance being plugged in or switched on. Often a kettle, a heater, an old fridge, a dishwasher or something with an element.
2. Cumulative earth leakage. The one nobody expects and probably the most common on commercial sites.
Every piece of electronic equipment leaks a small current to earth by design — the filters in switch-mode power supplies do this deliberately. Any one device leaks a trivial amount. Twenty computers, monitors, printers, chargers and a photocopier on one circuit can add up to a meaningful fraction of a 30 mA threshold.
Nothing is faulty. The circuit is simply carrying so much designed-in leakage that any small addition — one more device, a damp morning, a motor starting — pushes it over.
Signature: trips at seemingly random times, often in the morning when everything switches on at once, and no single appliance can be blamed. Nothing found when an electrician tests each item individually.
3. Moisture. Water in an outdoor socket, a junction box, an outdoor light, the gland on a piece of equipment, or a roof leak that has found a conduit.
Signature: tripping correlates with weather. Trips after rain, in heavy dew, in fog, or after a wash-down. This is the most diagnostic pattern in the whole subject — if it correlates with weather, look outside.
4. The wrong RCD type for the load. Increasingly common as buildings fill with electronics, and covered separately below.
Narrowing it down without an electrician
Before anyone bills you for diagnosis time, half an hour of observation will usually cut the problem in half. This costs nothing and does not require touching the installation.
Keep a log. Date, time, what was running, weather. Three or four events with context is worth more than any amount of guessing. Most nuisance tripping is solved by the pattern rather than by testing.
Test the pattern against these questions:
- Does it trip at the same time of day? Points to something on a timer — hot water, heating, irrigation, refrigeration defrost cycles.
- Does it trip when it rains? Outdoor circuit, moisture.
- Does it trip when a specific appliance starts? Start there.
- Does it trip at start of business, when everything comes on together? Cumulative leakage.
- Did it start after new equipment arrived, or after building work? That is your prime suspect, including work in an adjacent tenancy.
- Does it trip with nothing plugged in at all? Then it is in the fixed wiring or the device itself, and it is an electrician's job now.
Isolate methodically. Turn everything on the circuit off. Reset the RCD. Bring items back one at a time, leaving several minutes between each. Tedious, and it works — particularly for cause 1.
For cumulative leakage, you will not find a culprit this way, because there is not one. That is itself diagnostic: if the circuit runs happily with two thirds of the equipment on it and trips with all of it, you have your answer.
Do not open anything, and do not work on the board. Everything above is observation and switching. Anything beyond that is electrical work and needs a registered electrical worker.
When it is the RCD itself
Sometimes the device really has failed, and it is worth knowing what that looks like so you do not chase a fault that is not there.
- Trips immediately on reset with the circuit isolated and nothing connected. The device or the wiring in the board.
- Will not reset at all, with no load.
- Trips at wildly inconsistent currents when measured — this is where trip-time testing earns its keep. An RCD that trips at 12 mA on a 30 mA rating is over-sensitive and will nuisance trip forever.
- Very old device with no history. Mechanical devices age. A device that has never been measured has no baseline, which is the argument for measuring rather than button-testing.
A proper trip-time test settles this in minutes. Testing the device is almost always cheaper than another round of hunting for a fault in the building.
Wrong type for the load
This cause is growing quickly and is frequently misdiagnosed as a faulty device.
Equipment with electronics in it — variable speed drives, LED drivers, EV chargers, solar inverters, UPS systems, modern kitchen equipment — does not produce a clean sinusoidal fault current. A Type AC RCD confronted with pulsating or smooth DC content can behave unpredictably: nuisance tripping in one direction, or failing to respond at all in the other.
The amended regulations address this directly. From 12 November 2026, new work must use RCDs that are Type A, or Type F or B. There is also a new requirement that RCDs be selected to be compatible with the types of load intended to be used, with consideration given to pulsating and constant DC fault currents and waveform distortion.
If your tripping started after new plant went in, this is a strong candidate, and swapping like for like will not fix it. See RCD types and what changes in November 2026.
There is an inversion worth understanding here. A Type AC device on a circuit with heavy DC content can fail safe-seeming — not tripping when it should. Nuisance tripping is at least visible. The silent version is worse and nobody notices it, which is why type review is worth doing even where nothing is tripping.
The thing you must not do
Do not bypass, remove, or replace the RCD with a less sensitive one to make the tripping stop.
It happens. Someone is fed up, production is being interrupted, and there is a straightforward-looking way to make the problem go away. An RCD used for protection against electric shock is expected to have a rated residual current no greater than 30 mA — fitting something less sensitive to solve an operational annoyance removes the protection entirely, and leaves a board that looks protected.
If you inherit a site and find an RCD bypassed, treat it as urgent. Somebody made that decision for a reason, which means there is very likely still a fault on that circuit — and now nothing is watching it.
When to call someone
Call a registered electrical worker when:
- It trips with nothing connected
- It will not reset
- Tripping correlates with weather — that is a moisture path in the fixed installation or outdoor fittings
- You have isolated methodically and found nothing
- It started after new plant or building work
- Anyone has bypassed or removed an RCD
- You are getting shocks or tingles from any equipment, even mild ones — this is not a nuisance-tripping question, it is urgent
What to hand them when they arrive: your log, a list of what is on the circuit, when the problem started, and what changed around that time. Diagnosis is billed by the hour and that page of notes is usually worth more than the first hour of it.
Related: how often RCDs should be tested and when are RCDs required in New Zealand.