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[smiffy64]

Hello...first post here and hope someone can help.

I have an old MEM consumer unit in my garage where the RCD has started tripping even with all MCBs off.
The lights work as they are not on the RCD circuits.
The garage RCD trips then the MCB that is connected in the house consumer unit also trips.

A clue was that the also ancient garage alarm had stopped working where it usually activates on a power cut but did not.
I was wanting rid anyway so I removed it from the circuit but the garage RCD is still tripping.
The only MCB I would have on would be the sockets but as I said, it is tripping with the sockets MCB off anyway.

I used to be a kitchen fitter but my experience in electricals is limited although I know my limits even if I consider myself a competent DIYer.

Questions:
Why does it also trip the MCB in the house consumer unit, considering that as long as the RCD in the garage is off, everything in the house is fine?

Why does it trip even though all MCB's in the garage unit are off?

Could the RCD in what looks at least 25 year old consumer unit have failed?

The electrics are far too complicated in my garage with not needed intercoms, pit lights etc, so I might just upgrade to a simpler modern unit.

Thanks,
RS

[Mr White]

Changing the consumers unit is a waste of money and time if the reason your RCD trips is still there, the reason the RCD trips needs to be found and corrected. An MCB only isolates the Live, an RCD trips because of an imbalance, that means it will also detect a neutral to earth fault.

[smiffy64]

an RCD trips because of an imbalance, that means it will also detect a neutral to earth fault.

Ok, thanks. The consumer unit in the house that the garage is connected to does not have an RCD, so due to the garage having one, is it possible that it is tripping due to something anywhere in the circuit, which will include the house circuit that the garage connects to?
It only trips when I switch the garage RCD on.
Hope I explained myself enough there.

[Mr White]

An RCD protects anything that is connected to it "Down stream" of where it is connected. Since in this case the RCD is in the garage, there is nothing in the house that can trip it as the house is "Up stream" from the RCD

To find the actual cause you will need a Multifunction tester and the knowledge of how to use it, it will be a job for an electrician