Armoured cable to shed - advise please

[EFLImpudence]

Are you suggesting that the floor of the house would, by a similar argument, also 'need' bonding?

No. The opposite.

I wonder why such a fuss is made regarding sheds.
A large house (yours? or a country pile) could result in ground adjacent the building being further from the supply and any bonding than a shed in the garden of a more modest property.

 

[JohnW2]

No. The opposite. I wonder why such a fuss is made regarding sheds.

I'm not sure that there is a lot of 'fuss' - one of the main reasons for frequent discussion is that there is a lot of confused and/or conflicting advice out there is relation to earthing and bonding in outhouses.

A large house (yours? or a country pile) could result in ground adjacent the building being further from the supply and any bonding than a shed in the garden of a more modest property.

The proximity (or otherwise) of the house and shed is not necessarily all that relevant in relation to the issue I was talking about (and, in any event, sheds are not uncommonly 'at the furthest possible point from the house' in relation to many 'modest' properties).

If one takes a hypothetical extreme case of an outhouse to which a TN-C-S earth was exported, and which had no floor, just bare (sometimes wet) soil on which people might stand with bare feet (not unknown in gardens) then, if that soil were close to 'true earth' potential, even in 'normal service' there could be a few volts between their feet and any exposed-c-p they touched and, in those very rare case of network faults, that PD could rise to dangerous levels. In practice, there will usually be some floor, but the closer it comes to being similar to 'bare soil', the closer will it come to that situation.

Is it your feeling that if the outhouse is close to the main house, and if the main house has some bonded extraneous-c-ps, then the soil in the vicinity of the house will (by virtue of bonding) be at a similar potential to the TN-C-S 'earth', and therefore the same would be true of the floor of a nearby outhouse (hence minimising the above-mentioned problem)? If so, other than in some urban areas, I would suspect that a high proportion of shed, even of 'modest' houses, are far enough from the house for that not to be a strong or reliable argument. Looking out of a top floor window here (good view of the village) a high proportion of the sheds I can see are probably at least 15-20m (in many cases more) from the associated house, and I'm not at all sure that the 'field' from the house's bonded extraneous-c-ps will necessarily extend (significantly) that far (obviously depends upon where the extraneous-c-ps 'go'). I suppose that, for aesthetic reasons, people will tend to locate shed 'at the end of the garden'.

Kind Regards, John

 

[EFLImpudence]

I am just saying that I don't see the difference between a shed 20m from a house; or just outside a house which may be 20m, or more, from the supply and bonding, but no one suggests that anything (TT or earth mat) should be done in the latter circumstances.

Put it another way - if a house has the supply and bonding at the rear, what is the difference between a shed 20m farther to the rear and the front garden which is 20m away?

OR

The same house with the supply and bonding at the front - is the relevant figure that the shed is 20m from the rear of the house, or 40m from the supply and bonding?

 

[JohnW2]

I am just saying that I don't see the difference between a shed 20m from a house; or just outside a house which may be 20m, or more, from the supply and bonding, but no one suggests that anything (TT or earth mat) should be done in the latter circumstances.

Oh, I see. I don't think you can necessarily count me amongst those "no ones". What I would 'suggest' will 'depend' upon various things, but, as I said, I don't see that distance is really a relevant issue (other in relation to the 'ground fields' issue I mentioned). The arguments/viewpoints for/against various approaches to bonding/earthing are surely exactly the same whether the shed is 1m or 100m from the house, aren't they?

Put it another way - if a house has the supply and bonding at the rear, what is the difference between a shed 20m farther to the rear and the front garden which is 20m away?

Exactly - no difference at all (other than bonding conductor resistance, which is trivial).

... OR ... The same house with the supply and bonding at the front - is the relevant figure that the shed is 20m from the rear of the house, or 40m from the supply and bonding?

Again, as far as I am concerned, the distances have nothing to do with the arguments for/against various approaches to earthing/bonding.

In other words, if I understand correctly, you appear to be arguing against something I have never suggested.

Kind Regards, John

 

[EFLImpudence]

The arguments/viewpoints for/against various approaches to bonding/earthing are surely exactly the same whether the shed is 1m or 100m from the house, aren't they?

Yes, exactly, but is that how people view it?

That a shed is 25m away tends to elicit suggestions that would not be considered in a kitchen - even with damp flagstones.
That there are no extraneous-c-ps leads to suggestions that one be introduced.

In other words, if I understand correctly, you appear to be arguing against something I have never suggested.

I am not arguing; you asked me what I meant.

 

[John D v2.0]

There's no point debating whether or not something is extraneous based on how practical it is to bond, or whether people forget to bond extraneous parts sometimes.

1 The damp floor is the most likely extraneous part in most domestic outbuildings.
2 You could bond it with mesh if you want

If you don't bond it, you'll likely survive regardless, especially if you have a plastic cu and use only class ii equipment. But the shed wouldn't be an earth free or equipotential zone.

 

[JohnW2]

Yes, exactly, but is that how people view it?

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?

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.

That a shed is 25m away tends to elicit suggestions that would not be considered in a kitchen - even with damp flagstones.

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).

I am not arguing; you asked me what I meant.

The English language (or, at least, my use of it) has slightly failed us again. I did not mean to suggest that you were 'being argumentative' - I was trying to refer to 'the arguments' that you were presenting. I could probably have worded it better.

Kind Regards, John

 

[ban-all-sheds]

Does the 18th introduce any requirements for another ME to be installed if a PME supply is exported more than a given distance? Or if it's a sub-main to a CU in an outbuilding?

 

[bernardgreen]

Lay in two ducts, one for the LV (230 volt) cable and one for any ELV cables ( data telephone alarm etc ) that ypu may want to install now or later.

 

[ban-all-sheds]

There is a case to be made for installing a pair of ducts - one for the power cable, one for phone/data, so that future changes can be done by pulling a new or additional cable through.

 

[bernardgreen]

Sorry BAS. there are some posts that I don't read.

 

[ban-all-sheds]

Another thing to consider is how to power the shed. Ideally you don't want the garage feed sharing an RCD with the house circuits- have a chat with your electrician about Henley blocks and standalone MCB at the house end and 'garage' CU at the shed end ideally with RCBOs.

Although it's more usual to have a switchfuse at the house end rather than an MCB, I think.

 

[ban-all-sheds]

Sorry BAS. there are some posts that I don't read.

Not always the best policy.

 

[JohnW2]

There's no point debating whether or not something is extraneous based on how practical it is to bond, or whether people forget to bond extraneous parts sometimes.

I certainly have not been debating that. For example, in the case of my daughter's kitchen floor, I believe that there is a strong argument for regarding it as an extraneous-c-p, which therefore should be bonded. A decision has been made (strictly speaking, in violation of regs) not to attempt to bond it, on the basis that to do so would be a major undertaking (and might even require a battle with the Listed Buildings folk, who might get emotional about their 'Listed quarry tiles' !), not because of a denial that it should be regarded as an extraneous-c-p.

.. The damp floor is the most likely extraneous part in most domestic outbuildings.

I think here we see the difference between BS7671's definition of an "extraneous-c-p" (which, strictly speaking, regards something as an extraneous-c-p even if the path from it to earth is well over 1 MΩ!) and EFLI's 'measurement-based' view that it only needs to be regarded as an extraneous-c-p if it offers a path to earth of resistance/impedance that could result in a significant current travelling through a person.

To my mind, EFLI's view is the more sensible, and, for the majority of sheds (whose floor is not 'wood direct onto soil'), I would suspect that his measurements would in many/most cases indicate that bonding was not required (electrically-speaking, regardless of regs).

Kind Regards, John

 

[bernardgreen]

A decision has been made (strictly speaking, in violation of regs) not to attempt to bond it, on the basis that to do so would be a major undertaking (and might even require a battle with the Listed Buildings folk, who might get emotional about their 'Listed quarry tiles' !), not because of a denial that it should be regarded as an extraneous-c-p.

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