help please 2393/2395

[JohnW2]

[recovery and rehydration break between bouts of extensive mowing ]

You know my views on the OSG.

Indeed I do, which made me mildly surprised that you were taking the 'OSG approach' However ....

I would think that reducing the maximum Zs value is easier than correcting all the measurements taken, bearing in mind that the meter can only measure what is there.

Very good point. If one is talking about measurements (taken at ambient temperature), rather than calculations, then I have to agree/concede that the 'OSG approach' does make more sense, for the reason you give.

However, in practice it is not so often necessary. ... When calculating or measuring the Zs it is far below the BGB given maximum maximum, for example, my own guide is that if it is less than a quarter that figure then I do not need to do any adjusting as all will be well regardless.

Fair enough, but I would think you could actually go much further than that - even the difference between 10° and 70° is only a factor or 1.24 (or 0.81 if you want it 'the other way up') and the effect on total Zs will be less than that (because of Ze), so you could probably be confident that any measured Zs which was less than about three-quarters of the BGB figure would be OK.

I presume that the maximum maximums in the BGB are so that the installer can calculate whatever allowance for temperature is seen fit. ...None at all for a 6A lighting circuit, for example ???

Perhaps, although the OSG's (very 'conservative') view is clearly that one should work on the basis of what the R1+R2 would be at 70°, and the BGB itself is pretty silent about what assumptions one is meant/allowed to make about temperature.

Kind Regards, John

 

[Spark123]

The big green book covers a whole range of different temperature rated cables which is probably why they don't stick to 70°C.
It is up to you to design the circuit taking into account the maximum temperature rating of the cable.
I'm not even certain that BS7671 quotes resistance figures.

 

[JohnW2]

The big green book covers a whole range of different temperature rated cables which is probably why they don't stick to 70°C. It is up to you to design the circuit taking into account the maximum temperature rating of the cable.

That's exactly the point I've been making. If one is designing (i.e. calculating), rather than measuring, then the requirement appears to be to satisfy the 'maximum Zs' requirements (per BS7671 tables) at the rated ('operating') temperature of the cable (i.e. at a resistance per VD tables in App 4 of BS7671), so it makes sense to use the actual ('unadjusted') BS7671 'max Zs' figures and the actual 'at operating temp' resistances from BS7671's VD tbulations - rather than using the OSG's resistivities at 20° and hence have to use the OSG's 'adjusted' max Zs figures (in order ti get the max Zs at 70°, which the OSG seems to be assuming {not necessarily correctly} will always be the 'operating temp'/rating). [As I conceded in my reply to EFLI, when one is talking about actual measurements of R1+R2, at ambient temp, (rather than design/calculation), the OSG approach is probably simpler]

I'm not even certain that BS7671 quotes resistance figures.

I don't think it does, in quite those words. However, anyone who doesn't understand that the "mV/A/m" figures (always quoted at 'operating temp' for the cable type concerned) they give in their VD tables in App 4 is the same as mΩ/m (and/or doesn't understand that they have to halve those tabulated figures to get the resistance of a single conductor) probably should not be reading the BGB until they've first read a few other books

Kind Regards, John

 

[Spark123]

The volt drop figures are for live conductors, as you know the live conductors in most of twin and earths cables are a different size to the CPCs. So unless you want to correct for temperature and halve the mV/A/m figures for both 1.5mm and 2.5mm as in the OP I'd avoid using the figures from BS7671.
As opposed to the on site guide I'd recommend the OP gets a copy of Guidance Note 3.

 

[Risteard]

Instead of multiplying the permitted Zs by 0.8 you can multiply the (R1+R2) figure by 1.2 to adjust for 70 degrees and then add this to the Ze.

 

[JohnW2]

The volt drop figures are for live conductors, as you know the live conductors in most of twin and earths cables are a different size to the CPCs. So unless you want to correct for temperature and halve the mV/A/m figures for both 1.5mm and 2.5mm as in the OP I'd avoid using the figures from BS7671.

Fair enough, if you want to avoid some very simple mental arithmetic. That's what I did for 2.5/1.5 T&E ... [(18/2) + (29/2)] is hardly mathematically taxing, and takes only a second or two to work out in one's head.

As opposed to the on site guide I'd recommend the OP gets a copy of Guidance Note 3.

Agreed - but, IIRC, it takes essentially the same approach to these things as does the OSG.

Kind Regards, John

 

[JohnW2]

Instead of multiplying the permitted Zs by 0.8 you can multiply the (R1+R2) figure by 1.2 to adjust for 70 degrees and then add this to the Ze.

You can. The OSG approach (which assumes that Zs is measured at 10 °C) uses a factor of 1.24 (equivalent to multiplying permitted 0.806).

As I've said, if one is designing [i.e. calculating (R1+R2), rather than measuring it], it seems to get a bit more messy with the OSG, since they give resistance figures at 20°C but give give figures for 'maximum Zs, corrected to 70 °C' on the assumption that one is using 10 °C resistances. Hence, if one uses the OSG resistance figures, if one wanted accurate answers, one would therefore have to apply a further correction factor to their 'maximum Zs' figures, to account for that 10/20 °C difference.

Kind Regards, John

 

[robertseymour]

Thanks for your help lads ,after hours of reading books I'm just about there cheers