bah, beaten by adam
Ze is the external loop impedance. It can be determined by measurement, calculation or by enquiry. R1+R2 is the resistance of the phase and earth wire at the furthermost part of a circuit. This can also be determined by measurement or by calculation, see table 9A in the OSG. The only thing to be careful of is BS7671 max efli tend to be at the max design temp of 70ºC and the figures in the OSG are the measured figures @ 20ºC.
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