What is this?
When a circuit breaker (MCB) or RCD will not reset at all — it springs back to the off position immediately or will not latch — it indicates either an active fault on the circuit that is still present, or the protective device itself has failed. This is more serious than a circuit that trips occasionally, because it means the fault is persistent.
Common causes
- Active short circuit on the circuit (live touching neutral or earth)
- Severe earth fault with very low insulation resistance
- Water ingress into wiring, sockets, or fittings on the circuit
- Mechanically failed MCB or RCD that can no longer latch
- Melted or damaged cable creating a permanent fault
- Pest damage to cables (rodents chewing through insulation)
Is it dangerous?
A circuit breaker that will not reset is a high-priority issue. The device is telling you there is an active, serious fault. Do not attempt to force it into the on position or hold it in place. Do not bypass it or use any method to keep it switched on. The protection is preventing a fire or electric shock.
Can I fix it myself?
Switch off all MCBs protected by the device and try resetting it. If it resets with all MCBs off, turn them on one at a time to find the faulty circuit. On the faulty circuit, unplug everything and try again. If the MCB physically will not latch (the mechanism feels broken), the device itself has failed and needs replacing by an electrician. Never force a trip switch or wedge it in the on position.
When to call an electrician
Call an electrician immediately. A circuit breaker that will not reset means there is either an active serious fault or a failed protective device. Both scenarios require professional attention. If the affected circuit includes essential services like your fridge or heating, explain this when booking so the electrician can prioritise.
What will an electrician do?
Test the protective device to determine if it has mechanically failed
Disconnect the circuit and test for short circuits and earth faults
Carry out insulation resistance and continuity testing to find the fault location
Inspect the circuit for physical damage, water ingress, or pest damage
Replace the faulty MCB or RCD if the device itself has failed
Repair the circuit fault and verify the installation is safe before restoring power
Typical cost
£100 – £300
MCB replacement itself is inexpensive (£10–£30 for the device). The main cost is fault-finding and repair of the underlying issue.
