Yet more on PME earth export

[JohnW2]

Hi folks,

As often discussed here, my TT installation appears to ‘enjoy’ a connection to a PME earth in an adjacent property, via bonding and water supply pipes. For that reason, given the theoretical hazards of exporting a PME earth (even an ‘unintended’ one!), I have not exported the house’s earth to the circuits supplying outside lights, outside sockets and an outbuilding (a greenhouse).

Instead, the dedicated (RCD) CU supplying just those circuits (which is in the cellar) has been provided with a separate TT electrode (on the opposite side of the house from the main TT electrode), and this CU (hence all it’s final circuits) is totally ‘isolated’ from the house’s earthing system.

As far as I am concerned, this is fine, and achieves what I want. However, an inevitable consequence of this arrangement is that this part of the installation cannot have any main protective bonding. As far as I can see, the only exposed part (within the cellar) connected to the ‘garden circuits earth’ is the ‘MET’ of this part of the installation (an exposed ‘earth block’). As far as the rest of the house is concerned, this constitutes an extraneous-conductive-part. It’s also undeniably true that the cellar contains many things (electrical and pipework) which are connected to the house’s main MET. Hence, technically-speaking, my cellar is not a completely equipotential zone.

I was wondering whether people thought that a Jobsworth might have a problem with this lack of bonding – and, if so, what on earth one could do to resolve the issue without totally destroying the whole point of deiberately not having bonding. Would Mr Jobsworth perhaps be happier if I put the “garden installation’s ‘MET’” inside an insulating enclosure?

Kind Regards, John

 

[securespark]

Your rods are connected through the general mass of earth, no?

 

[JohnW2]

Your rods are connected through the general mass of earth, no?

Of course (and to every other earth rod, including TN-C-S PME ones, in the world), but the impedance between them will be relatively high.

What is your point?

Kind Regards, John

 

[Spark123]

It is an exposed conductive part, technically it is a no-no as you have two earthing systems which are simultaneously accessible within your equipotential zone.
If you wanted to you could use a Henley block for the earthing terminal, then it isn't an exposed conductive part.

 

[JohnW2]

It is an exposed conductive part, technically it is a no-no as you have two earthing systems which are simultaneously accessible within your equipotential zone.

As I said, I think it's theoretically a bit 'worse' than that. Being 'liable to introduce a potential' (from its own earth rod), it's an extraneous-conductive-part as far as the rest of the installation is concerned.

If you wanted to you could use a Henley block for the earthing terminal, then it isn't an exposed conductive part.

Indeed. As I said, it would be easy enough to insulate the earthing terminals (whether by using a Henley Block or other means). In fact, I could probably do away with the 'external' earth block altogether, and just use the earth bar of the CU. Are you implying that you think the absence of any bonding would then be Jobsworth-proof (common sense says it ought to be - but, almost by definition, Jobsworths do not necessarily deal with common sense!)

Kind Regards, John

 

[securespark]

What supply type next door has is out of your control, as is the "unintended" PME connection.

You should not have two rods for your installation, unless closer together and connected to each other. There could be a pd between them.

 

[EFLImpudence]

Isn't this separate earth an extraneous-conductive-part in the cellar and should be bonded along with the pipes?

As a method for alleviating the danger of a lost neutral would be the installation of an electrode (of which you now have two but have one anyway) I don't see what you have achieved.

Is not the lack of bonding in the cellar a greater hazard than your neighbours neutral disappearing?

 

[JohnW2]

What supply type next door has is out of your control, as is the "unintended" PME connection.

No argument with that - but I have to assume that my earthing system is probably connected to a PME earth.

You should not have two rods for your installation, unless closer together and connected to each other. There could be a pd between them.

What I am trying to achieve is two totally separate 'installations', the only common factor being that they are fed from the same cutout and meter. If, hypothetically, I moved this 'garden CU' into an enclosure on the other side of the wall (i.e. no longer in the house), would you then accept it as a separate 'installation'? If so, if I were to ensure that there were no exposed conductive parts within the house associated with the 'garden supply' (i.e.if I got rid of the exposed earth block) would you consider that as effectively the same as if it were all 'outside'?

The fact that the two earth rods could be at different potentials (if one rose to close to the potential of a faulty PME 'earth', whilstt he other remained close to true earth potential) is the very reason I have tried to create these two totally separate installations. Joining the earth rods would totally defeat the whole point fo the exercise.

Kind Regards, John

 

[JohnW2]

Isn't this separate earth an extraneous-conductive-part in the cellar and should be bonded along with the pipes?

That's what I said, and it's that which creates the dilemma - since if I did bond it, there would be no point in having it (other than lowering my Ze)!

As a method for alleviating the danger of a lost neutral would be the installation of an electrode (of which you now have two but have one anyway) I don't see what you have achieved.

You're average rod, per se, would have little impact on a serious rise in the (neighbour's) PME 'earth' potential that had been transmitted to my earthing system.

I don't understand why you don't see what I've achieved. The (incredibly small) risk I'm trying to guard against is that of a rise in the neighbour's PME 'earth' potential being transmitted to exposed-conductive-parts in the garden, where people could be standing (maybe with bare feet) on ground at approximately true earth potential. If I completely isolate my 'garden installation' from anything which could possibly be connected to the neighbour's earth, that risk surely vanishes?

Is not the lack of bonding in the cellar a greater hazard than your neighbours neutral disappearing?

No, IMO. For a start, the one and only exposed part of the garden installation (the earth block) is nowhere near anything else of relevance (electrical or pipework) in the cellar, hence absolutely no chance of both being touched simultaneously.

It seems that the (simple) thing I should do is get rid of this exposed earth block (I should have thought of that in the first place, years ago, but it's only recently occurred to me). The situation will then effectively be no different from that would exist if I moved the CU (hence this whole 'installation') to 'outside the house'. What would you then say?

Kind Regards, John

 

[Spark123]

It is an exposed conductive part, technically it is a no-no as you have two earthing systems which are simultaneously accessible within your equipotential zone.

As I said, I think it's theoretically a bit 'worse' than that. Being 'liable to introduce a potential' (from its own earth rod), it's an extraneous-conductive-part as far as the rest of the installation is concerned.

It can become live under fault conditions hence I still think it will be an exposed conductive part.
I don't think it will be worse whether you call it exposed or extraneous, the only real danger is having contact with both at the same time as a fault occurs on one.

 

[JohnW2]

It can become live under fault conditions hence I still think it will be an exposed conductive part.
I don't think it will be worse whether you call it exposed or extraneous,

I guess it depends exactly what one is talking about. The incoming earthing conductor certainly qualifies as an extraneous-conductive part, and without invoking 'the vanishingly improbable' that is never, per se, going to become live (it's only connected to an earth rod, to which nothing else is connected) - in that sense, it's no different from an incoming water supply pipe. However, I agree that the earth block could become live (hopefully only for as long as it took an RCD to operate) as a result of a fault on one of the circuits - so I suppose you could also regard it as an exposed-c-p. Whatever, as you say, I don't think it matters much what one calls it.

the only real danger is having contact with both at the same time as a fault occurs on one.

Exactly - and, I've said to EFLI, there is no possibility of that. There is at least 2 or 3 metres separation between this earth block and anything connected to the house's MET. However, as I've said, I think I'll get rid of the exposed earth block, anyway, if not only to avoid the possibility of any comments! Once that's done, do you see any other issues or problems?

Kind Regards, John

 

[EFLImpudence]

No, IMO. For a start, the one and only exposed part of the garden installation (the earth block) is nowhere near anything else of relevance (electrical or pipework) in the cellar, hence absolutely no chance of both being touched simultaneously.

Oh. Fair enough. I thought the CU was in the cellar with the pipes.
You did say "many things (electrical and pipework) which are connected to the house’s main MET"

It seems that the (simple) thing I should do is get rid of this exposed earth block (I should have thought of that in the first place, years ago, but it's only recently occurred to me). The situation will then effectively be no different from that would exist if I moved the CU (hence this whole 'installation') to 'outside the house'. What would you then say?

With the above clarification, I suppose it is alright.

I shall have to think more. It doesn't seem right.

 

[THRIPSTER]

Interesting John. If you feel you have to cover up the MET, does that mean you have to cover up anything else it is bonded to?

Regards

 

[JohnW2]

Oh. Fair enough. I thought the CU was in the cellar with the pipes. You did say "many things (electrical and pipework) which are connected to the house’s main MET"

The CU is, indeed, in the same cellar as the pipes (and exposed-conductive-parts of the cellar's electrical installation) but, as I've now clarified, far too far away from any of them for both 'earths' to be touched simultaneously. I only mentioned the "many things (electrical and pipework) which are connected to the house's main MET" because of my fear that Mr Jobsworth might still have a problem (with the lack of bonding) despite the fact that none of that could be touched simultaneously with the earth block of the 'garden supply'. Apologies if I was not clear enough.

It goes without saying (I hope!) that I would not have created a situation in which the garden supply's earth and house-MET-connected items could be touched simultaneously - since that would have created a replication (in the cellar) of the very (extremely small) risk I was trying to avoid in the garden!

It seems that the (simple) thing I should do is get rid of this exposed earth block (I should have thought of that in the first place, years ago, but it's only recently occurred to me). The situation will then effectively be no different from that would exist if I moved the CU (hence this whole 'installation') to 'outside the house'. What would you then say?

With the above clarification, I suppose it is alright. I shall have to think more. It doesn't seem right.

I'll be interested to hear why you feel that it "doesn't feel right" after you have thought further. If I moved this CU just a couple of feet so that it was outside of the building (in some sort of enclosure), or installed it in an outbuilding, you presumably would have no problem. Indeed, it would then be the very arrangement for "TTing an outbuilding" which many people (perhaps including you) regularly advocate. If the whole of this part of the installation (including the earth block) is hidden by insulation, I don't see why being in the cellar makes it any different from being 'outside the house', or in an outbuilding.

No-one has yet asked me why I did it like this. The main reason is that there are several final circuits in that CU, supplying various outdoor sockets and lighting, as well as the greenhouse. To have a CU, with its RCD and own TT electrode in a central place (which happened to be the cellar) seemed to be by far the simplest way to do it. What would you have done?

Kind Regards, John

 

[JohnW2]

Interesting John. If you feel you have to cover up the MET, does that mean you have to cover up anything else it is bonded to?

Yes, of course I would. However, the whole point is that the 'local MET' (the MET of the garden installation) in question is not connected/bonded to anything else in the house (or cellar). If it were, the whole point of the exercise would have been destroyed.

As I've been saying, think of that CU (plus earth electrode and 'MET') as being in an outhouse, and fully 'isolated' from the house's earthing system. I don't think anyone would then be arguing about anything - as I've said to EFLI, it's the very arrangement that many here would advocate. All I have done is physically moved that into my cellar. Provided everything about it (including its 'MET') is insulated and hence not touchable, I don't see that the change in location has any significance, does it?

Kind Regards, John