Discuss SWA as cpc to submain-Suitability, & PC's on RCD's. in the Electrical Forum area at ElectriciansForums.net

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I think I’m with you here. If we had two adjacent houses, the first TNCS then a looped supply to the 2nd which is TT, we wouldn’t dream of saying that the gas pipe entering the 2nd house is only an exposed conductive part because it’s connected to the MET next door.
Having two supplies is a totally different situation as they are outwith your control so the DNO could change one, even to put it on a totally different sub-station, etc.

Really this thread highlights that there should be greater consideration when multi building installations sharing a common supply have their earthing characteristics changed by the DNO.

By a strange coincidence this month's IET magazine has this article covering the rather real dangers of TN-C-S bonding:


Mackenzie, the former SPEN employee, first became aware of a potential public safety issue in 2014 when he was alerted to an incident in a property in Galashiels, Scotland. A resident’s coat had fallen onto the property’s gas meter and had caught fire. Readings taken at the scene detected 35 amps of current flowing through the metallic gas service pipe entering the property. Mackenzie said it was fortunate the resident was at home at the time.

I always though the bonding to the customer's side of the gas meter was bonkers from an electrical point of view, but that is what the regs say to do, presumably due to the legal ownership change (supply pipe & meter are Transco's property, etc, but pipe after the meter belongs to the installation).
 
Great article. I also feel fortunate to have managed to see the training video it refers to before it was taken down.
Was there anything controversial in it, or just taken down as part of general updates to all training material, etc?

I find it a bit disturbing that neither the HSE nor SPEN were willing to discuss the matter with the IET magazine.
 
Was there anything controversial in it, or just taken down as part of general updates to all training material, etc?

I find it a bit disturbing that neither the HSE nor SPEN were willing to discuss the matter with the IET magazine.
It referred to "the Galashiels situation" several times, which until the article you've just posted had no context, so nice to finally understand that reference. Maybe a public video effectively admitting liability was too risky for the lawyers.

Basically they mocked up two cut-outs on different phases with a shared service pipe between them. There was a single halogen floodlight load on each.
They demonstrated how breaking the PEN on one of them (no noticeable result) and then disconnecting the main earth (and MET and bonding) on the second one would "turn off" the first one and leave you holding a live conductor.

They also demonstrated that neither drummond lamps, contact voltage detectors, nor non contact voltage detectors could pick up anything wrong at the second install while the MET was connected (the one the current was being diverted through), and only a clamp meter picked up any hint of a problem. It said this was going to become a standard additional test.

Finally I can't remember how but they unbalanced the phases and demonstrated (briefly!) that the floodlight became a little too bright for comfort!
It was a good video and a real shame it's no longer available.
 
It referred to "the Galashiels situation" several times, which until the article you've just posted had no context, so nice to finally understand that reference. Maybe a public video effectively admitting liability was too risky for the lawyers.
OK.
They also demonstrated that neither drummond lamps, contact voltage detectors, nor non contact voltage detectors could pick up anything wrong at the second install while the MET was connected (the one the current was being diverted through), and only a clamp meter picked up any hint of a problem. It said this was going to become a standard additional test.
Now if only "smart" meters had a small CT on the main earth, they could provide real-time warning of PEN faults back to the DNO and even bleep to warn the householder / sparky attending that something was not well...
 
Hello all. I do apologise for just getting back on this thread.......had a few things going off and only just back to work.

The thread has made tremendous reading and i thank all for their contributions. I will try and clarify the situation and if anyone wants to contribute further it would be great. I have been back to the job today as customer is still wanting doing.

Main warehouse 3 phase supply 400A TNCS with 35mm bonding to water/ gas and structural steelwork. Submain to separate stand alone office building is 16mm SWA with armour as the cpc. Inside the office is the single phase CU that customer wants changing. From that CU we have 16mm bonding inside the office to the water and gas pipes.

Issues so far are that the submain swa armour is inadequate as a bonding conductor back to the main MET and also as i understand it the office bonding should be 35mm not 16mm.

To clarify the service pipes situation, the property has one water meter in the driveway outside which has 2 supplies from it in what looks to be MDPE type pipe, one to the main warehouse, one to the office. AFAICS the MDPE makes the bonding of these pipes irrelevant.

The gas is interesting on further investigation. In the warehouse the bonding is to the gas mains valve union which only enters the property about a metre. After this union the meter has been removed (property no longer needs gas). The meter removal has left two separate pieces of unconnected pipework........one piece is the feed around the warehouse and one piece goes off underground to feed the office building.

The warehouse feed is fixed to the structural steelwork of the building so will be 'accidentally' bonded if you like although im saying it would no longer need to be anyway as its no longer introducing a possible earth potential.

The underground feed to the office is now only bonded at the office although it originates from the main warehouse. I have continuity between these two unconnected feeds (probably back through the submain and armour).

Hope this clears up the situation. I agree the submain feed armour is inadequate as a bonding conductor and the existing setup is not compliant. TT'ing the office would bring its own problems as getting a rod in would prove challenging. Also convincing the owners to dig up the concrete to run in a new bonding conductor is a hard sell. I guess an insulated piece of pipe in the gas line as it enters the office may be a solution if they will go for it..............again it will seem strange to them to have work done on a redundant gas pipe!!

Not sure this job will come off which is a shame as it leaves an old CU with 3036's in it. Interesting topic though

Incidentally, what would people's thoughts be on re-instating the bonding to the branch of gas pipe which is in the main warehouse but which leaves underground for the office.
 
It’s possible but where it comes into the office is a concrete floor in a kitchen area so they aren’t going to be keen on that either!
Could it be capped (just in case) and permanently covered with something insulating?
 
Could it be capped (just in case) and permanently covered with something insulating?

Yeah should be able to sort that, seems a bit less of a mess than getting it below ground. I suppose it throws up the question of whether we need a gas safe guy to cap it off considering its disconnected at the other end?

Regarding my point on the break in the gas pipe in the main warehouse..........it is essentially now two separate pipes both of which go underground at some point (1 feeding in as the mains supply now capped, the other as the feed to the office)......both of these are presumably now separate extraneous conductive parts and therefore both require bonding at the main warehouse?
 
Yeah should be able to sort that, seems a bit less of a mess than getting it below ground. I suppose it throws up the question of whether we need a gas safe guy to cap it off considering its disconnected at the other end?
If it is disconnected possibly not, but just in case I would get a gas safe person to do the work.

But if it really is an unused section of pipe and so ends could be open once established, is there any chance you could draw along 35mm tri-rated as a CPC?

My suspicion is no as it might have 90 deg bends that are too tight a radius shortly below the surface at each end, but if it were possible to dig down to any bends then you might have a useful section of "duct" for a CPC.
Regarding my point on the break in the gas pipe in the main warehouse..........it is essentially now two separate pipes both of which go underground at some point (1 feeding in as the mains supply now capped, the other as the feed to the office)......both of these are presumably now separate extraneous conductive parts and therefore both require bonding at the main warehouse?
I would bond them at the supply end at least.

The other thought is if this now-unused buried gas pipe is no longer carrying any flammable substance, and it is fully on their site so under their control, could it be consider an earth rod for TT-ing the out-building?
 
The other thought is if this now-unused buried gas pipe is no longer carrying any flammable substance, and it is fully on their site so under their control, could it be consider an earth rod for TT-ing the out-building?

No because it still enters the main installation and is bonded there. It wouldn't act so much as an earth rod as it would a direct connection between the two installations.

At which point we could start to talk ourselves round in circles.
 
No because it still enters the main installation and is bonded there. It wouldn't act so much as an earth rod as it would a direct connection between the two installations.
If it is unused it could be capped close to ground and covered with insulation at the main building.

I.e. Opposite to first thought of bonding at main building and insulate at out building.
 

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