you measure the Zs or Ze with an earth loop impedance meter.. with the power on
you calculate it by obtaining the Ze value from the supplier ( they should have it on record ) and using a low ohms meter to measure the R1+R2 value, which you then add to the Ze to get the Zs..
you calculate it by obtaining the Ze value from the supplier ( they should have it on record ) and using a low ohms meter to measure the R1+R2 value, which you then add to the Ze to get the Zs..