RCD earth potential

Has anyone ever encountered a fault with a RCD which causes a potential on earth in respect to neutral?

I am a trainee electrician, working with an experienced electrician. We encountered a fault today, earth had a potential of 238V in respect to Neutral and a 0 V potential in respect to line.

We did an insulation resistance test at 500V on all circuits from line to neutral, line to earth and neutral to earth. All circuits gave a resistance of >999M ohm. So there is no way line and earth can be crossed anywhere.

The fault didn't trip the RCD or the MCB, yet there was a potential difference between neutral and earth.

We got really strange voltage readings while probing around, differences between earth and neutral ranging from 100v to over 300v, how can this be as nominal voltage was 238V.

The potential difference between neutral and earth was greater when the RCD was in the closed position.

All I can possibly think of is the somehow the line windings around the iron core in the RCD have made contact with the Neutral windings. Could a fault like this possibly create a voltage on Neutral, and cos it is a TN-C-S system, Neutral is connected to earth at the incoming supply, so is it possible this voltage could be seen on the earth bar in the consumer unit?

I would suggest you have a floating earth, i.e it is not connected to anything. As well as doing insulation tests did you do any continuity tests to check your earth was actually connected anywhere?
 

ProDave said:

I would suggest you have a floating earth, i.e it is not connected to anything. As well as doing insulation tests did you do any continuity tests to check your earth was actually connected anywhere?
 




 

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Yes we connected a long lead to the MET and checked continuity to the earth bar in the consumer unit. We had a positive result from said test.

Could a floating earth reach a voltage of 238V?

Stringy75 said:

Yes we connected a long lead to the MET and checked continuity to the earth bar in the consumer unit. We had a positive result from said test.

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And how about upstream of that? Every time I have had spurious readings and voltages where they shouldn't be it has been a DNO issue.

Stringy75 said:

Could a floating earth reach a voltage of 238V?

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A floating anything could reach a potential of the supply voltage if it is somehow connected to it (directly or indirectly).

So you have continuity between the MET and the earth bar in the CU.

But what about continuity between the earth bar in the CU and the earth wire that you are reading 238V on?

Think about it. If that earth wire IS connected how can it possibly be at 238V? The only explanation is that it's floating.

Perhaps it's neutral that's floating? but then surely nothing would work? or was that the fault you were investigating. It would not be the first time N on a supply has gone open circuit.

EDIT:

The more I read this, the more I think you have an open circuit neutral on a TNC-S supply, so no neutral, no earth. but then nothing would appear to be working (though it will still be "live") a very dangerous situation. But surely if nothing was working you would have mentioned that minor little detail?

 

Did you test Ze? That would be a reasonable place to start but surely your experienced mate did that?

What was your Ze? Did you verify all the CU terminations are tight and all cables properly inside their connections correctly?

Doc H.

We didn't actually do a Ze test. We had continuity to the MET, maybe we should have, but our thoughts were elsewhere, it was a bit crazy, there were two electricians and me a trainee all having different opinions. I will suggest a Ze test tomorrow. We actually took the consumer unit off the wall, disconnected everything and took it off the wall and did insulation resistance tests on all the circuits, then put the consumer unit back on the wall and wired it up. The fault was still there.

Why take it off the wall?....just curious

what "fault" were you originally trying to rectify ?

i would have thought a Ze would be one of the first tests....but it does tend to get confusing when three different opinions are floating about

"when you are up,to your nads in crocodiles it is difficult to remember that the original objective was to drain the swamp"

Why did you take the cu off the wall? Seems the wrong first step to me.

• Taking it off the wall was not the 1st step. We had already found another fault, it had 3 ring mains and the ends of the loops were in different brakers. One of them had been there on and off over the past week and for some reason wanted to get the plasterboard off from behind the consumer unit, I don't know why, it didn't reveal anything in the end.
  

The board was a rats nest, so it was decided to re-wire it, while the conductors were out one of the electricians took the opportunity to unscrew the plasterboard behind the consumer unit

Can we go back to the beginning? Why were you there and what were you doing?

why were you called to look at this (what was the reported fault)?

Stringy75 said:

, earth had a potential of 238V in respect to Neutral and a 0 V potential in respect to line

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what was the voltage from line to Neutral

I think that pro Dave is along the right line's, look at the MET

ProDave said:

So you have continuity between the MET and the earth bar in the CU.

But what about continuity between the earth bar in the CU and the earth wire that you are reading 238V on?

Think about it. If that earth wire IS connected how can it possibly be at 238V? The only explanation is that it's floating.

Perhaps it's neutral that's floating? but then surely nothing would work? or was that the fault you were investigating. It would not be the first time N on a supply has gone open circuit.

EDIT:

The more I read this, the more I think you have an open circuit neutral on a TNC-S supply, so no neutral, no earth. but then nothing would appear to be working (though it will still be "live") a very dangerous situation. But surely if nothing was working you would have mentioned that minor little detail?

 




 

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Yes we had power to the sockets and lights worked

Stringy75 said:

Yes we had power to the sockets and lights worked

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Those working does not prove you have an earth,!

Murdoch said:

Can we go back to the beginning? Why were you there and what were you doing?

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It had been re-wired, the "electrician" who installed it would not go back, some kind of falling out. It is the worst job I have ever seen. Everything is  just chock blocked and not even taped. The guy I am working for was called in because people were getting shocks from sockets, the sockets are not screwed to the back boxes 

Murdoch said:

Those working does not prove you have an earth,!




 

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No there is no working earth, the earth has a potential of 238V in respect to Neutral

Stringy75 said:

Yes we had power to the sockets and lights worked




 

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There is a potential of 238V on the earth bar in respect to Neutral

poni said:

why were you called to look at this (what was the reported fault)?

what was the voltage from line to Neutral

I think that pro Dave is along the right line's, look at the MET




 

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I will suggest a Ze test tomorrow, but I really do not think it is a fault on the distributors side

I think the customer needs an actual electrician rather than 2 "electricians" and a trainee.

I cannot believe that some basic common tests have been ignored in favour of removing random bits of plasterboard. Furthermore if the board has been removed and refitted then someone should have done some basic tests before making the installation live.

Something doesn't sound right, so either the Chinese whispers method of relaying this info has made it sound worse than it is or the 2 electricians on site are incompetent.

Lurch said:

I think the customer needs an actual electrician rather than 2 "electricians" and a trainee.

I cannot believe that some basic common tests have been ignored in favour of removing random bits of plasterboard. Furthermore if the board has been removed and refitted then someone should have done some basic tests before making the installation live.

Something doesn't sound right, so either the Chinese whispers method of relaying this info has made it sound worse than it is or the 2 electricians on site are incompetent.




 

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How rude!! Thank you for sharing your opinion, Not very helpful though!

Stringy75 said:

Everything is  just chock blocked and not even taped

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sound's like a top electrician, saving on tape!

maybe not the MET, but something to do with the final connection's to it, tail's or earth bond,