Extraneous conductive parts in outbuildings

[D Skelton]

With the 45KΩ you are aiming for a 5mA safe current, what is the basis of this level of current?
Perception is lower and let go is higher, is this just a very safe level?

10mA is seldom likely to prove fatal to a human, it is also said to be the let go threshold. 5mA is never going to prove fatal to a human unless applied directly across the heart and will not cause involuntary muscle contraction.

There are no hard and fast rules when it comes to determining a safe level of shock current but this is the value I was taught to work to and I still do.

 

[Engineer54]

Well that was kind of my point lol.

It would be supplementary if already bonded and yes, you would be a fool not to IMHO

Well as in this case of separate buildings, any extraneous metal pipe or other metalwork entering a building, would actually require bonding, i wouldn't necessarily be calling that bonding supplementary either.

 

[D Skelton]

Well as in this case of separate buildings, any extraneous metal pipe or other metalwork entering a building, would actually require bonding, i wouldn't necessarily be calling that bonding supplementary either.

Technically it is supplementary as it is in addition to. Bonding is required between the MET and each extraneous conductive part within the installation. If the installation is formed of multiple buildings then this requirement has to be applied to each seperate building of the installation. IMHO it still wouldn't be main bonding though as you would be installing supplementary bonding to the same requirements of the main bonding in the main building. This is my take on it anyway, but we're arguing semantics here though and it all hinges on definitions not being clearly defined and our interpretation of the word 'supplementary'.

The main point though has been addressed, and that is that bonding should be installed at the outbuildings if there are extraneous conductive parts present.

 

[Engineer54]

In the case of extraneous metallic water/gas pipes entering a building whether those pipes are continuous or not, and virtually all will be. The fact that they are entering the building from the ground should mean that it would be main bonding because no-one can guarantee that pipework will not be altered in the future by insertion upstream of plastic piping, which means it will be imperative that main bonding of that extraneous pipework via the buildings EMT back to the MET is carried out.

Which was not wholly what you were stating in post #3 and #6 of this thread. The same reasoning applies to multi occupancy buildings, eg, blocks of flats etc. Semantics it might be, but an important issue, that's best done right, just same...

 

[D Skelton]

That's why I was saying I would supplementary bond at the outbuildings, but I'm not too proud to be corrected on my interpretation of my definition. All that said, we're still singing from the same hymn sheet as the reason I would bond is because of the very reasons you have stated.

With regards to multiple dwellings within one building, I view them as being one installation in one building, therefore my interpretation would mean that you only needed to bond the once rather than at each dwelling, although personally, I would still bond at each dwelling. I may stand to be corrected on this also.

 

[spark 68]

If you look at the GN8 diagram, it only shows the MPB path, it even says that the supply (to the outbuildings) cables CPC "may" also fulfil this role (depending on min csa etc.).

Another thing not shown on that diagram is that all the CPCs for that out/building would also be required to be connected to the EMT, or EMB (Earth marshalling Bar) on that drawing.

Exactly the same as you would for flats and the like, so I am sure that just bonding straight back to the MET at the origin is not what is intended, more to create an equipotential zone in each building is what is required.

I have only added this just for completeness, not aimed at anyone in particular.

 

[Silly Sausage]

There's a row of 15 old terraced houses, each with lead water pipes entering each one, presumably fed off the same water main.
You're called out to carry out some electrical work at one of the properties, which one is irrelevant.
You find on initial inspection that there is no main bonding in place.

I'll let you finish this off Mr Skelton!

 

[Engineer54]

If you look at the GN8 diagram, it only shows the MPB path, it even says that the supply (to the outbuildings) cables CPC "may" also fulfil this role (depending on min csa etc.).

Another thing not shown on that diagram is that all the CPCs for that out/building would also be required to be connected to the EMT, or EMB (Earth marshalling Bar) on that drawing.

Exactly the same as you would for flats and the like, so I am sure that just bonding straight back to the MET at the origin is not what is intended, more to create an equipotential zone in each building is what is required.

I have only added this just for completeness, not aimed at anyone in particular.

It's not, as you say, each flat is classed as a separate installation in it's own right, and has it's own equipotential zone, connected to the flats MET, which in reality is a building EMT, that is then connected via the flat's incoming supply tails/cable back to the buildings MET!!

 

[D Skelton]

There's a row of 15 old terraced houses, each with lead water pipes entering each one, presumably fed off the same water main.
You're called out to carry out some electrical work at one of the properties, which one is irrelevant.
You find on initial inspection that there is no main bonding in place.

I'll let you finish this off Mr Skelton!

Put main bonding in

 

[Engineer54]

That's why I was saying I would supplementary bond at the outbuildings, but I'm not too proud to be corrected on my interpretation of my definition. All that said, we're still singing from the same hymn sheet as the reason I would bond is because of the very reasons you have stated.

With regards to multiple dwellings within one building, I view them as being one installation in one building, therefore my interpretation would mean that you only needed to bond the once rather than at each dwelling, although personally, I would still bond at each dwelling. I may stand to be corrected on this also.

Sorry, but yes you need to be corrected on this one too i'm afraid!!...lol!!

 

[spark 68]

It's not, as you say, each flat is classed as a separate installation in it's own right, and has it's own equipotential zone, connected to the flats MET, which in reality is a building EMT, that is then connected via the flat's incoming supply tails/cable back to the buildings MET!!

Yes I know that E54 lol, just the diagram is not that clear, and could be somewhat misleading by the way it is drawn I just added some clarification, even the text in GN8 is not very clear. lol

 

[D Skelton]

Sorry, but yes you need to be corrected on this one too i'm afraid!!...lol!!

Can you clarify?

IMHO, until the meaning of the words 'building' and 'electrical installation' are clearly defined in the regs, the answer is about as clear as mud lol.

Does the introduction of a meter in what is essentially a submain for all intents and purposes split an installation in two?

 

[Geoffsd]

Each flat may classed by the regulations as a separate installation but in reality they are not, are they?
They may have been converted from what used to be one house.

Although 'main bonding' in each flat is required, this is in fact, as has been said, 'supplementary' much the same as a house's garage fitted with a separate CU and water pipe from the house.

Isn't the requirement to main bond each flat merely because it may be uncertain whether the real main bond is satisfactory and to save hunting for it?

 

[D Skelton]

Each flat may classed by the regulations as a separate installation but in reality they are not, are they?
They may have been converted from what used to be one house.

Are they though, this is my point, because from what I see, there is no clear definition of what an electrical installation actually is, where it starts and where it ends? I would go further and suggest that not only in reality are they not seperate electrical installations, but that they are not even defined as such in any regulation whatsoever?!

Although 'main bonding' in each flat is required

As above, is it???

I would do it, but out of nothing more than caution. It states clearly that bonding needs to be applied in seperate buildings, which as we agree on is techically supplementary, but it says nothing about seperately occupied spaces within the same building, metered or not???

The words 'grey' and 'area' spring to mind!

 

[spark 68]

GN8 gives some guidance wrt flats, each *installation* should have it's own bonding arrangement, the picture example given in GN8 shows offices, however the same would apply to individual flats.

It tells you that this would equally apply to seperate installations in a multi-occupancy building.

Edit: the other scenario would be where you had one supply/Origin with sub-mains then this would follow the same scheme as given earlier for multiple buildings served by one supply, of course there is scope for other variations, I suppose the basic idea is to create an equipotential zone in each dwelling/installation.

 

[Geoffsd]

Are they though, this is my point, because from what I see, there is no clear definition of what an electrical installation actually is, where it starts and where it ends? I would go further and suggest that not only in reality are they not seperate electrical installations, but that they are not even defined as such in any regulation whatsoever?!

Confused - I thought I was agreeing with you.

As above, is it???
I would do it, but out of nothing more than caution. It states clearly that bonding needs to be applied in seperate buildings, which as we agree on is techically supplementary, but it says nothing about seperately occupied spaces within the same building, metered or not???

Isn't the word - premises. 544.1.2

So, it depends what now are 'premises'.

I would consider flats or offices to be separate premises.

 

[D Skelton]

GN8 gives some guidance wrt flats, each *installation* should have it's own bonding arrangement, the picture example given in GN8 shows offices, however the same would apply to individual flats.

It tells you that this would equally apply to seperate installations in a multi-occupancy building.

It gives guidance yes, but it is also trying to make 'clear' something that is not. Quite rightly it says that "It is clear that BS 7671 requires bonding in each installation" because this is clear, but then it goes on to say "And this would equally apply to seperate installations of a multiple occupancy building".

Well hang on a minute there.. It still hasn't defined what an electrical installation actually is, where it starts, or where it ends. It points out that the origin of an installation is where electrical energy is delivered to an electrical installation, but where is the point of delivery? At the meter? At the service head? At the TX?

The fact remains, it is not clear, and I know that GN8 has tried to make sense of it, but it simply doesn't cut the mustard in my mind I'm afraid. Like I said, I would always install bonding in a seperate occupied space within one building, but could I condemn someone for not doing so? Hmm, I'm not so sure I could.

I remember discussing this very issue a few years back with a very qualified engineer, his answer was essentially "cover your ar$e!". He also rambled on a bit about the IEE and how they couldn't organise a pi$$ up in a brewery, let alone write a book. I wouldn't go that far personally, but I can sure as hell see where he was coming from!

 

[spark 68]

Edit: the other scenario would be where you had one supply/Origin with sub-mains then this would follow the same scheme as given earlier for multiple buildings served by one supply, of course there is scope for other variations, I suppose the basic idea is to create an equipotential zone in each dwelling/installation.

I was editing the previous post, I agree that GN8 does not quite clear the waters much,

 

[D Skelton]

Confused - I thought I was agreeing with you.


Isn't the word - premises. 544.1.2

So, it depends what now are 'premises'.

I would consider flats or offices to be separate premises.

Haha, we are agreeing, I was just pointing out that even though we are agreeing, the regs are even vaguer still!

The Oxford English Dictionary defines the word 'premises' as a house or building. Just thought I'd point that out to muddy the water even further

 

[spark 68]

Ha ha, that is why I used the word "dwelling". lol