Why does my new fuse board keep tripping?

If your new fuse board keeps tripping, it's very rarely because the board is faulty. Much more often, it's doing exactly what it was fitted to do. Modern boards have protection that older fuse boxes didn't, and it picks up small faults, such as a damp outside light or a worn appliance, that were there all along but went unnoticed.

This post explains why a new board can trip when the old one never did, the most common causes, the checks you can safely make yourself and when to call an electrician. Never take the cover off the fuse board, and don't hold a switch up to stop it tripping. It's switching off to protect you.

Why the old fuse box never tripped

Many older fuse boxes use rewireable fuses (a length of fuse wire in a holder) or basic circuit breakers. These only switch off when a circuit draws far too much current, for example if a cable is cut through or too many heaters are plugged in at once.

A modern board adds RCD protection. An RCD, or residual current device, constantly compares the electricity going out on a circuit with what comes back. If even a small amount is leaking away to earth, through damp, damaged insulation or a person, it cuts the power in a fraction of a second. Most new boards use RCBOs, which combine an RCD and a circuit breaker for each circuit, so a fault on one circuit only switches that circuit off.

So when a new board starts tripping, it's usually finding a leak the old fuses simply couldn't detect.

The most common causes

An appliance with a worn heating element

Kettles, ovens, washing machines, dishwashers and immersion heaters all have heating elements, which can start to leak a little current to earth as they age. On an old fuse box this goes unnoticed. With RCD protection, the circuit trips, often when that appliance switches on or heats up.

Outside lights and sockets letting in water

Garden lights, outdoor sockets and wiring to sheds and garages are common culprits, especially in autumn and winter. If the board trips mainly when it rains, or on damp mornings, water getting into an outdoor fitting is the likely cause.

Two circuits sharing a neutral

In some older homes, two circuits were wired so they share a neutral, the wire that carries current back to the board. This is called a borrowed neutral. It may work on an old fuse box, but a board with RCD protection sees the current going out on one circuit and coming back on another, and trips. It's a wiring fault an electrician needs to find and fix.

Damaged or deteriorating cables

A screw or nail through a cable, a cable crushed under flooring, or very old insulation that has broken down can all leak current to earth. These faults are worth finding, as they can be a shock or fire risk.

Safe checks you can make yourself

You can narrow the problem down without touching any wiring:

Note which switch tripped. Each RCBO is usually labelled with its circuit, such as kitchen sockets or upstairs lights.
Note when it happens. On a particular appliance? Only when it rains? At the same time each day?
Unplug everything on that circuit, switch the RCBO back on, then plug appliances back in one at a time. If it trips when one goes back in, that appliance is likely the cause. Stop using it until it has been checked or replaced.
If it trips with everything unplugged, or the switch won't stay on at all, the fault is in the fixed wiring. Leave that circuit off and call an electrician.

If you smell burning, see scorch marks or hear crackling from the board or a socket, switch off at the main switch if it's safe to do so and call an electrician straight away.

Our post on why your electrics keep tripping covers tripping more generally, including what to do when the whole house goes off.

What testing should show before and after

Before fitting a new board, an electrician should test the existing circuits, including an insulation resistance test, which checks whether current is leaking from the cables. This can show up faults that will make a new board trip, so they can be discussed and dealt with as part of the job, rather than coming as a surprise afterwards.

After fitting, every circuit is tested again, and you should receive an Electrical Installation Certificate with the results. If your new board starts tripping soon after it's fitted, contact the electrician who installed it first. Their test results are a useful starting point.

How we fit and check fuse boards

We test before and after every fuse board upgrade, fit RCBOs for each circuit, label every circuit clearly and add surge protection where appropriate. When we've finished, you get an Electrical Installation Certificate, and we notify Building Control where it's required. If a board is tripping, our fault finding service tests circuit by circuit to find the cause.

If your fuse board keeps tripping, or your old fuse box is due for an upgrade, Prolectrical Southern can test the circuits and explain what's needed. Find out more on our fuse board upgrade page.