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.