Out of interest, multi TN-C-S supplies in metal framed and metal clad buildings.

[ericmark]

I am trying to get my head around multiple TN-C-S supplies where metalwork of some sort, connects the buildings, or supplies. What stops excessive current through the PEN conductor if the PEN of one supply is lost?

Or what stops excessive current through connecting metalwork, for example an air line.

Where I work there is a collection of buildings, not sure how many supplies, but at least four. At least 3 into one metal clad steel framed building. And due to that building having more supplies than required, the EV charging point and compressor are connected to that building, and there is a large pipe connecting it to the workshop, this passes the accommodation block under a steel bridge, and likely touches hand rails and fences, possibly also connects to the railway lines which run for 8 miles, and there are further buildings along the line also with their own supplies, at the half way point I know there is an aerial mast with a massive copper earth at base, and a large coax into the signal box where the repeater is, which goes under the track.

I see no chance of the buildings becoming live, and in the main the connecting metal work could take 100's of amps without a problem, but it is more down to what happens with a loss of PEN to the other PEN's? And I have seen no bonding cables between the air line and other metal, so the connection between the PEN's is unknown, it may be just the supply cable to the compressor.

Is there a RCM on the DNO supply to show imbalance or some monitoring of the PEN current?

 

[Guitarguy]

I’m not sure if there is anything that ‘stops’ this, however TNCS (PME) supplies are not allowed in steel frame buildings for this reason, as in the case of the combined neutral/earth being lost, the metalwork would become live. There are places that have these sort of supplies though. Nowadays only TNS supplies are permitted, I’ve been on jobs putting new supplies into steel frames industrial units and it has all been split concentric or 4 core wavecon cables installed to keep the neutral and earth conductors separate at the supply point. I don’t know when this changed

 

[plugwash]

but it is more down to what happens with a loss of PEN to the other PEN's?

I strongly suspect the answer is they pick up the slack and noone notices.

And I have seen no bonding cables between the air line and other metal, so the connection between the PEN's is unknown, it may be just the supply cable to the compressor.

Yeah, that sounds bad.

There is a reason there is a minimum size for main bonding cables on TN-C-S suplies. Even under non-fault conditions they can carry current between the PEN and "actual earth" or between the PEN of one installation and another.

 

[ericmark]

It does seem a PME supply is not permitted, it seems Colinette yarns dies their last batch in 2001, and the railway bought their premises, the idea was to make the building into their new workshop, but getting the railway lines into the building presented a problem, so the electrics were stripped ready for the workshop but then it was realised it would not work, the building was three bays, and the first bay was made into a museum. The next two bays became a storage area and a location for the mid week gang to do under cover work.

So the supplies were not really required, however to get them removed would cost, so best option was to use them. So one supplied the museum, the next supplied a EV charging point, and the next the compressor. The small power requirements of the mid week gang is shared between two supplies and is 3 kW or less.

I had not even considered there may be a problem other than lack of RCD protection, and we were well aware of that problem.

I had asked about the EV charging point 2 x 22 kW and was told it was TT which did raise questions as to how close to the main building, the launch looked good, but the location so close to the personnel entrance and the EV users abusing the system has raised issues. At the moment turned off due to the abuse.

I do not work as an electrician, my qualifications have been photostatted and put into the system so I can use my skills, but in the main I paint things, and have not really looked at the electrical installation. I had just accepted that the clever guy in charge and he like me has a degree so no fool, had looked at all the risks, he was also the safety officer, so had no reason to question the electrics.

So this is something I and likely him had not thought about. After all it is the DNO who provide the supplies, and up to them to decide if PME is acceptable. And if not, why has it been supplied for the last 19 years or more?

 

[Guitarguy]

You’d have to get in contact with the DNO really, there are plenty of places around with PME supplies that shouldn’t be. The problem is that there isn’t anything stopping someone using the PME earthing terminal on a cutout regardless whether it is permitted due to the building type etc, originally it’s at of been TT and over the years someone has ‘helped themselves’ and used the supply earth, I’ve seen that before a few times

 

[ericmark]

If something goes wrong, it will most likely be the DNO equipment, even at 100 amp I don't think it would worry the steel frame of the building or the air line, and there is that much earthed unlikely it would ever cause the voltage to rise in the PEN even if broken. The weakest link is likely the PEN on the DNO's side. Which if we start to use more power, one could see with a loss of one PEN a chain reaction with the load being transferred to the remaining PEN's and them becoming overloaded.