Some advice ahead of running wiring to shed

[westie101]

Which is why we differentiate between outbuildings and agricultural buildings.

The choice of what to do is really dictated by the use of the enclosed space(?).
E.g. there is a world of difference between a substantial stone & brick outbuilding that has a a fridge, freezer, dryer, washing machine, water heater and general power & light and an aluminium framed greenhouse where it is known that water will get sprayed around.

A snippet I turned up suggests that in some farm situations if a concrete floor has steel reinforcement that this must be bonded if a PME earth is used!!

Regarding the earth rod vs metal piping issue.
I can see your point John but as I said it is that the earth rod is by design providing an earth, the metal pipes are not designed to provide an earth (did I not see that the 14th edition recognised this), that they do is coincidence but not to be relied upon.

In reality having dealt with a number of loss of neutral faults on both 3 and 1 phase networks no one has ever died or been injured. (that's not to say that a few cows haven't been affected often as the bonding and wiring has borne no resemblance to any regulations).

 

[Electrifying]

Okay, John,

If you wish to utilise the PME supplied TN-C-S system for your outbuilding you have to ensure that:

1/ The CPC of the distribution cable satisfies the regulations (544.1.1) with regards to bonding conductors under PME conditions.
So you can't run a 4mm armoured and utilise the CPC - it won't comply.
It's going to be 10mm minimum and, as 'Westie' pointed out, is sized in accordance with the CSA of the 'Supply' neutral conductor.

2/ Any 'Extraneous Conductive Parts' in the outbuilding should then be 'bonded' to the EMT (think of it as a 'remote' MET) - these 'bonds' should also be sized for PME conditions.
If they become 'accidental' earth paths, it's neither her nor there - they are 'bonds' first and foremost.

3/ The structure of the outbuilding should not be metal - this may itself require bonding and this could present a hazard to persons standing outside the building (in contact with true earth) - if they were to touch the structure.
In this instance the outbuilding should have a 'seperate' TT earthing system.

4/ The same could apply to un-insulated flooring within the outbuilding - could be safer to TT.

Now, if 1 and 2 above are followed - and the outbuilding isn't metal and the floor is suitably insulated, then it should be OK to utilise the 'Supply' earth.

If not, then you should terminate the 'supply' earth at the end of the 'distribution' cable and install a TT system for the outbuilding.

In doing this, you could, perhaps, have a 6mm supply - but you can't connect it's CPC to the outbuilding earthing system......as stated, it won't comply.

With a 6mm supply, I think you could use a 6mm 'Earthing Conductor' to your 'rod' (I haven't checked)........again, you can't connect a TN-C-S (PME) earthing system to this, it won't comply.

You could, if you satisfied all requirements with regards to PME conditions (i.e. CSA of protective conductors), connect the 'supply' earth to the outbuildings TT system, because then, as you stated, it will be no different to that of 'bonding' a gas pipe.

But if you're going to do that, you might as well just use the supply earth in the first place.

 

[JohnW2]

Electrifying, please don't think I've ignored most of your post because I'm only quoting and responding to a very small part of it. That's because I'm totally happy with, and completely agree with, virtually everything you've written. The 'issue', which perhaps relates to a misunderstanding on my part, relates to:

You could, if you satisfied all requirements with regards to PME conditions (i.e. CSA of protective conductors), connect the 'supply' earth to the outbuildings TT system, because then, as you stated, it will be no different to that of 'bonding' a gas pipe.

If that's the case, then there's no anomoly and I've essentially been wasting my time in this discussion Since there had been no discussion about the sizes of protective conductors, I thought westie was saying that what you describe there (the connection of an earth rod) was prohibited, per se (regardless of conductor sizes) - and I could not understand that since, as you say, it would be no different from bonding to an incoming pipe.

But if you're going to do that, you might as well just use the supply earth in the first place.

Indeed. However, I was not suggesting that one did that. What I would want would be the bonding to incoming pipes etc. - and I couldn't understand how that could be allowed if connection to an earth rod wasn't - but it seems as if maybe I was misunderstanding westie.

However, it still sounds to me as if the practical solution, particularly for modest supplies over reasonable distances, is to TT the outbuilding - otherwise one ends up having to run an excessively meaty cable to the outbuilding to satisfy the bonding CSA requirements.

Kind Regards, John.

 

[ban-all-sheds]

With a 6mm supply, I think you could use a 6mm 'Earthing Conductor' to your 'rod' (I haven't checked)........again, you can't connect a TN-C-S (PME) earthing system to this, it won't comply.

Say, by virtue of distance and/or load you're using a 10mm² supply cable, and say you used a 10mm² conductor from the rod.

What then?

 

[westie101]

Just as an aside I wonder what the copper equivalent csa of the steel is for various cable sizes?

 

[JohnW2]

Regarding the earth rod vs metal piping issue.
I can see your point John but as I said it is that the earth rod is by design providing an earth, the metal pipes are not designed to provide an earth (did I not see that the 14th edition recognised this), that they do is coincidence but not to be relied upon.

Yes, I fully understand that, and have never suggested that there should be any reliance on metal pipes for earthing.

You will have seen from my latest exchange with Electrifying that there has maybe been a misunderstanding. When I asked you ....

• 1...If the outbuilding has an earth rod, which is connected to the CPCs within the outbuilding and bonded to metalwork within the outbuilding, is it (as I thought you were saying) required that this earth be isolated from the PME earth?

... you replied Yes, and I thiught you meant 'always Yes'. However, do you you feel that Electrifying is right in saying that the answer would be No (i.e. no requirement for isolation of PME earth from outbuilding earth) if the size of the 'CPC' carrying the supply to the outbuilding had an adequate CSA for a 'main bonding' conductor?

Kind Regards, John.

 

[Electrifying]

However, it still sounds to me as if the practical solution, particularly for modest supplies over reasonable distances, is to TT the outbuilding - otherwise one ends up having to run an excessively meaty cable to the outbuilding to satisfy the bonding CSA requirements.

Kind Regards, John.

This I would agree with - I think someone mentioned a shed 350 metres away - I'd like to be a pound behind the person who owns that property.

Once again a design decision - as is the earthing arrangements.

As for the other issue - if PME conditions are met - i.e. CSA s of all protective conductors comply. - then there's no reason why you couldn't 'rod' the outbuilding as well as using the supplied earthing system........I do know of people who would rather do this if running a PME supplied earth to an outbuilding - sink a rod as well.

Some people believe we should have a rod at or near the cutout on PME supplies - I think it's done like this in other countries.

 

[ban-all-sheds]

Just as an aside I wonder what the copper equivalent csa of the steel is for various cable sizes?

See Tables 43.1, 54.2, 54.6

 

[JohnW2]

Just as an aside I wonder what the copper equivalent csa of the steel is for various cable sizes?

You can get a handle on that by looking at the relative 'k' values (for use in an adiabatic equation) specified in 543.1 of BS7671.

Of most interest are a k value of 51 for (70 degree PVC) SWA as compared with a k value of 115 for a copper conductor in a cable. There is thus a 115:51 ratio, such that, at least as far as that part of BS7671 is concerned, 10mm² copper would be equivalent to about 22.5mm² of steel armour. If you then look in the wiki here (here), you can find the armour CSA of various sizes of SWA.

Note that if you try working out copper/steel CSA equivalents simply on the basis of resistivities, you'll get totally different answers from these.

Kind Regards, John.

 

[westie101]

See Tables 43.1, 54.2, 54.6

Thanks, but as: -
I don't do wiring
I ain't buying a copy

could you perhaps enlighten me for say 4, 6, 10 & 16mm2

 

[Electrifying]

With a 6mm supply, I think you could use a 6mm 'Earthing Conductor' to your 'rod' (I haven't checked)........again, you can't connect a TN-C-S (PME) earthing system to this, it won't comply.

Say, by virtue of distance and/or load you're using a 10mm² supply cable, and say you used a 10mm² conductor from the rod.

What then?

Already answered in my above post, I think.

As I said, some people actually prefer to 'rod' the outbuilding as well.

I suppose you could look at it as a TN-C-S (PME) earthing system with an additional 'rod' - as opposed to a seperate TT earthing system.

 

[JohnW2]

However, it still sounds to me as if the practical solution, particularly for modest supplies over reasonable distances, is to TT the outbuilding - otherwise one ends up having to run an excessively meaty cable to the outbuilding to satisfy the bonding CSA requirements.

This I would agree with - I think someone mentioned a shed 350 metres away - I'd like to be a pound behind the person who owns that property.

Glad we agree!

As for the other issue - if PME conditions are met - i.e. CSA s of all protective conductors comply. - then there's no reason why you couldn't 'rod' the outbuilding as well as using the supplied earthing system........I do know of people who would rather do this if running a PME supplied earth to an outbuilding - sink a rod as well.
Some people believe we should have a rod at or near the cutout on PME supplies - I think it's done like this in other countries.

Well,as I said before, I may have misunderstood westie when I thought he was saying that this was 'prohibited'. As you've seen, I've asked him to confirm/clarify, and if he gives the 'wrong' answer, I'll leave you to do battle with him

Kind Regards, John.

 

[Electrifying]

Well,as I said before, I may have misunderstood westie when I thought he was saying that this was 'prohibited'. As you've seen, I've asked him to confirm/clarify, and if he gives the 'wrong' answer, I'll leave you to do battle with him

Kind Regards, John.

I hope he agrees, otherwise I'll have to have a re-think.....although I don't do too many TT installs to worry about it.

At least he'll point out any concerns he may have with my way of thinking.....which always helps.

Can't see him having an issue with it, though, but you never know.

 

[JohnW2]

I hope he agrees, otherwise I'll have to have a re-think.....although I don't do too many TT installs to worry about it.
At least he'll point out any concerns he may have with my way of thinking.....which always helps.
Can't see him having an issue with it, though, but you never know.

You're probably right. I think I probably made the mistake of interpreting his 'Yes' answer (about the requirement to isolate PME earth from an earth rod) to mean "always Yes", rather than "Yes, unless the supply CPC CSA is adequate as a main bonding conductor". We'll see!

Kind Regards, John.

 

[westie101]

John. thanks.

It would suggest that the smallest size if just the armouring is used would be around 6mm2. though I suppose if a 3 core was used 4mm2 could be used if core 3 and the swa were in parallel.