On attempting to measure the earth electrode resistance on a single-phase TT installation using an AVO earth loop impedance tester, the instrument indicated over limit voltage of 290v and would not permit the test to proceed. The P-N voltage was 235v. Unfortunately the operative did not measure neutral to earth voltage. A contractor on site at the same time as our operative confirmed the 290v using a different voltage indicator. I was intrigued and some days later I visited the site. Voltages seem to have returned to normal, 233v between live conductors and between phase and earth and 0 v between neutral and earth. The supply to the installation is fed via a local three phase power company tx.
Trumpy, Sorry Trumpy, misinformation! The instrument was actually a Robin loop tester. It appears that the indicated voltage can be out by 10% although this wouldnt have necessarily been the problem. The installation was not connected to the supply at the time. The NIE engineer suggested capacitance which disappeared when the installation was connected. Not sure about that one! Many thanks for your replies guys!