One ring main *intermittently* trips RCD

Is it possible that a rodent has partially eaten through the ring mains cable? It could have been electrocuted and its body was the "leak" to earth. It might now have dried out enough so that it's body is no longer capable of carrying sufficient current to trip the RCD.

Depending on where it is, you might or might not get an unpleasant smell as it decomposes.

One of these

An RCD's purpose, is to trip, should there be more 30mA difference, between the current flowing in the L versus the N - which usually means that >30mA, is finding an alternative route to earth, possibly via you.

So a small leak from L to E, or N to E, can cause the RCD to trip. Even a bit of moisture, is enough to cause a trip.

An MCB (your CB), is designed to trip if the current marked upon it (likely 32amp for your ring), is exceeded in the L. When an MCB trips, it only breaks/isolates the L. The N remains connected.

As you managed to reset the RCD, by turning off that MCB, it suggests your leakage was L to E. As it eventually cleared, and you were able to switch that MCB back on, it suggests the issues either cleared itself, or perhaps the moisture dried out. It would be reasonable to expect it happening again.

I always thought the MCBs were dual-pole switches. Seems I was wrong.

We had a short 3-second power cut this afternoon which affected the whole house. No MCB or RCD tripped. As far as I can tell, none of our neighbours were affected, and the electricity distribution company had no record of an HV trip. (I misunderstood people's responses on the village Facebook and thought other people had been affected, which is why I reported it.)

I've phoned an electrician to investigate. Could be an expensive one: a few minutes to fix but hours tracing wiring to find out where the dead rodent or the moisture is. :-(

Then its not inside your house

I finally got confirmation from several other people on our village Facebook that they too had the same brief outage. The lack of response to my earlier "Has anyone else had it or is it just me" made me suspect that it *was* just me, but I couldn't think of a mechanism that could explain it.

I'm still going to get some advice about the RCD trip on one ring main which was 100% reproducible for about half an hour as I unplugged a few appliances at a time and retried, hoping to identify a rogue appliance. And then all of a sudden, it didn't happen at all.

I suspect water has somehow how into the wiring somewhere (but where?) or else rodent attack (likewise, where?). At least a rodent can be detected by the smell, though I don't fancy a repeat of the rodent that died up in the loft of our previous house and died (not of electrocution). The smell was appalling. I discovered that all the chipboard flooring in the loft had been glued to the rafters as well as being screwed down, so lifting the boards one my one was not an option. Eventually, with the aid of a mirror on a stick and a torch, I found Ratty right in the eaves: I had to lie on the floor and reach under the edge of the boarding with a plastic bag over my hand and Ratty was

Come to think of it, that ring main is carrying a fair old current at peak load: dishwasher, washing machine, tumble drier and electric oven (though the oven is just there for emergencies if our gas Aga is out of action). I wonder how close to 32 A we are with dishwasher, washing machine and drier on at the same time... Not that this would trip the RCD, only the MCB over-current. And the tripping started at 6 AM when the only things on that circuit would have been two deep freezes which are normally about 60 W each, with very occasional peaks of 1400 W when they do their frost-free cycle.

More to the point, all those appliances have heating elements that can get a bit earth-leaky ...

Ah. 3 seconds outage is typical for a 'branch across the lines' short that is automatically tripped, and tries to reconnect later, and succeeds.

Many years ago when I was only at home for weekends I too noticed a 'smell'. After about 6 weeks it went away. It was only when I mouse and it had crawled to the only safe place it could find, and died.

TBH leave the buggers where they are. I am not so squeamish. I just drag them out by whatever is accessible and hope they don't fall in half.

My Ex left a dead one in the water softener. Bitch.

It had stopped working anyway so I had to replace it

That depends. RCDs can trip from over current as well. And high current will trip an RCD *if you have an earth neutral short*. It doesn't detect the ratio of libe to neutral, it detects the difference.

The definitie test is to remove the ring neutrals from the MCB and test leakage to earth

That does sound more mouse than direct short. 6 a.m. is PMT. Peak Mouse Time.

And it might not be obvious. Our fan-assisted (bottom) oven has worked perfectly for many years. The grill in the top oven didn't seem to be glowing as much as it should, so I thought I'd try the fan oven grill. That was really bright, and would have done the chestnuts really quickly, except that the RCD tripped after only a couple of minutes. :-(

I've tried the fan oven several times since and it continues to work perfectly, but then the top element doesn't need to get really hot.

Yes, in our last house the element in our oven failing would take out the earth leakage breaker. It happened every 6 years, and each time I forgot about what had happened 6 years ago, and reset the breaker being baffled about what had caused the trip, until 15 mins later when Mrs C would shout from the kitchen that the oven wasn't heating up.....