Lack of supplementary bonding - what’s the danger ?

[JohnW2]

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.

I can sympathise with that view and, again, certainly would not knock it.

In this rather confusing thread in which I, for one, may appear to have been contradicting myself at times, I have suggested that we don't often discuss (or install) SB these days because it has largely became 'unnecessary' because of RCD protection.

However, I have also pointed out that only way to avoid high touch voltages during the period before the fault is cleared is to have local SB.

It therefore follows that if one is not happy to trust an RCD to always do what one should, then one should have SB.
[ Conversely, of course, some might argue that if one has SB, one does not need RCD protection, at least for the purpose we are discussing! ]

 

[JohnW2]

I think the 50v or less is for normal dry conditions, special locations need a bit more protection.

It's obviously essentially arbitrary, but one has to pick some number.

Even with wet skin etc., <50V will probably avoid death in most people, but not all. In fact, no figure would be guaranteed to avoid death in all people - there are plenty of people out there who (often without knowing it) have hearts that are so electrically unstable that there are constantly at risk of spontaneously developing lethal rhythm disturbances i.e. even with no electric shock at all) - so, in them, a shock resulting in less than 1mA though their body, for a very short time, could sometimes be the 'final straw' that resulted in their death.

However, if we required touch voltages a lot less than 50V, it could result in practical problems. Even with SB, the touch voltages are determined by the length (and CSA) of the SB conductors, so we could end up with prohibition of installing things with exposed-c-ps "further than X cm from a pipe to which had to be bonded" (and/or a requirement to use massive bonding conductors)

All in all, I suspect that 50 V is not an 'unreasonable' figure to go with - but the precise number chosen is obviously essentially arbitrary.

Has anyone ever been touching a 50v bog standard phone line whilst ringing signal is active? It does bite even in normal dry conditions!

Indeed so However, although, as above, it theoretically could kill, I wonder if that has ever happened (in the last century or whatever)?

.... I became a fan of supp bonding and remained so even when RCDs were mandated on most circuits. Even the speed of working RCDs could still leave one uncomfortable. Then factor in the possible failure rate of RCDs ....

See what I've just written to secure. I agree that if one is not happy to trust an RCD to always clear a fault fairly quickly, then local SB is the only way of preventing high touch voltages.

(not just complete failure to actually disconnect at all but the failure to do so within the required time.

In terms of 'real' L-E faults in a circuit (rather than just an L-E path trough a victim), an RCD does not really have to perform 'on spec' to achieve what one wants (clearance of the fault before anyone has a chance to get a shock). Even if it took 10 or more times the In to make it triop, or even if it took 10 or more times longer to trip, it would still serve that purpose, and only 'complete failure to trip' would really be an issue.

As I see it, the only situation in which 'out-of-spec' could be a problem is if the only residual current was that flowing through a victim - and, as you know, I do wonder how often RCDs actually do trip as a result of that.

Putting aside the age old game of "catch this capacitor" I have only had a mains shock once at about 12 years old and a BT line zap once in adult life.
Both incidents taught me to avoid such stings.

Ignoring 'tingles', it's also literally many decades since I last had a significant shock of any sort, but my experience was a bit worse than yours during my (probably foolish- or, at least, 'less cautious') youth ! Back then I had several mains voltage shocks and at least some that would today be classed as "HV" ones (>1,500 V DC) - but, like you I soon learned the desirability of behaving in a manner which avoided such events!

 

[JohnW2]

I support that idea, but RCB's/RCBO's can fail to operate, so call SB a first/second line of defence.

True, per what I've recently been saying. As I've also observed, if one has SB (which is exceedingly unlikley to 'fail') some may arue that, at least from the viewpoint of touch voltages in the room concerned, RCD protection is 'unnecessary' if there is SB!

It's interesting that people often raise the issue of RCDs 'failing' but we very rarely see/hear mention of the possibility of MCBs failing. Particularly when RCDs are present, MCBs are not crucial to'personal protection',but they remain important in relation to 'fire prevention' in the face of (L-N) over-currents that an RCD would obviously not detect.

 

[Harry Bloomfield]

True, per what I've recently been saying. As I've also observed, if one has SB (which is exceedingly unlikley to 'fail') some may arue that, at least from the viewpoint of touch voltages in the room concerned, RCD protection is 'unnecessary' if there is SB!

Unnecessary or not, I still think it advisable - Plumbers have been known to disconnect the SB, and/or insert sections of plastic pipe, without giving thr bonding a thought.

 

[JohnW2]

Unnecessary or not, I still think it advisable - Plumbers have been known to disconnect the SB, and/or insert sections of plastic pipe, without giving thr bonding a thought.

Yes, plumbers (and other people!) have been known to do those things and, certainly in the case of 'disconnecting' (and not reconnecting' the SB, that could obviously be a problem if there were no RCD protection.

'Inserting a bit of plastic pipe' is also a possibility that is usually mentioned, although I'm not sure if that would often (if ever) be a problem, since the effect, if any, on the issues we are discussing would usually be to render a touchable part 'floating', therefore not a 'touch hazard'.

As always, one does have to consider the question of much one should do in anticipation opf possible incorrect things that could be done in the future.

Ironically, I think it has been (and still is, in some cases) plumbers who have usually been responsible for the 'cross-bonding' of pipes, which proliferates in some installations, and which is very often unnecessary and unhelpful!

 

[securespark]

Not helped by manufacturers' instructions asking for "cross bonding" at the boiler.

 

[JohnW2]

Not helped by manufacturers' instructions asking for "cross bonding" at the boiler.

Quite so - maybe written by plumbers!

 

[EFLImpudence]

Perhaps this will persuade @equitum that SB makes a difference - and what a difference; it alters just about everything.

I think I have calculated it all correctly but if anyone sees an error please let me know.

 

[JohnW2]

Perhaps this will persuade @equitum that SB makes a difference - and what a difference; it alters just about everything.
I think I have calculated it all correctly but if anyone sees an error please let me know.

I essentially agree with your calculations, other than, trivially, I make the fault current with SB about 362 A (rather than your 360 A), which alters the answer fractiionally.

Of course, the touch voltage without SB would be appreciably lower higher than 115 V if you had chosen any cable other than 1mm² and lower if the Ze had been appreciably above 0.2Ω.

Your diagrams an calculations serves to confirm what I wrote at the very start, in post #5 (of 173 ), when I wrote:

As you say, an RCD, if present, will hopefully limit the duration of any electric shock. However, what I've written above reminds me that nothing other than local supplementary bonding can prevent a shock occurring (during the period until a device clears the fault) ....

The question, which we have debated, is whether or not, when an RCD is present (and working correctly),one needs to be concerned about a high touch voltage which persist for only a couple of dozen milliseconds (and/or whether one trusts an RCD to clear the fault that quickly, if at all).

Kind Regards, John
Edit: crucial typo corrected (thanks to EFLI for noticing!)

 

[EFLImpudence]

Of course, the touch voltage without SB would be appreciably lower than 115 V if you had chosen any cable other than 1mm²

Don't you mean higher?

and/or if the Ze had been appreciably above 0.2Ω.

Yes, there are infinite possible values of every part.

Your diagrams an calculations serves to confirm what I wrote at the very start, in post #5 (of 173 ), when I wrote:

The question, which we have debated, is whether or not, when an RCD is present (and working correctly),one needs to be concerned about a high touch voltage which persist for only a couple of dozen milliseconds (and/or whether one trusts an RCD to clear the fault that quickly, if at all).

Yes, I was going to write something along the lines of: given the choice of either SB or RCD I might (now) choose the SB.

 

[JohnW2]

Don't you mean higher?

I did, or should have! I will correct. Thanks for noticing!

Yes, there are infinite possible values of every part.

Indeed. Without SB, in a TN-S installation, the touch voltage could be a lot lower than 115V and, of course,would probably be very low in a TT installation.

Yes, I was going to write something along the lines of: given the choice of either SB or RCD I might (now) choose the SB.

That's certainly a valid viewpoint. As I've been repeatedly saying, SB is the only way of avoiding significant shocks (even very brief ones, when RCDs are present)). SB is therefore the 'surest' approach and doesn't rely on an RCD working as it should - but, as I said, it all depends upon how concerned one is about very brief shocks and/or the fallibility of RCDs.

An alternative, of course, would be to change to TT. With a Ze of, say, 50Ω, touch voltages would probably struggle to 'get into double figures', even without SB

Kind Regards, John