Short answer
In almost every case a tripping safety switch means it is doing its job. An RCD trips because current is leaking to earth somewhere on that circuit, and the usual culprits are a faulty appliance, moisture in an outdoor fitting or a damaged cable. The switch is rarely the problem. Repeatedly resetting it without finding the cause defeats the protection it exists to provide.
What a safety switch is actually detecting
An RCD monitors the current flowing out on a circuit against the current coming back. In a healthy circuit those match.
When some current takes another path to earth, the two stop matching and the device cuts supply within milliseconds. That other path could be a degraded appliance element, water, damaged insulation, or a person.
The most common causes
These account for the large majority of nuisance tripping:
- A failing appliance, particularly anything with a heating element: kettles, toasters, dryers, hot water systems
- Moisture in outdoor powerpoints, garden lighting or an outdoor fitting after rain
- Damaged cable insulation, sometimes from a nail, screw or rodent
- Water ingress into a light fitting, especially in bathrooms and outdoor areas
- Accumulated small leakage across several appliances adding up past the trip threshold
- A genuinely faulty RCD, which is possible but much less common than the others
How to narrow it down yourself
Unplug everything on the affected circuit, then reset the switch. If it holds, plug items back in one at a time, leaving a few minutes between each. The item that trips it is your likely culprit.
If it trips immediately with nothing plugged in, the fault is in the fixed wiring or a hardwired fitting rather than an appliance, and it needs an electrician.
If it only trips in wet weather, that points strongly at moisture in an outdoor fitting or a damaged external cable.
When to stop resetting it
If the switch will not stay reset, stop. It is holding back a fault, and forcing it repeatedly is not a fix.
If there is any burning smell, scorching, buzzing or a shock or tingle from an appliance, tap or metal fixture, turn the power off at the main switch if it is safe to do so and get a licensed electrician out. If there is smoke or sparking, call 000 first.
Why replacing the switch is usually the wrong move
Swapping an RCD that is correctly detecting a fault removes the warning without removing the danger. The leakage is still there.
Fault finding traces the actual cause first. Sometimes that is a twenty minute job, sometimes it takes longer, but the outcome is a circuit that is genuinely safe rather than one that has stopped complaining.
Sources
General information only. Do not attempt to repair electrical faults or damaged equipment yourself. If there is immediate danger, call 000.