Lack of supplementary bonding - what’s the danger ?

[trojanhawrs]

Yes you'd bond from towel rail to cast iron bath for instance.

 

[EFLImpudence]

" 701.415.2 Supplementary protective equipotential bonding
Local supplementary protective equipotential bonding according to Regulation 415.2 shall be established connecting
together the terminals of the protective conductor of each circuit supplying Class I and Class II equipment to the
accessible extraneous-conductive-parts, within a room containing a bath or shower, including the following:"

 

[JohnW2]

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 ?

How close does 'what' has the be from the appliance CPC - do you mean the point of bonding to the pipe/whatever?

If so, I don't think the regs say anything directly or explicitly about that but, as I've been saying, if you want the lowest possible touch voltage (until the fault is cleared), the point of bonding to the pipe should be as close as possible to the exposed-c-p (not the CPC, although that will probably be the same) of the item concerned.

 

[trojanhawrs]

" 701.415.2 Supplementary protective equipotential bonding
Local supplementary protective equipotential bonding according to Regulation 415.2 shall be established connecting
together the terminals of the protective conductor of each circuit supplying Class I and Class II equipment to the
accessible extraneous-conductive-parts, within a room containing a bath or shower, including the following:"

I stand corrected

 

[JohnW2]

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 ?

That seems to be the implication of the regs.

I'm not sure what people do if, say, there are multiple Class I items with exposed-c-ps on the same circuit. There will obviously be a CPC joining all their exposed-c-ps, but that would probably not satisfy the requirements for an SB bonding conductor - but I'm far from convinced that everyone would necessarily formally 'bond' all of the items (multiple Class I downlights on a low ceiling would be an example, if that ever happens )..

 

[JohnW2]

" 701.415.2 Supplementary protective equipotential bonding
Local supplementary protective equipotential bonding according to Regulation 415.2 shall be established connecting
together the terminals of the protective conductor of each circuit supplying Class I and Class II equipment to the
accessible extraneous-conductive-parts, within a room containing a bath or shower, including the following:"

That's true in bathrooms etc., but, for what it's worth, in a more general sense, the wording is slightly different...

415.2.1 Supplementary protective equipotential bonding shall include all simultaneously accessible exposed conductive-parts of fixed equipment and extraneous-conductive-parts including where practicable the main metallic reinforcement of constructional reinforced concrete. The equipotential bonding system shall be connected to the protective conductors of all equipment including those of socket-outlets.

 

[EFLImpudence]

As for it not mattering where the SP conductor is connected to the pipe; surely the farther it gets from the, for example, tap in a bathroom the less effective it must become until at the extreme it just becomes another CPC for the faulted appliance rather than an SB conductor for the tap.

 

[equitum]

That is what I was trying , clearly not that well, to express. Electrically no different to another cpc the further it gets away from the location , albeit significantly reducing the overall resistance of the actual cpc by being a parallel resistance . If the actual cpc is of negligible resistance say because it is going to a very near location, such as the CU in the understairs cupboard below the bathroom would you still add SB ?

 

[JohnW2]

As for it not mattering where the SP conductor is connected to the pipe; ....

I have subsequently clarified what I was intending to say - not that the point of bonding "did not matter" (which is what I erroneously wrote) but, rather, that what matters is the resistance (hence length of a given CSA) of the SB conductor - such that the lower that resistance (hence shorter the SB conductor) the better - and that means that the optimum point at which to bond the pipe (for lowest touch voltage until fault is cleared) is the point on pipe closest to the exposed-c-p being bonded, not the point closest to whatever is touchable (e.g. tap) on the end of the pipe.

.... surely the farther it gets from the, for example, tap in a bathroom the less effective it must become until at the extreme it just becomes another CPC for the faulted appliance rather than an SB conductor for the tap.

What you say is obviously true.

However, given that the pipe itself is a conductor (and usually one with a much larger CSA than CPCs or SB conductor, you could say the same of any supplementary bonding - namely that the SB was merely providing "another CPC", since it provides an additional path from the exposed-c-p to MET in parallel with the circuit's CPC. I therefore don't think it is a very useful concept to invoke.

In any event, as I keep saying, I cannot agree that it is necessarily true that [paraphrasing you] "....as the point of bonding gets further from the tap, the effectiveness of the SB 'must become less' ". The "effectiveness of the SB" is at its greatest (i.e. lowest touch voltage) when the SB is the shortest possible (i.e. when bonding is to the point in the pipe closest to the exposed-c-p. If the point of bonding moves away from that point, in either direction (e.g. to get it 'closer to the tap'), such that the SB conductor becomes longer than 'the minimum possible', the 'effectiveness' of the SB will decrease (touch voltage will increase). Do you disagree with that?

 

[JohnW2]

That is what I was trying , clearly not that well, to express. Electrically no different to another cpc the further it gets away from the location ,

As I've just written in response to EFLI, whilst that's certainly true, very much the same can be said of any supplementary bonding. Conductors do not 'know what they are' or 'why they are there', and the electrical effect of installing any SB will usually be to create an additional path in parallel with the circuit's CPC.

Do you disagree with what I've just written to EFLI - namely that if one moves the point of connecting SB to a pipe from 'the closest point to the exposed-c-p'' to a more distant point that is 'closer to the tap', that the 'effectiveness of the SB will decrease (i..e. touch voltage will increase), albeit probably only slightly?

I will concede that there is one reason why one might want to bond 'as close as possible to the tap' and that would be anticipate the possibility that, in the future, the pipework is modified such as to interrupt electrical continuity between bonding point and tap. However, as above, I believe the price [paid for that attempt to pre-emet 'possible (although improbable) future events' could, in some cases, be theoretically 'less effective' SB.

 

[ebee]

That seems to be the implication of the regs.

I'm not sure what people do if, say, there are multiple Class I items with exposed-c-ps on the same circuit. There will obviously be a CPC joining all their exposed-c-ps, but that would probably not satisfy the requirements for an SB bonding conductor - but I'm far from convinced that everyone would necessarily formally 'bond' all of the items (multiple Class I downlights on a low ceiling would be an example, if that ever happens )..

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.
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.
It might even end up being directly outside the bathroom.
Every case can be different.
You are making the potentials as near as possible the same i.e. the PD near zero.
So whatever happens on those circuits is as close to zero as possible.
If every circuit is correctly installed and intact then you reach that near zero.
You might need to move your bond a few inches due to practicalities, cosmetics being one of them.
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.
You might decide that somewhere between items 2 & 3 to be the best choice, is it doable? does it make the bond unnecessarily long or awkward?
Bonding is there to keep those voltages near zero, fortuitously it helps keep the R2 value of all those circuits to a minimum and give protection of a cpc disconnection too

 

[equitum]

So I’ve calculated taking the example of an exposed part of a lighting circuit that has an R1 R2 of 0.8ohms .The touch voltage at the exposed point would be approximately 208V in the event of a live earth fault before the breaker trips. Assuming a resistance as low as one thousand ohms for the victim touching that point. The current flowing through the human would be 0.05A without any supplementary bonding in place and no other parallel paths. This would be in practice slightly lower (I think) taking into account the resistance of the path from the human through the pipe and the main bond back to the MET making the parallel path of the human slightly higher. Enough to give you a good bolt ! (Assuming no RCD) for 0.4 secs or less since the fault current I have calculated as 104A in total - correct ?

 

[trojanhawrs]

Not sure your maths is adding up there. To get the prospective fault current you need to know the Ze and R1 R2, the Zs.

To work out the touch voltage at that fault you would need to know R1 and R2 independently e.g. if they were both 0.4 ohms the touch voltage would be 230/2 = 115V. I'm not sure how you got to the 208V number but assuming that was right, the current flowing through a 1000 ohm person would be 208/1000 = .208A, not .05A.

The reality is a breaker will operate as quickly as a RCD in a dead short situation, the touch voltages are likely to be much lower where it's not dealt with instantly.

 

[equitum]

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



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

208V because voltage drop across r1 is 32V approx and across the r2+rhuman 208V

The voltage drop will be slightly different with ze included but doesn’t make much difference to IHuman it’s still in the same ballpark

 

[EFLImpudence]

However, given that the pipe itself is a conductor (and usually one with a much larger CSA than CPCs or SB conductor, you could say the same of any supplementary bonding - namely that the SB was merely providing "another CPC", since it provides an additional path from the exposed-c-p to MET in parallel with the circuit's CPC. I therefore don't think it is a very useful concept to invoke.-

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.

In any event, as I keep saying, I cannot agree that it is necessarily true that [paraphrasing you] "....as the point of bonding gets further from the tap, the effectiveness of the SB 'must become less' ".

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

The "effectiveness of the SB" is at its greatest (i.e. lowest touch voltage) when the SB is the shortest possible (i.e. when bonding is to the point in the pipe closest to the exposed-c-p. If the point of bonding moves away from that point, in either direction (e.g. to get it 'closer to the tap'), such that the SB conductor becomes longer than 'the minimum possible', the 'effectiveness' of the SB will decrease (touch voltage will increase). Do you disagree with that?

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.