and only have the 18th edition blue book at minute
It's possible but rather hard work from the blue book (see page 409)
Volt drop per meter is given in miliVolt / Amp / metre
Volts = Current X Resistance.
Resistance = Volts / Current.
So that volt drop number is
also resistance / meter. But it's for Line and Neutral and you want Line and Earth. It's also not temperature corrects.
So for 2.5mm twin and earth....
Take the 2.5 number (18), divide by 2 as you only want one core's worth of resistance.
Then take the 1.5 number (29) and also divide by 2.
Add that all together. Then divide by 1.2 (temperature correction). That then gives you the number in the OSG
The MUCH easier way is to invest in buying the ProCerts app. Seriously recommended to the degree that I barely use the OSG.
im used to doing both dead and live and been able to see if R1 + R2 is right by the live the testing side of things.
It's early and I'm not caffeinated but that is not making too much sense. The common factor to both tests is the resistance of the cable, with or without faults. Maybe I'm missing you're point.