I have a 60A current operated circuit breaker (RS 339-960) randomly tripping in the night, and wondered

  1. what actually is this device (?RCD)
  2. what electrically causes it to trip

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.

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  • Your analysis of the unit is correct, and the likeliest explanation of the trips is a ground fault where current is leaking causing imbalance. On that yellow foamy stuff, you may want to chip off a little bit of it, take it outside and flame-test it. If that's what we Yankees call "Great Stuff" (a brand), it's basically napalm and will catch easily and burn very aggressively. Not stuff you want near electrical. When it burns, it emits toxic fumes, which will incapacitate people trying to escape. Commented Apr 6, 2023 at 18:33
  • It's been like that at for multiple decades, why now? I'll flame test it as that's a valid point.
    – reggie
    Commented Apr 6, 2023 at 19:49
  • Equipment ages, wires degrade, parts fail. Commented Apr 6, 2023 at 21:48

3 Answers 3


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.

  • My main fuse is 60A
    – reggie
    Commented Apr 6, 2023 at 18:18

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.

  • thanks for your response, I cannot actually find anything about this device on the net, probably as its so old. How can I tell if its a RCD? I suspect it has a toroid in it, to measure the difference between live and neutral, but not sure. I need to understand what it is to find out what could be tripping it. its weird, as its only happens at night. I'm thinking, central heating pump, or maybe water ingress on outside light, but need to understand how it works to decide if this could do it.
    – reggie
    Commented Apr 6, 2023 at 9:36
  • @reggie So all you need is to find what is causing a tripping current >25mA... How it works is immaterial.
    – Solar Mike
    Commented Apr 6, 2023 at 9:49
  • Could it be the device sensitivity? how would I know?
    – reggie
    Commented Apr 6, 2023 at 9:53
  • If you can, try unplugging different things plugged into its circuit. If the tripping doesn't happen, you've identified a cause. Also, did the tripping start recently; is it associated with some change?
    – Armand
    Commented Apr 6, 2023 at 10:23
  • It's happened the past 2 nights & once in the day with no obvious cause. Nothing has chainged, or new appliances being added. We don't have any timer switches that come on in the middle of the night. When I reset the switch it's OK for the rest of the day and evening.
    – reggie
    Commented Apr 6, 2023 at 18:05

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.

  • I'm afraid I don't have a clamp, but I understand what you're approach.
    – reggie
    Commented Apr 6, 2023 at 19:45

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