Why Does My Fuse Box Keep Tripping?

A tripping breaker is a safety device working correctly. Something on that circuit drew more current than it should, or leaked current to earth, and the breaker cut the power in a fraction of a second. The question is what caused it.

First, work out what actually tripped

Open the consumer unit and look at which switch has moved. Three things live in there and they mean different things.

An MCB protects one circuit against overload and short circuit. If a single MCB has dropped, the fault is on that circuit alone.

An RCD protects against current leaking to earth, which is the fault that electrocutes people. An RCD usually covers several circuits at once, so when it goes, half the house goes with it.

An RCBO does both jobs for a single circuit. Modern boards increasingly use them, which is why a newer installation tends to lose one circuit rather than half the house.

Write down which one tripped before you reset anything. It is the single most useful thing you can tell an electrician on the phone.

The kettle test

Most repeat trips come from one faulty appliance. You can usually find it without any tools.

Unplug everything on the affected circuit. Reset the breaker. If it holds, plug things back in one at a time, waiting a minute between each. The item that trips it is your culprit.

Kettles, washing machines, dishwashers, fridges and immersion heaters are the usual suspects, because heating elements fail by leaking to earth. Outdoor equipment is next: a garden light with water in it or a mower that has cut its own cable will trip an RCD every time.

If the breaker will not reset with everything unplugged, the fault is in the fixed wiring and it needs an electrician.

When it trips only sometimes

Intermittent trips are harder and more revealing. A pattern is worth more than a description.

• Goes when it rains: suspect outdoor wiring or water reaching a fitting.
• Goes when the heating fires up: suspect the immersion or boiler circuit.
• Goes when someone showers: suspect the shower element.
• Started after work was done in the house: suspect a nail or screw through a cable.

Keep a note of the time and what was running. That log turns a two-hour hunt into a twenty-minute one.

Accumulated leakage

Every appliance leaks a tiny amount of current to earth. That is normal. An RCD trips at around 30 milliamps, and enough appliances on one RCD can add up to close to that on their own. The board then sits on a hair trigger and one more thing tips it over.

This is why a board with everything behind a single RCD causes so much trouble, and why splitting circuits across RCBOs usually fixes a house that trips for no obvious reason.

What we do

We measure rather than guess. Insulation resistance testing on each circuit finds the leak, and earth leakage clamp readings show which circuits are contributing what. That tells us whether the answer is a faulty appliance, a damaged cable, or a board that needs its protection split up.

When to stop resetting

Reset a breaker once. If it trips again straight away, leave it off and call someone. A breaker that will not stay in is reporting a live fault, and repeatedly forcing it back is how a fault becomes a fire.

Call immediately, and leave the power off, if you can smell burning, if there are scorch marks around a socket or the board, if anything is warm to the touch, or if you have had a shock from an appliance or a tap.