random tripping

[Tom__40]

Hi. Yesterday my ring main MCB just tripped randomly and I've no idea why. What seems more odd though is there is an RCD socket (for the washing machine) spurred off the ring main located in the kitchen - that didn't trip. What's more, the RCD the bathroom fan upstairs is wired off of did trip (but the MCB for that circuit didn't), and that's on a separate upstairs lighting circuit (none of which was in use at the time).

I do have plans to get a new consumer unit installed though probably not for a couple of years (money/other work to do first). The existing one is an old push button type. I do know they share a neutral bar at the consumer unit.

When it tripped there really wasn't much in use, my computer, toaster, fridge/freezer were all plugged in...that's about it. It reset just fine, and everything I did have plugged in has been used since with no issue. I have had very occasional (once every few months maybe) tripping before. Fridge freezer is old and uses a 13A fuse....from reading other threads I'm wondering if it went to cycle the compressor on and tripped it? But surely it would trip every time if it was that?

cheers

 

[Taylortwocities]

An MCB trips on current overload, an RCD trips if there is an earth fault.

Therefore, for some reason, the MCB thought you were pulling too many amps. May be a transient problem. If it’s all reset then see how it goes for the next few days.

 

[Tom__40]

Hi. I've since tried a couple of things:-

The (old) consumer unit had an identical 30A MCB not being used, so I swapped it with the existing one on the ring main. Problem persisted, so unlikely to be a faulty MCB.

I think the problem is on the upstairs lighting circuit - reason being it happened this afternoon when I turned on the upstairs light switch (upstairs and downstairs lighting are definitely on separate circuits).

Investigated a bit further and checked the utility, the lights for which appear to be on the upstairs circuit. The cupboard for the utility had a light wired off the live and neutral of the main utility light (using very old/not even sheathed, just two single insulated wires twisted together) cable. What I hadn't spotted before was they'd drilled through the top of the door frame and shoved it through, and just on the other side untwisted said wire next to a bit of terminal block, one end screwed in other one joined right next to it with what I assume was a bit of old heat shrink. But they untwisted a bit soon, I hope this is what the issue was and it was causing a short between live and neutral. Now ripped it out as it looked so dodgy (I can live without the cupboard lighting)....what I don't get is why would a short on that circuit would trip the ring main (as well as an RCD on the upstairs lighting circuit, which I does make sense). Any ideas why a short on one circuit would make another trip welcome!? It is an old 50s house if it makes a difference. TN-S supply according to an inspection cert I have from a year or two ago.

Now sitting here with the upstairs lighting circuit turned off to see it I still get any issues!

 

[ericmark]

30 amp MCB on lighting circuit, clearly very old can't remember when we went to 32 amp, and lighting circuits would normally be 6 amp, doggy is a bit of an under statement.

Try some pictures, as some thing seems to have been missed in the telling.

 

[Tom__40]

Sorry, I wasn't clear. 6A on lighting circuit, 30A on ring main. There's only 3 circuits in use -upstairs lighting (6A), downstairs lighting (6A), ring (30A). Pics of the removed wire, and terminal block next to hole in doorframe attached! One end of this was wired to the main utility light live and neutral bars. The other end one bit went to the terminal block, and the other to another piece of wire using the presumed heat shrink (which practically fell off). Next to the terminal block where they were stripped, you could see they looked to be touching (or very nearly). The rest of the wire to the light itself and the lightswitch further down the doorframe was of the same type. No earthing to the cupboard light at all!