Voltage up to 100v on CPC even when disconnected

[chrisses]

Oh dear!, so instead of investigating this issue (above 50V touch ?), the installation was put into service with a possible inherent defect.

Yes oh dear I have dealt with the issue accordingly

 

[chrisses]

so, the stray voltage must be coming from that MCB. dis all that circuit's cables from CU. do continuity and then IR tests on that circuit with all loads disconnected. bet you the IR tests will show a L-E fault on that circuit.

Yeah I think your right but am I not right in thinking if that was the case then I would be able to disconnect the earths and only one would still be live?

 

[telectrix]

not if the fault is transmitting through a load to N and then onto the earth via the TNC-S.

 

[Octopus]

Oh and to clarify as well when I switch off the breaker to this circuit only the earth voltage disappears

In which case I'd be back with a copy of the EIC you did for the rewire and comparing the results with what you find now.

 

[Octopus]

so, the stray voltage must be coming from that MCB. dis all that circuit's cables from CU. do continuity and then IR tests on that circuit with all loads disconnected. bet you the IR tests will show a L-E fault on that circuit.

But wouldn't the rcd trip?

 

[spark 68]

I would be more concerned about how the Earth is rising above 50V, and why the ELI test could not be performed first and foremost, then try to find the other fault.

Something doesn't add up here, I would be inclined to start re-testing from the beginning.

 

[chrisses]

not if the fault is transmitting through a load to N and then onto the earth via the TNC-S.

I've arranged to go there tomorrow I'll let you know how I get on.

- - - Updated - - -

Thanks to everyone who has taken the time to give advice on this I will let you know how I get on

 

[Des 56]

You need to clear your head with the confused state you are at


Approach the job with a fresh mind as if it was the initial verification,do the appropriate tests and in the correct sequence
The fault or faults can and should become apparent

Personally I would find faffing around taking voltage readings a waste of time and effort and it will likely confuse ( as it has done with your investigating)

Ignore these voltage readings that you seem fixated on,clear the system using insulation.continuity and loop impedance tests
Get those done correctly with acceptable results and you will sort the problem yourself

 

[Marvo]

I would be more concerned about how the Earth is rising above 50V, and why the ELI test could not be performed first and foremost, then try to find the other fault.

Something doesn't add up here, I would be inclined to start re-testing from the beginning.

I agree, start from scratch and be methodical. Test the incoming supply to prove all the faults are on the premises, find out why the earthing is high impedance hance allowing a rise in touch voltage and find the IR fault(s). Finally test the RCD to make sure it's still functional.

 

[HandySparks]

TN what though ? TN-S or TNC-S...

I may have missed it, but have we got an answer to this?

 

[HandySparks]

I like a bit of fault-finding.

I plan to be in Guildford tomorrow. PM me if you like and I may be able to put a second pair of eyes on the job.

 

[Engineer54]

I then disconnect all the cpc's from each other at each switch (fed at switches not 3 plate) I still have voltage on the earth on the incoming and outgoing feed even though they are disconnected from each other

I wouldn't mind betting that at least one fault, is in one of the switch back boxes that's jammed packed full of cables!!

 

[Deleted member 72666]

Re: Voltage up to 100v on CPC even when disconnected

Originally Posted by spark 68
Oh dear!, so instead of investigating this issue (above 50V touch ?), the installation was put into service with a possible inherent defect.



Yes oh dear I have dealt with the issue accordingly




How, may I ask?​

 

[chrisses]

I may have missed it, but have we got an answer to this?

TNC-S sorry forgot to answer that

 

[HandySparks]

TNC-S sorry forgot to answer that

OK. I think it's worth checking the supply first. If that's not right, nothing else will make sense if you're live testing.

What's the resistance between incoming neutral tail to the CU main switch and the earth bar in the CU?

And worth checking polarity on the supply. Does a volt stick light up on the line, but not on the CU neutral or earth bars?

 

[chrisses]

I cracked it
the problem was actually quite simple in the end, the solution was a bit tougher though
i approached it as a new installation and tested the whole lot, turns out the guy who said he'd done r1 + r2 obviously hadn't
there was a break in the earth somewhere in the circuit leaving most of the lighting circuit unearthed!
with no earth the cpc was getting endured current from the live
couldn't find the break in the earth but managed to refeed the circuit from the board!
thanks to everyone for the helpful advice I'll try and return the favour in the future

 

[Octopus]

Trying to do a Zs on the circuit may have helped highlight the fault. Worth remembering for future.

 

[Des 56]

I cracked it

guy who said he'd done r1 + r2 obviously hadn't

These apparent complicated faults,they generally have a very basic cause
Well done,

 

[Engineer54]

I cracked it
the problem was actually quite simple in the end, the solution was a bit tougher though
i approached it as a new installation and tested the whole lot, turns out the guy who said he'd done r1 + r2 obviously hadn't
there was a break in the earth somewhere in the circuit leaving most of the lighting circuit unearthed!
with no earth the cpc was getting endured current from the live
couldn't find the break in the earth but managed to refeed the circuit from the board!
thanks to everyone for the helpful advice I'll try and return the favour in the future

Another valuable lesson learned, ... ''Never'' take as gospel what others tell you, Always check for yourself!! That is especially the case when in fault finding mode!! lol!!

 

[GT1]

Had this sort of induced current before with similar voltages.
What you have to factor in is not just the voltage but the current behind it too.
It may well be 100 odd volts or so but may only be milliamps 'worth..
Satisfy yourself that it's all compliant, that OCPDs and RCDs are working correctly then, let the earthing system do it's job and dispose of it.