Intermittent RCD tripping - how to diagnose?

[NotSoSmart]

I've had a look at other similar threads on this subject but can't find anything that describes my problem so here goes...

I'm not an electrician so may get some of the terms wrong - please just ask for clarification and I'll do my best to explain what I mean...

We have an old house (1850+) which is undergoing renovation. We have a new consumer unit with 2 sides to it - the left side has all the sockets on individual MCB's and an RCD; the right side is just lights on individual MCB's.

We get an intermittent issue of the main RCD tripping - no MCB's trip ever. 'Intermittent' in this case means up to 4 times a day, at various times during the day, or night.

At night we switch off and unplug everything we can, except the fridge, washing machine (too difficult to reach) and boiler. It still trips.

We have been able to make the main RCD trip by plugging in a TV or printer, but since it still trips overnight when both of these are unplugged, we assume the problem must be elsewhere.

From other posts it suggests that the problem is leakage to earth - is that correct, and how does one trace where the leak is?

Could we have a faulty RCD, and how does one check this?

As I mentioned earlier, we are in the middle of renovations. Floorboards are being lifted & replaced, walls moved, and general chaos is ensuing. I'm guessing that a likely culprit is a nail through a cable, but how does one check for this?

Many thanks for your help!

 

[JohnD]

unplug the washing machine and turn off the boiler at the wall

leakage is most commonly on "watery" appliances such as washing machines, immersion heaters, kettles, steam irons, tea urns, central heating boilers and pumps, hot tubs, garden fountains, fishtanks, garden lights and sockets exposed to rain.

Intermitted tripping is often due to a boiler heating and cooling over long periods and turning its pump on under the control of a timer and thermostats.

Also heating elements such as cookers.

Any of these that you cant unplug should have dual-pole isolators or FCUs on the wall.

 

[Taylortwocities]

As you saym the RCD may just be tripping outside its specified range. You would need a specialised RCD tester to check this.

Your local friendly sparky will have one...

 

[Steve]

Fridges are prone to causing nuisance trips, especially as they age.

 

[loosenup]

I agree with steve .. most likely the fridge . I had this prob a coupla months back with a brand new fridge .. condensation was forming on the motor cos of lack of ventilation around appliance .. the terminals were shorting . vented fridge and cured problem .
Phill

 

[NotSoSmart]

Hi folks,

sorry for the delay but much testing has been taking place...

The fridge is brand new, as is the boiler, so unlikely to be those we think. Anyway, here is the breakdown of symptoms:

Overview - the RCCB trips (it controls just the sockets MCBs on the left sie of the consumer unit - the right side is the lights circuits and the main breaker) intermittently.

Load: we can 'make' the RCCB trip, but not always in the same manner. Sometimes it will trip if we plug in the telly, or a computer, or a printer, but not always. It's not specific to one (or more) sockets, and sockets that do trip it don't do it all the time (i.e. plugging in a computer on one specific socket won't always trip the RCCB) It can trip when there's hardly anything plugged in, and doesn't trip even when I plug in all my DIY appliances (jigsaw, circular saw, tile cutter, battery chargers, etc)

Random: it can trip at anytime, not just when appliances are being plugged in, or switched on, or coming on on a timer etc. It can trip overnight or when we're not in (lost the contents of the fridge & freezer the other weekend!). Overnight we unplug all appliances except the fridge and turn off the boiler. It still trips.

Often when it trips we have to reset the RCCB three times (i.e. when we flip on back on, it won't stay on - not that it trips again, but that it won't stay in the on position)

To me this sounds like a fault in the RCCB or consumer unit itself - would that be right? Any other suggestions as to what might be the fault?

 

[JohnD]

You say "we unplug all appliances except the fridge and turn off the boiler. It still trips"

So I think you are optimistic in thinking that because they are new they can't be at fault.

It is very common for a boiler, or its pump, to have earth leakage due to water getting onto an electrical part.

I hope your boiler has an FCU with a DP switch that enables you to isolate it fully. in many cases, just switching things off leaves the neutral and earth connected, and a neutral/earth fault can cause a trip. This is why switching off circuits at the MCB doesn't always help.

The pattern you describe, seemingly random, can happen if you have an N-E fault, which will cause a trip as you increase load; or of a general "background" leakage of, say, 25mA which is not enough to trip the RCD, but tales only a little extra leakage to reach the tripping point. Background leakage, if not due to damaged cables including those which have been gnawed, usually comes from "watery" appliances like washing machines and other water heaters; and sometimes from outdoor sockets or lights exposed to damp and rain.

The randomness can arise if it is caused by something with a timer or a thermostat... like your freezer and heating.

Do you have a cooker outlet with a socket, which is not on the RCD? If so, try plugging the freezer into it.

If you can lay your hands on one of those RCD adaptors as used with lawnmowers, you could plug the freezer in through it.

 

[DESL]

We have an old house (1850+) which is undergoing renovation.

There could be a clue here.

Had any plastering or rendering done during renovation?

Wall paper stripping?

Plumbing work that might have leaked into electrics somewhere?

Any likelihood of damaged cables

 

[securespark]

I had a fault on the parents-in-law's install.

RCD tripping. Isolated it to a circuit which was just a 20A radial with a towel rail on it. The fault turned out to be a pinhole in the cable, letting in moisture.

Who'd have thought it? But that's the sort of thing you're up against.

 

[Tim 77]

I had a fault on the parents-in-law's install.

RCD tripping. Isolated it to a circuit which was just a 20A radial with a towel rail on it. The fault turned out to be a pinhole in the cable, letting in moisture.

Who'd have thought it? But that's the sort of thing you're up against.

I came across a similar fault the other week. Narrowed it down to the leg of a ring between a fused spur in the kitchen and a socket in another room. Looked above the spur and saw a shiny new clock on the wall, took it off the nail it was hanging on and clipped my meter onto the nail in the wall and the neutral of the ring, got a dead short between them

 

[NotSoSmart]

All good suggestions - thanks. There's always the possibility that its damaged cables, but how do I isolate which circuit it's on? Turn off all but one circuit and wait for it to trip?

 

[JohnD]

If it is a neutral/earth fault, you can't turn it off. the MCBs in the CU only break the Live (phase) connection.

what make is your consumer unit? Has it got any spare ways in it? Can you post a pic? How do you feel about taking the front off and tinkering about inside?

 

[DESL]

Switching off each cct at CU will not eliminate a N-E fault but will eliminate L-E faults.

With all intermittent faults a good book is required and a large helping of patience

 

[NotSoSmart]

The CU has no identifying marks or makers name, model etc. on it other than a small symbol in the top left corner - how do I post a pic?

 

[JohnD]

might be a Chinese one

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