New to the forum, trying to learn about neutral-earth fault troubleshooting!

[jezcunningham]

Let us know how you get on. Its a good job you did not mention any outside lights,,, eeek

well... there are a couple or three, but as I mentioned I've not got around to the lighting circuits yet - they didn't show any leakage current

 

[SJD]

But the fault might still be there...

Yes the fault might still be there, but at the moment it seems you don't know how many faults you have, or where they might be. The whole point of an RCBO board is to make each circuit independent, a fault on one won't affect another. I'd say it is a worthwhile improvement irrespective.

That said, I've not seen details of the supply (TN-S, or TN-C-S, vs. TT) which might have some bearing on any consumer unit change.

 

[plugsandsparks]

If you are looking for an LE fault, its one process, for a NE fault it is another. The trick is to reliably get readings, its also the hard bit. Without getting into the difficult forum issues of safety etc etc. Your best bet is to provoke an NE fault by drawing lots of current, does not matter where from, i.e. kettle, immersion heater and all that. Then isolate L and N from suspecting areas. i.e. your double pole garage isolator. Otherwise its get a pro in to start to pull circuits out of the board to test, assuming the fault wants to play.

 

[nicebutdim]

I'd ditch the Crabtree board and start again.

Might not be a popular suggestion, but there are always plenty of Crabtree RCBOs at reasonable prices on ebay. Many seem to be sold by people in the trade.

Chances are it'd still be cheaper to replace the lot with another brand.

 

[jezcunningham]

Yes the fault might still be there, but at the moment it seems you don't know how many faults you have, or where they might be. The whole point of an RCBO board is to make each circuit independent, a fault on one won't affect another. I'd say it is a worthwhile improvement irrespective.

That said, I've not seen details of the supply (TN-S, or TN-C-S, vs. TT) which might have some bearing on any consumer unit change.

I’d love to just rip it out but to spend the best part of a grand to narrow it down for fault finding seems a bit drastic. It may come to that...
It’s T-N-CS by the way.

 

[jezcunningham]

If you are looking for an LE fault, its one process, for a NE fault it is another. The trick is to reliably get readings, its also the hard bit. Without getting into the difficult forum issues of safety etc etc. Your best bet is to provoke an NE fault by drawing lots of current, does not matter where from, i.e. kettle, immersion heater and all that. Then isolate L and N from suspecting areas. i.e. your double pole garage isolator. Otherwise its get a pro in to start to pull circuits out of the board to test, assuming the fault wants to play.

Well I think what you said about getting reliable readings is key. I've got numbers coming out of my ears but no smoking gun.
Today I can't reproduce the effect of reducing the overall leakage current (at the meter tails) by isolating the garage. And I can't provoke any fault to trip the RCD. My wife's had both the wash m/c and tumble dryer running and I've whacked on the toaster and induction hob at max and although I can see an increase in overall leakage current, isolating the garage has no effect.
I think the poor IR on the garage lights (and visibly rusty 20-year-old battens) is something worth fixing in its own right, but it doesn't appear to be playing a part in my RCD tripping. (Which now hasn't happened since I started investigating 11 days ago.)
This amateur has now pulled a lot of wires out of breakers and busbars to measure IRs, unscrewed numerous faceplates looking for nicked neutral wires, done leakage and/or IR tests on loads of appliances, and I've reassured myself about the general health of the installation. I've found one or two weak points, but "the fault doesn't want to play".
I think I'm going to leave it a day or three and mull it over. I've got a new tv coming that will need some other diy efforts, and then an updated iphone to sort. I could be on a different forum trying to sort those out...
Thanks for all the interest and suggestions guys and I'll see you in a few days.

 

[davidanthony41]

Well, you asked for details - sorry if this is too long...

I have suffered random RCD tripping since I moved into this house 8 years ago. It happens infrequently - about once every 3 months on average - and I have struggled to find any pattern to it. Perhaps it is worse when the weather has been wet – but this isn’t a hard and fast conclusion.

For the initial house insurance I needed an inspection certificate (and every 5 years) and I asked the first electrician to advise on the tripping. He did all the standard measurements, found nothing wrong (insulation-resistance-wise) and replaced the RCD. Didn’t seem to solve it (still tripped occasionally) so we’ve lived with it, assuming it might be a mouse in the loft or…

A year later we had the kitchen replaced and some changes to the consumer unit – new induction hob with new radial circuit and disconnection of old wiring, repositioned oven and microwave with new radial circuit. Time rolls on and it’s still tripping now and again.

Year 5 comes and a different electrician does the inspection; some low IR measurements but no appliances/lighting disconnected so not considered a fail. (I’ve done a census – I have a total of 95 devices permanently powered – but that’s a whole other thread…)
He notes that “RCD cannot be reset without turning off all the MCBs”. Advises that I probably have a neutral-earth fault, which is notoriously time-consuming to troubleshoot – he’ll do it for a daily rate but can’t estimate how many days it might take.

We live with it a while but in October it tripped 4 times in as many days. So I decide it’s a suitable task for lockdown and I can do some of the grunt work myself. I buy myself a basic insulation resistance tester, a current clamp ammeter that reads down to 1mA, and I make a 13A extension cable with the cores separated so I can check each appliance. I spend a lot of time checking the appliances – nothing shows anything other than 0mA – but I’ve yet to check lighting…

So I move on to leakage current at the meter tails – it’s fairly steady at a minimum of 11mA but jumping up to 18mA or even 29mA occasionally – and I flick off the MCBs one-by-one to see which circuit might be contributing most. Strangely there are two MCBs which each reduce the 11mA to zero – one of the ring mains (not the kitchen) and the radial circuit to the garage.

So now I do insulation resistance (just at 250V) on those two – the ring gives 87Mohms and the garage gives 14Mohm. Same result on both L-E and N-E of course as loads are still in circuit between L and N.

Advance 3-4 days and it’s been raining and I repeat the garage tests and now I see less than 1Mohm so I suspect damage to the underground armoured cable. I turn off the 2-pole isolator in the garage (so I’m only measuring the cable) and – at 500V this time - I get >200M N-E and 70M L-E. I turn to the other side of the isolator and measure the garag e (radial power and radial lighting) and find it is the lighting circuit measuring under 1Mohm – it’s very simple – two switches and two fluorescent strips. They’re rusty and probably >20 years old so I assume the ballasts are shot.

I’m now at the point that I expect to replace the fluorescent fittings with IP-rated LED battens but I’m not convinced I’ve really found the root cause. For example, why did the leakage current drop to zero when I flick off the ring MCB _or_ the garage MCB? And why does the wet weather affect the results? Maybe I’m just living life on the edge at 11mA and any old transient pushes the RCD over the limit?

And it's not tripped for 10 days now...

Outside lights are notorious and oven. Elements which will clear when heated up.

 

[plugsandsparks]

Similar issue, 14 lights to choose from, went for the one with some internal condensation, found an Ants nest, my readings were all over the place, some of those Ants took a direct hit from IR testing, lol

 

[jezcunningham]

Let us know how you get on. Its a good job you did not mention any outside lights,,, eeek

I promised an update after mulling over the advice (and trying to get a few more measurements).

I decided to follow the idea of a N-E fault. I did it all at 250v because I didn't want to try to unplug everything, even though (of course) I'd got all the power fully off. So one-by-one I pulled the neutrals out of the busbar, flipped off the MCB for the same circuit, and did N-E IR measurements. I was hoping to find a short somewhere (I'd already pulled the fronts off almost every faceplate) but in fact I found nothing really wrong. I found the kitchen ring measuring only 5Mohm, the downstairs ring at 7Mohm and the garage at 14Mohm. The rest were at >200.

So I investigated those three circuits a bit further, with some issues that I have now fixed. In the kitchen the offending appliance was the TV. Although only a class2 device (no earth wire) there is significant leakage and it turned out to be via the aerial wire. I suppose there is a lightning protection circuit that discharges into Live/Neutral...

In the garage, it was the lighting circuit with the issue - two old fluorescent battens (old = age of the house? i.e. 28 years?) They looked ok inside but I assume it's the magnetic ballasts that are leaky so I decided to just junk them anyway and I've replaced them with IP66 led battens.

Based on your advice re: outside lights I decided the two that I had installed when we moved in 8 years ago were probably past their best, so they've now been replaced with decent IP66 fitments and led bulbs.

While checking out the faceplates I had to wait for the laundry to finish but I then found the plug for the tumble dryer was rather warm. I couldn't be sure if it was the plug or the twin 13A socket so I replaced both - but where do you get a decent 13A plug from nowadays? I used to use only MK which were so well-designed - those nylon fingers in place of a cable clamp, all the wires cut to same length, nice captive washers on the screws, ... Seem unobtainable now so I chose an MK Duraplug but it's just not the same.

And while I was doing all this stuff I decided to add another USB 2-gang 13A socket in the kitchen - I chose MK again (at vast expense) but one of the screws was seized so it went back for a refund and I'll do without it for now. Who's best nowadays - it's not MK...?

So that left the downstairs ring with 7Mohm to be investigated. To be honest I haven't done anything about it because considering all the electronic gear that's connected (including surge-protected strips for the computers, a couple of TVs, broadband stuff, etc) that it's probably just normal.

And I can add to the story that, touch wood, I've not had the RCD trip out since October 30 which is before I started all this investigation. I'll keep my fingers crossed, and maybe save up to have the CU replaced, using RCBOs, but I suspect we'll be talking close to 4 figures for the cost. I'm fully confident in the state of all the circuits now having inspected more thoroughly than ever before and having made several improvements. Perhaps when the next EICR is due (and depending what the results are) will be time for me to make the decision.

For now I just want to thank all of you who have steered me in the right directions. It's been great to have a set of experienced professionals to guide me.