Voltage on PME

[Des 56]

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

 

[DaveyD]

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

 

[Chr!s]

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

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

 

[DaveyD]

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

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.

 

[Chr!s]

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.

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

 

[DaveyD]

Thanks again for your valued comments Chris. Will work on that.

 

[DaveyD]

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

 

[IQ Electrical]

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

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.

 

[DaveyD]

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.

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 ?

 

[pushrod]

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?

 

[IQ Electrical]

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 ?

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.

 

[DaveyD]

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?

Thanks for your reply, yes I have a reading.

 

[DaveyD]

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.

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.

 

[DaveyD]

Update :: Plastic installed to new sink - voltage still at 10 vac. I have an inspection tomorrow! Darn.
Ideas ?
TIA (again)
Kind Regards, David

 

[pushrod]

Metal waste pipe?