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.

 

[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.

 

[Knobhead]

Could be PNB?

It could be, but PNB is usually associated with a dedicated transformer. All the OP has said is the transformer is close by, it could be supplying other feeds in which case the DNO won’t use PNB.


From SP Power Systems Limited Low Voltage Earthing Policy.

The earthing of Low Voltage networks and the provision of earth terminals to customers can generally be achieved in one of the following ways:

• Combined neutral and earth conductors (CNE) earthed at several points (protective multiple earthing – PME), known as TN-C-S
• Separate neutral and earth conductor (SNE) bonded and earthed at or near the substation
known as TN-S
• A phase and neutral conductor is provided to the customer’s supply terminals (a supply with the transformer earthed as normal, however, no network earth) and the customer is responsible for providing a suitable connection to earth and the fitting of an appropriate Residual Current Device (RCD), known as TT
• Combined neutral and earth conductor earthed adjacent to the customer’s supply terminals (protective neutral bonding – PNB – applies to a dedicated transformer supplying a single customer)



I find it strange that there’s a 200A and a 300A feed.

 

[Jm1980]

Could be PNB?

That is a possibility. TNCS was stated on the PIR that was performed a number of years ago with a footnote of 'by enquiry'. Having seen the cabinet though it could well be PNB. I'll clarify a few points, apologies if this wasn't stated in my first post. The cables from the tx are waveform and run into there own DNO cabinet which contains some big fuses. The neutral splits off and connects to the side of the cabinet via a stud which carries through to the outside of the cabinet for connection of the earthing conductor. Also onto the stud on the inside is a copper/brass bar which connects to the framework of the cabinet. So this could be the PNB.

If that is the case then ive heard seperate arguments about whether PNB is actually TNCS or indeed a TNS :-|

Im still intrigued as to the role of the 'wire rope' that connects to one cabinets earthing conductor/PNB stud................could it be some kind of source earth for the DNO equipment, it looks very deliberate.

The TX supplies only this installation and nothing else. The TX apparently has a fuse cabinet on the side of it where these waveform cables are fused and run out to the cabinets, distance of about 10 metres. It is likely to be 2 x 400A supplies i would think, my bad was referring to the 300A and 200A as this is what the customers switchfuses are.

Regards

 

[Knobhead]

That is a possibility. TNCS was stated on the PIR that was performed a number of years ago with a footnote of 'by enquiry'. Having seen the cabinet though it could well be PNB. I'll clarify a few points, apologies if this wasn't stated in my first post. The cables from the tx are waveform and run into there own DNO cabinet which contains some big fuses. The neutral splits off and connects to the side of the cabinet via a stud which carries through to the outside of the cabinet for connection of the earthing conductor. Also onto the stud on the inside is a copper/brass bar which connects to the framework of the cabinet. So this could be the PNB.

If that is the case then ive heard seperate arguments about whether PNB is actually TNCS or indeed a TNS :-|

Im still intrigued as to the role of the 'wire rope' that connects to one cabinets earthing conductor/PNB stud................could it be some kind of source earth for the DNO equipment, it looks very deliberate.

The TX supplies only this installation and nothing else. The TX apparently has a fuse cabinet on the side of it where these waveform cables are fused and run out to the cabinets, distance of about 10 metres. It is likely to be 2 x 400A supplies i would think, my bad was referring to the 300A and 200A as this is what the customers switchfuses are.

Regards

The two coloured sections hardly inspire confidence.

There are documents available from the various DNO’s regarding intake earthing arrangements, I would suggest you download them and do a bit of studying.
The one I’m thinking of describes the situation you’ve found perfectly.

Happy hunting.

 

[Jm1980]

I will take a look Tony, thanks.

Nice little steady number replacing and adding some DB's i thought.............why is it never that simple.

 

[Knobhead]

Because life ain’t like that.

I’ll give you a hint now you’ve given a bit more information about the transformer. It is liable to be PNB up to the intake point, after that you treat it as TN-S. That should give a hint about the “wire rope”.

 

[Jm1980]

I’ll give you a hint now you’ve given a bit more information about the transformer. It is liable to be PNB up to the intake point, after that you treat it as TN-S. That should give a hint about the “wire rope”.

Ive had a quick scan of the DNO earthing arrangment downloads, not exactly light reading and one for tonight! As i remember it, PNB is used mainly when the tx supplies one customer only so that would fit and with the setup i have encountered. The CNE conductor is carried through to the meter cabinet where it is split there. So that would make the 'wire rope' the connection to source earth (performed at this end rather than at the tx end) much like a TNS. Thats my take on it.

To me that explanation makes it much like a TNS yet would it still be officially classified as a TNCS?, ive heard debates on this in the past, think GN8 even classifies it as a TNCS.

.......and then onto the fact that the second supplies meter cabinet has no such 'wire rope' connection. If that too was PNB then would you not expect to see that connection too?

Regards

 

[Engineer54]

Just means that instead of earthing the Neutral at the Transformer, it's earthed at the the main LV switchboard, which in your case sounds like they have made the connection at the DNO's Service Position.

If this is the case it's not a TNC-S distributed system, it's going to be TN-S!! All TN-S systems start off as TNC-S at the origin, be it the TX or Main Switch board where the neutral is earthed!!
I've no idea why the DNO didn't originally provide a single source 800A supply into the building, and let the customer provide his own industrial/commercial distribution system.

 

[Jm1980]

I've no idea why the DNO didn't originally provide a single source 800A supply into the building, and let the customer provide his own industrial/commercial distribution system.

From looking around the place and at the age of stuff i think that the second supply was run in maybe 10-15 years after the first as the premises and building grew. Id guess the original stuff is maybe mid 70's with the second supply being put in around the mid 90's.

 

[Engineer54]

From looking around the place and at the age of stuff i think that the second supply was run in maybe 10-15 years after the first as the premises and building grew. Id guess the original stuff is maybe mid 70's with the second supply being put in around the mid 90's.

What size (Kva) is the supplying DNO TX, and are you positive this TX only supplies the one building??

 

[Knobhead]

A photograph of this setup would be useful and of interest. Especially the LV side to see if there’s a feeder pillar. I would be very surprised if WaveCon is fed direct from the transformer LV to two independent supplies. This would point to there being a fused feeder pillar, the N→E link will be in there.

It’s looking like it could very well be PME as the CNE is linked to earth at the intake.