MCB tripped, and now RCD won't reset (2025 Update)

What happens if you disconnect the output of the RCD?

Leakage between neutral and earth. Typically your MCB is single pole and interrupting the live but not the neutral. Try disconnecting the neutrals of each circuit one at a time until you find which is causing the trip. (it may not be the garage)

Theo

Unlikely. More likely you have a genuine earth to neutral short

If it is neutral to earth it needs a bit of current to be flowing anyway else it won't trip..

Although with TN-S that current could be in another property... (causing a voltage rise on neutral WRT to earth).

A bit if an update. I had to leave the house to run an errand and when I got back about 25 minutes later, the RCD reset normally and everything is working again.

Not sure what to do now.

Tim

RCDs do sometimes get 'nervous' in my experience and, after a rest, will reset.

I'd do nothing for the present to see if the problem recurs at all.

True, but that must be happening or the RCD wouldn't keep tripping would it?

Ive seen that occasionally.

Try turning each appliance on until it pops again.

If its earth neutral short each circuit will need a load on to pop it Sonething like an electric drill works well - nice high startup current

Yebbut you can still get that if the MCB to the circuit is turned off.

ie:

live - RCD - MCB for circuit 1 - light bulb - neutral for circuit 1 - neutral busbar in CU - neutral for circuit 2 - neutral/earth leak - earth.

ie the leak is on circuit 2, but it doesn't need any power to be on the live side of circuit 2 to cause an RCD trip. Turning off MCB 1 doesn't tell you anything about the location of the fault.

If instead there was one RCBO per circuit then the RCBO for circuit 2 would have tripped, telling you where the fault was.

Theo

Sounds like the simplest approach for the meantime. ;-)

Tim

Some sort of capacitance at work ?

Could be something on a timer or switch? eg immersion heater is on a timer, but it's only powered when the tank stat says the water is too cold. So it'll only cause a trip during timed periods but also only when hot water has been drained.

The next day it's cold so the boiler runs the central heating and heats the water, and the immersion doesn't fire so no trip. It'll only trip on warm days when somebody also had a bath.

(you can construct similar scenarios for various other appliances)

Theo

How?

He said he turned off EVERTYHING and it still tripped

You can also construct scenarios where several items together will cause enough earth leakage, but one alone will not. That is particularly true of an earth neutral short

Ah, I missed that. If every single MCB in the installation is off there could still be a leakage path from neutral to earth but the current would have to come from somewhere external, eg from a neighbour. I would be checking the supply N-E impedance as it might suggest there isn't a good bond between them.

If it is a TT supply, the neutral is only bonded to earth at the transformer so there could be a decent voltage between N and the customer's earth rod. If there was N-E leak downstream of the RCD that could cause a N-E current to flow through the RCD and trip it.

Theo

Actually, TT could be the source of randomness too. If it's dry, the ground dries up and the earth impedance to the substation increases, meaning the N from the transformer floats higher above local earth. A higher voltage means more current can flow into a N-E fault, perhaps enough to trip the RCD.

Then it rains, and the ground gets wet. E impedance to the transformer decreases (ie the quality of the earth bond improves) and there's less potential difference between N and local E. That could reduce the current flowing into a local fault and could fail to trip the RCD.

Would need some measurements to tell if this is at all plausible...

Theo

Amusing nervous the RCD! Perhaps what is happening is that there is possibly a degree of leakage thats residual, like the leakage of a PC power unit caps in it on the incoming supply line. Had this at a local radio station years ago would take a large rewire to make it better as more staff stated up their PC's the leakage rose, and sometimes the next PC online too much for the RCD who said "stuff that I'm off"!.

RCBO's wouldn't have helped that as there was only the one ring main in use in the building, the owners flogged the licence and the building is now a Pizza hut!..

Had this at our office building years ago. 35PCs simply tripped a 60A mains incoming if they all powered on at once, We had to turn them on one at a time., We fiitted 100A eventually