James, your right, but in measuring ze you are trying to eatablish that there is a satisfactory external loop impedance. Dont forget that any zs measurment is likely to take the shortest possible path, exactly the same as an earth fault will, any fault will not travel the length of the cpc back to the dis board. If the building has a steel frame with metal conduit strapped to it it becomes a parallel path and will affect your readings, as will any water heaters or central heating connected. The only true way is to measure Ze with everything disconnected and then add R1+R2 to give you Zs. invariably the measured Zs will always be lower due to these parallel paths
