Consumer Unit Replacement issue

[P&Q]

Hi Guys,

I got a little problem that’s a bit confusing and I’d appreciate your help on it.

Firstly, from the picture I have attached, what type of earthing system do you reckon this is? There’s an individual cable coming from the ground going into a metal trunking as you can see, which looks like TNS, however on the existing CU the main earth and neutral have been combined which looks like TNC-S, there is no relevant paper work available at all, I called the DNO and they suggested sending out an electrician for £180 + VAT to tell me what the earthing system is, apparently they have no records of what the system is in this building – is that usual?

This picture is the main supply in a building of six flats. I’m trying to replace the consumer unit in one of the flats on the second floor. There is over 10m long of 16mm tails. The max demand on the CU in this flat is 66.4A, the main cut out fuse is a 60A, so by my calculations the cut out fuse and tails need upgrading but the problem is, upgrading the tails is a big job going from ground floor up to the second floor.

So, I was wondering if you guys with more experience had any solutions that could help me like always?

Thank you in advance for your help.

 

[telectrix]

measure the Ze, PSSC and PEFC. if the latter 2 are the same, you've most likely got TNC-S. if PSSC is higher than PEFC, and the Ze is below 0.8ohms, it's likely to be TN-S.

 

[P&Q]

measure the Ze, PSSC and PEFC. if the latter 2 are the same, you've most likely got TNC-S. if PSSC is higher than PEFC, and the Ze is below 0.8ohms, it's likely to be TN-S.

Thanks for your reply, I don't have the PSSC but in my notes I can see the PEFC is 1.2, the Ze is 0.21 - I forgot to compare them like that - thanks for the reminder, I'll definitely do that. any ideas with regards to the rest? tails?

 

[telectrix]

stick to 16mm fused at 60A or 80A at most.

with that Ze it could be either TN=S or TNC-S. i would think the latter is more likely.

 

[ijustwannbeawin]

measure the Ze, PSSC and PEFC. if the latter 2 are the same, you've most likely got TNC-S. if PSSC is higher than PEFC, and the Ze is below 0.8ohms, it's likely to be TN-S.

wouldnt lower than 0.8 ohms suggest TN-C-S?

 

[cadspark]

I believe the DNO are not allowed to charge to confirm type of earthing or size of cutout fuse. If they have to send someone out to do this then it should be free.

 

[P&Q]

I believe the DNO are not allowed to charge to confirm type of earthing or size of cutout fuse. If they have to send someone out to do this then it should be free.

I agree actually, which is why I asked everyone here. What does everyone else think?

 

[telectrix]

wouldnt lower than 0.8 ohms suggest TN-C-S?

no, TNC-S is below 0.35. TN-S max. is 0.8, but a TN-S can be lower than 0.35. so <0.35 could be either.

 

[lamb]

It is a TNC-S installation, with the supply neutral and earth combined in the ryefield board (this is quite common, ryefield boards come with a CNE link in them anyway, to be removed for SNE services).
The earth cable coming from the ground and terminated onto the neutral side of the cutout with a mechanical shearbolt lug will go to an earth electrode buried near the cable entry point to the building with denso tape wrapped around the rod clamp. I have seen this done on dozens of newbuild flats I have worked on with exactly the same set up.

 

[mogga]

This picture is the main supply in a building of six flats. I’m trying to replace the consumer unit in one of the flats on the second floor. There is over 10m long of 16mm tails. The max demand on the CU in this flat is 66.4A, the main cut out fuse is a 60A, so by my calculations the cut out fuse and tails need upgrading but the problem is, upgrading the tails is a big job going from ground floor up to the second floor.

WHY THE HELL would YOU want to upgrade the DNO submains??? if the DNO supply from the street into a house was 16mm would you want to upgrade it as well??

 

[P&Q]

It is a TNC-S installation, with the supply neutral and earth combined in the ryefield board (this is quite common, ryefield boards come with a CNE link in them anyway, to be removed for SNE services).
The earth cable coming from the ground and terminated onto the neutral side of the cutout with a mechanical shearbolt lug will go to an earth electrode buried near the cable entry point to the building with denso tape wrapped around the rod clamp. I have seen this done on dozens of newbuild flats I have worked on with exactly the same set up.

Many Thanks for your reply,

P&Q

 

[superspark]

It is a TNC-S installation, with the supply neutral and earth combined in the ryefield board (this is quite common, ryefield boards come with a CNE link in them anyway, to be removed for SNE services).
The earth cable coming from the ground and terminated onto the neutral side of the cutout with a mechanical shearbolt lug will go to an earth electrode buried near the cable entry point to the building with denso tape wrapped around the rod clamp. I have seen this done on dozens of newbuild flats I have worked on with exactly the same set up.

Are you saying that the link is made in the Ryfield board, via the CNE block, to make this a TNCS, then back down via the tape to the rod.

 

[lamb]

There is usually a CNE link in the LV panel of the HV/LV transformer room, with an earth electrode (usually buried mat) to tie the neutral to earth potential.
Then at the service end, there will be a CNE link in the cut-out or ryefield board (sometimes a LUCY distribution board), with an earth cable lugged on that goes to a large rod in the ground just outside the premises, which also ties the neutral to true earth potential.
Hence multiple earth electrodes > protective MULTIPLE earthing.

Also, overhead supplies with ABC cable (insulated singles twisted together) will normally be PME. On every third pole, an earth cable runs down the pole from the neutral core to a buried earth rod at the bottom (usually 70 or 95mm green yel or black single core cable). This is because PME needs multiple earth electrodes along a supply.

 

[Engineer54]

Are you saying that the link is made in the Ryfield board, via the CNE block, to make this a TNCS, then back down via the tape to the rod.

That seems to be exactly what he's saying!! My only problem with that is, ...if this is a DNO supplied PME rod, it would almost certainly be connected directly to the cut out neutral, not via a galvanised metal strap that terminates at the steel trunking, ...Whether there is a flying lead bolted to the termination within the trunking going into the ryefield panel or not!!

 

[Noton1205]

16mm tails are fine with an 80a fuse