A bit of bondage!
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Yes, until Mr Cockburn actually lives in the same time frame as the rest of us then he won't get it! I wonder when he will get back to us on the weegie board?
When needed - to reduce potential between exposed and extraneous conductive parts, not to ensure operation of the CPD.
Not just because it's metal and a live wire may jump out and touch it.
Supplementary bonding may be used between the MET and an exposed conductive part to ensure that where a protective device does not operate in time that the maximum touch voltage does not exceed 50v, however this method isn't that common. I think I put a bit about it in the wiki.
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But not between NON-extraneous/exposed conductive parts
which is what I am trying to dissuade John from doing.
which is what I am trying to dissuade John from doing.
There was a question in a past 17th edition 2391 exam about this, I'll try to dig it out (the paper) when I get a chance (when toddlers are all in bed).
Bet Mr C can't answer it as he has not read past the 16th ed it seems.
If I post the question I would invite everyone to refrain from answering it and invite Mr C to answer it 1st. Let see if he can, and if he can how he reconciles it with his other "views"
Sure - that's essentially the definition of the purpose of supplementary bonding.
However, I can but repeat my comment above - that I'm starting to wonder whether you believe that the need ever exists.
If you do, I can but presume that it must relate to voltage drops in protective conductors or, say, pipework. Otherwise, there is seemingly no way that a conductive part contained wholly within a building can be an 'extraneous-conductive-part' (as defined by regs) if all the required main protective bonding is in place. As I wrote before, if MPB is in place, then any metal whiolly contained within the building is either floating (in which case it can't be an e-c-p, and therefore does not need bonding) or else it part of the building's equipotential zone (i.e. effectively connected to the MET, and hence CPCs), and cannot introduce a potential other than that of the MET - hence again, not really an e-c-p.
So, if you do feel that the need for supplementary bonding ever exists is it perhaps because you think there may be a significant potential difference between the ends of the CPC which earths the exposed conductive part (i.e between the exposed conductive part and MET) and/or because you think there may be a significant potential difference between the metalwork which is not an exposed conductive part (I hesitate to call it an 'e-c-p') and the MET (despite main bonding and pipework continuity)? .... or what?
Kind Regards, John.
I don't know why the topic has changed to MY supplementary bonding but -
I have never said that it is never needed. It is needed when it is needed.
I don't understand this part. If the part is not extraneous then what has the MPB to do with it?
Correct. I would rather say 'should not be bonded' but as I wrote before 'What has the MPB to do with it?
That depends on why it's connected to the MET. Are you talking about the incoming services? In which case it WILL be an extraneous-c-p or do you mean an exposed-c-p.
The MET could be a fair distance away. It depends what you mean by significant but there may be a PD between a metal light fitting and a water pipe. This is what we have to determine.
If it's not an extraneous-c-p either then no.
I found that somewhat confusing but I have tried to answer.
You will also note I have reinserted the most important part of my original quote (in bold) which you omitted.
The original discussion was to dissuade you from bonding all metal parts (out comes the green and yellow) whether necessary or not or dangerous.
Talked to an old hand electrician ( mostly industrial ) who remembers ( like me ) the days of electric irons with bayonet "plugs" that "plugged" into the lamp holder hanging from the ceiling.
He has the same opinion as me. TT suplies with a true ground rod used as the safety earth are much safer than systems that rely on an "earth" that is not a true ground,
The modern earthing and bonding is a best fit between safety and the problems that "earths" imported from modern cost cutting supply system where the "earth" is derived from the network neutral.
Any imported true ground such as a metallic service pipe presents no hazard as long as the imported "earth" is at a potential close to true ground.
The reason to bond incoming service pipes to the imported "earth" is to prevent the service pipe ( and pipe work connected to it ) from creating a hazard when the imported "earth" is no longer close to true ground.
When that happens the real hazard is the imported (safety ?) "earth" which could create an electric shock hazard to any one holding an earthed item when also in contact with true ground either directly by standing on it or by touching pipe work in contact with true ground via a service pipe. The ground is safe, the (safety ?) "earth" is the real hazard. That is why electric garden tools are now almost always double insulated and thus banned from having an "earth" wire. If the "earth" was not the problem then they could be double insulated with the option of "earthing" the metal parts like the handle. But because the "earthed" handle may become a hazard to someone in contact with true ground the used of "earth" in most electrical garden tools has to be banned.
He has the same opinion as me. TT suplies with a true ground rod used as the safety earth are much safer than systems that rely on an "earth" that is not a true ground,
The modern earthing and bonding is a best fit between safety and the problems that "earths" imported from modern cost cutting supply system where the "earth" is derived from the network neutral.
Any imported true ground such as a metallic service pipe presents no hazard as long as the imported "earth" is at a potential close to true ground.
The reason to bond incoming service pipes to the imported "earth" is to prevent the service pipe ( and pipe work connected to it ) from creating a hazard when the imported "earth" is no longer close to true ground.
When that happens the real hazard is the imported (safety ?) "earth" which could create an electric shock hazard to any one holding an earthed item when also in contact with true ground either directly by standing on it or by touching pipe work in contact with true ground via a service pipe. The ground is safe, the (safety ?) "earth" is the real hazard. That is why electric garden tools are now almost always double insulated and thus banned from having an "earth" wire. If the "earth" was not the problem then they could be double insulated with the option of "earthing" the metal parts like the handle. But because the "earthed" handle may become a hazard to someone in contact with true ground the used of "earth" in most electrical garden tools has to be banned.
I've been acknowleding that, and have merely being trying to understand when you felt supplementary bonding may be needed.
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What I was trying to say was that once all required MPB (of incoming services etc.) is in place, the entire building becomes an equipotential zone, so there is not really any opportunity for 'extraneous-conductive parts' to exist within the building. Since it's all about establishing an equipotential zone, when the regs define an e-c-p as a conductor 'liable to introduce a potential', I think it's very reasonable to take that to mean a potential other than that of the equipotential zone (i.e.MET). Once all MPB is in place, no conductor within the building (other than an exposed-c-p) has the capacity to do that.
I was really making a general statement about all exposed metal other than exposed-c-ps within a building. As I see it, any metal is either floating or part of the equipotential zone (connected to MET) - I see no other possibilities. Even exposed-c-ps are also more-or-less part of the equipotential zone (connected to met via CPCs) - but you may argue that they may not be quite equipotential because of the relatively small CSA of CPCs.
That sounds like a 'Yes', then.
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Here, I was merely referring to the corresponding 'pipe side' of the situation. If the impedance of the path from the pipe (via pipework and MPB) to MET was zero, and the impedance of the path from the exposed-conductive part (via CPC) to MET was also zero, then there clearly could be no pd between pipe and exposed-c-p, so no supplementary bonding would be required. You seem to have indicated (above) that you might feel that supplementary bonding was required if there were a 'long CPC' from exposed-c-p to MET, and my 'and/or' statement referred to the corresponding situation if the path from pipe (via pipework and MPB) to the MET might have 'appreciable' impedance.
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Yes, we've moved a fair bit from that original discussion. Looking back, my "out comes the G/Y" statement was far too much of a generalisation, and I have subsequently qualified that a fair bit in subsequent discussions. I think that I am now pretty close to you in terms of that original discussion.
One interesting point. Consider again the infamous metal bath with plastic plumbing. You have indicated that you believe it should not be bonded, because you feel that earthing the bath via a bonding conductor would make the situation less safe (more dangerous). You say that because you believe that it is more likely that someone would touch the bath whilst also in contact with electricity (e.g. frayed appliance cable) than that electricity would come in contact with the bath and that someone would touch it and something earthed smultaneously. Is that a fair statement of your position? If so, and if you feel that having the bath earthed (via explicit G/Y bonding conductors) creates a more dangerous situation, do you also feel that there should be a requirement for metal baths (and kitchen sinks etc. etc.) to be electrically isolated from any paths to earth (i.e. via metal pipework)?
Kind Regards, John.
It's not up to me, per se, if conditions require it then Supplementary Bonding shall be applied.
Well, yes; you are saying once all extraneous-c-ps are bonded there are no more.
That is when Supplementary Bonding may be required, i.e. if conditions for its omission cannot be met.
Only because you seemed to have thought I was vehemently opposed to Supplementary Bonding because I have been trying to dissuade you from Bonding NON extraneous parts.
Obviously if the PD is zero all is well but if not...
OK.
You still sound as if you disagree.
Yes. I think it extremely unlikely electricity would come into contact wit the bath. Where would it come from?
I would not necessarily advocate such (I am not a great fan of the nanny state) but obviously if everything was plastic the situation would not arise.
One thing you keep saying in your posts is "at the MET" .
I am sure no PD exists at the MET and simultaneously accessible objects, not that ordinary people often touch the MET.
However at the other end of the building things may be different.
Of course, but I've been trying to learn, by asking you what conditions would require Supplementary Bonding to be applied.
OK - so you're basically just talking about the criteria for ommission in 701.415.2? Provided there is an RCD, it seems hard to believe that these criteria for ommision will ever not be satisfied; 411.3.2 appears to allow the automatic disconnection required by ommission criterion (i) in 701.415.2 to be provided by an RCD, and I would presume that the Zs required for that will always be achieved - is that your experience? If so, in practice, it would seem that there is effectively never going to be a need for Supplementary Bonding, provided that there is MPB and RCD protection - is that how you see it?
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No, I never thought you were 'vehemently opposed' to it. Rather, as I've tried to explain above, it seemed that you were effectively taking a view ('the word of the regs.') which, in practice, would mean that it would never be required (so long as there is RCD protection).
No, I didn't intend my words to indicate any disagreement - I was merely stating my understandanding of what you believe.
I'm not a believer in the Nanny State, either, but, for better or for worse, that's essentially what a lot of the regs are about.
Interestingly, supplementary bonding is the default of the regs, which merely indicate situations in which bonding may be omitted - which falls far short of saying that it must be omitted if those conditions are satisfied - which I think is a little less strong than your feelings.
Agreed, but I was simply using 'the MET' to indicate a point at which the alleged 'equipotential zone' (which, as you suggest, is not always going to be equipotential) exists (because that's where MPB is required to be connected). Under no-fault conditions, there presumably will not normally be any any appreciable pd between CPCs and bonded pipework anywhere (even very distant from the MET), since there should not be a significant current flowing in either. Under fault conditions, there could, of course, be an apreciable pd if a very high fault current was flowing through the CPC.
Kind Regards, John
Well, there are few requirements with the introduction of RCDs but without them, without quoting every regulation, the equation of 415.2.2 is a good test, although GN3 recommends 0.05Ω.
Also if disconnection times cannot be met. A better term for this would be Supplementary Earthing, but that would be another kettle of worms.
Yes, but you say it as if it were a bad thing. If it is not required why would you fit it. If you disagree (don't trust the RCD) then you can ignore it and bond as before.
If so then you also can deduce when it is required. The regs. do not say it MUST be omitted on plastic pipe, merely not required. That is their way. The On-site guide 4.5 and 4.6 is much more succinct.
Is that not what I said?
Exactly.
Agreed. The GN3 recommendation is a bit more demanding, but have you, in practice, found (m)any installations in which the 415.2.2 test is not satisfied?
Very true. This (and, indeed, the 'bath scenario') are examples of those situations in which, whether they like it or not, people simply have to accept that the concepts of bonding and earthing can get totally muddled up with one another. In the 'floating bath scenario' it is not 'bonding' which creates a hazard, it's the earthing which comes with the bonding
Not at all. I am again simply trying to ascertain whether you feel that, in practice, there is every going to be a need for Supplementary Bonding (given the presence of 'mandatory' MPB and the near certainty of an RCD - at least, in recently designed circuits).
It is indeed. Are you surpised that I am agreeing with you?
Kind Regards, John.
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