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

[westward10]

Main protective bonds are required in this building (see Regulation 411.3.1.2) and the armour of your supply cable is inadequate to support PME bonding.

 

[davesparks]

Can you verify that it is the same gas pipe in the main MET location? If so you can make sure it is bonded there (you really can't relay on steel work supports, etc) and the problem is dealt with.

The problem would not be dealt with. The gas pipe, whether it is bonded in the main building or not, needs to be bonded at the point of entry to the outbuilding.

The needs to be adequately sized for the incoming supply, 35mm has been mentioned by the OP, and be at least that size all the way back to the MET. This bond can be a continuous conductor back to the MET but would normally connect to an earth marshalling terminal at the outbuilding which is then connected back to the MET via a combined CPC and bonding conductor as part of the submain.

 

[timhoward]

I wonder what the practicalities would be of getting an insulated section of gas pipe fitted where it enters the outbuilding, and whether this is more or less attractive than upgrading the bonding back to the origin.
I have a feeling it's going to be a hard sell to convince anyone that this problem needs solving too, so best of luck.

 

[pc1966]

The problem would not be dealt with. The gas pipe, whether it is bonded in the main building or not, needs to be bonded at the point of entry to the outbuilding.

Very true, but if it is the same metal that is already bonded to the MET then the job here is really ones of supplementary bonding, and for that the SWA armour is adequate.

Of course that is not how the regulations, or GN8, treats it as it assumes any extraneous conductive part in a different building might not be part of some structure that always has an identical electrically potential. Hence the very onerous demands such as here where (probably, guess based on 300A supply) 35mm bonding would be specified and that is not in any way reasonable to expect for a cable of 16mm.

If running a new cable is really out of the question then the obvious solution would be to TT the outbuilding and put in a 300mA delay RCD incomer or similar. Putting in a suitable rod might prove tricky without hitting any buried service pipe of course!

TN-C-S - a novel way for DNOs to push hidden costs to the customer...

 

[pc1966]

I wonder what the practicalities would be of getting an insulated section of gas pipe fitted where it enters the outbuilding, and whether this is more or less attractive than upgrading the bonding back to the origin.

That might well prove to be the best solution by far. Probably cheaper than running in a new cable if the original was buried direct and not ducted, and it also avoids the issue of TT-ing the building (safe install of a rod needed, neutral-switching RCBOs if proper selectivity needed, danger of any control/signal cables between buildings becoming the "main bonding conductor" and going up in flames, etc)

I have a feeling it's going to be a hard sell to convince anyone that this problem needs solving too, so best of luck.

Indeed, here is a case where the implications of a TN-C-S supply really make trouble for any existing installation. Of course it fails the current regulations, but I would doubt it is actually unsafe if the gas pipe is already bonded and the one and only common thing.

 

[timhoward]

If running a new cable is really out of the question then the obvious solution would be to TT the outbuilding and put in a 300mA delay RCD incomer or similar.

Now this is hurting my brain.
At first glance you end up back where you started, just with a copper pipe instead of a CPC?
And an RCD that won't trip if the neutral currents are diverted.

I'm obviously missing something!

 

[pc1966]

Now this is hurting my brain.
At first glance you end up back where you started, just with a copper pipe instead of a CPC?
And an RCD that won't trip if the neutral currents are diverted.

I'm obviously missing something!

The risk that this argument on PME bonding it trying to alleviate is the case where the two buildings are in fact on different pipes. So of the incoming supply has an open-PEN fault and only the out building's pipe is a good earth (probably via other TN-C-S bonding elsewhere...) and thus you get all of the faulted segment's neutral current flowing along the sub-main's armour to this pipe and it overheats.

By making that out-building TT you no longer have that risk as no CPCs between them (overlooking the issue of any control/signal cables that might be present).

Now if both buildings are on the same pipe then your out-building Zdb is going to be very low as the parallel earth of the pipe is there, but you don't have any risk of high CPC currents, even if you do get high service pipe currents.

 

[pc1966]

Looking at GN8 is helpfully expands the reg's definition of an extraneous conductive part as:

1. A conductive part;
2. Liable to introduce a potential, generally Earth potential; and
3. Not forming part of the electrical installation

Back to the OP's situation, if I understand it this out-building is on the main site and fed from the warehouse. The question for me is if the gas pipe is also fed from the same warehouse where it is bonded to the MET (i.e. not a separate supply, and not the point of incoming supply that then goes to the main building).

If so then I would argue that a metal pipe from the main building, largely on and under the control of the overall property, once bonded to the MET is part of the overall installation (point 3 above) so it is no longer an extraneous conductive part but is instead an exposed conductive part.

In that case it still needs bonding, but that could be anything meeting supplementary bonding regulations. Though personally I would still use 10mm as the steel armour is around 5-6mm equivalent and a bit better at dissipating heat due to larger cable surface area.

So after 3 pages back at what @westward10 said in post #2 !

 

[westward10]

Not since I read Regulation 411.3.1.2.

 

[westward10]

The risk that this argument on PME bonding it trying to alleviate is the case where the two buildings are in fact on different pipes. So of the incoming supply has an open-PEN fault and only the out building's pipe is a good earth (probably via other TN-C-S bonding elsewhere...) and thus you get all of the faulted segment's neutral current flowing along the sub-main's armour to this pipe and it overheats.

By making that out-building TT you no longer have that risk as no CPCs between them (overlooking the issue of any control/signal cables that might be present).

Now if both buildings are on the same pipe then your out-building Zdb is going to be very low as the parallel earth of the pipe is there, but you don't have any risk of high CPC currents, even if you do get high service pipe currents.

Strange you should mention control/signal cables. A fair few years back I was working in a metal framed warehouse where they grew of all things dandelions for their seeds to be used by chemical companies for weed killer production. They had made it TT but there was no electrode, the main protective bond to the structure provided a somewhat tenuous earth, it was all but non existent due to the concrete base. A couple of Zs tests took out the data cables connected back to the owners house as their return to earth was better than the steel structure.

 

[pc1966]

Not since I read Regulation 411.3.1.2.

On my first reading of that it appears to rule that out on the statement "the above requirements shall be applied to each building".

Which is correct, extraneous parts much be bonded and in a manner to safely carry any fault currents.

But my argument is whether or not a service pipe from the main building, and bonded there, is an extraneous part. If the only potential it can introduce is that of the supply MET then it is no different from, say, an exterior light circuit, or some conduit between buildings carrying network cables, etc, which would normally be supplementary bonded if any doubts about touching two parts simultaneously.

Of course critical to that argument is knowledge of the pipe and its relationship to the supply point so the default to bond everything with 35mm or whatever is one approach, but if it does meet the criteria of originating (electrically speaking) from the main supply point then I don't see that as necessary as I see it could be classed as for conduit, cable tray, etc, going between buildings.

 

[pc1966]

Strange you should mention control/signal cables. A fair few years back I was working in a metal framed warehouse where they grew of all things dandelions for their seeds to be used by chemical companies for weed killer production. They had made it TT but there was no electrode, the main protective bond to the structure provided a somewhat tenuous earth, it was all but non existent due to the concrete base. A couple of Zs tests took out the data cables connected back to the owners house as their return to earth was better than the steel structure.

I have also past experience of signal cables being damaged!

In my case between buildings in Malaysia which has big lightning storms practically every 2nd afternoon leading to damaged electronics. Since then I try to have either high isolation voltages (optocouplers or fibre optic networking, etc) or to have metal cable tray or conduit from apparatus A to apparatus B so there is a good alternative earth path for any unwanted current.

 

[pc1966]

From the "too much idle hands" afternoon, here is an example for discussion:

Here example 'A' is some conduit between buildings, on its own it would be extraneous as it is in contact with the earth, but in reality if bonded at the supply building 1 then I think most folks would not consider it necessitated the 35mm or whatever bond at building 2 (assuming they are not miles apart) on the grounds that the few ohms Ra is only going to cause a few / low tens of amps to flow, and most of that would be at building 1 where it is fully bonded.

Bond case 'B' is what I think the OP might have, and again my argument is that under open PEN conditions almost all of the fault current would go via the bond at building 1 to the service network (if conductive) and possibly a bit via the buried pipe between buildings. Again it is hard to see why a great deal of current would flow via CPC to 'B' even if a few amps are going in to the ground between buildings.

However, case 'C' shown here is one I would consider in need of the full PEN bonding on the grounds that if the (this example water) supply network is low impedance to ground then you could see a very high fault current and it is possible/likely that the inter-building CPC will be a lower impedance than the pipe so it could well carry the majority of that current. If any of the pipe was disconnected or replaced by plastic then clearly C is the exit point for all related fault current on that service network.

Basically in case 'C' I would not class the pipe as originating from the supply MET building in any electrical sense.

Over to you folks for discussion...

 

[davesparks]

From the "too much idle hands" afternoon, here is an example for discussion:
View attachment 107042
Here example 'A' is some conduit between buildings, on its own it would be extraneous as it is in contact with the earth, but in reality if bonded at the supply building 1 then I think most folks would not consider it necessitated the 35mm or whatever bond at building 2 (assuming they are not miles apart) on the grounds that the few ohms Ra is only going to cause a few / low tens of amps to flow, and most of that would be at building 1 where it is fully bonded.

Bond case 'B' is what I think the OP might have, and again my argument is that under open PEN conditions almost all of the fault current would go via the bond at building 1 to the service network (if conductive) and possibly a bit via the buried pipe between buildings. Again it is hard to see why a great deal of current would flow via CPC to 'B' even if a few amps are going in to the ground between buildings.

However, case 'C' shown here is one I would consider in need of the full PEN bonding on the grounds that if the (this example water) supply network is low impedance to ground then you could see a very high fault current and it is possible/likely that the inter-building CPC will be a lower impedance than the pipe so it could well carry the majority of that current. If any of the pipe was disconnected or replaced by plastic then clearly C is the exit point for all related fault current on that service network.

Basically in case 'C' I would not class the pipe as originating from the supply MET building in any electrical sense.

Over to you folks for discussion...

My understanding, which may be out of date as I haven't dealt with this in a couole of years, is that the water pipe, gas pipe and network conduit all need 35mm bonding at building 1 and building 2.

 

[nicebutdim]

Would be great if the OP could confirm whether buildings are have separate gas supplies or, as I understood, had supply pipe linking them.

My initial thoughts had been TT earthing to second building to remove necessity of new bond, but stopped typing when that gas pipe came to mind.

Las page or so has made great reading.

 

[pc1966]

My understanding, which may be out of date as I haven't dealt with this in a couple of years, is that the water pipe, gas pipe and network conduit all need 35mm bonding at building 1 and building 2.

If they are extraneous (as in conductive & going somewhere unknown) then yes, as they could end up carrying the neutral current of any faulted segment and that could be well over a hundred amps in the worst case. No doubt that at building 1 that applies.

For example, a 500kVA substation so around 750A/phase and one phase at two-third or so load, and assume the extraneous connection is of low enough impedance, such as a bonded service pipe linking two TN-C-S supplies either side of the PEN break.

But the argument here is at what point does another link between buildings transition from being just an exposed conductive part (such as a cable or conduit above ground linking the two buildings) where its potential is always close to the MET and supplementary bonding is sufficient, to an extraneous conductive part that can introduce not just another potential (e.g. true Earth) but can also do so with a low enough impedance to need the Table 54.8 sized conductors due to the very high fault currents that might persist.

That is kind of my view of A & B above where even with 230V on the MET (so really a worst-case fault with only one phase running) and the 10-20m of buried pipe being, say, Ra = 5 ohms as an example you would see 46A and the CPC linking building likely to carry less than half of that, making even 4mm supplementary bonding safe.

It is probably an academic debate really, as the easier route for the OP to achieve compliance might be to TT it (signal cables considered...), to persuade them to put in a 35mm CPC, or to look at an isolating joint for the gas pipe. I have never had to get one fitted but presumably the gas supplier (DNO equivalent, as I guess it should be before the meter) can do so?

For general info:

IJAS Limited Stoclkist

IJAS Limited was formed in 1996 to act as the UK distributor for Alfa EngineeringIJAS is the main supplier of Isolation Joints to the UK market, in particular to British Gas Transco and its subcontractors.

www.ijasltd.co.uk

Insulation inserts in metallic gas service pipes to consumers’ premises

Recently, a technical question was raised about the use of insulating inserts/insulation joints in metallic gas service pipes to consumers’ premises in order to electrically insulate one section of pipe from another.

tinyurl.com

 

[westward10]

It is academic the Regulations require main protective bonds and the armour of the supply cable is inadequate to support these bonds.
You can over think these things or just accept it.

 

[davesparks]

If they are extraneous (as in conductive & going somewhere unknown) then yes, as they could end up carrying the neutral current of any faulted segment and that could be well over a hundred amps in the worst case. No doubt that at building 1 that applies.

If you test them at each building they would be identified as extraneous parts in both buildings. Bearing in mind that correctly testing for an extraneous part is done with the installation isolated and disconnected from the means of earthing, so any existing bonding will not affect the test.

For example, a 500kVA substation so around 750A/phase and one phase at two-third or so load, and assume the extraneous connection is of low enough impedance, such as a bonded service pipe linking two TN-C-S supplies either side of the PEN break.

But the argument here is at what point does another link between buildings transition from being just an exposed conductive part (such as a cable or conduit above ground linking the two buildings) where its potential is always close to the MET and supplementary bonding is sufficient, to an extraneous conductive part that can introduce not just another potential (e.g. true Earth) but can also do so with a low enough impedance to need the Table 54.8 sized conductors due to the very high fault currents that might persist.

That is kind of my view of A & B above where even with 230V on the MET (so really a worst-case fault with only one phase running) and the 10-20m of buried pipe being, say, Ra = 5 ohms as an example you would see 46A and the CPC linking building likely to carry less than half of that, making even 4mm supplementary bonding safe.

I agree that this is all true in the real world and reflects what would actually happen in a fault. However installing in accordance with this would be a departure from BS7671 so you would have to be able to justify that departure and prove that it is equally as safe, or safer, than complying with BS7671.

Of course you would likely only be required to prove this during an investigation if something goes wrong.

 

[loz2754]

I don't agree that connecting an extraneous conductive part (gas pipe) to the MET changes it to an exposed conductive part. As others have said, the requirements of BS7671 are pretty clear - all extraneous conductive parts need to be bonded to the MET at the main intake and also in any outbuildings. The outbuildings may take advantage of the distribution circuit protective conductor if that protective conductor is of sufficient size. Otherwise, a new bonding cable would need to be installed back to the MET. Or create a TT system for the outbuilding and bond the services present in the outbuilding to the TT MET. Or remove the need for bonding the services in the outbuilding by introducing an insulated section in the pipework.
In my humble opinion, that is.

 

[timhoward]

It’s been a fascinating thread and I’ve learned a lot.
But trying to prove beyond doubt that services between buildings are the same pipes, and that nothing will ever change feels like territory we shouldn’t be getting into.

A mid-pipe repair using plastic would return to the risk of the submain bonding being a lower Ra than the origin and the need for the larger conductors returns.

I don't agree that connecting an extraneous conductive part (gas pipe) to the MET changes it to an exposed conductive part

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.

I agree and understand that the actual risks and effects considerably vary but the regs can’t really address this as the potential for the arrangements to change is too high.

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.