GN3 states that direct measurement of Zs can only be made on a live installation, neither the connection with earth nor bonding conductors are disconnected. Readings given by the loop impedance tester may be less than Ze + (R1+R2) because of parallel earth return paths provided by any bonded extraneous-conductive-parts.

In my opinion you should disconnect the main equipotential bonding conductors from the incoming gas and water service pipes if they are metallic in order to obtain a realistic value for Zs. (bear in mind that no other bonding conductors should be removed)

If the incoming gas and water service pipes are metallic and the test is carried out with the main equipotential bonding conductors connected, the measurement of Zs becomes worthless and the risk of shock induced into another electrical system in close proximity is un-quantifiable, In my opinion unsafe, uncontrolled and completely out of the hands of the inspector and defiantly in breach of H&S.

2008-03-07 Testing Electrical Installations.