I assumed that when I wrote "introduce a potential (into the cage)" that it was implicitly clear that I was talking about a potential different from that of the cage!
Kind Regards, John.
cross bonding
- Thread starter dag1167
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Kind Regards, John.
The original question was about bonding which ensures there is not a potential difference between extraneous conductive parts.
It seems you are not distinguishing between bonding and earthing. Indeed you seem to treat the difference and the regulations, which will ensure a safe installation, with some contempt.
Bonding is not to protect against the pipe becoming live. That is earthing and will ensure the cpd disconnects the power.
I personally can't help thinking that an awful lot of heartache and (sometimes heated) discussion would be avoided if we did not attempt so hard to make and maintain this distinction; few topics create such lengthy threads here, and elsewhere.
Sure, CPCs and bits of G/Y cable (and even bits of metal pipe) serve two different functions but, except in the extraordinarily rare context of a earth-free environment, the two functions cannot really be separated (i.e. you can't tell a bit of copper to conduct under some circumstances but not others).
If all metalwork is bonded (and at least some of it is bonded to MET) it will, in practice, also be 'earthed'. If all metalwork is 'earthed', it will certainly be bonded. The only thing one has to watch is that the conductors concerned need to be adequate to fulfill both functions.
That's how I see it, anyway.
Kind Regards, John
As most premises have plastic gas and water pipes any connection between the DNO earth, where it exists, and any metalwork is bonding. It is not allowable to use water/gas to provide an earth so there is no confusion. So any reference to earthing can only be erroneous.
As for the regulations they are what they are and based on a far greater set of inputs than just the authors, I am, however, concerned that there is too much reliance on the various books and guides than a full understanding of what the regulations mean.
Bear in mind that I work in a slightly different part of the industry and have been trained to look deeply at regulations and in a lot of cases design my own solutions to comply. Not to use on site guides etc. (as basically they don't exist)
I'm a bit confused here. Although, as I recently wrote, I think the distinction (between bonding and 'earthing') is probably fairly unhelpful, why can one not 'earth' something to the DNO earth? Given that the DNO earth is all that many/most people have, you almost seem to be saying that 'earthing' is not possible.
Kind Regards, John.
Is it an earth connection or is it a bond?
So in normal circumstances that bit of 10mm connects a metal pipe to the DNO earth point, the E core of the T & E connects (via the 13 A plug and flex) connects the metalwork of, say a washing machine, to earth - agreed?
So for a L - metalwork fault (and some N - metalwork as well) protective devices will operate
In some fault conditions all these green/yellow wires bond everything together to avoid a potential difference between adjoining pieces of metalwork - agreed?
So what is it?
So in normal circumstances that bit of 10mm connects a metal pipe to the DNO earth point, the E core of the T & E connects (via the 13 A plug and flex) connects the metalwork of, say a washing machine, to earth - agreed?
So for a L - metalwork fault (and some N - metalwork as well) protective devices will operate
In some fault conditions all these green/yellow wires bond everything together to avoid a potential difference between adjoining pieces of metalwork - agreed?
So what is it?
You seem to be re-inforcing my point that attempts to make this distinction are really not at all helpful. In other words, the short answer to your question is 'who cares?'. As I said, one cannot tell G/Y cables to cause a protective device to operate when there is a L-E fault, but not to prevent potential differences arising between the things they are connected to, or vice versa!
Kind Regards, John.
We don't make a distinction, they are different.
We earth parts which may, because of a fault, become live so that the cpd operates and they are no longer live. This would rarely be the pipework.
We bond parts that are, in their own right, already earthed to equalise the potential between them. This is most often the pipework.
Now you are admitting there are different functions. Perhaps we should use different coloured cable for bonding.
Yes, they can - see above.
No, but if it shouldn't be connected then it will never conduct or if it can never conduct then it shouldn't be connected (bonded nor earthed).
Because it may already be earthed on its own is why it needs to be bonded.
Perhaps it should, for safety, be neither earthed nor bonded.
They are different by definition only, in reality and electrically they may be the same piece of conductor serving the two purposes under different conditions.
Agreed but it was you who introduced laundry rooms and metal machines when discussing bonding.
The reality of installations and in an effort to discuss why
As long as the electrons know where to go and what to do when there is a fault all should be OK.
But as has been pointed out the "bonding" cables and the "earthing" cables or wires are all metallically connected together and are ( hopefully ) connected to a reasonable route back to the star point of the supply transformer. That route being via the neutral in a PME system or via a ground rod and the mass of the earth in a TT system.
The cable to the earth rod in a TT system. Is this cable "bonding" as required for any service pipe coming into the building or is it "earthing" as required for the operation of the RCD protective devices. ?
Yes I am splitting hairs ( or is it splitting copper strand ).
But as has been pointed out the "bonding" cables and the "earthing" cables or wires are all metallically connected together and are ( hopefully ) connected to a reasonable route back to the star point of the supply transformer. That route being via the neutral in a PME system or via a ground rod and the mass of the earth in a TT system.
The cable to the earth rod in a TT system. Is this cable "bonding" as required for any service pipe coming into the building or is it "earthing" as required for the operation of the RCD protective devices. ?
Yes I am splitting hairs ( or is it splitting copper strand ).
I don't disagree with any of that, but that doesn't alter the reality that the two functions are all muddled up with one another. The fact is that whilst, as you say, the exposed-conductive-parts would rarely be pipework, by 'earthing' them you are electrically connecting them to the pipework (since that also has connections to 'earth') - and thereby 'equalising the potential between them' (i.e.'bonding').
Kind Regards, John.
Well, I don't think this is 'splitting hairs'. It's precisely the point.[/b]
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