2 supplies into same building, earthing.

[Jm1980]

Hi guys

Im after some advice on an installation im doing some work on. The installation has 2 supplies into the same building. The supplies are side by side and both TPN, one 300A and the other 200A. They are TNCS and supplied from the same tx although im unsure if its PME or just TNCS as there is no sticker and the tx is literally a few meters away.

My concern regards the earthing. Each supply's meter cabinet has a terminal on the side for connection of an earthing conductor. One supply has an earthing conductor present that runs to the buildings local structural steelwork. The other supply cabinet has no earthing conductor present although there is a lugged 'wire rope' connected to the terminal which disappears underground with the supply cable and is obviously something to do with the DNO. It could possibly be the old remnants of a TNS supply i guess. There is no MET at all for the installation. There is one set of (undersized) bonding in place which also runs back to the structural steel.

It appears therefore that the install is only using the earthing conductor of one supply via the one cabinet. Supplies can be simultaneously touched at many parts throughout the install. Steel nature of the building means the earthing of everything is effectivelly all connected together anyway. All Zs readings fine. But this surely isn't right and the whole setup comes across as a lash up with no thought or regs in mind.

So, is this something i really need to take up with the DNO and request an engineer visit? In my experience they are generally hard work. Has anyone any suggestions as to the solution? I think both supplies would require bonding together. Would this be best achieved with a single MET with 2 earthing conductors run to it (one from each supply) or is it better to have 2 MET's (one for each supply) and then connect these together with what would probably be a decent size cable.

Its a situation ive never come across and as im replacing a couple of DB's and moving some circuits about, i feel obliged to sort the earthing out as i have scope to do this. Appreciatte the view's on this (long) post guys.

Cheers.

 

[Farmelectrics]

thought pme was a tncs

 

[Farmelectrics]

personally id request a site visit from a dno engineer

 

[Jm1980]

thought pme was a tncs

As i understand it PME is tncs but not all tncs are PME. Protective multiple earthing so basically they spike the CNE to earth at various points amongst its length. I would query if this is done because the run from the tx is so short, hence it might be straight forward tncs.

I think my understanding is correct. Im thinking a DNO call might have to be made as im not going to know the supply conductors size (they look to be maybe 185mm) so difficult to size the earthing. I would appreciatte comments on how the earthing would be best achieved though.

 

[Farmelectrics]

yes wouldnt like to call that one let the dno do that side

 

[Sintra]

Had this a few months back on a job in London. Short story is there should be one MET linked to both supplies and all bonding and earthing taken from that.

 

[Jm1980]

Had this a few months back on a job in London. Short story is there should be one MET linked to both supplies and all bonding and earthing taken from that.

It does seem logical Sintra and the way i would imagine it should be, thanks. Was it something you had to consult your DNO over?

 

[Sintra]

It was in consultation with the DNO. It was either do the one linked MET or they where going to disconnect one of the supplies as under their new policies it's only permitted to have one supply per building unless they are specifically in seperate areas ie. Landlords and tennents.

 

[Jm1980]

Sounds like this could open a can of worms. Ive heard the DNO can get a little narky regarding 2 supplies into the same building which seems crazy seen as they are the ones who run them in in the first place.

 

[ImpededLoop]

Hi Jm1980. It's not that uncommon as sometimes two buildings are knocked into one and both supplies utilised.

I'd agree with Sintra that creating one MET linked to both supplies is the best way if you need to retain both supplies.

Out of interest, have you assessed the max demand? It may be that you only need one of the supplies anyway.

 

[Jm1980]

Hi loop

Im going to try contacting the dno see if they can advise. Would they be the ones to specify one MET or 2......or is that purely a decision for myself? I only ask because regs dont really go into this but then again, the dno have no responsability for MET's and earthing conductors ect.

ive already assessed maximum demand and loading and both supplies are indeed required.

 

[Sintra]

If they are both already in place and have meters installed I would not get them involved, just carry on and fit a suitably sized MET. Unfortunately in my case the job had been designed using the existing 2 x 400A supplies but unluckily for me they were isolated with fuses and meters removed so I had no choice but to get them involved. Believe me when I say my battle with them lasted nearly 7 months before I had power on.

 

[rapparee]

It was in consultation with the DNO. It was either do the one linked MET or they where going to disconnect one of the supplies as under their new policies it's only permitted to have one supply per building unless they are specifically in seperate areas ie. Landlords and tennents.

Why do they not like two supplies, are they afraid the two supplies may be on seperate phases and introduce the possibility of a dead short between the two phases if a circuit was interlinked onto two supplies

 

[ImpededLoop]

I'd agree with Sintra (again!). I don't think the DNO can really help you. I would create an MET connected to both supplies with labels so the next electrician knows what the set up is.

 

[Sintra]

Why do they not like two supplies, are they afraid the two supplies may be on seperate phases and introduce the possibility of a dead short between the two phases if a circuit was interlinked onto two supplies

Nothing to do with phases as both where 3 phase in my case. Their argument was that there could be different potentials to earth hence the one MET. It really did take take some persuasion to get them to install the meters and get the power on. I'm actually going there tomorrow or Thursday so I'll take a couple of pictures of the set up. Then I can start another thread showing just how daft these people can be.

 

[Jm1980]

Appreciatting the input guys.

Getting the DNO involved is something i really wouldnt want to do although im not sure i can avoid. I could do with knowing the supply neutral size so i can size my earthing accordingly otherwise id be guessing. Im also intrigued as to what the 'wire rope' is thats connected to one supplies earth/neutral bond and disapears underground.
i would hope that i wouldnt have as much hassle as you sintra given that its an existing supply already in use......although i wouldnt count on it!

 

[davesparks]

You could always calculate the earthing conductor size!

 

[Engineer54]

If as you say the supply TX is literary a just few metres away, then DNO need their pants pulling down if they are supplying TNC-S and not TN-S. They are effectively creating a problem that doesn't need to there, end of!!!

 

[netblindpaul]

Could be PNB?

 

[Knobhead]

Treat it as you would any switchroom with multiple inputs. A single MET for all supplies.

About the only time you’ll run in to problems is if you have a MV supply.
Depending on the local earthing arrangement for the LV the HV and LV have independent MET’s. ≤1Ω for the LV the two MET’s can be linked >1Ω they have to be separate.