Amd3 Transitional Period

[EFLImpudence]

How often is a conductor actually able to be fully loaded?

Is there any published data on how long a cable takes to reach 70° when fully loaded?

My point was not to do with the time taken but (in domestic) how many circuits are ever fully loaded.

At the extreme, having a Cmin for a domestic lighting circuit is hardly a consideration.

 

[JohnW2]

How often is a conductor actually able to be fully loaded?

Is there any published data on how long a cable takes to reach 70° when fully loaded?

Quite possibly, but certainly none that I have ever seen. More to the point, although we tend to assume it, I've not seen it actually stated (in relation to the BS7671 tabulation) that conductors necessarily will reach 70° if loaded to the CCC (for the installation method concerned) as indicated in the tables - there could well be 'margins' built in.

Furthermore, the CCC of a run of cable might be reduced because just a very small proportion of it was subject to a particular installation method. Even if the temperature of that small bit of conductor did reach, or approach, 70° at that ('de-rated') CCC, most of the conductor would not - thereby seriously complicating attempts to adjust R1+R2 (or R1+RN) (for the entire cable run/circuit) for conductor temperature.

Kind Regards, John

 

[JohnW2]

We're not talking about a situation in which "the voltage drops to a minimum" - but, rather, of an installation in which the supply voltage usually/always is close to the minimum permitted.

Why or what's the difference?

I think you may have to answer that one - it was you who wrote "...at the same time as the voltage drops to minimum".

but they have not singled out industrial installations as being the only ones that will be affected - it is you who (possibly correctly) have done that.

Well, in a way. It's just that long distances will have to account for it.

Maybe - but as eric is always pointing out, as cable lengths get longer then VD (if one worries about such things!) will become a limiting factor long before Zs does.

Kind Regards, John

 

[EFLImpudence]

I think you may have to answer that one - it was you who wrote "...at the same time as the voltage drops to minimum".

The voltage is allowed to vary; you seem to imply that the minimum will constant.

Maybe - but as eric is always pointing out, as cable lengths get longer then VD (if one worries about such things!) will become a limiting factor long before Zs does.

True - but not always otherwise wouldn't that mean the Cmin would never actually be reached?

 

[JohnW2]

At the extreme, having a Cmin for a domestic lighting circuit is hardly a consideration.

True, but that's primarily because such circuits are, in electrical terms, so over-engineered. Logically, we would probably be using something like 0.4mm² cable for a 6A circuit (VD aside!).

However, I think the principle remains. IF, as is the case, the regs specify a 'maximum Zs' (even if as high as 7.67&#937 required to achieve acceptable disconnection times with a B6, then that 'maximum' surely ought to result in acceptable disconnection times in all installations, not just those whose supply voltage was at or above 'nominal', shouldn't it?

Kind Regards, John

 

[JohnW2]

I think you may have to answer that one - it was you who wrote "...at the same time as the voltage drops to minimum".

The voltage is allowed to vary; you seem to imply that the minimum will constant.

Sure, it varies a little. My point was that I (and I suspect JPEL/IET) was not really thinking about wildly varying supply voltages, so much as one that was 'usually' fairly close to the permitted minimum.

Maybe - but as eric is always pointing out, as cable lengths get longer then VD (if one worries about such things!) will become a limiting factor long before Zs does.

True - but not always otherwise wouldn't that mean the Cmin would never actually be reached?

I think that we are probably largely into that "otherwise". Isn't eric's point that, if one strictly applies the 3%/5% VD guidelines, then one is unlikely to get to the current 'maximum Zs' (particularly if Ze is low), and that the same will undoubtedly remain true even with the slightly reduced maxima that will result from application of Cmin?

Kind Regards, John

 

[EFLImpudence]

At the extreme, having a Cmin for a domestic lighting circuit is hardly a consideration.

True, but that's primarily because such circuits are, in electrical terms, so over-engineered. Logically, we would probably be using something like 0.4mm² cable for a 6A circuit (VD aside!).

Precisely, but it isn't; that is what I'm saying.

However, I think the principle remains. IF, as is the case, the regs specify a 'maximum Zs' (even if as high as 7.67&#937 required to achieve acceptable disconnection times with a B6, then that 'maximum' surely ought to result in acceptable disconnection times in all installations, not just those whose supply voltage was at or above 'nominal', shouldn't it?

It should or may.

However, you seem to be missing my point.

We already limit Zs by derating it for temperature rise to 70° (multiply by 1.2). If the temperature is not going to reach this figure because of overengineering, spare capacity above OPD rating or safety tolerances then it probably would compensate for not applying a Cmin.

A blanket derating of x1.2 and Cmin is just a OSG-like instruction for not thinking.

 

[ban-all-sheds]

How often is a conductor actually able to be fully loaded?

Is there any published data on how long a cable takes to reach 70° when fully loaded?

My point was not to do with the time taken but (in domestic) how many circuits are ever fully loaded.

I know, but it's related, and could be relevant. If a cable is fully loaded, but takes an hour, say, to get to 70° then concerns about what R1+R2 maxes out at are different than if it will get to 70° in a minute.

 

[JohnW2]

True, but that's primarily because such circuits are, in electrical terms, so over-engineered. Logically, we would probably be using something like 0.4mm² cable for a 6A circuit (VD aside!).

Precisely, but it isn't; that is what I'm saying.

Quite. I think we are agreed that, because of that over-engineering, Zs and R1+R2 'limits' are irrelevant in such circuits (not worth measuring?!), other than perhaps in relation to VD - if that interests you.

However, I think the principle remains. IF, as is the case, the regs specify a 'maximum Zs' (even if as high as 7.67&#937 required to achieve acceptable disconnection times with a B6, then that 'maximum' surely ought to result in acceptable disconnection times in all installations, not just those whose supply voltage was at or above 'nominal', shouldn't it?

It should or may. ... However, you seem to be missing my point. ... We already limit Zs by derating it for temperature rise to 70° (multiply by 1.2). If the temperature is not going to reach this figure because of overengineering, spare capacity above OPD rating or safety tolerances then it probably would compensate for not applying a Cmin. ... A blanket derating of x1.2 and Cmin is just a OSG-like instruction for not thinking.

I'm not missing your point. However, I stick to my view that IF one is going to have 'maximum Zs' figures specified (no matter how irrelevant), then they should be figures which 'work' for any permitted supply voltage. I do not find it intellectually or logically satisfactory to have maxima which only work for supply voltages of nominal or higher and to justify it by saying that something else 'compensates for that' - and, in any event, it is not really true 'compensation' since it does not 'compensate' any more with low supply voltages than high ones. You appear to be saying that because we are being 'conservative' in other aspects of the calculations, it is permissible to have 'maximum Zs' figures which are not correct for all households - which seems wrong to me.

Kind Regards, John

 

[EFLImpudence]

I'm not missing your point. However, I stick to my view that IF one is going to have 'maximum Zs' figures specified (no matter how irrelevant), then they should be figures which 'work' for any permitted supply voltage. I do not find it intellectually or logically satisfactory to have maxima which only work for supply voltages of nominal or higher and to justify it by saying that something else 'compensates for that' - and, in any event, it is not really true 'compensation' since it does not 'compensate' any more with low supply voltages than high ones. You appear to be saying that because we are being 'conservative' in other aspects of the calculations, it is permissible to have 'maximum Zs' figures which are not correct for all households - which seems wrong to me.

Yes, that's all correct.
Perhaps 'compensate' was not the correct word - 'incude' ?

Therefore should the new Cmin regulation be accompanied by more realistic values of Ct rather than the blanket x or /1.2 ?
If not, then the foreseen occurrence of EICRs unnecessarily faulting installations will happen unless it is unrealistic because of volt-drop and will never be encountered.

 

[Risteard]

The BYB (aka Amd 3 of BS7671:2008) is published on Monday, and has an 'effective date' of 1st July.

It must have been published already as my copy was dispatched yesterday.

 

[Risteard]

That clearly means that, as of 1st July, all new installations and new work will be required to be compliant with Amd3

Only if designed after the cut-off point.

 

[JohnW2]

The BYB (aka Amd 3 of BS7671:2008) is published on Monday, and has an 'effective date' of 1st July.

It must have been published already as my copy was dispatched yesterday.

I understand that mine was also posted yesterday, although the official 'publication date' is Monday

Kind Regards, John

 

[JohnW2]

Yes, that's all correct. Perhaps 'compensate' was not the correct word - 'incude' ?

Whatever word one uses, I do not personally find it satisfactory to use one bit of 'conservatism' to 'make up' for the fact that some stated 'maxima' do not achieve what they're meant to for a proportion of installations! I therefore welcome the concept of "Cmin" (although I personally would probably have phrased it simply in terms of "Umin"

Therefore should the new Cmin regulation be accompanied by more realistic values of Ct rather than the blanket x or /1.2 ? ... If not, then the foreseen occurrence of EICRs unnecessarily faulting installations will happen unless it is unrealistic because of volt-drop and will never be encountered.

It doesn't really need to be 'accompanied by' that, does it, since there is not really a 'blanket "x or / 1.2", is there? - don't forget Appendix 14.

Kind Regards, John

 

[JohnW2]

That clearly means that, as of 1st July, all new installations and new work will be required to be compliant with Amd3

Only if designed after the cut-off point.

Probably. As I discussed with EFLI earlier, I don't know for certain, but I would imagine that it has such a provision - although, as I said, I personally think it's probably over-generous to allow that 'design to old regs' to take place after publication of the new ones!

Kind Regards, John