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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.
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Discuss SWA as cpc to submain-Suitability, & PC's on RCD's. in the Electrical Forum area at ElectriciansForums.net
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.
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.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.
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 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.
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.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.
Now this is hurting my brain.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.
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.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!
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.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.
On my first reading of that it appears to rule that out on the statement "the above requirements shall be applied to each building".Not since I read Regulation 411.3.1.2.
I have also past experience of signal cables being damaged!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.
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...
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.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.
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 don't agree that connecting an extraneous conductive part (gas pipe) to the MET changes it to an exposed conductive part
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