The guide
Short answer: a safety switch trips when it detects current leaking to earth. Nine times out of ten that's one faulty appliance or water getting into something outside. Unplug everything, reset the switch, then plug things back in one at a time until it trips again. If it trips with nothing plugged in, the fault is in the wiring and you need an electrician.
What a safety switch does
A safety switch, or RCD, compares the current going out on a circuit with the current coming back. They should match. When they don't, the difference is going somewhere it shouldn't, through a damaged cable, a wet fitting or a person, and the switch cuts the power in about 30 milliseconds.
That is why it trips and a normal circuit breaker doesn't. A breaker only cares about too much current, from an overloaded circuit or a short. A safety switch cares about current going to the wrong place, which is the one that kills people.
How to find what's tripping it
Work through it in this order. It takes ten minutes and usually finds the problem.
- Go to the switchboard. Find the switch that's down or sitting in the middle. Note which circuits it covers, if the board is labelled.
- Unplug everything on those circuits. Not just switched off at the wall: unplugged. Many appliances leak current through the plug even when they're off.
- Reset the safety switch. Push it firmly up to on. If it trips straight away with nothing plugged in, stop here and call an electrician.
- Plug things back in one at a time. Wait a minute between each. When it trips again, the last thing you plugged in is your suspect.
- Confirm it. Leave the suspect unplugged, reset, and plug everything else back in. If the switch now stays on, you've found it.
Don't hold the switch up against a trip, and never tape or wedge it on. If it won't stay on, it's protecting you from something.
The usual suspects
Appliances with heating elements. Kettles, ovens, dishwashers, washing machines and old fan heaters are the most common. As an element ages its insulation breaks down and starts leaking current. The fix is a new element or a new appliance.
The fridge or freezer. An old fridge can trip a safety switch intermittently, often at night when the defrost element cycles on. If your switch trips at 3am with nobody up, suspect the fridge.
Water outside. Outdoor power points, garden lights, pool pumps and spa heaters all trip safety switches when water gets in. If yours trips after rain or after the sprinklers run, start outside. Close to the coast and the lake, salt speeds up the corrosion that lets water in.
A leaking roof. Water running down onto a light fitting or a ceiling junction box trips the lighting circuit. If the lights go off during a storm, look up.
Extension leads and power boards. A cheap power board left on a wet floor or a damaged lead under a rug can trip a switch on their own.
When it's the wiring, not an appliance
If the safety switch trips with every appliance unplugged, the fault is in the fixed wiring. It might be a cable nicked by a screw during a renovation, a rodent-chewed cable in the roof, water in a wall socket or a failing old cable in a 1960s house. You can't fix any of those yourself, and you shouldn't try.
An electrician will test the insulation on each circuit with a megger, which finds the fault without opening every wall. Most wiring faults are found and fixed in a single visit.
When the safety switch itself is the problem
Safety switches do wear out. One that trips at random with nothing obvious on the circuit, or one that won't reset at all, may need replacing. Test yours every three months with the button marked T: it should trip straight away. If it doesn't, it isn't protecting you, and it needs replacing whether it ever tripped or not.
If your house still has only one safety switch, or none on the lighting circuits, it's worth adding more. Our switchboard upgrades page explains when a board needs replacing to take them, and the electrician cost guide has what a new safety switch usually costs.
When to call an electrician
Call straight away if the switch trips and you smell burning, see scorch marks on a power point or switch, or hear buzzing or crackling. Don't keep resetting it. If it trips with nothing plugged in, or keeps tripping after you've removed every suspect, book a fault-finding visit.
If half the house is off and no switch has tripped, check with your neighbours. If they're off too, it's a network fault, so report it to Ausgrid on 13 13 88. If it's only you, ring us and we'll talk you through it.
FAQ
Common questions
Why does my safety switch keep tripping?
Most often it's one faulty appliance leaking a small current to earth: a kettle, a fridge, an oven element or an old heater. The next most common cause is water, in an outdoor power point, a garden light or a light fitting under a leaking roof. If it trips with everything unplugged, the fault is in the wiring and needs an electrician.
How do I reset a safety switch?
Turn off or unplug everything on that circuit, then push the safety switch back up to on. If it stays on, plug things back in one at a time until it trips again, and the last one in is your culprit. Never hold the switch up against a trip or tape it on.
What is a safety switch?
A safety switch, also called an RCD, watches the current going out on a circuit and the current coming back. If some of it leaks to earth, which is what happens when someone gets a shock, it cuts the power in a fraction of a second. It is not the same as a circuit breaker, which only trips on overload.
Why does my RCD trip when it rains?
Water is getting into something on that circuit, usually an outdoor power point, a garden light, a pool or spa connection, or a light fitting under a leak. It's worth finding before it gets worse, because the next fault might not be a small one.
Why is half my house without power?
Usually one safety switch has tripped and taken the circuits it protects with it, while the rest of the house stays on. Check the switchboard for a switch in the down or middle position. If nothing has tripped and half the house is still off, it may be a supply fault, so call an electrician or Ausgrid.
Is it dangerous if my safety switch keeps tripping?
The switch tripping is it doing its job, so in that sense you're being protected. But the fault behind it is real, and a safety switch that trips again and again is telling you something is leaking current. Find the cause rather than living with it.