should it not be only applied to the R1+R2 ,as the ZE part may be at a different temperature

It did used to be written that way in BS 7671, but it seems to have disappeared. I guess there's an argument that while the supplier's conductors (the Ze part) may well not be entirely cold, it may not be exactly at full load either - say you did a Ze test during the day in early summer where there's little heating or air conditioning load, the DNO's conductors might well be cooler than say late afternoon in in mid winter, when it's dark and a lot of heating is in use. So some adjustment for the supplier's conductors may be justified - although perhaps not quite as much as 20% - but as we don't really know what, 20% is probably a good rule of thumb and errs on the side of safety.

   - Andy.

should it not be only applied to the R1+R2 ,as the ZE part may be at a different temperature

It did used to be written that way in BS 7671, but it seems to have disappeared. I guess there's an argument that while the supplier's conductors (the Ze part) may well not be entirely cold, it may not be exactly at full load either - say you did a Ze test during the day in early summer where there's little heating or air conditioning load, the DNO's conductors might well be cooler than say late afternoon in in mid winter, when it's dark and a lot of heating is in use. So some adjustment for the supplier's conductors may be justified - although perhaps not quite as much as 20% - but as we don't really know what, 20% is probably a good rule of thumb and errs on the side of safety.

   - Andy.

The biggest elephant after all that is that the day after tests, the DNO may replace the street transformer or main feeders and you would be none the wiser.
Some countries that have rather wilder electrical installations than the UK require an earth fault trip/ giant RCD at the incomer for just that sort of reason, and in many cases over and under-voltage lock off as well.

Mike.

mapj1 said:

The biggest elephant after all that is that the day after tests, the DNO may replace the street transformer or main feeders and you would be none the wiser.

That happened in our street a few weeks ago, but one could hardly fail to notice. My first thought was whether they might have dropped the voltage, but 241 V in my office just now, so apparently not.

The new transformer's labels mention its neighbours, so they must be connected in some sort of ring or grid, in which case if you changed the voltage at one, you would have to change the lot.