Exporting TN-C-S to outbuilding - again!

[JohnW2]

So, as well as being the means of exporting the PME earth, it is also providing more-than-adequate bonding back to the bonded pipework (hence MET) of the house.

You are not allowed to use that pipe as a conductor.

That may be true in terms of the regs (although I'm not quite sure what reg explicitly prohibits it) but, in the OP's situation, in electrical terms it would (IMO) be fine.

In the OP's situation, in which the outhouse has been isolated from the house's PME earth in all other senses, a need for main bonding of the outhouse arises only IF the copper pipe provides an electrically unbroken path back to the MET in the house (via pipework and house's main bonding). If that electrical path is (or ever becomes) broken, then the need to main bond the outhouse vanishes. Whenever the need for bonding exists (due to that path being present) there is, by definition, a connection of at least 22mm² CSA from the outhouse to the house's main-bonded pipework. Hence, whether or not it is compliant with regs, I personally think that it is electrically fine, and even 'fail safe' (if that bonding path were ever broken, the need for bonding would also disappear).

However, as I've said to the OP, there are reasons which make some people feel that it is better/safer to keep the PME earth totally away from the outhouse. If he wants to do that, then his plan to interupt the electrical path through the pipework is probably the best way for him to go.

Kind Regards, John

 

[Chri5]

Has the pipe been tested? I'd is really metal all the way, or might the UG bit be plastic?

Bernard suggestion of isolating the pipe seems logical.

 

[JohnW2]

Has the pipe been tested? I'd is really metal all the way, or might the UG bit be plastic?

We haven't been told, nor even whether any of it is underground, but the OP was clear in saying that he believed it was representing an electrical connection between the METs.

Bernard suggestion of isolating the pipe seems logical.

Indeed - and I think the OP has already decided to do that.

I have a TT supply to my house, so I don't have to worry about this, but if it were PME, I personally would not want to export the PME earth to an outhouse, so would do everything (including interupting any continuous metal pipework) necessary to isolate the outhouse from the PME earth (and then TT the outhouse).

One can similarly argue that it is desirable to insert a plastic interuption in the pipework feeding an outdoor water tap if the house has a PME installation.

Kind Regards, John

 

[RF Lighting]

John how do you know that the water pipe or gas pipe supplying your house isn't connected to a PME supply further up the line? Would you do anything differently if you knew it was?

 

[JohnW2]

John how do you know that the water pipe or gas pipe supplying your house isn't connected to a PME supply further up the line? Would you do anything differently if you knew it was?

I was afraid someone would ask that! As you probably recall, I know that my house almost certainly is connected to a neighbour's PME earth via the water supply pipe, as witness the fact that my 'TT' installation has an EFLI (with main bonding connected) at its origin of around 0.25Ω. So, yes, I ought to regard it just as if it my house had an explicit PME supply.

The fact is that my outbuildings (which were wired up before most of the village was PME'd, not too long ago) were, for whatever reason, locally TTd and isolated from the house earth.

As for your 'how do you know...?" question, I suppose that the loop impedance is a useful guide. In my case, it makes it fairly clear that I do have a connection to a TN earth; if the loop impedance were very much higher, then that would make such a connection much less likely.

However, you make a good point. Apart from anything else, things change (like the fairly recent PMEing of my village). I suppose that one therefore should always assume that a TT installation could have (or could acquire in the future) a connection to a PME earth, and therefore take steps to avoid exporting the 'TT' earth (if one's view is that PME earths should not be exported).

Kind Regards, John

 

[THRIPSTER]

BAN, as OMS has pointed out on the IET forum - have a look at 543.2.6 - you might change your mind. As stated there, I am not happy that the unsightly pipe will not be changed ans will go with TT.

I, m going with TT and a plastic section in pipe before building.

Regards

 

[THRIPSTER]

The copper pipe runs from the house cellar where it is bonded to the house MET, under the floors and out along the terraced wall into the outbuilding. It is electrically continuous.

 

[bernardgreen]

John how do you know that the water pipe or gas pipe supplying your house isn't connected to a PME supply further up the line? Would you do anything differently if you knew it was?

This is the situation I fell into when I bought this cottage. The adjacent retail unit has a TT supply and I have a new PME supply. The two have a single water meter and the inter connecting pipe has to have an isolating section. At some point in the future I will be going TT.

 

[JohnW2]

This is the situation I fell into when I bought this cottage. The adjacent retail unit has a TT supply and I have a new PME supply. The two have a single water meter and the inter connecting pipe has to have an isolating section. At some point in the future I will be going TT.

As I said, I'm also in this position, my TT installation seemingly 'enjoying' a connection to the neighbour's PME earth, I assume via the (iron/steel) water supply pipe (I can think of no other possible explanation).

I do wonder whether my water meter inherently has electrical conductivity across it - the casing is certainly plastic - as appear to be the integral pipe unions. The water supply company religiously installed a (≤4mm ² !) G/Y strap across it, and it could be that this is the only connection between my installation and the PME earth - which, if true, would also be a bit worrying (although maybe the ≤4mm strap would serve as a 'fuse'!). One of these days I may 'tamper with' that strap and discover whether there is continuity across the meter itself. If there isn't, then removal of the strap would presumably return my installation to being truly TT.

Kind Regards, John

 

[bernardgreen]

It might be interesting to put a clamp meter on that strap across the meter.

A fuse made out of 4 mm will create quite a flash if it has to blow. Though I expect a prolonged heavy earth current fault could be indicated by fresh warm water out of the cold tap.

 

[JohnW2]

It might be interesting to put a clamp meter on that strap across the meter.

Already done that - and, presumably to no-one's surprise, found no measurable current - but one wouldn't expect to see any significant current under no-fault conditions.

A fuse made out of 4 mm will create quite a flash if it has to blow. Though I expect a prolonged heavy earth current fault could be indicated by fresh warm water out of the cold tap.

As my "≤" implied, I'm not even convinced that it's 4mm² ! However, I'm not sure where you think high currents would flow to - even if the potential of the incoming water pipe were to rise to 240V above true earth, you'd be hard-pressed to get much more than about 3A-4A down my earth electrode, and I don't know where else it could go!

Kind Regards, John

 

[THRIPSTER]

BAS - you have not commented on my post above - do you still say that (extraneous) pipe cannot be used to link MET's ? Would be interesting to hear your views.

Regards

 

[bernardgreen]

It was the DNO's engineer who pointed out the dangers of allowing my MET ( PME system ) to be joined to the MET of the adjoining shop ( TT system ) in anyway at all.

If there was any link then a fault in the "earth" source to my property ( the network neutral ) would affect the earth to their installation. It was compounded by the fact that their supply is from a different branch of the local network than my supply. If they upgraded their system to PME then the link between the two METs would become a link between two different network neutrals. If one of the neutrals faulted to open circuit then the link could be carrying very high currents as it would be in the return path to the substation for the branch whose neutral had failed.

 

[ban-all-sheds]

BAS - you have not commented on my post above - do you still say that (extraneous) pipe cannot be used to link MET's ? Would be interesting to hear your views.

Away from home and without the regs at the moment...

 

[JohnW2]

It was the DNO's engineer who pointed out the dangers of allowing my MET ( PME system ) to be joined to the MET of the adjoining shop ( TT system ) in anyway at all. If there was any link then a fault in the "earth" source to my property ( the network neutral ) would affect the earth to their installation.

I'm sure no-one would argue with that. If the properties' earthing systems were linked and your (PME) property sufferered from a supply fault affecting the PME 'earth' potential, then that would obviously equally affect the other (TT) property.

What I was asking about was your statement that such a link could result in a 'prolonged heavy earth current', sufficient to heat the water in the pipes.As I said, with one of the properties being TT, one would be hard pressed to get more than 2 or 3 amps flowing. With a 15mm pipe (CSA about 22mm², about 1mΩ per metre), that would amount to a power dissipation in the pipe of something less that 10mW/metre, hardly enough to do much water heating

It was compounded by the fact that their supply is from a different branch of the local network than my supply. If they upgraded their system to PME then the link between the two METs would become a link between two different network neutrals. If one of the neutrals faulted to open circuit then the link could be carrying very high currents as it would be in the return path to the substation for the branch whose neutral had failed.

Yes, but that's totally moved the goalposts, since it is an extremely small risk which exists if both of the properties have (or are connected to) their own, DNO supplied, PME earths (i.e. neither is straight TT). As I wrote recently in the thread on gas/electricity separation,perhaps the greatest (but incredibly unlikley) 'electrical risk' to a gas pipe probably arises from the presence of main bonding. Under a very unlikely combination of faults/conditions (network arrangement), this could result in the bonding representing a connection between a high voltage PME 'earth' (due to fault) and an intact, low resistance, PME earth - with the possibility of extremely high currents flowing. The same would occur if the connection between the two (faulty and non-faulty) PME 'earths' was via a pipe, rather than a main bonding conductor. However, this risk does not exist so long as one of the properties in TT.

Kind Regards, John