SWA as cpc to submain-Suitability, & PC's on RCD's.

[Jm1980]

Been looking at a CU change in a small office and would appreciate comments on the following.

Office is supplied from the mains intake in the warehouse via a 2 core 16mm SWA buried under concrete with the SWA used as cpc (60A BS88 is the protection). Mains intake in the warehouse is a 3 phase 300A TNCS supply.

Within the office is 16mm bonding to the incoming water and gas services which runs back to the CU i am looking at changing. My question is about the suitability of the armouring as the CPC for the office supply, especially as it branches out into bonding after. Any thoughts from anyone on the setup?

The 16mm SWA is probably a bit small in all honesty if your looking at belt and braces approach but it has existed like this for years with no issues and loading is fairly light. Replacing the cable would be a bit of a nightmare.

The old board is plug in wylex mcb's and a few 3036's protecting a couple of lighting circuits and computer equipment. Obviously i am required to RCD the socket outlets now. I have 3 ring circuits, each with 4 PC's running from it. In terms of build up of leakage current, what's your guys experience of how many PC's should be the max we put on a circuit to avoid nuisance trips?
I will modify the circuits to high integrity cpc connections.

Also looking at AFFD's for the sockets, new amendment has them as a recommendation for socket outlets but not a must for this type of building. What's everyone doing regarding them as they have left a bit of a grey area with that one!

Cheers

 

[Jm1980]

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.

 

[westward10]

Can the gas pipe if redundant be cut off below ground level and permanently buried.

 

[Jm1980]

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!

 

[pc1966]

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?

 

[Jm1980]

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?

 

[pc1966]

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?

 

[davesparks]

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.

 

[pc1966]

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.