Slightly unusual mains cold water supply

[bernardgreen]

Do the regulations say anything about connecting the MET of a PME system to an earth rod ?

I can recall being told that connecting the neutral derived "earth" of a PME supply to an earth rod was not permitted.

So what is this lead pipe other than a very effective earth rod that also delivers water. ?

Depends on whether

[1] the other end of the lead pipe connects to a metal water main that is bonded to the sub-stations neutral and hence not an earth rod.

[2] the other end of the lead pipe connects to a plastic metal water main and hence an earth rod

[3] the lead pipe connect to any other metallic pipes that feed other houses and are in metal to metal contact with the bonded pipe work in those houses.

My opinion is to treat it as a plastic pipe and bond only to the copper pipe work if it is a PME supply or if it is a TT supply then consider using the lead water pipe as a SUPPLEMENTARY earth "rod", supplementary to the main earth rod.

 

[westie101]

I can recall being told that connecting the neutral derived "earth" of a PME supply to an earth rod was not permitted.

I can see the point, but it is a bit mute, it is a water pipe and used as such, it bears no physical resemblance to an earth rod so it isn't one.

OK electrically there is no difference!


(loads of PME supplies with metal water supply pipes without insulated inserts)

 

[JohnW2]

Bonding internal metal work is an absolute requirement of the PME regs, I'll get some quotes from them at work tomorrow.

That certainly makes sense. It sounds as if someone probably needs to tell BS7671 about this! As you go on to say ...

That is in effect the most dangerous system if things are done wrong.
Think of the electrical effect of the loss of a neutral/earth prior to the mainfuse.
With any load switched on, any metalwork connected to the earth becomes live (draw it out on paper) (I've seen it)
Now if it could be proven that the water & gas systems in a property are totally insulated from earth, not bonding them would not be an issue.
However that is rarely (if ever the case) so there is potential difference and a current path from the live metalwork to earth via the person who is touching both!!
So if they are bonded to the metalwork that has become live there is no potential difference so no risk of shock.

Exactly, and that's the very reason why I keep saying that I'm much more comfortable with the idea of the internal pipework being bonded (to MET), even if it is 'apparently' insulated from earth (i.e insultated from any incoming extraneous conductive part). However, every time I say it, there are certainly detractors, and sometimes comments verging on ridule and suggestions that I "don't understand what bonding is about".

Kind Regards, John.

 

[westie101]

There could be a case for differing colours for cables that connect metalwork to earth for reasons connecting with the correct operation of protective devices and those used for equipotential bonding.

Just as a switchwire is marked in a ceiling rose!

 

[JohnW2]

There could be a case for differing colours for cables that connect metalwork to earth for reasons connecting with the correct operation of protective devices and those used for equipotential bonding.

I suspect that would escalate the confusion!

A lot of the problem and confusion surely derives from the fact that a piece of cable connecting the a piece of metalwork to the MET (directly or indirectly) is a piece of cable connecting the piece of metalwork to the MET. Since you cannot tell a piece of wire that it is only to serve as a means of equipotential bonding, or that it can only serve as a means of facilitating operation of a protective device, the reality is that any/all such cables will carry some or all of the currents associated with either of those 'purposes'!

The constant reminders that 'earthing' and 'bonding' are not the same seems to derive from the fact that DNOs provide something which is at least colloquially called an 'earth terminal' which is not necessarily always at earth potential, particularly under (supply) fault conditions. If DNOs provided a 'true earth' (e.g. a local TT electrode!), the distinction between earthing and bonding would really be meaningless - and the sort of main 'bonding' that BS7671 talks about (in relation to extraneous conductive parts) would only be necessary if 'they' felt that the extrordinary unlikely possibility of an extraneous conductive part introducing a non-earth potential into the building had to be provided for.

Kind Regards, John.

 

[westie101]

extraneous conductive parts

A mouse designed by a committee is an elephant!
A most confusing set of words that is more than likely the cause of confusion

 

[JohnW2]

extraneous conductive parts

A mouse designed by a committee is an elephant!
A most confusing set of words that is more than likely the cause of confusion

I couldn't agree more. However, I think the issues, discussion and disagreements would still be with us even if they had chosen a phrase which makes more sense in terms of Plain English - since they would still presumably be talking about an electrically conductive object which entered a building/whatever from outside and which had (or might have) the ability to introduce a potential (almost certainly earth potential) into the building.

Kind Regards, John

 

[westie101]

electrically conductive object which entered a building

Ah! It refers to any electrically conductive object within a building which is a huge difference.

So for a PME supply it would include the steel frame if there was one (I once had an interesting discussion about the correct size cable to use when I installed a 300mm2 aluminium supply to a steel framed building) metallic gas and water systems, a bizarre one - but the reinforcing rods of a concrete floor in a milking parlour and other such stuff.
It's all about the risk that these could provide a current path to earth in the event of a fault as discussed

 

[1john]

RF, you may find with that type of transition fitting that the copper pipe is touching the lead pipe within the fitting, if the blinking plumber had used a compression lead-loc then you'd just be proceding as normal!

 

[JohnW2]

electrically conductive object which entered a building

Ah! It refers to any electrically conductive object within a building which is a huge difference.
So for a PME supply it would include the steel frame if there was one (I once had an interesting discussion about the correct size cable to use when I installed a 300mm2 aluminium supply to a steel framed building) metallic gas and water systems, a bizarre one - but the reinforcing rods of a concrete floor in a milking parlour and other such stuff.
It's all about the risk that these could provide a current path to earth in the event of a fault as discussed

I am impressed with the fact that you are usually very down-to-earth, but I think you're on the verge of quibbling now; maybe some of the other influences in this forum are infectious

I agree with everything you say. However, to be able to introduce an earth potential into a building, a conductor obviously has to, in some senses, originate outside of that building (otherwise the 'into' cannot be satisfied) - and the structural elements you speak of come into that category. I would also remind you that we are (and nearly always are, in this forum) talking in terms of domestic installations - in which any conductive elements of the type you mention are pretty rare.

Kind Regards, John.

 

[JohnW2]

RF, you may find with that type of transition fitting that the copper pipe is touching the lead pipe within the fitting, if the blinking plumber had used a compression lead-loc then you'd just be proceding as normal!

Indeed. As you will have seen, I pointed that out in the third message of this thread

Kind Regards, John.

 

[westie101]

Sorry John but I feel it important, the structure of say a house, is built on the ground and as such is at earth potential (though not a good conductor).
Anything therefore that may be in contact with that structure again is at earth potential so a risk in the described situations.

A metallic object does not need to go outside a property to be a risk

(we actually do get calls where folk get shocks from contact with walls in some circumstances)

 

[JohnW2]

Sorry John but I feel it important, the structure of say a house, is built on the ground and as such is at earth potential (though not a good conductor).
Anything therefore that may be in contact with that structure again is at earth potential so a risk in the described situations.
A metallic object does not need to go outside a property to be a risk
(we actually do get calls where folk get shocks from contact with walls in some circumstances)

I'm not disagreeing with the 'electrics' of what you are saying, merely the language. For a building to be sitting on the earth is of no consequence unless there is something which conducts (aka 'a conductor', hence 'an extraneous conductive part' in BS7671-speak), at least to some extent, between that earth (outside of the building) and the inside of the building (where people might potentially be at risk).

However, there is no limit to how pedantic this line of thought could get. To the best of my knowledge, no material has an infinite resistivity, so any standard building materials have a finite resistance, and therefore could, if one were silly enough, be regarded as an 'extraneous conductive part' for the purpose of BS7671!

Kind Regards, John.

 

[ban-all-sheds]

extraneous conductive parts

A mouse designed by a committee is an elephant!
A most confusing set of words that is more than likely the cause of confusion

Extraneous-conductive-part. A conductive part liable to introduce a potential, generally earth potential, and not forming part of the electrical installation.

 

[ban-all-sheds]

With any load switched on, any metalwork connected to the earth becomes live (draw it out on paper) (I've seen it)

In which case I'd rather not deliberately make my radiators and taps etc become live when there was no need to because they were not, until I made them so, part of the electrical installation.