Current on earth?

[jelliottelectrical]

Okay, trying to get my head round this

Commercial site (farm converted to shops, supply still from main farmhouse)

100amp 3 phase TNCS supply

Issues started 3 weeks ago, cutout fuses blowing, replaced by DNO twice and then last week again, however this time arcing was observed from the main distribution board, and on inspection the busbar section of the board was visibly charred/melted. Distribution board is only 7 years old, Hager invicta 3.
All isolated overnight and then all sub boards connected to a generator the next day to restore supplies, so submains were physically disconnected from the house to allow the repair works to take place.

Today, finished fitting the distribution board prior to testing and reconnecting the submains. The board has a direct connection to a local rod, but also to the cutout. When I went to join the DNO earth to the board, there was a lot of sparking. Disconnected any other earths to check it wasn’t current from anywhere in the Installation and basically current was flowing from between the dno neutral/earth.

DNO attended, didn’t even come in and look, when I described the issue, he just said ‘join them together, it’ll be fine’ I’m not too happy with that. As i see it, a fault on their network is using this earth rod as a return path.

Not too sure what to do next, other than to get the DNO out again and hope someone more helpful turns up. I’m not comfortable with the idea of leaving it like this and also my customer is now saying he thinks that this caused the other issues! (I’m not in agreement necessarily with that one)

 

[JohnW2]

Today, finished fitting the distribution board prior to testing and reconnecting the submains. The board has a direct connection to a local rod, but also to the cutout. When I went to join the DNO earth to the board, there was a lot of sparking. Disconnected any other earths to check it wasn’t current from anywhere in the Installation and basically current was flowing from between the dno neutral/earth. .... DNO attended, didn’t even come in and look, when I described the issue, he just said ‘join them together, it’ll be fine’ I’m not too happy with that. As i see it, a fault on their network is using this earth rod as a return path.

How big are these 'sparks? Do you have a clamp meter that would enable you to measure the current when the 'earths' are joined?

Assuming that you are talking about TN-C-S, and that the DNO are satisfied that there is no fault in their network, then the 'DNO earth' will inevitably be at least a little (perhaps 'significantly') above true earth potential. Hence, if you connect it to an earth rod of fairly low resistance/impedance, an appreciable current may flow, perhaps enough to result in some 'sparks' when the two 'earths' initially come together.

Kind Regards, John

 

[jelliottelectrical]

Sorry should have mentioned that. Measured with a clamp meter, fluctuating rapidly, but around 20amps was the highest I saw.

I did test the rod and that gave me a reading of 47 ohms.

The substation is literally across the road, so I wouldn’t have thought there would be that much difference in earth potential between there and the rod I have.

Not sure if I’m just not thinking clearly on this

 

[JohnW2]

Sorry should have mentioned that. Measured with a clamp meter, fluctuating rapidly, but around 20amps was the highest I saw. ... I did test the rod and that gave me a reading of 47 ohms.

Hmmmm. 20A though 47 would imply a potential difference (between DNO earth and your earth rod) of 940V, which is hardly credible!

Have you, without the 'earths' joined together, measured the voltage between your earth rod and the DNOs incoming Ls and N? It almost sounds ass if the incoming "N" is not actually a neutral!

Kind Regards, John

 

[jelliottelectrical]

I measured for a voltage between the two earths but got nothing. I didn’t measure live to earth or neutral to earth though so I’ll do that tomorrow when I’m there.

 

[JohnW2]

I measured for a voltage between the two earths but got nothing.

That's odd/'wrong' - since you wouldn't get current (let alone 20A) flowing between two things at the same potential!

I didn’t measure live to earth or neutral to earth though so I’ll do that tomorrow when I’m there.

Yes, I think you need to do that (measuring to your rod, of course) - although if there is no potential between the earths, that would seem to rule out any 'polarity' issues with the supply.

Kind Regards, John

 

[plugwash]

I did test the rod and that gave me a reading of 47 ohms.

You've mentioned the DNO earth, and the rod, but what about main bonding? main bonding could well present quite a low impedance to earth depending on what it's connected to.

 

[JohnW2]

You've mentioned the DNO earth, and the rod, but what about main bonding? main bonding could well present quite a low impedance to earth depending on what it's connected to.

Fair questions, although I took (perhaps wrongly!) the "any other earths" in ..

... there was a lot of sparking. Disconnected any other earths to check it wasn’t current from anywhere in the Installation and basically current was flowing from between the dno neutral/earth.

... to probably include main bonding.

So, another question for the OP - have you yet measured (with both 'earths' connected) the current in the DNO's earthing conductor (from cutout to MET/board) and also, separately, that in the conductor of your earth rod?

Kind Regards, John

 

[jelliottelectrical]

You've mentioned the DNO earth, and the rod, but what about main bonding? main bonding could well present quite a low impedance to earth depending on what it's connected to.

sorry bonding was disconnected eventually as were any other earths to avoid parallel paths, but the issue presented itself both with and without bonding connected

 

[jelliottelectrical]

That's odd/'wrong' - since you wouldn't get current (let alone 20A) flowing between two things at the same potential!
Yes, I think you need to do that (measuring to your rod, of course) - although if there is no potential between the earths, that would seem to rule out any 'polarity' issues with the supply.

Kind Regards, John

Which confused me. I would have expected to see some voltage on one of the earths. But something is clearly amiss if there is at times 20amps flowing between the two earths.

 

[JohnW2]

sorry bonding was disconnected eventually as were any other earths to avoid parallel paths, but the issue presented itself both with and without bonding connected

Fair enough - that's what I had suspected.

You clearly need to do some more measuring, since what you've told us so far simply doesn't 'add up' ... 20A through a 47Ω earth rod (with no psrallel paths) makes no sense, and nor does any current (let alone anything like 20A) flow between two points (like your 'two earths') between which there is no measurable potential difference!

Kind Regards, John
Edit: missing word added

 

[EFLImpudence]

Is it just a poor choice of words or am I missing something?

How or where do you measure to find a current between to earths?

 

[JohnW2]

Is it just a poor choice of words or am I missing something? How or where do you measure to find a current between to earths?

I understood that it was being said that, when the two 'earths' (DNO TN-C-S 'earth' and local earth rod) were joined, a current (up to 20A) could be measured with a clamp meter around some part of that path (and with all know possible 'parallel paths' disconnected). It was also said that, with those two earths not connected to one another, no voltage could be measured between them.

As I said, that doesn't 'add up', nor does (in the absence of parallel paths) 20A through a 47Ω earth rod.

Edit: I've just realised the problem to which you were referring - a typo (missing word) in my (missing word now added in red) ...
".... and nor does any current (let alone anything like 20A) flow between two points (like your 'two earths') between which there is no measurable potential difference!"
Apologies. Missing word in original now corrected.

Kind Regards, John

 

[jelliottelectrical]

I understood that it was being said that, when the two 'earths' (DNO TN-C-S 'earth' and local earth rod) were joined, a current (up to 20A) could be measured with a clamp meter around some part of that path (and with all know possible 'parallel paths' disconnected). It was also said that, with those two earths not connected to one another, no voltage could be measured between them.

As I said, that doesn't 'add up', nor does (in the absence of parallel paths) 20A through a 47Ω earth rod.

Edit: I've just realised the problem to which you were referring - a typo (missing word) in my (missing word now added in red) ...
".... and nor does any current (let alone anything like 20A) flow between two points (like your 'two earths') between which there is no measurable potential difference!"
Apologies. Missing word in original now corrected.

Kind Regards, John

wot he said!

 

[bernardgreen]

nor does (in the absence of parallel paths) 20A through a 47Ω earth rod.

It may be that the Ground rod's impedance to AC voltage is a lot lower than it's resistance to a DC test voltage.

This difference between impedance (AC) and resistance (DC) is the result of electro-chemical reactions between the metal of the rod and the chemicals in the moisture in the ground.