Gas pipe bond (IET vs Corgi)

[JohnW2]

I think that phrase effectively connected to the MET is only used regarding the omission of supplementary bonding when RCDs are protecting the circuits of the bathroom.

I also think that's true - but, as I've just written, I think that same concept is, implicitly (and, IMO should be explicitly), part of the definition of an extraneous-c-p - i.e. something can only be an extraneous-c-p if it is not 'effectively connected' to the MET.

Kind Regards, John

 

[Malc]

The CH will have a lot of plastic pipework so the bathroom radiator will be isolated by default. If it was all copper then it would already be connected to MET by boiler E connection so bonding under the bathroom floor wouldn't be needed or an issue.

I see what you're saying re extraneous-conductive-part, something can be extraneous and conductive but if there is no risk of it bringing in a potential (different from MET) then it isn't an extraneous-conductive-part as meant by the regs.

 

[JohnW2]

I see what you're saying re extraneous-conductive-part, something can be extraneous and conductive but if there is no risk of it bringing in a potential (different from MET) then it isn't an extraneous-conductive-part as meant by the regs.

Exactly.

Kind Regards, John

 

[EFLImpudence]

I also think that's true - but, as I've just written, I think that same concept is, implicitly (and, IMO should be explicitly), part of the definition of an extraneous-c-p - i.e. something can only be an extraneous-c-p if it is not 'effectively connected' to the MET.

Yes, but that is obviously less restrictive for an RCD and omission of SB, to which the phrase refers - presumably much less than 1666Ω and difficult not to meet, unless I'm missing something.

The phrase is not used with regard to fitting SB where, with a 40A shower, a combined impedance of two parts back to the MET may have to be less than 0.25Ω.

If all parts could be connected to the MET with zero impedance then supplementary bonding would not be required ever.

 

[JohnW2]

Yes, but that is obviously less restrictive for an RCD and omission of SB, to which the phrase refers - presumably much less than 1666Ω and difficult not to meet, unless I'm missing something.

Of course - but I was using MY phrase 'effectively connected to the the MET' to explain what I understood to be the intended (but inadequately worded) meaning of the definition of an extraneous-c-p (not anything to do with what the regs say about SB). As you say, the 'effectively connection' I'm talking about would be 'close to zero impedance'.

As we've often discussed, the reality is that the resistance/impedance between a pipe and the MET will, in practice, be either 'close to zero' or 'close to infinity' - all this talk about substantially 'intermediate' possibilities is really just hypothetical.

Kind Regards, John

 

[EFLImpudence]

Isn't the phrase "effectively connected to the the MET" NOT mentioned in SB rules because it is the impedance between it and other parts which is important.
That one part may have a zero impedance to the MET does not stop it being an e-c-p.

I presume "effectively connected to the the MET" where omission of SB where RCDs are concerned really means low enough that other parts' impedances do not render it ineffective for the operation of the RCD.
Because ALL parts must be equally "effectively connected to the the MET".

Whilst it doesn't define "effectively connected to the the MET", do you think it is really the case that it ought to say that all parts must be "effectively connected to each other via the MET" for operation of the RCD?

 

[Malc]

You don't need to be connected to MET for an RCD to work. Or to put that another way, if the RCD works then there is another path. That path could be direct to earth and then to the power cable in the ground so not via MET. Otherwise RCD would not protect you mowing outside.

 

[EFLImpudence]

That's true but in this case we are talking about the current through a person when touching two exposed and extraneous-c-ps and SB.

There may be no other earth contact by the person.

 

[JohnW2]

Isn't the phrase "effectively connected to the the MET" NOT mentioned in SB rules because it is the impedance between it and other parts which is important.

I am starting to regret that I used the phrase "effectively connected to the MET" when explaining my understanding of what an extraneous-c-p is - with the benefit of hindsight, I should have simply said something like "having a very low impedance path to the MET". I certainly did not intend to provoke this discussion about the regs' use of that particular phrase.

That one part may have a zero impedance to the MET does not stop it being an e-c-p.

That I don't understand. As I've said several times, it is my belief that the full definition of an extraneous-c-p should be something like "A conductive part liable to introduce a potential (usually Earth potential) different from the potential of the MET (and not forming part of the electrical installation)". If the part has a zero (or very low) impedance connection to the MET, then it cannot introduce any other potential (hence cannot produce any hazard), and therefore, I believe, cannot qualify as an extraneous-c-p.

Whilst it doesn't define "effectively connected to the the MET", do you think it is really the case that it ought to say that all parts must be "effectively connected to each other via the MET" for operation of the RCD?

I don't think I fully understand the question, or the thinking behind it - but, obviously, "no".

Kind Regards, John

 

[EFLImpudence]

You may regret using it but surely you are using it in the same way.
If not, I don't understand your meaning of it - apart from what the words say.

This is a feature of discussions with you.
It starts out fine, interesting and beneficial - and then you introduce something (an impossibility) I've never thought about before and also say you don't understand that which to me is blatantly obvious and simple.

That I don't understand. As I've said several times, it is my belief that the full definition of an extraneous-c-p should be something like "A conductive part liable to introduce a potential (usually Earth potential) different from the potential of the MET (and not forming part of the electrical installation)". If the part has a zero (or very low) impedance connection to the MET, then it cannot introduce any other potential (hence cannot produce any hazard), and therefore, I believe, cannot qualify as an extraneous-c-p.

I understand that as far as main bonding is concerned but -
for supplementary bonding that is not the only consideration, is it? The MET is not in the bathroom.
If it were not connected to the bonding, it would just be a zero impedance path to earth.

Its relevance is with its relationship with an exposed-c-p which when becoming live by a fault should be limited to 50V to the pipe.
The impedance of the CPC (of the faulted fitting to the MET) is then added to the zero (without SB) for determining the voltage.

I don't think I fully understand the question, or the thinking behind it - but, obviously, "no".

As above. It is not alone.

 

[EFLImpudence]

Further:

If the MET itself were in the bathroom - and simultaneously accessible with an exposed-c-p - then IT would be an extraneous-c-p, wouldn't it?

 

[JohnW2]

You may regret using it but surely you are using it in the same way.
If not, I don't understand your meaning of it - apart from what the words say.

I've just explained what I meant - i.e. ""having a very low impedance path to the MET".

I understand that as far as main bonding is concerned ....

Well, for a start, I was primarily talking about main bonding - given that is what this thread is all about. However ....

but - for supplementary bonding that is not the only consideration, is it? The MET is not in the bathroom. If it were not connected to the bonding, it would just be a zero impedance path to earth.

I was talking just about one pipe but I suppose that, for completeness, I should have talked in terms of all metal things which enter the room. If all pipes entering the room are 'isolated', then there is no need to bond anything. If at least one of the pipes does have a very low impedance path to the MET, then any other pipes which not not have a similarly low impedance to the MET would (at least under some fault conditions) be potentially able to "introduce a potential" (different from that of other pipes into the room) and hence would qualify as extraneous-c-ps. If all pipes had very low impedance paths to the MET, none of them would be "liable to introduce" potentials into the room which were different from other potentials present in the room, so none would count as extraneous-c-ps. That's how I see it, anyway.

Don't forget that the definition of an extraneous-c-p is in terms of the liability to introduce "a" potential, regardless of the magnitude of that potential.

I suppose that another way of looking at this is that it is primarily about the desire for an equipotential zone. Given that it is very likely (at least, with metal plumbing) that at least something in the bathroom will be at (or very close to) MET potential, the only way that it can be an equipotential zone is if every touchable bit of metal is either 'isolated' or has a very low impedance path to the MET.

Kind Regards, John

 

[JohnW2]

Further: If the MET itself were in the bathroom - and simultaneously accessible with an exposed-c-p - then IT would be an extraneous-c-p, wouldn't it?

Interesting question! However, by definition it presumably could not be an extraneous-c-p, since it is "part of the electrical installation"?

Kind Regards, John

 

[EFLImpudence]

Oh. That's a good point. It would have the same effect as one, though, wouldn't it?

It would still require bonding to the exposed-c-ps so what would it be called?

 

[stillp]

It would still require bonding to the exposed-c-ps so what would it be called?

An exposed conductive part.