Lack of supplementary bonding - what’s the danger ?

[JohnW2]

If there's a parallel path (the A-C) then yes the resistances matter, the lower A-C is as a percentage of the whole fault path (A-MET) will determine the potential difference between A and B.

Indeed so- and, as I have suggested, all that seems to matter is the total resistance of the 'wired' A-C path (a bit of CPC plus the SB conductor). I have also suggested that that remains true regardless of where (on the pipe) point C is relative to point B, but you now go on to say ...

John was also wrong to suggest it doesn't matter where the pipe is bonded.

Do you therefore disagree with what I have written and illustrated in post #87 (as well as above) and, if so, why?

You seem to be assuming that there will (or could be) be an appreciable PD between B and C but, as I've said, since that conductor (pipe) will only be carrying a tiny amount of current (that passing through the victim), I really don't see how that could be the case.

 

[trojanhawrs]

You seem to be assuming that there will (or could be) be an appreciable PD between B and C but, as I've said, since that conductor (pipe) will only be carrying a tiny amount of current (that passing through the victim), I really don't see how that could be the case.

No, B and C will be the same but by changing the bonding point you're changing the relationship of A-C and C-MET. You want the resistance of A-C to be much lower than C-MET, if it's higher you'll drop significant voltage by the time it gets to C.

 

[JohnW2]

You seem to be assuming that there will (or could be) be an appreciable PD between B and C but, as I've said, since that conductor (pipe) will only be carrying a tiny amount of current (that passing through the victim), I really don't see how that could be the case.

To illustrate, I think that 15mm copper pipe has a CSA of about 22.8 mm², from which I'd guess that it's resistance is around 0.96 mΩ/metre.

Hence, if the pipe feeding a tap was supplementary bonded, say 10 metres from the tap (unlikely) then, if a very nasty shock current of, say, 200 mA was passing through the victim, I reckon that the PD between the tap and point of bonding would be around 0.0019 V - which, if correct, I would suggest qualifies as 'negligible'. Indeed, even if the current through the victim were ridiculously high, high enough to 'fry', let alone merely kill, the victim, that PD would still be 'negligible'.

 

[equitum]

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[JohnW2]

No, B and C will be the same but by changing the bonding point you're changing the relationship of A-C and C-MET.

The potential of C-MET, per se, is surely irrelevant ? What (all that) matters is the PD experienced by the victim (A-B), and you have now agreed that's essentially the same as A-C, and that remains the case regardless of the potential of C relative to MET.

Am I missing something?

You want the resistance of A-C to be much lower than C-MET, if it's higher you'll drop significant voltage by the time it gets to C.

Isn't that essentially what I've said - namely that one wants to minimise A-C, even if (because of the 'geography') achieving that requires increasing the distance from B to C (i.e. bonding the pipe further from the tap) ... (which may sometimes be the case) ?

 

[equitum]

Does /is supp bonding always made directly from the appliance/light fitting ?

 

[trojanhawrs]

The potential of C-MET, per se, is surely irrelevant ? What (all that) matters is the PD experienced by the victim (A-B), and you have now agreed that's essentially the same as A-C, and that remains the case regardless of the potential of C relative to MET.

Feel like a stuck record here. Heres a picture.

If you were to swap the resistances in the below picture you would have 80 volts between A and C rather than the 20 illustrated.

 

[JohnW2]

Feel like a stuck record here. Heres a picture. If you were to swap the resistances in the below picture you would have 80 volts between A and C rather than the 20 illustrated.

Sorry if I'm frustrating you! Needless to say, I am also getting a bit frustrated, but am trying to remain patient! I suspect that we are either talking somewhat at 'cross-purposes' or, perhaps more likely, that we are both saying much the same thing, but not fully understanding/realising that.

Is the problem perhaps that you are talking, theoretically, about resistances, whereas I am talking about 'practicalities'?

I have repeatedly said, and think you agree, that 'touch voltage' (A-B) is minimised by minimising the restance of the supplementary bonding path. (from exposed-c-p to bonding point). Is that the case?

If so, my (really only) point is that "minimising the resistance of the SB path" does not necessarily equate to bonding a pipe as close as possible to the potential point of contact (e.g. a tap on the end of the pipe).

If some point on the pipe (feeding a tap) is closer to the exposed-c-p than to the tap, then the resistance of the SB path, hence touch voltage', will be lower if one bonds at the point than if one bonds the pipe closer to the tap, won't it?

In other words, as I've said a number of times, it's essentially the resistance (hence length, for a given CSA) of the SB path that matters, such that the lowest 'touch voltage' will result from bonding to point on the pipe which is closest to the exposed-c-p, even if that means bonding an appreciable distance from the 'touchable part' - i.e. that the best point at which to bond the pipe is not necessarily 'as close as possible to the tap' in our example.

I don't have any SB in my house but, if I did, I can think of several places in which the 'lowest resistance SB path' (hence, by my reckoning, the lowest 'touch voltage') would result from bonding pipes an appreciable distance from 'the touchable part'.

 

[trojanhawrs]

I'm using resistances to demonstrate the theory and why supplementary bonding is used. You may be right about making a lower resistance path but don't forget you're shortening the path to the MET, and all that really matters is the ratio between sup bond : path back to earth - theoretically this could still work out favourably but practically you should be bonding as close to the exposed conductive part as feasible. Saying things like "it doesn't matter where the pipe is bonded" is not helpful.

 

[JohnW2]

I'm using resistances to demonstrate the theory and why supplementary bonding is used. You may be right about making a lower resistance path but don't forget you're shortening the path to the MET, and all that really matters is the ratio between sup bond : path back to earth

Indeed, and I haven't even bothered to mention that - but, as you imply, if one bonds the pipe further from it's end (e.g. from the tap), that will usually mean 'closer to the pipe's main bond to the MET - so that might be an additional benefit (of bonding at a distance from the touchable part at its end (e.g. tap), in addition to (in some cases) a reduced length (hence resistance) of the SB conductor itself.

- theoretically this could still work out favourably but practically you should be bonding as close to the exposed conductive part as feasible.

That's what I've been saying. The bonding conductor should be bonded to the pipe/whatever as close as possible to the exposed-c-p being bonded - which (as below) does not by any means always translate to "closer the better" (to the tap) necessarily being the optimal approach.

In fact, the more I think of my house, the fewer are the cases in which the "as close as possible to the touchable part" would actually seem to be the optimal approach - exposed-c-ps that would need to have SB are in many cases much further from 'touchable parts' (like taps) than they are from the nearest point of the pipe (to which the exposed-c-p could be bonded through an SB conductor of appreciably less resistance than if it were bonded to 'close to the tap/whatever)..

Saying things like "it doesn't matter where the pipe is bonded" is not helpful.

Yes, I don't know why I wrote (and to some extent expanded upon) "doesn't matter", since it clearly does matter, so my apologies for any confusion I caused. However, I have subsequently clarified that what I was trying to do was to disagree with (as a generalisation) ...

That’s assuming the bond is made at the tap itself other than in reality some other point in the pipe work

Closer the better, but any link is going to reduce the potential between the 2 points.

... since, as I have explained, "closer the better" (to the tap) is not necessarily the best approach, since it will sometimes result in people being exposed to unnecessarily high (but probably not very high) touch voltages.

I think the main lesson is that one needs to consider the actual practical situation, rather than to attempt to apply theoretical generalisations to real-world situations.

 

[equitum]

Rather than focus on where on the pipe the s. bond needs to be on the pipe, how close must it be to the appliance CPC ?

 

[trojanhawrs]

Rather than focus on where on the pipe the s. bond needs to be on the pipe, how close must it be to the appliance CPC ?

It's only required for parts that can be simultaneously touched if that answers your question. Suppose it depends on if you're wiring the BFG's house or not.

 

[equitum]

It's only required for parts that can be simultaneously touched if that answers your question. Suppose it depends on if you're wiring the BFG's house or not.

No not really , for parts that can be touched simultaneously, does the s.bond have to be made directly from the electrical item, say a class 1 light fitting or towel rail ?

 

[trojanhawrs]

Why would you bond a class 2 fitting?

 

[equitum]

Sorry typo class 1 obvs