Voltage on PME

DaveyD

DaveyD

-
Arms
Hi

Just investigating a tingle - on our own utility sink! Its a TNC-S system, all circuits check out, bonding to utilities new and good and checks out OK, check earth contuinity on all appliances. Tingle still there when all circuits de-energised for over half and hour. ZE = 0.20 but has a voltage of 10v through me at the sink, that's without shoes on a slate floor. Can anyone help?
TIA
Regards, David
 
Could be a neutral breaking down somewhere in the installation or even external.

I would have said if your measuring with a digital meter you could be geting a "phantom Voltage"
 
malcolmsanford said:
Could be a neutral breaking down somewhere in the installation or even external.

I would have said if your measuring with a digital meter you could be geting a "phantom Voltage"
Click to expand...
Yes it's a fluke 1651 but just re calabrated and a decent set of new kewtec leads. When I wear rubber shoes the volts drop to 1.2.
 
Sorry Davey just re-read your post and your geting it off a sink I see, which in all honesty most likely rules out phantom voltage as this is a capacitance occurance.

It would seem you may have a neutral breaking down somewhere either in your installation or external. Though with the bonding in place as you say you should not be introducing a potential from outside.
 
malcolmsanford said:
It would seem you may have a neutral breaking down somewhere either in your installation or external. Though with the bonding in place as you say you should not be introducing a potential from outside.
Click to expand...
It's Saturday so please excuse my brain, can you please help me comprehend how this is so if all power is off? TIA
 
if it's the DNO neutral

you should notice it on other exposed/extraneous metalwork

a simple check for voltage drop might indicate that problem

there should be no tingle with main switch off-in that case
 
Last edited by a moderator:
Is it possible that given you have your installation isolated at the incomer, that bonding conductors of your installation are recieving the neutral-earth potential present with true earth, off an adjascent installation, through their bonding and a perhaps shared metallic service supply

The neutral earth potential then present on your installation and the effects felt when you bridge that potential
 
Update: Rod put in 40 ohms. Connected to MET. Little difference.
Tried this:
PME disconnected from MET - so just Utility bonding and TT conected to MET. Energised. Voltage through me (Sink to meter to me to earth) 1.2 VAC
Tried This:
PME still disconnected from MET, Voltage through me to ground 8.2 vac.

Any idea?

TIA

Regards, David
 
Update: Rod put in 40 ohms. Connected to MET. Little difference.
Tried this:
PME disconnected from MET - so just Utility bonding and TT conected to MET. Energised. Voltage through me (Sink to meter to me to earth) 1.2 VAC
Tried This:
PME still disconnected from MET, Voltage through me to ground 8.2 vac.

Any idea?

TIA

Regards, David
Click to expand...

Hi David

I still believe your issue is the difference in potential between pme earth and true earth.

Recommendations in this instance are, TT the installation, isolate the area say using plastic plumbing or to install a electrode connected to the MET.The main risk being if the PEN breaks then theres a risk of electric shock in that location.

Now when the load is greatest is when the potential difference between the earths will be greatest due to the volts dropped.

If we assume a Load of 7KW then the RA of the electrode will need to be 2.1 ohms to limit touch voltage to 50v under broken pen conditions.

Now by lowering the 40 ohms Ra you have will lessen the sensation between the two earths and limit the touch voltage upon a broken PEN.

Regards Chris
 
Chr!s said:
Hi David

I still believe your issue is the difference in potential between pme earth and true earth.

Recommendations in this instance are, TT the installation, isolate the area say using plastic plumbing or to install a electrode connected to the MET.The main risk being if the PEN breaks then theres a risk of electric shock in that location.

Now when the load is greatest is when the potential difference between the earths will be greatest due to the volts dropped.

If we assume a Load of 7KW then the RA of the electrode will need to be 2.1 ohms to limit touch voltage to 50v under broken pen conditions.

Now by lowering the 40 ohms Ra you have will lessen the sensation between the two earths and limit the touch voltage upon a broken PEN.

Regards Chris
Click to expand...

Thanks for your reply Chris. So you dont think its an issue for the DNO ? Just adding the rod back to the MET did not lower, I guessed the utilities underground were also bringing that to the system to some extent so not too suprised. So you think removing the PME and changing to TT system? That did reduce the voltage to 1.2 vdc.
 
Thanks for your reply Chris. So you dont think its an issue for the DNO ? Just adding the rod back to the MET did not lower, I guessed the utilities underground were also bringing that to the system to some extent so not too suprised. So you think removing the PME and changing to TT system? That did reduce the voltage to 1.2 vdc.
Click to expand...

Well your dropping 15 volts, now how much of that 15v is dropped across the supply neutral.

If you have a weak tnc-s say a L - N loop of .0.6 ohms, .3 supply line .3 cne/pen, if we apply 35 amps we would get a volt drop across the neutral of around 10 v.

Your volt drop of 15v suggest a higher loop than the .2 measured.

Well you need to bare in mind two things, one you obviously want to remove the tingling sensation and secondly you need to consider the risk in that location due to a broken PEN.

If you have hit 40 ohms Ra will relative ease you could go down the additional electrode route, getting the Ra down to 2 ohms would reduce the sensation and limit touch voltage to around 50 V.

You could isolate the metal work in the location say by using plastic plumbing.

Or you could TT the installation which remove the link to the neutral.

By all means ring the dno, if a fault exists on the neutral supply this may increase the volts dropped and increase the sensation, though the fact that you have felt the sensation highlights the fact that the locations floor is earthy.

Regards Chris
 
Update: OK, called DNO just to ask about this. They took it seriously and send a man round, then another two. All here for a couple of hours. Thoroughly decent chaps BTW. So they got a good loop of 0.23 at the cut out. But with all consumer side completely disconnected and many phone calls later they tested cut out to my temp rod and got between 6 to 11 vac. I showed them where the supply continued up the wall to next door and how this live cable was capped off since the new build and supply. They cut this off at our eves and said joiners would contact me to remove old cable (shame I gould have joined my tails to that! ). They finally checked connection at the sub station. All ok.

10:30 tonight a joining team came from 40 miles away as they were told we had no power! Oh dear. "Sorry guys"

Upshot is 5 men and the 4 people they spoke to had no idea why a voltage there but thought it should not be. However as the loop is fing and the voltage at 238 they are tied as to what they can do.

Is it just Chris and me to have encountered this small voltage?

Kind Regards, David
 
DaveyD said:
Update: OK, called DNO just to ask about this. They took it seriously and send a man round, then another two. All here for a couple of hours. Thoroughly decent chaps BTW. So they got a good loop of 0.23 at the cut out. But with all consumer side completely disconnected and many phone calls later they tested cut out to my temp rod and got between 6 to 11 vac. I showed them where the supply continued up the wall to next door and how this live cable was capped off since the new build and supply. They cut this off at our eves and said joiners would contact me to remove old cable (shame I gould have joined my tails to that! ). They finally checked connection at the sub station. All ok.

10:30 tonight a joining team came from 40 miles away as they were told we had no power! Oh dear. "Sorry guys"

Upshot is 5 men and the 4 people they spoke to had no idea why a voltage there but thought it should not be. However as the loop is fing and the voltage at 238 they are tied as to what they can do.

Is it just Chris and me to have encountered this small voltage?

Kind Regards, David
Click to expand...

Absolutely not, I've seen this on a property with sandstone floors on a PME supply, a voltage of 5-15 volts between the PME terminal and 'true earth' at the floors.

We abandoned the PME and created a TT system which solved the problem completely.
 
IQ Electrical said:
Absolutely not, I've seen this on a property with sandstone floors on a PME supply, a voltage of 5-15 volts between the PME terminal and 'true earth' at the floors.

We abandoned the PME and created a TT system which solved the problem completely.
Click to expand...
Thank you too for your valued input IQ. Did the supplier suggest that or did you just change it over and mark on the EIC as TT not TNC-S ?
 
I know you mentioned that main bonding was ok but out of interest have you checked if the sink has become isolated from it (maybe ptfe on compression joints) by checking resistance between it and a cpc?
 
DaveyD said:
Thank you too for your valued input IQ. Did the supplier suggest that or did you just change it over and mark on the EIC as TT not TNC-S ?
Click to expand...

We did a periodic inspection first as that was what the client asked for but the suppliers took no interest whatsoever beyond the service head so we changed the installation to a TT system and issued an EIC to suit.
 
pushrod said:
I know you mentioned that main bonding was ok but out of interest have you checked if the sink has become isolated from it (maybe ptfe on compression joints) by checking resistance between it and a cpc?
Click to expand...
Thanks for your reply, yes I have a reading.
 
IQ Electrical said:
We did a periodic inspection first as that was what the client asked for but the suppliers took no interest whatsoever beyond the service head so we changed the installation to a TT system and issued an EIC to suit.
Click to expand...
Thank you. I was thinking they might do that change for me. If I had not pointed out the test from cut out to rod they would have just gone away happy with the ZE.
I am dropping a new RFC for the kitchen and Utility on a spare way for this Napit inspection on Tuesday. I know PME is better than TT. I did not want to change TT and him check EIC, note a TT system and ask why I thought the voltage was there as I have no real answer.
 
Update :: Plastic installed to new sink - voltage still at 10 vac. I have an inspection tomorrow! Darn.
Ideas ?
TIA (again)
Kind Regards, David
 
Can you stick a screwdriver as a temp stake in the garden and measure the resistance between the sink and true earth
It does sound to me as if the potential is between the Pme and true earth

How the sink top is getting the continuity to true earth would then be the next step to investigate
 
I have put a rod in, no voltage between sink and rod.
Napit inspector came today said their would only be a potential voltage between the two earths if their was a voltage on our earth or the PME.
The DNO guys did agree we havd a PME voltage when they measured from the cut out to my earth.

Regards, David
 
DaveyD said:
I have put a rod in, no voltage between sink and rod.
Napit inspector came today said their would only be a potential voltage between the two earths if their was a voltage on our earth or the PME.
The DNO guys did agree we havd a PME voltage when they measured from the cut out to my earth.

Regards, David
Click to expand...

Well what would we do without them?!

Was that the extent of his priceless advice?

Did the assessment go ok other than that anomoly?
 
He said the DNO should really determine if I can TT but although the lads that came said good idea I have no way of contacting them to get official information.
Re assesment - It did thanks for asking, I appreciate your interest. He was really nice. I did not know the AG rating on a fountain and we had a debate on why the answer to a question on max ZS on a B 32A 60898 was for the tabulated rather than x 0.8 adjusted value I gave. But otherwise fine. No IP codes and no Zones directly. I think I talked too much when I showed him the job and mentioned things then.
Regards, David
 
IQ Electrical said:
Absolutely not, I've seen this on a property with sandstone floors on a PME supply, a voltage of 5-15 volts between the PME terminal and 'true earth' at the floors. We abandoned the PME and created a TT system which solved the problem completely.
Click to expand...
Update :: Apologies for the delay. (Rather ironic as it's a pet hate of mine when people post and dont have the decency to update)This situation is now resolved. Thanks for all your advise - there are so many helpful members here. I had been using a rod but now back on P.M.E. and vac of 0.2. A month or so ago the whole street lost power for a 7 hours. DNO came out and dug the road up overnight and fixed something! It could be a coincidence but the potential vanished when the job was done. It's a shame I did not have the presence of mind to ask them what they did. What do you think?Kind Regards, David
 
Probably repaired a joint and possibly added an electrode which might explain the PME terminal becoming closer to 'true earth' potential.
 

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DaveyD

DaveyD

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Voltage on PME
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Periodic Inspection Reporting & Certification
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