Lack of supplementary bonding - what’s the danger ?

[JohnW2]

Glad I’ve got you all thinking - it’s not as cut and dried as the initial responses is it …

It's nothing new. We started thinking, and exposing the apparent flaws in the regs as regards supplementary bonding, a good few years ago.

One of the reasons it's not discussed a lot more these days is that it has become almost moot, because there are few real-world situations in which the regs will still require SB, even in bathrooms - so the issue is no longer of much other than academic interest.

Hmm but How/in what scenario could the fault current achieve 500 A ?

In the example I gave, if the R1+R2 of the circuit with a fault was about 0.16Ω - but you need to realise that I just pulled numbers out of the air, 'as I was typing', to illustrate the point I was making. The conclusions would be essentially gthe same even if I had used more frealistic figures.

 

[JohnW2]

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

Indeed so - and that's another reason why the discussion is essentially academic.

In the case of 'a fault of negligible impedance', and assuming a correctly designed circuit, even OPD-mediated (let alone RCD-mediated) ADS will clear the fault so quickly that the changes of someone bring in contact 'with the wrong too things' during that tiny span of time is close to zero.

The problem arises, in the absence of RCD protection) when the impedance of the fault is 'a bit more than zero', since the fault can then persist for quite a long time, greatly increasing the chance of someone 'touching the wrong two things' whilst the fault is still present. RCD protection addresses that issue, even for faults of 'quite considerable impedance'.

 

[JohnW2]

... 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.

There is no good reason for SB in the situation you describe - which is presumably the reason why the situation you describe is the very one in which the regs no longer require SB, even in bathrooms.

As I've just written, any discussions about SB are now essentially academic, since it is generally no longer required, anywhere - and that's why we don't discuss it much these days - which I suppose tempts me to ask why you started this discussion.

 

[Harry Bloomfield]

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.

So, the greater risk, is of some otherwise isolated bit of metal, becoming accidentally live. Perhaps a washbasin tap, where someone, as often happens these days, has had a section of it's copper supply pipe, replaced with plastic, and due to some fault the tap becomes live. Someone takes hold of both the H and C taps, thus finds themselves with 240v across the heart.

 

[equitum]

There is no good reason for SB in the situation you describe - which is presumably the reason why the situation you describe is the very one in which the regs no longer require SB, even in bathrooms.

As I've just written, any discussions about SB are now essentially academic, since it is generally no longer required, anywhere - and that's why we don't discuss it much these days - which I suppose tempts me to ask why you started this discussion.

The regs no longer require it in general because of RCD protection. Before that, or where RCd protection is not provided ,and except for in the unlikely event of a fault while you happen to be hanging on to multiple objects, I asked the question what was/is the benefit. Clearly after sp many contradictory thoughts and opinions, all of your well documented and thorough analysis and debate didn’t exactly come to a black and white conclusion on the subject , did it ?

 

[Harry Bloomfield]

As I've just written, any discussions about SB are now essentially academic, since it is generally no longer required, anywhere - and that's why we don't discuss it much these days - which I suppose tempts me to ask why you started this discussion.

My bath taps are bonded close to the taps, my metal bath is bonded, the metal handles on the bath are bonded, likewise the washbasin and radiator. All metal pipes entering the bathroom are bonded, and all of the bonded items are connected together. In fact every tap, sink, pipe, and radiator in the house is very carefully bonded.

 

[EFLImpudence]

The regulations only require SB where a spurious R <50V/Ia.

With an RCD that is virtually impossible not to be the case.
While still possible without an RCD, thanks to John we have realised that the formula applied to the situation in the regulation 415.2 does not make sense anyway.

So, who knows? Perhaps Equitum is right.

 

[equitum]

My bath taps are bonded close to the taps, my metal bath is bonded, the metal handles on the bath are bonded, likewise the washbasin and radiator. All metal pipes entering the bathroom are bonded, and all of the bonded items are connected together. In fact every tap, sink, pipe, and radiator in the house is very carefully bonded.

Or you could just bond the water pipes and install an RCD - or as I said , maybe green and yellow cable is your “thing”

 

[Harry Bloomfield]

Or you could just bond the water pipes and install an RCD - or as I said , maybe green and yellow cable is your “thing”

I did not install it, but none of it is visible, however I am perfectly happy with the situation and yes, everything is RCD protected, apart from fridges and freezers.

 

[JohnW2]

Glad I’ve got you all thinking - it’s not as cut and dried as the initial responses is it …

For what it's worth, I still stick by one of the first things I wrote, but maybe not everyone thought enough about the totality of what I was saying (highlighted here) when I wrote something like:

The only way to avoid high 'touch voltages' (until the fault is cleared) is to have local supplementary bonding.​

I still believe that to be true, but one has to question why anyone would be concerned about that (provided there was RCD protection) since, as Harry has said the probability of someone 'touching the two wrong things' during the couple of dozen milliseconds that the fault was present (assuming RCD protection) must be very close to zero

 

[JohnW2]

.... I asked the question what was/is the benefit. Clearly after sp many contradictory thoughts and opinions, all of your well documented and thorough analysis and debate didn’t exactly come to a black and white conclusion on the subject , did it ?

As I've just written, my personal view is, and always has been, that in the presence of RCD protection, supplementary bonding overs no significant benefit - and the regs now seem to essentially reflect that view.

 

[JohnW2]

So, the greater risk, is of some otherwise isolated bit of metal, becoming accidentally live. Perhaps a washbasin tap, where someone, as often happens these days, has had a section of it's copper supply pipe, replaced with plastic, and due to some fault the tap becomes live.

It would require a pretty unusual fault in something to result in an electrically floating tap to become live, wouldn't it? What sort of thing are you thinking of - the infamous 'frayed lead' (does PVC 'fray'? of an electrical kettle touching a tap or metal draining board/sink, perhaps?

Someone takes hold of both the H and C taps, thus finds themselves with 240v across the heart.

well, as we've been discussing, a portion of 240V

 

[JohnW2]

My bath taps are bonded close to the taps, my metal bath is bonded, the metal handles on the bath are bonded, likewise the washbasin and radiator. All metal pipes entering the bathroom are bonded, and all of the bonded items are connected together. In fact every tap, sink, pipe, and radiator in the house is very carefully bonded.

... and such was 'how it was' in thedays of the 16th ed.

However, if you were starting from scratch today, would you have any of that bonding?

 

[securespark]

By connecting metal items that do not form part of the electrical installation or are not extraneous conductive parts to earth your are introducing voltage potentials under non fault conditions. Woukd you/have you also bonded your window frames , kitchen sink , metal fire surround etc etc ?

In the 15th Ed., yes!!

As for RCDs, there is a great reliance on them, to do their job, especially in this context of supplementary bonding, touch voltages, etc, but these things do fail regularly.

I'm in the bond & RCD camp.

 

[JohnW2]

The regulations only require SB where a spurious R <50V/Ia.

They do, although we (at least I) don't really understand why. However, if there is no RCD protection (which would, in itself, bew non-compliant in such a room), then, in a bathroom, SB is required even if the condition you mention suggests that it is not required.

With an RCD that is virtually impossible not to be the case. While still possible without an RCD, thanks to John we have realised that the formula applied to the situation in the regulation 415.2 does not make sense anyway.

Exactly. However, as I've been saying the question is becoming increasing moot, hence the discussions essentially academic, given that RCD protection effectively removes any requirement for SB. So, yes, equitum is probably right in suggesting that (when there is RCD protection, which is required almost everywhere these days) SB offers no appreciable benefit - but the regs seem to have essentially beaten him/her to that one!