Is the earthing system TNCS or TT

[bennie]

Carrying out a EICR in a commercial setting. In the main intake room the DNO supply comes in and it states clearly this is a PME system. 5 years ago it had a 120mm earth from the head going on to the MET which then had separate earths to each isolator on the panel. Within that time a new pumping station has been put in and the 120mm main earth has been taken out and a TT system has been made with two 70mm earths to an earth stake. When testing for Ze I removed the earths to the stake from the MET and got the results I thought I would. I left these off and tested at the incomer to see why they're made it in to a TT, may be the reading was too high, but it was 0.15 on each phase. I attached the earths from the stake but they are not doing anything as the reading stayed at 0.15. Should this go down as a TT system with the Ze results from the earth stake? Or should it go down as a TNCS with the main earth missing which I'd put down as a C2 as its clearly still getting a return path through the neutral and earth from somewhere.

 

[davesparks]

It will have been converted to TT for a good reason, I suspect it would be due to the pumping station being unsuitable for connection to a PME earth.

Have you looked at the paperwork from when the pumping station was installed to try and shed any light on this?

 

[bennie]

It will have been converted to TT for a good reason, I suspect it would be due to the pumping station being unsuitable for connection to a PME earth.

Have you looked at the paperwork from when the pumping station was installed to try and shed any light on this?

Unfortunately like with a lot if installs there is no paperwork apart from a dubious test sheet. Surely this would have been done at the pumping station DB? As stated the TT system isn't doing anything because of the low resistance at the sub mains. So far I've put it down as a TNCS but with a C2 for the missing main earth but I've also put down that there is TT system and given the reading for that as well with a note saying further investigation is required to find out why this has been done.

 

[timhoward]

....I tested at the incomer to see why they're made it in to a TT, may be the reading was too high, but it was 0.15 on each phase. I attached the earths from the stake but they are not doing anything as the reading stayed at 0.15

Are you saying that at the origin, you tested the original suppler earth terminal and got 0.15 ohms.
Then with no conductor between the origin and the MET you still get 0.15 ohms at the MET?
How many connections were still in the MET, was there any bonding still connected?

Do the earthing arrangements as they stand meet the standards in BS7671 or not? If they do I don't see a reason to C2 anything, but before compiling the report I'd try and get an understanding of the rationale for the change.

 

[bennie]

Are you saying that at the origin, you tested the original suppler earth terminal and got 0.15 ohms.
Then with no conductor between the origin and the MET you still get 0.15 ohms at the MET?
How many connections were still in the MET, was there any bonding still connected?

Yes that's right 0.15 at the origin & 0.15 at the MET & like I said the earth that went from the supply to the MET has been removed. What i should explain is this is a large commercial/industrial setting on the railway so the chances of removing parallel paths are slim, there's track bonds, sump pumps, pumps, transformers, structural bonds, lightning bonds and the list goes on. I suppose it's not a C2 but if I can't find why they have done this I think it needs looking in to further and the TNCS earth reinstalled as that's the fault path anyway.

 

[bennie]

I must say as well that I can only get a loop reading on the two wire hi setting, on the 3 wire low the readings are all over the place from <0.01 to 0.89. I think the new pumping station is causing this and that's possibly why they changed the earthing system but surely that should've been done in the pump house leaving everything else supplied from the sub mains on the TNCS.