Intermittently tripping RCD - any ideas?

Nickthedentist · 19 Nov 2005

Hello everyone,

My parents' house was re-wired about 10 years ago, and has a MK Sentry CU with a 30mA RCD as the main incomer.

Until last month, the RCD has only tripped once - when I drilled through a lighting cable causing a N-E short

. But it's now happened about 4 times in the last month and resetting all their clocks etc. is becoming a bore now.

It seems to happen at roughly the same time every day, say, between 06:00 and 07:15. They get up at about 6:30, so in other words, sometimes it's before they run a bath/shower and sometimes it's afterwards. It's probably not the central heating, as it's a combi boiler and the problem happened before they switched the CH on for the winter a couple of weeks ago.

My main suspect was the outside lighting circuit which feeds 3 halogen PIR lamps, but isolating those completely has made no difference

Someone suggested that their may be a slight leakage present all the time, and that the voltage surges that occur in the early morning might be the "final straw", causing the RCD to trip.

Any ideas, anyone? I'm tempted to replace the RCD next, just to rule it out of the equation.

Look forward to your clever suggestionsn

Nick.

kai · 19 Nov 2005

Have you checked the immersion heater?

Nickthedentist · 19 Nov 2005

kai said:
Have you checked the immersion heater?

Nice idea, but they have a combination boiler with no tanks and no immersion heater

Nick.

crystal ball · 19 Nov 2005

If as you say the lighting circuits are via the RCD, with the darker mornings upon us, maybe it is a lighting circuit that is the problem

JohnD · 20 Nov 2005

When I checked my earth leakages prior to fitting a whole-house RCD, i fount there was a detectable constant circuit on the immersion heater, and a larger one from the washing machine. Not enough to trip, but perhaps enough to use up the slack. Everything else was very high resistance. Borrowed the tester, can't remember any figures.

Couldn't help noticing they were both on watery things.

Lectrician · 20 Nov 2005

When you isolated the outside circuits, did you also isolate their neutrals??

To locate a fault like this, you need an insualation tester, and the ability to interpret its readings.

Nickthedentist · 20 Nov 2005

crystal ball said:
If as you say the lighting circuits are via the RCD, with the darker mornings upon us, maybe it is a lighting circuit that is the problem

Yep, everything goes through the RCD. That's one reason why I suspected the outside lights, which are Screwfix HiLo PIR types, in other words they're on dimly all night and come up to full brightness when someone approaches. But disconnecting them has made no difference

To clarify what I was trying to say earlier, this problem seems to sometimes occur before anyone gets up or the boiler comes on, which presumably means it's a problem with the house's wiring, some appliance that's still plugged in connected to the mains even when it's apparently off (e.g. fridge, video, computer), or even the RCD itself

Nick.

Nickthedentist · 20 Nov 2005

JohnD_ said:
When I checked my earth leakages prior to fitting a whole-house RCD, i fount there was a detectable constant circuit on the immersion heater, and a larger one from the washing machine. Not enough to trip, but perhaps enough to use up the slack. Everything else was very high resistance. Borrowed the tester, can't remember any figures.

Couldn't help noticing they were both on watery things.

I think you're barking up the right tree there, John, there's probably a small "steady state" fault which only has to be increased slightly by a "transient" one to trip the RCD.

But I think all our "watery things" (e.g. kettle, washing machine, dryer) have double-pole switches so shouldn't be responsible. Having said that, I'll try actually unplugging them to see if it makes any difference.

Nick

Nickthedentist · 20 Nov 2005

Lectrician said:
When you isolated the outside circuits, did you also isolate their neutrals??

To locate a fault like this, you need an insualation tester, and the ability to interpret its readings.

Good point, but I did - I know (from bitter experience!) that N-E leakage can sometimes trip RCDs too.

Your insulation tester suggestion is probably the way to go. I've got a fairly good knowledge of electrical test procedures, so maybe it's time to look on eBay for a Megger or similar - the old "hand-cranked" ones are less convenient than the nice new electronic ones, but they should do the trick and seem to go for under £20

Nick.

Lectrician · 20 Nov 2005

Yup - turn off all MCB's, take ALL neutrals out of the N bar, and test each circuit to earth seperatley ( L to E, N to E). No need to split the earths - best to leave them in place, as the leak could easily be to a different earth than the circuits CPC.

A 30mA RCD as main incommer is more than frowned upon

What type of Earthing arrangement do you have?? PME with a 30mA RCD incommer can cause nuisance tripping due to a fault in a neighbouring property under certain conditions.

Nickthedentist · 20 Nov 2005

Lectrician said:
Yup - turn off all MCB's, take ALL neutrals out of the N bar, and test each circuit to earth seperatley ( L to E, N to E). No need to split the earths - best to leave them in place, as the leak could easily be to a different earth than the circuits CPC.

Thanks for that, it's nice to know how the pros do it

Lectrician said:
A 30mA RCD as main incommer is more than frowned upon

Yeah, I read that somewhere else too, but until a few years ago, all the CUs I saw on sale (admittedly in DIY shops) were like this, with no split-load provision.

Lectrician said:
What type of Earthing arrangement do you have?? PME with a 30mA RCD incommer can cause nuisance tripping due to a fault in a neighbouring property under certain conditions.

The earth busbar in the consumer unit is linked to the sheath of the supply cable, so presumably it's TN-S. Could what you describe still happen with this system?

Cheers,
Nick

JohnD · 20 Nov 2005

Lectrician said:
PME with a 30mA RCD incommer can cause nuisance tripping due to a fault in a neighbouring property under certain conditions.

What time do the street lights go off outside the house? i don't know if it's an urban myth, but I heard a long time ago about a site having a peaky surge every day from that very thing.

Nickthedentist · 20 Nov 2005

JohnD_ said:
What time do the street lights go off outside the house? i don't know if it's an urban myth, but I heard a long time ago about a site having a peaky surge every day from that very thing.

That's a good one, John, I'll keep an eye out.

But last time it happened, it was still pretty dark outside

Cheers,
Nick.

Spark123 · 20 Nov 2005

Nickthedentist said:
Until last month, the RCD has only tripped once - when I drilled through a lighting cable causing a N-E short .

How did you repair the cable? Is this a recent event?

PartPapproved · 20 Nov 2005

Nick you didn't use amalgam to repair it did you? its not a filling you know!

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Intermittently tripping RCD - any ideas?

kai

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