Overhead Supply TNC-S?

[lee staf]

Hi
I am about to test a rewire of a bungalow and have started by taking readings at the CU, the problem is that testing for Psc i get a measurement but when i test Pfc i get open circuit >200Mohms suggesting no earth. I have an overhead supply but the suppliers head shows what looks to me as TNC-S? as there is a main earth conductor 16mm from MET to earth terminal attached to suppliers head on neutral side? When doing a quick test at socket readings of 128ohms taken. Should i treat this as TT system and stake or contact Supplier. All circuits are RCD protected on 17th Ed CU.
Any help would be much appreciated.
Cheers
Staf

 

[irishsailor]

Is the main switch RCD'd that may indicate TT

 

[Guest123]

Hi.

What was your actual Ze reading at the origin???

The 128 ohm reading at the socket may be through the bonding conductors.

 

[lee staf]

Hi guys
The origin when eating Ze is showing >200Mohms with main bond disconnected from CU I havechecked continuity of main earthing conductor and this is 0.01 and is terminated into the earth block attached to the neutral incomer however I tried testing a socket for Zs and got 128 ohms so I presume with bonding conductors in place i am getting some earth through pipework to general mass of earth. The CU doesn't have a main switch RCD so doesn't suggest TT system and why have main earth to suppliers neutral? If supplier is providing PME then at origin I know I should have max of 0.35 ohms,I have dead tested installation and all good all bonds continuos all upto supplier block.
I rang EDF and tried to explain problem to them but Lady on line started talking to me about green and yellow wires!!
Thanks for help so far.

 

[Guest123]

First thing I would do as you obviously have no earth and not getting any sense from EDF is get an earth rod in and make the install a TT.

 

[flukey]

hi staf,
try speaking to the supplier as EDF are useless. round here its normally siemens that will sort your problem

 

[Monkey1984]

Don't the main cut outs on a TN-C-S system have a little bit of plastic that basically stops the connection and that needs to be removed. Once done it acts correctly.

Might be way off the mark but sure thats the case - Not everyone needs a TN-C-S arrangement when the main cut out is changed and i think that is what stops it.

Probably wrong lol!

 

[Deleted member 9648]

There's a link between the neutral and the earth block on some service heads ,if tncs was not originally requested at the time of installation this link was left out.I'ts easy to just assume that the earth block is actually earthed. Seen this on two jobs in recent times,one was a recently (pre 17th) extended and rewired property with a split load board which I did some minor works in and got no earth when I tested. Sure enough connected as a tncs and no link in the head.....clearly the guy who done the installation didnt test,and only the socket circuits were RCD'd!!
Getting EDF to understand the problem was just impossible.all they would say was 'yes,PME is available'. It stayed on a temporary connection untill we gave up with EDF and went back and done it ourselves.

 

[Monkey1984]

There's a link between the neutral and the earth block on some service heads ,if tncs was not originally requested at the time of installation this link was left out.I'ts easy to just assume that the earth block is actually earthed. Seen this on two jobs in recent times,one was a recently (pre 17th) extended and rewired property with a split load board which I did some minor works in and got no earth when I tested. Sure enough connected as a tncs and no link in the head.....clearly the guy who done the installation didnt test,and only the socket circuits were RCD'd!!
Getting EDF to understand the problem was just impossible.all they would say was 'yes,PME is available'. It stayed on a temporary connection untill we gave up with EDF and went back and done it ourselves.

That is basically what i was trying to say but was a bit drunk last night.

 

[JUD]

Hi guys
The origin when eating Ze is showing >200Mohms with main bond disconnected from CU I havechecked continuity of main earthing conductor and this is 0.01 and is terminated into the earth block attached to the neutral incomer however I tried testing a socket for Zs and got 128 ohms so I presume with bonding conductors in place i am getting some earth through pipework to general mass of earth. The CU doesn't have a main switch RCD so doesn't suggest TT system and why have main earth to suppliers neutral? If supplier is providing PME then at origin I know I should have max of 0.35 ohms,I have dead tested installation and all good all bonds continuos all upto supplier block.
I rang EDF and tried to explain problem to them but Lady on line started talking to me about green and yellow wires!!
Thanks for help so far.

You won't get a reading if you ate the Ze

 

[ezzzekiel]

if they state pme or tns and you are getting no ZE reading then they are required to sort out asap, normally nedl/yedl would be there within an hour under these conditions unless of course they stated tt

could be disconnected clamp from sheath or as said the knockout lug inside the service head to link accross n/e has not been fitted but it is still there probs. (temp rod)

 

[m4tty]

You won't get a reading if you ate the Ze

You'd get a reading through your tounge wouldnt ya

 

[trebor]

Hi
I am about to test a rewire of a bungalow and have started by taking readings at the CU, the problem is that testing for Psc i get a measurement but when i test Pfc i get open circuit >200Mohms suggesting no earth. I have an overhead supply but the suppliers head shows what looks to me as TNC-S? as there is a main earth conductor 16mm from MET to earth terminal attached to suppliers head on neutral side? When doing a quick test at socket readings of 128ohms taken. Should i treat this as TT system and stake or contact Supplier. All circuits are RCD protected on 17th Ed CU.
Any help would be much appreciated.
Cheers
Staf

Yoo will obviously get a PSCC reading as this is between line and neutral, the PEFC value if the earthing conductor is connected to the neutral would have to be the same not taking into account any parallel earth paths which would make it higher, the earting conductor is is there fore not connected to the neutral, if it was pme there should always be a label on the cut out, the other reason could be that the link has not been closed in the ciut out between neutral and earth terminals, that is down to the dno im afraid

 

[Darkone]

I recently spoke to an DNO inspection team with regards to PNE and the guy was supprised how many earths were missing between supply poles, either not reinstated when line work had been carried out or the now more popular, theft. Apparently the criminal minded are now cutting the eathing cables from the first two meters on a pole, rendering a bad earth to properties.