Touch voltage

[JohnW2]

Yes, I think that is what I said. ... My point is that the max.Zs does not alter with temperature. It is the same whatever the temperature; it does vary with voltage. ... It is the measured reading that has to be modified to compensate for the conditions. ... 1.1Ω is not the max.Zs for B32A MCBS; it is 1.37Ω - at 218.5V.

All agreed.

However, as I've said, what it appears that 1.1Ω "is" is the maximum acceptable Zs measurement at ambient temperature on a circuit protected by a B32 which one thinks that sometimes might be loaded to the extent of having a conductor temp of 70°. Indeed, even if one doesn't think the conductor temp will ever be that high, it will obviously rise to above ambient temp in-service, so that it's unlikely that 1.37Ω at ambient will ever be theoretically 'low enough' (given that we now want ADS to work down to supply voltages of 218.5V).

... but, as I keep saying, I seriously doubt that (m)any electricians (or others) have ever thought of this! Are we perhaps both 'getting it wrong'?!

Kind Regards, John

 

[EFLImpudence]

However, as I've said, what it appears that 1.1Ω "is" is the maximum acceptable Zs measurement at ambient temperature on a circuit protected by a B32 which one thinks that sometimes might be loaded to the extent of having a conductor temp of 70°.

Agreed, but it is possible for that to be unnecessary in some circumstances, BUT the max.Zs for ADS is always 1.37Ω

Indeed, even if one doesn't think the conductor temp will ever be that high, it will obviously rise to above ambient temp in-service, so that it's unlikely that 1.37Ω at ambient will ever be theoretically 'low enough' (given that we now want ADS to work down to supply voltages of 218.5V).

Perhaps for typical 32A circuits, but for others it might be so.

... but, as I keep saying, I seriously doubt that (m)any electricians (or others) have ever thought of this! Are we perhaps both 'getting it wrong'?!

I don't think we are getting it wrong and I don't think it is so rare to think about it.
I have often seen mentioned the 80% - 0.80 or conversely 1.2 - mentioned for temperature correction. Indeed the volt drop values in Appendix four, which actually state they are for 70°, already include the correction.

I agree we cannot ignore the temperature correction for most circuits but it does cater for the very, very worst of all circumstances.

 

[JohnW2]

Agreed, but it is possible for that to be unnecessary in some circumstances, BUT the max.Zs for ADS is always 1.37Ω

I'm never attempted to disagree with that, and have explictly agreed more than once.

Perhaps for typical 32A circuits, but for others it might be so.

It obviously depends upon how 'close to the brink' one is, but regulations and practices should take into account things sometime are 'that close'.

As you often point out, there really is no sensible meaning of 'only just compliant' since, with a B32, a Zs of 1.3700000Ω simply IS compliant. Conversely, 'just not compliant', since 1.37000001Ω IS NOT compliant.

If one's Zs measurement at ambient temp is very close to (but under) 1.37Ω, then any load will result in at least some rise in conductor temp, and if that temp rise is only a degree or two, the Zs will then come to exceed 'the maximum', hence making the circuit non-compliant. It is for that reason that I'm suggesting that to think of a threshold of 1.37Ω (in this case) for a measurement at ambient temp is probably '#unsafe' for anything other than a lighting circuit (for which, at least these days, the cable is so over-specified)

I don't think we are getting it wrong and I don't think it is so rare to think about it.

Hmmm. Assuming we are right (and I'm pretty sure that we are) then anyone who did think about it would, presumably, never accept a (non-lighting) circuit whose Zs measured at ambient temp was anywhere near the (one and only) 'maximum', would they?

I have often seen mentioned the 80% - 0.80 or conversely 1.2 - mentioned for temperature correction. Indeed the volt drop values in Appendix four, which actually state they are for 70°, already include the correction.

Yes, those figure presumably do (as it says) relate to resistance/VD at 70°, so it would be fine to use those figures to calculate whether a circuit's Zs was below 'the maximum permitted' (and would remain so, even when conductor temp were 70°). As we've been discussing, the problem arises when one measures (at ambient temp) the Zs, rather than calculating it.

I agree that very few 32A circuits will get to anything like 70° in practice, but many might get to, say, 50°, so maybe people should regard a measurement of around 1.2Ω or 1.3Ω as the maximum acceptable Zs when measured at ambient temp? (but, again, I'd be prepared to bet that very few do!)

Kind Regards, John

 

[JohnW2]

Hmmmm! Part 2 will follow immediately after this ...
Edit: where did it go. I'll have to send it several small bits, it seems!

 

[JohnW2]

Part 1 ...

 

[JohnW2]

Part2:

 

[JohnW2]

Part3:

 

[EFLImpudence]

That sort of illustrates my point.

In fact neither is compliant. The figure is actually 1.365625.

1.37 is only good down to 219.2V.

 

[JohnW2]

As you often point out, there really is no sensible meaning of 'only just compliant' since, with a B32, a Zs of 1.3700000Ω simply IS compliant. Conversely, 'just not compliant', since 1.37000001Ω IS NOT compliant.

That sort of illustrates my point. In fact neither is compliant. The figure is actually 1.365625. 1.37 is only good down to 219.2V.

All true, but it's not those 'borderline' situations which concern me.

I'm sure that there are countless B32-protected circuits out they which have been declared to be 'fine' (either at installation or subsequent EICR etc.) because they have a Zs, measured at essentially ambient temperature, of, say, 1.2Ω, or even 1.3Ω - and it would not take an unthinkable amount of load on such circuits for their in-service Zs to rise to above 1.365625Ω (or whatever). If you agree with me, do you regard that situation as 'satisfactory'?

Of course, in practice the calculated 'max Zs' will often be pretty conservative, since I imagine that many Type B MCBs trip magnetically at currents appreciably less than 5 x In. However, given the spec of such devices 'as is', it would not be safe to 'assume' that the trip threshold would always be less than 5In.

Kind Regards, John

 

[JohnW2]

This is getting tiring!
Mods, can you not exercise some control over the behaviour of the forum software? Anyway ...

 

[EFLImpudence]

I think it is very rare that a circuit is anywhere near the limit so most people never have to worry about it.

 

[JohnW2]

I think it is very rare that a circuit is anywhere near the limit so most people never have to worry about it.

Is that really true, particularly with TN-S (and, even more so, TN-S in a large house)? If Ze/Re is something approaching 0.8Ω, that does leave much scope for R1+R2 before one gets appreciably over 1.1Ω total (at ambient temp).

I don't have immediate access to the figures but, even with my 'borrowed TN-C-S' (Ze ~0.3Ω), I'm pretty sure that I have a good few 32A circuits with Zs of >1.2Ω.

It's worth remembering that faults are probably most likely to occur when a load is present, so most faults probably arise when conductor temp is appreciably above ambient.

Kind Regards, John

 

[EFLImpudence]

I don't have much experience of houses as large as yours, but looking at it the other way round: with a Ze of 0.8 and a Zs requirement of 1.1, not much is going to be possible.

RCDs.

 

[JohnW2]

I don't have much experience of houses as large as yours, but looking at it the other way round: with a Ze of 0.8 and a Zs requirement of 1.1, not much is going to be possible. ... RCDs.

If I understand you correctly, are you agreeing with me that (at least with TN-S) many of the circuits deemed (at least by implication)(on EICs or EICRs) to be 'OK' (compliant), in terms of ADS via MCBs, will, in reality, probably not be compliant in that sense (and, as you say, would be relying on RCDs for ADS)?

As for house size, I think it's probably the 'medium-sized' ones, with relatively long final circuits, that will be the 'worst'. Like any 'large building', my (admittedly large) house has CUs scattered all over it, such that the final circuits are probably generally shorter than those in a medium-sized house, and distribution to those scattered CUs is via 16mm² or 25mm² cables, hence not adding much to the Zs figures.

Kind Regards, John

 

[EFLImpudence]

If I understand you correctly, are you agreeing with me that (at least with TN-S) many of the circuits deemed (at least by implication)(on EICs or EICRs) to be 'OK' (compliant), in terms of ADS via MCBs, will, in reality, probably not be compliant in that sense (and, as you say, would be relying on RCDs for ADS)?

Only IF, as your example, Ze is 0.8Ω. Most these days have to be considered as TN-C-S despite appearing TN-S, so who knows?
I obviously have to agree that if only 0.3Ω is left for the circuits then they might not be compliant with the numbers; 45A shower circuit max.Zs- 0.78Ω; that does not mean they won't cause ADS satisfactorily in the actual circumstances. 240V, conductors not at 70°, MCB at 4 In. etc.
That must have been the situation before it was deemed necessary to make the requirements more stringent.
You seem to think the electricians do not know what they are doing. You might be right so where does that leave us?

As for house size, I think it's probably the 'medium-sized' ones, with relatively long final circuits, that will be the 'worst'. Like any 'large building', my (admittedly large) house has CUs scattered all over it, such that the final circuits are probably generally shorter than those in a medium-sized house, and distribution to those scattered CUs is via 16mm² or 25mm² cables, hence not adding much to the Zs figures.

Ok.