I have a 60A current operated circuit breaker (RS 339-960) randomly tripping in the night, and wondered
I suspect it trips on a differential of 25mA between live and neutral, hence fault. I know it's sensitive, as many years ago an electrician tested it and said its trip time was very quick (not sure on the ms figure)
I've searched the part number on the web, to no avail.
That looks like an old RCD, though difficult to be certain, definitely old enough to not be in internet searches. It looks like 30mA is more normal now for the trip rating. https://www.tlc-direct.co.uk/Technical/DataSheets/MK_Sentry/MK_17ED.pdf
Tripping at night suggests some device on a timer turning on and causing the trip. One reason for the allowed current rating to be larger than it was is that modern switch-mode power supplies there is generally a permanent leak of up to 3mA to earth, so plugging in another always-on device could mean that something that only comes on at night now goes over the limit.
One potential concern with your setup is that the tails going into and out of the device look much newer than the device, which means they have been replaced at some point. This would normally only happen if there was a problem, or when something was changed, most commonly the 'leccy board fuse or the meter. As the (possible) RCD is only rated for 60A, you should check that your main fuse is similarly rated (it could be as low as 30A, but very unlikely). If it's been upgraded to more than 60A, but the RCD wasn't then you have a dangerous situation on your hands. See https://innovation.ukpowernetworks.co.uk/wp-content/uploads/2021/03/Fuse-Upgrade-Guide-March-2021.pdf for more details on fuse upgrade. It shouldn't have been upgraded with that RCD in place, but mistakes do happen.
That installation is old, very old.
The label shows RS, so check their website: enter link description here
As for what may be tripping it, there can be many causes, you need to become a detective and track down which circuit, then which part of the circuit etc.
As an example, I bought a house and for weeks every morning the lights downstairs would not work while they worked the previous evening.
Disconnected each light, disconnected parts of the circuit...
When doing other work I had to lift some floorboards and found 1 nail going through the supply wire for the light. Benn there for years and the hole was just large enough that the nail did not touch - until I stood on the floorboard which happened as I got into and out of bed..
As for your old installation, I suggest that the fuse box is replaced and the continuity and earthing checked.
If you have a current clamp meter and can separate the liven and neutral wires from each circuit from the earth. Then put the clamp meter around those together. You can also put them around the feeder wires and turn on one circuit at a time.
That will show the current that could cause the RCD to trip. However requires the circuit to be energized. It will help you find circuits that are contributing to the leaking current and take steps to mitigate them.
