Armoured cable to shed - advise please

[JohnW2]

The stone walls of my cottage can be considered as an "extraneous-c-p"

By BS7671 definition, I'm sure that's true - but, as I've said, almost anything is an extraneous-c-p by that definition. Anything with a sub-infinite resistance/impedance to earth will qualify as being an extraneous-c-p by that definition!

However, as I've said, I regard EFLI's approach (derived from BS7671's view about Supplementary Bonding) as far more sensible, and I rather doubt that you'd ever be able to measure of path of less than 23k (if one wants to limit current to 10mA) or even 46k (for 5mA).

On the other hand, when (occasionally) the water table rises to above my daughter's kitchen floor, such that there is 'standing', mineral rich, water above the tiles, I imagine that EFLI's measurements would probably confirm that it should be regarded as an extraneous-c-p.

Kind Regards, John

 

[EFLImpudence]

Well, as I've said, it's certainly how I view it. You seem to be saying that (maybe most) 'other people' regard distance as being a major factor - is that what you believe?

In a way but, perhaps not in actual numbers.

A shed in the garden always raises these issues, whereas an attached garage does not.

There are those in another forum who just advocate TT for all such sheds - they just seem to like it.
However, even they would not suggest it for the attached garage.

There is one 'practical issue' which is to some extent distance-related (and which is really how this issue arose in this thread). If the shed's electricity requirements are very modest, such that SWA smaller than 10mm² is being considered, then if the shed has extraneous-c-ps which need bonding, and if a TN-C-S earth is being exported to the shed, then one has to either incur the additional cost of moving up to 10mm² SWA or else incur the hassle (and small expense) of running a separate 'bonding conductor' - and the greater the distance, the greater that increase in marginal cost. However, given the 'future-proofing' considerations, I would personally be inclined to suggest that at least 10mm² should be installed, anyway, regardless of current requirements.

Yes, agreed.
Isn't that the only debate as far as the earthing arrangement of a shed?
That is the cost of or difficulty of a suitable bond compared with installing TT.

Well, again, maybe I am an exception - since, as I wrote, I definitely did consider it in my daughter's kitchen (with wet quarry tiles), even if I 'try not to think about it'. My 'concern' was that the (sometimes) wet floor really constitutes an extraneous-c-p (that should be bonded), regardless of how close or distant it is to/from the origin of the electrical installation (in my daughter's tiny cottage, it's probably no more than about 3m).

Do you think anyone has ever "bothered" to satisfactorily resolve such a problem?

 

[bernardgreen]

In vino veritas ( ? maybe )

If an installation is TT then there cannot be any un-bonded extraneous-c-p as all incoming pipes etc etc are already bonded by the earth around them.....

except for that incoming Neutral

 

[JohnW2]

In a way but, perhaps not in actual numbers. A shed in the garden always raises these issues, whereas an attached garage does not.

If it's literally an 'attached' garage, then I would personal regard it as being part of the house (in some cases, that is obviously literally true), so I would not think of it any differently from any other 'rooms' in the house - i.e. earthing would be provided by the installation's CPCs in the usual way, and if any extraneous-c-ps enter the building through that 'room', they would be main bonded in the usual way.

There are those in another forum who just advocate TT for all such sheds - they just seem to like it. However, even they would not suggest it for the attached garage.

Indeed. I've dealt with e 'attached garage' above, but as far as TTing sheds in general, are they not perhaps considering issues other than bonding? ....

Yes, agreed. Isn't that the only debate as far as the earthing arrangement of a shed? That is the cost of or difficulty of a suitable bond compared with installing TT.

As I've hinted, there are other issues, such as people standing (maybe with bare feet) on wet ground outside the shed and reaching in and touching an exposed-c-p (maybe a lighht switch, commonly just inside the door) - analogous with the 'outside tap' scenario. In many senses, that hazard is at its greatest when there are no extraneous-c-ps in the shed to be bonded, since bonding, if present, will raise the potential of the nearby ground to closer to that of an exported TN-C-S earth which (very rarely) could be seriously elevated. Hence, some people (maybe bernard?) would favour always TTing outbuildings, even if (particularly if) there was nothing in the outbuilding to bond.

Do you think anyone has ever "bothered" to satisfactorily resolve such a problem?

Well, "there's always one (or a handful)" but, in general, I agree that it's likely that very few people would 'bother' (even if they thought about it, which many probably wouldn't). However, it's all a bit ironical and inconsistent, since the same people who would not bother bonding (or even think about bonding) my daughter's kitchen floor which has been known to be covered with half an inch of mineral-rich water would quite probably be capable of getting 'emotional' about a few inches of unbonded extraneous-c-p (maybe an incoming metal water pipe, before it joined plastic plumbing) in some hardly accessible corner of the room!

Kind Regards, John

 

[JohnW2]

If an installation is TT then there cannot be any un-bonded extraneous-c-p as all incoming pipes etc etc are already bonded by the earth around them.....

Is not your 'neighbour' an example of an exception to that generalisation? Don't forget that to be an extraneous-c-p, a conductor has to be "liable to introduce a potential, usually [but not always] earth potential". In your case, were it not for your 'insulating sections', pipes entering the (TT) neighbouring property from your (TN-C-S) cottage would be "liable to introduce" a potential that could be greatly above earth potential.

Kind Regards, John

 

[bernardgreen]

The neighbouring retail unit and by cottage are a special case of two separate supplies from two different street cables in one building.

For out buildings fed by a sub-main from a nearby building I would consider TT as preferable provided a reliable Ground rod could be installed. Damage to the submain cable could leave only the Live connected to the installation in the out building. YES I know the risk of that happening is small.

 

[JohnW2]

The neighbouring retail unit and by cottage are a special case of two separate supplies from two different street cables in one building.

Would not the potential problem I described still exist if both were fed from the same 'street cable' but with one being TTd whilst the other was using the neutral-derived earth of the supply?

For out buildings fed by a sub-main from a nearby building I would consider TT as preferable provided a reliable Ground rod could be installed.

As I recently wrote, I suspected that would be your preference.

... Damage to the submain cable could leave only the Live connected to the installation in the out building. YES I know the risk of that happening is small.

Are you talking about 'the usual' discussion topic of a (very rare) 'lost TN-C-S neutral', or something different?

If so, why do you regard the risk to be greater in an outbuilding than in the main house - just because of the 'outside tap scenario', or what?

Knowing your concerns about TN-C-S, one thing I've never really understood is why you chose to use the TN-C-S 'earth'. The fact that it was 'available' to you didn't mean that you had to use it (I've been offered TN-C-S, but said "no, thank you").

Kind Regards, John

 

[EFLImpudence]

If it's literally an 'attached' garage, then I would personal regard it as being part of the house (in some cases, that is obviously literally true), so I would not think of it any differently from any other 'rooms' in the house - i.e. earthing would be provided by the installation's CPCs in the usual way, and if any extraneous-c-ps enter the building through that 'room', they would be main bonded in the usual way.

No, but you think of it differently than a detached garage or shed.
What about 'not attached' but a foot away or three feet away?
Is there a distance at which circumstances change?

Indeed. I've dealt with e 'attached garage' above, but as far as TTing sheds in general, are they not perhaps considering issues other than bonding? ....
As I've hinted, there are other issues, such as people standing (maybe with bare feet) on wet ground outside the shed and reaching in and touching an exposed-c-p (maybe a lighht switch, commonly just inside the door) - analogous with the 'outside tap' scenario.

Does that not apply to the attached garage and even the house?

In many senses, that hazard is at its greatest when there are no extraneous-c-ps in the shed to be bonded, since bonding, if present, will raise the potential of the nearby ground to closer to that of an exported TN-C-S earth which (very rarely) could be seriously elevated.

How nearby given that the bonding can only be at one point per extraneous-c-p?
Again, at what distance do the circumstances change? Is it not a lot less than people think?

 

[bernardgreen]

Knowing your concerns about TN-C-S, one thing I've never really understood is why you chose to use the TN-C-S 'earth'. The fact that it was 'available' to you didn't mean that you had to use it (I've been offered TN-C-S, but said "no, thank you").

It was the quickest option at the time with the intention of getting a couple of rods in place along with other services work in the yard. So far that work has not been started.

 

[John D v2.0]

Do you think anyone has ever "bothered" to satisfactorily resolve such a problem?

Are you suggesting you agree it's a problem just one that's not worth resolving?

To help answer your question, it's not really dealt with a lot. it's only really mentioned in the literature here:
https://www.voltimum.co.uk/articles/q-day-extraneous-conductive-parts

re farm/livestock, so not really domestic:
http://www.installeronline.co.uk/wp-content/uploads/2013/05/Down-on-the-farm.pdf
"Wherever possible, metal grids should be installed within the floor and connected to the supplementary bonding. Where there is no such metal grid, the use of PME (or TN-C-S) is not recommended."

re hv->lv substations, clearly a lot more risky. bottom of p27 deals with bonding the ground

and pole mounted controls

theres a lot of interesting design of how to bond things that you wouldn't normally bond!
https://library.ukpowernetworks.co....istribution+Network+Earthing+Construction.pdf

 

[John D v2.0]

as an additional reference, it seems UKPN will recommend against PME when there's a flat with a shower downstairs, unless it has an "earth" grid under it
" Very often these properties have concrete floors and the ground floor flat may be equipped with a shower and it is often impractical for the builder to install a bonded earth grid in the shower area. Section 6.7 describes the risks associated with showers located at ground floor level and where an earth grid is required. The absence of an earth grid should be discussed with the customer, and a TT earthing system with an independent earth electrode and RCD protection considered as a more appropriate alternative to a PME earthing system."
http://library.ukpowernetworks.co.u...+Customer+LV+Installation+Earthing+Design.pdf

 

[JohnW2]

No, but you think of it differently than a detached garage or shed.

Not really, since ....

What about 'not attached' but a foot away or three feet away? Is there a distance at which circumstances change?

... as I've said, my view is that distance does not really come into it.

Does that not apply to the attached garage and even the house?

It can - but, as far as I am concerned, whether or not there is anything (for anyone) to be concerned about depends upon the situation, not the distance from anything - and whether one 'bothers' to do anything about it, even if one decides that there is a 'cause for potential concern', will depend upon one's view of the magnitude of the risk.

How nearby given that the bonding can only be at one point per extraneous-c-p?

It will only be bonded (connected to a bonding conductor) at one point, but it (say an underground metal pipe) may wander around all over the place underground, affecting local earth potential (if it is bonded to something which 'goes high voltage') wherever it goes.

Again, at what distance do the circumstances change? Is it not a lot less than people think?

Are you talking about the 'field'? If so, I suspect that you are right that it extends (significantly) far less from the actual extraneous-c-p (which may be an earth rod in some situations) than some people seem to think. In other words, that mitigation (due to a bonded extraneous-c-p) against the theoretical hazards of an exported TN-C=S earth is probably not all that great, even if something is bonded in the outhouse.

Kind Regards, John

 

[JohnW2]

It was the quickest option at the time with the intention of getting a couple of rods in place along with other services work in the yard. So far that work has not been started.

Oh, fair enough - so you plan/hope to get rid of the TN-C-S earth eventually?

Kind Regards, John

 

[JohnW2]

... re farm/livestock, so not really domestic: ...

As we've discussed in the past, livestock, particularly 4-legged ones, create a very different, higher risk, situation - but not something one usually encounters in teh domestic situation (hence a DIY forum)!

... "... Where there is no such metal grid, the use of PME (or TN-C-S) is not recommended.

... AND ...

" Very often these properties have concrete floors and the ground floor flat may be equipped with a shower and it is often impractical for the builder to install a bonded earth grid in the shower area .... The absence of an earth grid should be discussed with the customer, and a TT earthing system with an independent earth electrode and RCD protection considered as a more appropriate alternative to a PME earthing system."

It's interesting to see them seemingly acknowledging that the earth they supply is more hazardous than a TT installation. Whilst this difference in hazards is exacerbated in the presence of livestock or with showers on concrete ground floors, it is always there to some extent - which makes one wonder how well they sleep in the knowledge that (presumably primarily to save cost) they are providing millions of customers with earths that are more 'hazardous' than they need to be!

Kind Regards, John

 

[John D v2.0]

earths that are more 'hazardous' than they need to be!

I dunno... the earth they supply has a hazard cause by them, but TT also has a hazard, that the RCD may not operate. By providing an electrical supply at all they are causing a hazard, especially at perhaps 250v.

Everything comes down to a risk assessment, and whether a risk assessment is even worth it.

They are saying PME is not too risky in general, but if there is a GF shower, etc., the risk outweighs the benefit therefore they won't supply PME. Everything comes down to shades of grey, and the more sensitive the situation the more effort can go into risk assessment and bespoke design. So substations have some standard designs and very careful bespoke design based on risk assessment, but your house is pretty much left up to you. That seems to be working ok at the moment.