Confusing earthing issue

[J_T]

Hi
I am having a few issues with the earthing at my house. This is a bit long winded but hopefully you can understand.

I am not a time served electrician but I have my 17 edition.

The supply to the house from NIE is a TT system. There is 11 KVA over head going to the transformer pole which is about 50 metres from my house. On the transformer pole, from what I can make out there is a 3 phase transformer that is going to a neighbouring farm approx 200 meters away and 2 single phase transformers, 1 of which I am assuming is feeding my house.
Again from what I can see there is an earth spike at the transformer and there are 3 overheads coming away from the transformer pole. 2 of which are the live and neutral that supply my house and 1 which is an earth that travels 1 pole away from my house and runs down that pole to a 2nd earth spike.
It is a new build house that was wired 4 years ago. The live and neutral from the NIE pole runs through a 30 meter duct underground to my meter box in a small brick house 30 meters from my distribution box inside the house. The meter box and distribution board are connected using a 16mm 3 core SWA cable.

I have a 100mA RCD at the meter box which supplies a live an neutral ISCO and an earth spike here that is connected to a 3rd ISCO. My house connections are taken from here.
At my distribution board the house circuits are split into 2 groups, each fed off a separate 30mA RCD.

At the time of installation we tested the earth loop impedance and got a result of 133 ohms.
4 months ago after finishing the wiring upstairs I thought it would be a good idea to have the wiring tested and signed off as acceptable.

The guy carrying out the testing was happy with the internal circuits but when it cam to testing the speed of cut out on the RCDs, the results were intermittent. At 1 times the fault current it wouldnt trip every time and at 5 times the fault it wouldn't carry out the test and displayed <50V (I think it was < and not >).
After checking the onsite guide it does mention in section 11.5 about a 50V rise in the protective conductor potential not sure if this is what it refers to.
The earth loop impedance had risen to >400ohms.

I put in an additional earth closer to the transformer, less than 10 meters from it. I think the ground is quite stony. The earth loop reduced to 300 ohms and we seen the same results for the RCD tests.

As a test I have daisy changed this closer earth spike to the fore mentioned second earth that I am pretty sure is connected to the transformer and actually clamped it to the transformer earth.

Using a different meter to test the earth loop (Fluke) before calling the testing guy back it is throwing up an error 4 message and won't carry out the test.
On further testing there is approx 20VAC between the neutral and earth. No matter what I do this voltage is there. I have disconnected all but my original earth spike. I have tested with all load disconnected so it is just the NIE supply neutral to earth.
This is also tripping one of my 30mA RCDs at the dist board even with all supplied load circuits disconnected so I have temporarily replaced with a 2 pole isolator. I still have the 100mA protection at the meter.

Last night the 100mA at the meter tripped and would not reset. The normal 20VAC neutral to earth was still there, I disconnected the earth to see if that made a difference and isolated the house. The voltage difference had reduced to 0VAC. I though maybe it was something capacitive that was bouncing around between earth spike (grasping at straws).
Today the voltage was still 0VAC I thought I would chance putting the 30mA RCD back in. The voltage earth to neutral was 0VAC, as soon as I tried to connect the RCD it tripped, the voltage was 20VAC again!!

I am really stumped here, I have asked NIE to take a look as I am half suspecting the transformer. I have see it in the past having sparking issues. Maybe the earth I am connected to is from 1 of 2 transformers on the pole and my neutral is from the other..?
Could I have too many earths in? Or generating a potential difference between rods?? I know there is a minimum distance and I am sure I am exceeding this.

They have said they can supply me a dedicated earth but I would have to move my meter into the house which is a lot of work. I am not sure if this is only for TNS as I would be happy with TNCS if they could do that without relocation.
Besides I think I have generated a dedicated earth at present by connecting directly to the transformer spike.

I would really appreciate any feed back on this as I feel we have tried and tested everything we can.

Sorry for the long windedness of this post.

Thanks

JT

 

[Pat H]

My understanding for TT earthing is 200ohm or more is unstable.
But there is a lot of info on TT earthing and the issues the ground connection can make.
Sounds like you are getting a good ground connection and its changing as the ground conditions change.
I suspect you are going to need a supplied ground and move the meter or install a more robust local ground yourself.

 

[The Ghost]

Presumably you are getting the voltage of 20v ac from upstream ? Is there any voltage on N/E downstream i.e. from the house from something awry in the house wiring? Could you not try an earth stake closer to the RCD's in the house? Is there any possibility your RCD is malfunctioning, could you try swapping them over as you said one is not tripping ? Just conjecture as it is hard to visualise. Can you draw a diagram?

 

[Wilko]

Hi - sorry to hear about the earthing issues and service interruptions. Leaving aside fault finding for a tick, I would be inclined to take their offer of TNCS as it's just one less thing for you to worry about. Once it's done they will be obliged to maintain it going fwd. Do please be careful as N and E should be the same voltage unless there is a fault ... I would def. get help.

 

[J_T]

Thanks for the replies. I actually did sink a new rod at my back door as a test last week and connected it to the earthbar in my dist box. I cant remember if i disconnected the existing earth to the bar at the time but the voltage was still there.
I can appreciate this doesnt sound straight forward, I have taken a few pictures. I will try and put them and a few sketches in a word document tomorrow for reference.
I also am getting help from a colleague with more experience in testing than me and the correct testing equipment.

I am tending towards ruling out the house/upstream connection as i can see the same voltage on the supply side of the 100mA rcd when it is switched off i.e. not connected to the house.

Also the reason i am reluctant to take the TNCS option is that it will require alot of additional work to move the meter box and reroute the cable.

TBC

 

[Pat H]

Ok sounds like the 3 phase is unbalanced. An unbalanced load on one of the other phases will cause a neutral current to flow lifting the voltage of the neutral.
You can't fix that you need to raise it with the DNO.

 

[The Ghost]

But... but hes single phase ??

 

[Pat H]

No he is one leg of a 3 phase. So the neutral he has is shared by the other 2 phases. Thats why his neutral is changing. One or more of the other phases is being heavily loaded and causing the neutral to rise with respect to earth.

 

[westward10]

No he is one leg of a 3 phase. So the neutral he has is shared by the other 2 phases. Thats why his neutral is changing. One or more of the other phases is being heavily loaded and causing the neutral to rise with respect to earth.

I can't see that.

 

[Bobster]

Just to point out it is also not good practice to knock in rods so close to the HV transformers. Depending on their earthing arrangement your encroaching onto their sphere of influence.

 

[Pat H]

Read the original post. He states he can see a 3 phase transformer that he is fed from.

 

[westward10]

Read the original post. He states he can see a 3 phase transformer that he is fed from.

Even though he cannot be guaranteed the transformers are separate.

 

[The Ghost]

Maybe there is some damage or flora/fauna in the duct feeding your house causing this voltage thus some damage to your cables L-N? Do I smell a rat?

 

[The Ghost]

On the transformer pole, from what I can make out there is a 3 phase transformer that is going to a neighbouring farm approx 200 meters away and 2 single phase transformers, 1 of which I am assuming is feeding my house.

I see single phase supply?

 

[Pat H]

His symptoms fit with him being 1 leg of 3 phase. He has three phase on sale pole and used nearby. So my money is hes one leg of the farms 3 phase and its unbalanced.

 

[Pat H]

His symptoms fit with him being 1 leg of 3 phase. He has three phase on sale pole and used nearby. So my money is hes one leg of the farms 3 phase and its unbalanced.

 

[The Ghost]

I do agree with you as its very possible such could be happening

 

[J_T]

Thanks for the feed back. What you are saying about the three phase could certinly be the issue. I will get talking to my neighbour to see if he has added in any new equipment reacently.

I got speaking to NIE today, they are going to send someone out to have a look at the supplies so will keep you updated for anyone in the future having the same issues.

I have put a few photos together in a word document for reference to hopefully give a better idea of what I am seeing.

 

[J_T]

Thanks for the feed back. What you are saying about the three phase could certinly be the issue. I will get talking to my neighbour to see if he has added in any new equipment reacently.

I got speaking to NIE today, they are going to send someone out to have a look at the supplies so will keep you updated for anyone in the future having the same issues.

I have put a few photos together in a word document for reference to hopefully give a better idea of what I am seeing.

Sorry in addition to this I had sunk a spike between the meter box and the NIE spike at location 4.
This is about 10m from the transformer. I disconnected it on sunday and the reading earth to neutral has been 0vac since (any time ive checked). Watch this space.

 

[J_T]

Ok so i had NIE out and sods law everything looked fine. i was cinvinced i had wasted the guys time and removing 1 earth spike had made the difference.
This evening ahaing i tried to change the 2 pole isolator back to original 30mA RCD. Again i seen the earth neutral voltage was 0vac. as soon as i switched the supply at the dist board the 20vac reappeared, my 100mA RCD at the meter tripped and would not reset with the load connected. i had to switch off the 30mA rcd, reset the 100mA then put the 30mA back on.
The voltage went back to 0vac. i believe i could show the nie guy this if i was home when he was there.

 

[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