Some advice ahead of running wiring to shed

[westie101]

Interesting one. I would take the view that where two pieces of metalwork are adjacent and can be touched simultaniously the 0.05ohm valu makes sense as the aim is to prevent potential differences.
If you have the *extreme) situation of a shed 350m away where you cannot get into that position (unless you are Mr Elastic) the impedance value could be higher. however I would ensure that I bonded the metalwork in the shed as I would in the house (there is nothing to say that you cannot exceed minimum requirements)

 

[JohnW2]

Interesting one. I would take the view that where two pieces of metalwork are adjacent and can be touched simultaniously the 0.05ohm valu makes sense as the aim is to prevent potential differences.
If you have the *extreme) situation of a shed 350m away where you cannot get into that position (unless you are Mr Elastic) the impedance value could be higher. however I would ensure that I bonded the metalwork in the shed as I would in the house (there is nothing to say that you cannot exceed minimum requirements)

That all corresponds with common sense.

If there are no 'extraneous-conductive-parts' (i.e. things which could be at true earth potential) in a distant outbuilding, then I can't see that there is actually anything to 'main bond' - and I can't really see what is gained by making a distant outbuilding equipotential with the main house.

If there are extraneous-conductive-parts, then the situation obviously changes a little but, as you imply, the crucial thing then is that everything in the outbuilding is bonded together ('supplementary bonding', in BS7671-speak), and the connection back to the supply earth really isn't relevant to that.

The importance of the impedance of the connection back to supply earth really relates to the operation of protective devices (i.e.Zs), and I imagine that most installations would satisfy that, even at 350m.

However, one thing confuses me. In recent discussions you have indicated that if an outbuilding has a separate local earth electrode (to which everything within the outbuilding is 'bonded'), that the earthing within the outbuilding has to be isolated from the supply earth - which makes sense. If the path to true earth is 'incidental' (i.e. an extraneous-conductive-part is present, and bonded), rather than deliberate (i.e. an earth rod), that sounds exactly the same to me - so it would seem that one would always have to have isolation from the supply earth if any e-c-ps were present in the outbuilding. Is that your view?.

Kind Regards, John

 

[westie101]

Until I had reread the appropriate documents I had what could be called an older view.
In any design it is important to look at the "what ifs", why things are done a certain way and what needs to be achieved. I don't think there is "an answer" to each situation that fits all.
As regards the DNO supply and a fault on that there are a number of solutions. As for the operation of protective devices that is a seperate question. Obviously though each requirement needs to be met.

As with a lot of designs the personal preferences of the designer can often be part of the solution!
As in your case with the seperate earth rod, or another will prefer to use a DNO earth both are correct!

 

[JohnW2]

As with a lot of designs the personal preferences of the designer can often be part of the solution!
As in your case with the seperate earth rod, or another will prefer to use a DNO earth both are correct!

Sure, but maybe I've misunderstood something. I thought the ('your') regulation/COP you recently posted indicated that if an outbuilding had its own earth rod, the earth within that outbuilding had to be isolated from a DNO-supplied (e.g. PME) earth. Is that correct? If so, is the same true if there are ('bonded') extraneous-conductive-parts within the outbuilding (since that seems conceptually the same as an earth rod to me)?

Kind Regards, John.

 

[westie101]

Ah sorry.
Yes the reg suggests two situations where a property has a PME supply
The first as above, the second is to seperate the earth in the building and provide insulated sections in any common metallic services and use a seperate earth rod.
The bonding I would take as equipotential bonding, not the provision of an "earth"

Other services can/should not be used to provide an earth which is wjhat you last sentance suggests to me.

 

[JohnW2]

Ah sorry. Yes the reg suggests two situations where a property has a PME supply
The first as above, the second is to seperate the earth in the building and provide insulated sections in any common metallic services and use a seperate earth rod.
The bonding I would take as equipotential bonding, not the provision of an "earth"
Other services can/should not be used to provide an earth which is wjhat you last sentance suggests to me.

Sorry (too!), but I'm still a bit uncertain as to what is being said - and this is maybe at risk of turning into one of those 'bonding vs earthing' discussions! I'm probably being so dim that I hope you won't regard it as being patronising for me to try to reduce this to a couple (well, one and two halves, plus a 'supplementary'!) of questions to which you should be able to give Yes/No answers (in terms of your regs/COPs):

• 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?
  
  2...If there is no local earth rod, and the PME earth is connected to CPCs within the outbuilding, and bonded to any exposed metalwork within the outbuilding, then (a) is this allowed if there are 'extraneous-conductive-parts' (e.g. services, structure) which may introduce true earth potential into the outbuilding? and (b) if so, is it permissible to 'bond' those e-c-ps to the (PME) earth?

As a supplementary question, if your answers to (1) and (2b) are both Yes, can you explain why?

Kind Regards, John.

 

[westie101]

1. Yes as if a fault were to occur on the supply network this earth rod may carry the full load current of a number of properties, load which the associated conductors may not be big enough to carry!

2 Yes if it is felt that they introduce the risk of potential differences that could lead to danger.

I agree that in some cases this bonding could be to metal that is eventually buried in the ground and would act as an earth rod, but it seems that as it is not intended to use it as an earth rod that is acceptable. Whereas if it is an earth rod that is a deliberate connection which is frowned upon!

Consider the case of a single phase PME network supplied by a 50kVA transformer, if the neutral/earth is broken close to the transformer there is a risk of up to 200A of load current trying to find a path to earth, if it is via a 10mm2 cable and an earth rod...............

I'm not even going to go into the case of if a live to the detached neutral fault were to occur on the network and the subsequent levels of fault current!

 

[holmslaw]

..

 

[JohnW2]

1. Yes as if a fault were to occur on the supply network this earth rod may carry the full load current of a number of properties, load which the associated conductors may not be big enough to carry!
2 Yes if it is felt that they introduce the risk of potential differences that could lead to danger.
I agree that in some cases this bonding could be to metal that is eventually buried in the ground and would act as an earth rod, but it seems that as it is not intended to use it as an earth rod that is acceptable. Whereas if it is an earth rod that is a deliberate connection which is frowned upon!

Oh Dear!! As you probably realise, that's what I feared you would say. However, it's clearly a complete nonsense, and really is a prize example of the 'earthing vs. bonding' issue. It just doesn't make any rational sense that a connection to a 4 foot earth rod is 'prohibited' (for good reasons), yet connection to goodness knows how much underground water pipe (quite probably providing a better path to earth than the rod) is not only allowed, but probably 'required' by some regulations - the only difference being that one is called 'an earth rod', whereas the other isn't!! This really is (to my mind) an unnecessary obsession with the words 'earthing' and 'bonding', without any apparent consideration for the underlying principles or safety issues!

Consider the case of a single phase PME network supplied by a 50kVA transformer, if the neutral/earth is broken close to the transformer there is a risk of up to 200A of load current trying to find a path to earth, if it is via a 10mm2 cable and an earth rod...............

Indeed, but change 'earth rod' to 'underground water pipe' in that sentence and it's still more-or-less equally true. The only saving grace is that there is obviously no way that anything like 200A (probably not even 10A) is going to flow to earth through the average rod, or even probably supply pipe - but the price paid for that is that everything would rise to a high potential above true earth (albeit theoretically not hazardous if all, and adequate, bonding is in place - at least inside buildings).

This all goes to confirm my previously voiced view that if I had a PME supply (which I don't), I'd only feel safe with an isolated TT system in an outbuilding!

Kind Regards,

 

[westie101]

In reality it is a design choice, so if it can be shown to be safe nothing is really wrong!

I do wonder at times as it seems born from, on one part a huge assumption that all other services are or will be non-metallic and a realisation that they are not..
The biggest contention about the issue is that for a PME supply the bonding requirements are dictated by the needs of the supply network under fault conditions as well as the installation. The provision of an earth is easy, the DNO provides it!

(and we've not even got to oil pipes and LPG pipes)

 

[Electrifying]

This all goes to confirm my previously voiced view that if I had a PME supply (which I don't), I'd only feel safe with an isolated TT system in an outbuilding!

Kind Regards,

So you don't think the gas pipe in your house is going to do exactly the same thing as the gas pipe in your outbuilding??? under supply fault conditions?

It is the 'bonding' and 'earthing' issue - you're right.

These pipes are 'bonded ' to the MET because they are 'extraneous' - they introduce earth potential.
If they become 'accidental' earth paths - so what - can't be avoided - the bonding is more important because, while everything is at the same potential (be it 50v or 230v) - you are safe.

You seem like an intelligent fella, John, I don't know why you can't grasp this.

In the case of TT'ng the outbuilding, the associated conductor CSAs are a lot smaller and will not satisfy the 'Regs' if the outbuilding also remains connected to the supplied 'PME' earth.

 

[JohnW2]

In reality it is a design choice, so if it can be shown to be safe nothing is really wrong!

As a statement, that's obvioulsy true. However, one would hope that no designer would call a design 'safe' when a particular length of cable was called one thing and 'unsafe' if exactly the same cable were called something else - and that's not far off what we're talking about!

As I've said, the only approach which sounds safe to me is to keep a PME earth totally out of the equation in most domestic outbuildings. Even if there are no explicit "e-c-ps", assorted paths to true earth (particularly when everything is very wet) are hard to rule out in the majority of common types of outbuilding (sheds, greenhouses etc.),and there's a practical limit to what can be bonded.

Kind Regards, John.

 

[JohnW2]

These pipes are 'bonded ' to the MET because they are 'extraneous' - they introduce earth potential.
If they become 'accidental' earth paths - so what - can't be avoided - the bonding is more important because, while everything is at the same potential (be it 50v or 230v) - you are safe.

Indeed. That's why I would want them bonded.

You seem like an intelligent fella, John, I don't know why you can't grasp this.

The issue is not about the difference between earthing and bonding. Hopefully we all understand the difference of concept and purpose, even if the two functions can sometimes be a bit difficult to separate.

The thing I can't grasp is the apparent inconsistency in what westie has told me - that if I have an earth rod for an outbuilding, with everything (including incoming supply pipes) bonded to that, I have to isolate that local earth system from the supply's earth, yet, if I disconnect the earth rod, I am then allowed (indeed, required) to bond the supply earth to everything in the building, including metal tubes or rods emerging from the ground, provided I promise not to call them earth electrodes. I obviously would want to have the the e-p-cs bonded, for reasons we all understand only too well, but I just don't understand why I am allowed (per westie's COP) to do that (with a PME supply) when I'm apparently not allowed (per westie's COP) to connect the supply earth to something I choose to call an earth electrode.

In the case of TT'ng the outbuilding, the associated conductor CSAs are a lot smaller and will not satisfy the 'Regs' if the outbuilding also remains connected to the supplied 'PME' earth.

As above, what is confusing me is the apparent suggestion that it is acceptable to contect the 'PME' earth to the contents of outbuilding and also to a metal object emerging from the ground, so long as that metal object is called, say, a water pipe and not an 'earth electrode'. You also seem like an intelligent fella - can you not understand my difficulty with this suggestion?

Kind Regards, John.

 

[Electrifying]

We posted together again - reply following

 

[JohnW2]

John, we posted at the same time - you need to read my previous post and try, once again, to get your head around the difference.

I've just replied to it, slightly beating your latest post Hopefully you'll be able to understand the nature of my problem from that reply (and it's not a lack of understanding of the difference between earthing and bonding )

Kind Regards, John.