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

 

[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.

 

[pc1966]

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:

Industry must acknowledge dangerous network fault, warn experts

While networks and the government are well aware of the dangers of neutral current diversion, they have failed to publicly acknowledge the extent of the fault. This is delaying potentially lifesaving work, E&T has learned.

tinyurl.com

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).

 

[timhoward]

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

Great article. I also feel fortunate to have managed to see the training video it refers to before it was taken down.

 

[pc1966]

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.

 

[timhoward]

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.

 

[pc1966]

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...