Lack of supplementary bonding - what’s the danger ?

[trojanhawrs]

Good point forgot Ze. Was based on r2 of 0.5ohm r2 0.3ohm and RHuman 1000.

R1+R2 = 0.5 and Ze = 0.3?

Don’t forget human and r2 are acting in parallel so the path from the light point is 2.001 ohm. 240/2.3001=104.3A

I don't know where you got this number from. A parallel path reduces the resistance it doesn't increase it, regardless it's negligible.

 

[equitum]

R1+R2 = 0.5 and Ze = 0.3?


I don't know where you got this number from. A parallel path reduces the resistance it doesn't increase it, regardless it's negligible.

No I said r1 and r2 0.3 and 0.5 . I didn’t include Ze i forgot. Let me redo the math as yes you are correct 1000 and 0.5 in parallel is still just below 0.5

 

[equitum]

I’m cooking dinner maybe someone could do the maths. No supp bond. R1 of lights = 0.3ohm R2 of lights 0.5ohm Ze 0.2 ohm. Human of 1000ohm in contact with light fitting during live earth fault and tap. 240V supply. Main bond to water in place. What is fault current, current through human, voltage at touch point of light fitting ?

 

[EFLImpudence]

Using your numbers:

The voltage at the fault is ...143.75V (230 x 5/8)
Fault current is .........................230A ( 230 / Zs of 1Ω)

Current through person is ...143.75mA (143.75 / 1000)

 

[JohnW2]

I.ve been thinking about this overnight, and wonder whether some of the confusion has resulted from the fact that there are two different issues, which may have been a bit muddled up - and this may also partially explain the seemingly confusing/misleading/incorrect statement I made at the start. I just hope I've got my thinking a bit straighter in what I write below, but I'm sure you will all point out if I haven't

Using the terminology of our variously diagrams, what we are interested in is the 'touch voltage' (until fault is cleared), namely "A-B", so tjhat, in turn depends upon the potentials (relative to anything, say the MET) at points A and B (expose-c-p and point of contact {tap} respectively).

As far as the potential (say, relative to MET) at point B (point of contact with tap), that is going to be virtually the same as the potential at the point of (SB) bonding of the pipe ('point C'). In turn, the potential at that bonding point is, I would suggest, usually going to be 'not much above MET potential', since, although the pipe will be carrying a portion of the (possibly very high in total) fault current, the CSA of the pipe will be large (much larger than that of the CPCs or SB conductors). Hence I would suggest that, in practice, the potential at C, hence also at B, will usually be 'not much above MET potential', and that that largely remains true regardless of where point C is on the pipe - it is in that sense that I was probably thinking when I wrote that the location of the point of (SB) bonding 'did not matter'.

Moving to the potential at point A (the exposed-c-p) that is not appreciably affected by the location of point C (SB bonding to pipe), per se, being primarily dependent upon the resistance (hence length for a given CSA) of the SB conductor (again assuming that changes in VD along the {large CSA) will not be very significant). It is for this reason that the touch voltage (A-B) will usually be at its lowest when points A and C are as close as possible (hence SB conductor as short as possible) - i.e. when the exposed-c-pis bonded to the nearest possible point on the pipe (regardless of where the 'tap' is).

At first sight, SB would seem to be very simple, logical and 'obvious' ("join all touchable things together so that there can't be appreciable PDs between them") - and that is essentially correct. However, given that the reason for doing this is to reduce/minimise touch voltages, and in view of some of the things you go on to say, I would point out that one can achieve 'similar' simply by reducing the resistance of the circuit's CPC (e.g. by adding a further one in parallel with the existing one) ....

As we know, with 2.5/1.5 mm² T+E, if the point of contact with the 'pipe' (point B) is at MET potential, then, with a supply of 230V, the touch voltage will be about 144 V. If, instead of installing 4 mm² SB, one added a 4 mm² 'CPC' in parallel with the existing one (so that one effectively had a 2.5/5.5 mm² cable), that touch voltage would reduce to about 72V. If one added a 6 mm² CPC in parallel with the existing one, then that touch voltage would reduce to about 57.5 V.​

Of course, one consequence of reducing touch voltage (by any method) will be that the magnitude of fault current will increase, perhaps substantially.

That is true. I would not regard it as a concept but the nearer to the MET the SB connection is, the less the conductor is an SB until it becomes only another CPC.

Indeed, but that;'s not really any more than a matter of words. The SB conductor will achieve what it is designed to achieved, even if essentially the same would be achieved if you did something that you called "adding a parallel CPC". As I said,conductors do not know what they are called, or why they were installed - they just obey the Laws of Physics.

It is theoretically possible that exposed-c-p, potential point of contact and MET might be physically all very close one another, but if, for whatever reason, you installed SB in that situation, you wouldn't say that it "wasn't SB" (because it was 'virtually a parallel CPC') would you?

Let's face it, I don't think one can get away from the fact that if you connect a conductor from an exposed-c-p to (anywhere on) a pipe which provides a (I would suggest) very low resistance path to the MET, you will have, in effect, "added a CPC" in parallel with the existing one, even if you choose to call it something else, won't you?

Surely it must, or as above it is just another CPC (or unnecessary conductor).

As above, it is fulfilling the desired electrical role of a SB conductor, even if it could be viewed as a 'parallel CPC'. Again as above, it doesn't know whether you have chosen to call it SB or CPC or why you installed it.

I do not disagree but when moving farther away from the tap - or more importantly, nearer the MET, there must come a point when the conductor is no longer an effective SB.

What do you mean by "no longer an effective SB". As above, it will ensure that the potential of B ('point of contact') is virtually same as that of point C (point of bonding), which I'm suggesting will usually be 'not much above MET potential" - which is all one can ask of any SB conductor, isn't it.?

 

[EFLImpudence]

Very interesting.

"All" that is needed to reduce the touch voltage to an acceptable level is to hugely increase the csa (thus reducing the R2) of the CPC.
So - if there is only <50V touch voltage at the fault anyway then no SB would be required.

May I add another question:
Have you thought any further about your "debunking" of 415.2?
In view of what you have written in the post above, could it after all be correct?

 

[JohnW2]

My take is that you make a judgement call. ... Take all of the exposed conducive parts and decide which parts are most likely to be touched. ... A pipe with a tap on the end , well the tap is a sure bet, the pipework leading to it might or might not be easily touchable.

Fair enough, but I strongly suspect that everyone involved in this discussion has 'assumed' that the tap would be the most touchable part. If it was felt that someother part of the pipe were more likely to be touched than the tap, I imagine that those who have said "as close as possible" would then say "as close as possible to that point on the pipe", wouldn't they?

So you consider the likelihood and that`s where you would prefer to bond then you go to the nearest convenient place for your actual bond on the basis that ate the place you make the bond it is likely to be effective.

That would, at first sight, seem to be the most logical/obvious. However, as I have said repeatedly, as I see it if, on the sort of basis you suggest, you end up bonding the pipe in a place that involves a longer SB conductor than if you had bonded it somewhere else, you will (despite your well-intentioned approach) end up with a higher-than-necessary touch voltage.

You are making the potentials as near as possible the same i.e. the PD near zero.

Indeed but, as I've said, if I'm now thinking right, the 'touch voltage' will usually be nearest to zero when the SB conductor is of the shortest possible length, even if that involves bonding the pipe at some distance from what you feel is the 'most touchable point'.

If you have a circuit in a bathroom, example light fittings, and say there are 4 of them, you consider A the end of the circuit portion in the bathroom or B the first item or C half way between. I don`t think anyone would bond all 4 separately.

As I said, I also doubt that they would bond all four separately. However, as I illustrated in post #*****, the regs are somewhat lesss than clear about this. The 'general' reg, 415.2.1 (relating to any location, presumably including bathrooms) can be read to require that all (hence individually?) simultaneously-touchable exposed-c-ps must have SB. On the other hand, for just bathrooms (701.415,2) the requirement seems to be only for SB to be applied to each circuit, with no reference to 'simultaneously-touchable' - although t also refers ton"SB per 415.2' !'m therefore less than clear as to what actually is intended for a bathroom!

Bonding is there to keep those voltages near zero,

Indeed - and I have above explained what I believe that means in terms of the optimal place to bond a pipe (which I believe is not necessarily close to the most touchable part)

fortuitously it helps keep the R2 value of all those circuits to a minimum and give protection of a cpc disconnection too

Again, indeed. As I've recently observed, reduce R2 (e..g. with conductors called 'parallel CPCs", rather than "SB conductors" is a way of reducing touch voltage.

Kind Regards, John

 

[EFLImpudence]

Have you thought any further about your "debunking" of 415.2?
In view of what you have written in the post above, could it after all be correct?

No, it's still wrong isn't it?

The resistance between exposed-c-p and extraneous-c-p isn't a defining absolute value, is it.
It is dependent on the values of the other parts.

 

[JohnW2]

Very interesting. .... "All" that is needed to reduce the touch voltage to an acceptable level is to hugely increase the csa (thus reducing the R2) of the CPC.
So - if there is only <50V touch voltage at the fault anyway then no SB would be required. ... May I add another question: ... Have you thought any further about your "debunking" of 415.2? ... In view of what you have written in the post above, could it after all be correct?

This is where I'm getting pretty confused (and it's certainly relating to my "debunking of 415.2") - since, other than for the VD (which I think would usually be pretty small) due to part of the fault current flowing through what could be a short length of 'very large CSA' pipe, there is no electrical difference between installing SB (of a particular CSA) and increasing the CSA of the CPC by that same amount.

However, I think this is where I have to throw a large spanner into the works of the whole discussion, since everything we have been saying (and 'calculating') about touch voltages seems to be based on a false assumption....

... on the basis of a supply which, in the absence of a fault, has an L-MET PD of 230V, we have calculated touch voltages assuming that the L-MET PD remains at 230V during the fault - but this is obviously incorrect, since we are ignoring Ze.

If Ze is, say 0.3Ω and the fault current is, say, 500 A, then, during the fault, the L-MET PD will presumably fall to about 80V so that with 2.5/1.5 mm² T+E the touch voltage (between the exposed-c-p and something connected to MET which did not have fault current flowing through it) would only be about 50V - even without any SB.

Things would admittedly be less 'dramatic' (touch voltages higher than 50V) if Ze and/or fault current were appreciably lower than I have assumed, but I still feel that there must be something fundamentally wrong with what I've just written/suggested, since it would imply that, even in those 'worse' scenarios, one would probably only need a very modest increase in CSA of CPC (or very small CSA SB) to get touch voltages below 50V (rather than a "huge increase in CSA of CPC")

What is wrong with my thinking today? !

 

[JohnW2]

No, it's still wrong isn't it?

I believe it is ... but see my recent post about my current confusion

The resistance between exposed-c-p and extraneous-c-p isn't a defining absolute value, is it. It is dependent on the values of the other parts.

Exactly

 

[equitum]

Glad I’ve got you all thinking - it’s not as cut and dried as the initial responses is it … Hmm but How/in what scenario could the fault current achieve 500 A ?

 

[Harry Bloomfield]

Glad I’ve got you all thinking - it’s not as cut and dried as the initial responses is it … Hmm but How/in what scenario could the fault current achieve 500 A ?

Plus, what is the likelihood of anyone having themselves in parallel, at precisely the same instant as the occurrence of that fault current?

 

[equitum]

And I’m glad you asked that too - because other than the unlikely scenario that you happen to be in contact with an ECP, let’s say a tap, and an appliance at the exact time it creates a live/earth fault (unless that is you create the fault by touching and disturbance ), albeit for in all likelihood less than 0.4 s , then I’m yet to hear a good reason for supplementary bonding in the first place - disclaimer- in an installation with CPC’s connected and MEB in place and no RCD protection.

 

[trojanhawrs]

And I’m glad you asked that too - because other than the unlikely scenario that you happen to be in contact with an ECP, let’s say a tap, and an appliance at the exact time it creates a live/earth fault (unless that is you create the fault by touching and disturbance ), albeit for in all likelihood less than 0.4 s , then I’m yet to hear a good reason for supplementary bonding in the first place - disclaimer- in an installation with CPC’s connected and MEB in place and no RCD protection.

Yeah, apart from drowning I'm yet to hear a good reason for not falling asleep in the bath.

 

[equitum]

Yeah, apart from drowning I'm yet to hear a good reason for not falling asleep in the bath.

What the funk is that supposed to mean ?