Confusing earthing issue

[UKMeterman]

Do a video on your phone or camera and then stick it on You Tube and ask people to comment

 

[J_T]

Hi. Update.
NIE came out and did an extensive investigation. It lools like the spike at the transformer was damaged when the fieldnwas ploughed. At least there was a reason.
My earth loop has now gone from 300ish to 100ish on 1 RCD bank but 2 ohms for tge other.

I am still having an issue during testing. The house readings are fine but when we try to carry out the RCD test the RCD doesnt trip. Also when it is test tripped via button it dorsnt reset until ALL off the breakers on that RCD are switched off.

Any ideas? We are a bit stumped on this one when everything seems like it should be fine when the earth issue was resolved.

Thanks

 

[Lucien Nunes]

300ish to 100ish on 1 RCD bank but 2 ohms for tge other.

This doesn't make sense, as the main contribution to the 100 ohms is probably the rod, which is common to both circuit groups. They should be more or less identical, give or take the fraction of an ohm of cable splitting the feed to the two RCDs.

It suggests you have an N/E fault on one group, contrary to your 'readings are fine' and that one loop impedance test was made with one RCD off, isolating the fault. This would fairly well explain the failure to trip and/or reset under some test conditions. What is the aggregate L+N to earth insulation resistance for the installation, or downstream of each of the two RCDs? How confident are you of the IR readings taking in every circuit and appliance? Do the loop impedances still differ if they are taken with all circuits live?

If an explanation can be found for those different readings that is not due to an N-E fault, and the RCD is not faulty itself, then you are probably going to need a leakage clamp meter to do a differential leakage measurement on the L & N going through the RCD with each of the circuits energised, so that you can see what they are contributing to the total.

BTW, I have not seen this thread before. I hate to say it but from the symptoms, it was pretty obvious that there was a fault with the transformer earthing. The phase imbalance thing was a red herring for various reasons, and an Ra of 300 ohms as you achieved with your second spike, would have been sufficient to allow the RCD to function. What was also apparent was that it was quite likely there were two faults, one at the transformer earth grid and one in the house. You might now be on the trail of that second one...

 

[Midwest]

Good advice from Lucien. I've experienced similar issues with a neutral-earth fault (in house wiring), either preventing the RCD to function & Zs test causing RCD to trip.

OP, I think you mentioned the property has been rewired. If so with no appliances or luminaires connected, I would expect to see very high IR readings (>999).

 

[J_T]

Thanks for the input guys. Ill make some investigations and let yoi know how i get on

 

[J_T]

Sorry is a leakage clamp meter much different than a standard clamp meter? I have never came across 1.
Is it a device specifically for this type of issue?

 

[Bobster]

Sorry is a leakage clamp meter much different than a standard clamp meter? I have never came across 1.
Is it a device specifically for this type of issue?

works the same as an ordinary clamp meter, but it's range is set for a lot lower current, and higher accuracy. E.g. mA range rather than amps

 

[J_T]

Ok. Sods law. I got a loan of a fluke megger unit aith the intention of checking the circuits insulations but the fault seems to have disappeared i.e. when i trip the rcd it resets with all connected circuits on.
I had previoisly suspected an out side light that is fed from after the 100ma rcd at the pirelli box but before the 30ma rcds in the house.
When we last tested it was lashing rain outside but is dry now.

Would an earth fault at this point line up with the syptoms i have described?

 

[Strima]

Could well be water ingress in an external light fitting.

 

[telectrix]

Could well be water ingress in an external light fitting.

sounds likely.

 

[J_T]

Ok. I got a lend of a fluke tester.
I plugged in at a plug socket and carried out the loop impedance test (no trip) option. This gave a result of 1.5 ohms which doesnt sound right for tt. It also line up with the assumption that thete is a neutral earth fault somewhere.
I discinnected the outside lights. No difference then continued to disconnect circuits and perform the test untill i seen a different reading for a lighting radial... which was Err5.
I put my normal fluke multi meter across neutral and earth and measured 0.9ohms. I then disconnected the earths at different celing roses until i identified the fault (in the most awkward and out of reach fitting possible).
When i took the light fitting off the resistance went high (>1M).
I tightened up the wiring in the fitting (which is metal and earthed) and reconnected.
Everything seems ok now with the rcd tripping when tested and staying on when reset.
I think the Err5 is when you try and test with a load connected...
I didnt today but will do an insutalion test tomorrow to make sure there are no earth faults but hopefully that was the issue.
Does what im saying sound sensible?

 

[J_T]

Ok. I got a lend of a fluke tester.
I plugged in at a plug socket and carried out the loop impedance test (no trip) option. This gave a result of 1.5 ohms which doesnt sound right for tt. It also line up with the assumption that thete is a neutral earth fault somewhere.
I discinnected the outside lights. No difference then continued to disconnect circuits and perform the test untill i seen a different reading for a lighting radial... which was Err5.
I put my normal fluke multi meter across neutral and earth and measured 0.9ohms. I then disconnected the earths at different celing roses until i identified the fault (in the most awkward and out of reach fitting possible).
When i took the light fitting off the resistance went high (>1M).
I tightened up the wiring in the fitting (which is metal and earthed) and reconnected.
Everything seems ok now with the rcd tripping when tested and staying on when reset.
I think the Err5 is when you try and test with a load connected...
I didnt today but will do an insutalion test tomorrow to make sure there are no earth faults but hopefully that was the issue.
Does what im saying sound sensible?

 

[Octopus]

Seeing as this has been going on for nearly 1 year, would it be worth getting a decent spark involved who knows what they are doing ?

 

[J_T]

I appreciate it has been going on for a year but there have been 2 different faults here. Neither of which are run of the mill and 1 which required the suppliers intervention which is not always quick.
I was under the impression this was a forum for discussion/advice but if the advice is get an electrician i could have came to that conclusion myself rather than trying to understand the issue myself.

 

[Lucien Nunes]

Yes it does sound like the explanation, as per my previous post. Either your previous IR test was in error or quite possibly the fault was intermittent. E.g. when you took the IR test it was not quite in contact but by the time you were delving deeper into the Zs it had made contact. These things can be a royal pain to find, which serves as a reminder not to trust anything, anywhere. If I get conflicting or confusing results I always go back and recheck all measurements with extra care, avoiding the assumption that what seemed to be decent IR readings at the time remove all suspicion from the insulation as the culprit.

 

[Midwest]

Friggin MFT's. Whilst checking for extraneous pipework in bathroom today, I connect mine to a piece of copper pipe in a plasterboard wall, devoid of anything electrical and not physically connected to anything, just for the hell of it. Meter told me to pee off 'cos there was < 50v there

 

[UKMeterman]

Less than 50 volts? Check your symbols. But if you do find stray volts an insulation test might be a good idea.

 

[Midwest]

Less than 50 volts? Check your symbols. But if you do find stray volts an insulation test might be a good idea.

Apologies >50V