Main RCD trips but none of the individual MCBs?

[gregch]

Another day, another issue to deal with in the hovel.

Twice in the past couple of weeks we've had the power shut itself off (just the appliances and sockets, not the lights). When I check the consumer unit (photie attached), there's a "big" RCD that's tripped, but not none of the individual circuit breakers has tripped, so I have no clue what's going on.

When I switch it back on, everything's fine, so I thought it was a one-off until it happened the second time. It's a job for an electrician, but tricky because there's no individual tripped RCD/MCB to provide a clue, and it's so intermittent, so you can't check eg one socket or appliance at a time. I don't know how an electrician would be able to identify something that intermittent - but maybe they have ways?

Incidentally, the right hand RCD-protected MCB labelled "Garage" is an unused feed to a detached garage that no longer exists; I'm not certain that was ever properly disconnected at the consumer unit, just switched off (the cable to where the garage was is, I suspect, still there, but now buried under compacted MOT). If that's still actually connected to the consumer unit, could that cause the RCD to trip even though it's switched off at the circuit breaker?

[Regbuser]

There's a current leak to one of the MCB protected circuits; the RCD is activating due to this as the MCBs have no leakage current ability.
An electrician will check each MCB circuit to identify which one has the current leak. Likely to be a fixed appliance.

[ALPandy90]

Definitely one for an electrician to come and investigate as you have already suggested.

The RCD and MCBs are protecting against different things. Simply put, the RCD is protecting against electric shock, while the MCBs are protecting the connected circuit/equipment from damage. There's a bit more to it than that, but that is the gist of it. There will be scenarios where the RCD will trip, but the MCB will not - like you have here.

A electrician will work through the fault finding process methodically and can also test the devices to check they are functioning correctly.

While they're in there, ask them to check the garage feed and disconnect it if it is still there.

[gregch]

Regbuser said:

There's a current leak to one of the MCB protected circuits; the RCD is activating due to this as the MCBs have no leakage current ability.
An electrician will check each MCB circuit to identify which one has the current leak. Likely to be a fixed appliance.

Ah, I see, if I understand you correctly, the MCBs on the RCD-protected circuits will only trip if there's a short circuit, for example; but the RCD covering those circuits will trip if there's eg a leak to earth (which won't trip the circuit breaker)?

[Regbuser]

Yerp

[The Gauge]

gregch said:

Another day, another issue to deal with in the hovel.

Looks the same box that I have, and I have an RCB that often trips, has done for 17yrs and I have to keep resetting it. Problem is the box is out in my garage which is a pain. Never been able to find a sparky prepared to come and have a look, and as I don't know any sparkies I doubt one ever will.

[gregch]

Thanks folks, much appreciated; glad I asked as it's helped my understanding of this stuff. I'll call an electrician to come take a look.

I wonder if it might be a Neff oven that has a visibly warped and decidedly iffy-looking heating element and was possibly in use both times the trip occurred; but will get him to take a look at that 'garage' circuit too, and perhaps a quick look in the loft where all the electrics run and where the roof needs attention/re-roofing!