Tricksy electrical problem... please help!

[Dr Doofenshmirtz]

Your sparky will have an insulation resistance tester (at least, one would hope) - he should find the faulty item fairly quickly with that?
Check out John Ward's Youtube channel - he has a bunch of videos on how these tests are performed, how RCD's work etc.

[brman]

Salesy said:

Switch each circuit off at the MCB, remove the corresponding earth and then re energise the circuit. Test the voltage between the floating earth and the earth bar. Normal voltages to expect will be between 20-50V anything exceeding 50V will be suspect.

you may have used this before with success but it is
a) dangerous (although I would agree doable with care)
b) doesn't really tell you anything unless you use a low impedance volt meter. Unless your meter is specialist or a very old analog one then you can easily see over 50V just by pickup from adjacent cabling. (have seen this many times with a cpc break)

If you want to check for earth leakage on a a particular circuit, why not do it properly and just use a earth leakage clamp on that circuit

[brman]

Dr Doofenshmirtz said:

Your sparky will have an insulation resistance tester (at least, one would hope) - he should find the faulty item fairly quickly with that?
Check out John Ward's Youtube channel - he has a bunch of videos on how these tests are performed, how RCD's work etc.

Only if he is lucky! IR testing will likely find static problems eg water in a socket or junction box. But equipment problems often need the equipment to be energised and often working.
At a rough guess, 30% of such faults I am called to are found by IR testing. 30% can be found a with leakage clamp (or RCD ramp test at a push) and 30% require further investigation involving elimination by swapping circuits to RCBOs etc. Less than 5% would be due to a faulty RCD.

As examples, some of faults like this I have had to fix recently:
1) Kettle with cracked element. IR tested at +200M, no leakage measured. It was only after lots of discussion with the owner than I realised that it always tripped at about 2 in the morning. They then reset the RCD (which held), used the kettle for the morning cuppa then called me (at which point I could find no fault again). Turned out water was seeping into the element when it was cold (so overnight) and sometimes causing a trip. When they used the kettle in the morning it dried out the element internally so it was fine when I got to it. I have had similar with immersion heaters that only cause problems when they are not used for a while.
2) Outside light with water in it. An easy one. Owner reports it happens after rain and at night. So it didn't take a genius to suspect something outside that was only switched on at night Circuit identified as lighting by IR testing.
3) Junction box hidden behind a kitchen unit being dripped on by a leaky pipe. IR test showed the fault but I had to break the ring down to isolate where the problem was, then pull the kitchen cupboard apart to find the culprit.
4) Fridge that tripped the RCD but only when the owners were away for more than a week. They discovered that if they ran an extension lead from another socket to the fridge that it did not trip.
Trouble was, both sockets used were on the same circuit/rcd and that circuit tested fine so the fact it worked on another socket was a red herring. This one took me a few attempts but eventually realised that the difference in the two scenarios was that they left the fridge pulled out when they ran the cable from the other socket.This allowed more air to circulate behind the fridge. My theory being that, when they were alway the fridge was left shut so did not fire up as much, coupled with the heating being off etc allowed it to get a little damp being the fridge which was causing some leakage. I was never 100% sure about this one but a new fridge fixed it.
5) Fridge that tripped the RCD (sometimes) when it fired up. IR testing was fine and leakage clamp tests were fine with it running or not. Trouble was, my clamp meter does have a peak hold reading. It was only when watching the meter just as the fridge fired up did I spot a very short current spike. Sometimes (but not always) enough to trip the RCD. It took me a couple of hours testing everything else on the circuit before I narrowed it down to the fridge though.

I could give other examples but hopefully this shows that normal testing doesn't always find the problem so don't be too hard on a spark that struggles (assuming he does do the testing properly....). That said, it is not uncommon for me to have to find faults where other sparks have tried and failed so it never hurts to get a 2nd opinion.........

[hairyben]

brman said:

Salesy said:

Switch each circuit off at the MCB, remove the corresponding earth and then re energise the circuit. Test the voltage between the floating earth and the earth bar. Normal voltages to expect will be between 20-50V anything exceeding 50V will be suspect.

you may have used this before with success but it is
a) dangerous (although I would agree doable with care)
b) doesn't really tell you anything unless you use a low impedance volt meter. Unless your meter is specialist or a very old analog one then you can easily see over 50V just by pickup from adjacent cabling. (have seen this many times with a cpc break)

If you want to check for earth leakage on a a particular circuit, why not do it properly and just use a earth leakage clamp on that circuit

Yeah given the number of amatuer hour heros looking for ideas that frequent this kind of forum I thought thats a techique best not shared.

What mA meter do you use btw, have an alphatek but its quite erratic

[Salesy]

hairyben said:

Yeah given the number of amatuer hour heros looking for ideas that frequent this kind of forum I thought thats a techique best not shared.

What mA meter do you use btw, have an alphatek but its quite erratic

Ha Yeah agreed, totally forgot i was talking to a non spark. I use a Fluke for all voltage measurements and a Meggar MFT for all other stuff

[brman]

hairyben said:

brman said:

Salesy said:

Switch each circuit off at the MCB, remove the corresponding earth and then re energise the circuit. Test the voltage between the floating earth and the earth bar. Normal voltages to expect will be between 20-50V anything exceeding 50V will be suspect.

you may have used this before with success but it is
a) dangerous (although I would agree doable with care)
b) doesn't really tell you anything unless you use a low impedance volt meter. Unless your meter is specialist or a very old analog one then you can easily see over 50V just by pickup from adjacent cabling. (have seen this many times with a cpc break)

If you want to check for earth leakage on a a particular circuit, why not do it properly and just use a earth leakage clamp on that circuit

Yeah given the number of amatuer hour heros looking for ideas that frequent this kind of forum I thought thats a techique best not shared.

What mA meter do you use btw, have an alphatek but its quite erratic

alphatek 775 here too if I remember correctly. And yes I agree, it isn't great but is better than nothing and I have generally found it good enough to point me in the right direction for earth faults (which is all I use it for).

[Ultraviolet]

Murph7355 said:

Thirded.

OP, I would do this as a matter of course with the way you describe things. (In fact I did just that a few years ago ). It may not in itself fix the problem, but will be less of a ball ache when things trip and will give a stronger indication of what is actually failing. IIRC it wasn't that expensive to do.

Ref your oven and shower....how far are they/how long are the cable runs from the consumer unit?

Thanks - RCBOs definitely seem like the logical next step. The cable runs aren't massive - no more than 15 metres I would say...

[Ultraviolet]

brman said:

Only if he is lucky! IR testing.... so it never hurts to get a 2nd opinion.........

Cheers, sounds like you've been there and done that... I may PM you if you're able to help and near Bromley - the spark now seems to have gone AWOL...

The kitchen fitters have been back and checked the wiring... from their point of view, all the appliances are connected correctly.. so down to our wandering spark now to see what he suggests as next steps...

thanks

UV

[brman]

Ultraviolet said:

Cheers, sounds like you've been there and done that... I may PM you if you're able to help and near Bromley - the spark now seems to have gone AWOL...

The kitchen fitters have been back and checked the wiring... from their point of view, all the appliances are connected correctly.. so down to our wandering spark now to see what he suggests as next steps...

thanks

UV

I'm not near enough to Bromley I am afraid.