Hi Rob
The Zs of any circuit is a combination of the external earth loop impedance and internal earth loop impedance, given by
Zs =Ze+(R1+R2). In a perfect world this calculated Zs should equate to the measured Zs. However in an existing installation there are parallel earth paths which reduce the measured impedance on a live test. These parallel earth paths are not included on a R1+R2 dead test and so result in a slightly higher impedance and therefore an increased overall Zs.
whichever method is used the important point is that the overall Zs is appropriate to the required disconnection times.
Hope this helps
The Zs of any circuit is a combination of the external earth loop impedance and internal earth loop impedance, given by
Zs =Ze+(R1+R2). In a perfect world this calculated Zs should equate to the measured Zs. However in an existing installation there are parallel earth paths which reduce the measured impedance on a live test. These parallel earth paths are not included on a R1+R2 dead test and so result in a slightly higher impedance and therefore an increased overall Zs.
whichever method is used the important point is that the overall Zs is appropriate to the required disconnection times.
Hope this helps
