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Ok, I borrowed a megger from work and ran insulation resistance tests...
SFO to socket cable = 500v pass (>999MOhm or whatever it says)
CU to SFO cable = wouldn't ramp past about 2-3v, no resistance reading. Used the ohm-meter function (200ma), ~2.4-3k resistance between all 3 conductors. Resistance between 'earth' and a grounded water pipe ~4.4k.I assume that all tells the same story - fucked cable.
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OK. You need to do the following and tell me the readings. Turn off the CU at the main isolation switch first.
Disconnect the circuit live and neutral at the CU and test between them at 500V. Record the reading.
Then test between Live and Earth at the CU, don't disconnect Earth. Again at 500V, record the reading.
Same thing with neutral and earth.
Don't confuse IR tests with Continuity tests and don't think a water pipe is an actual ground.
Gas and water are bonded to the supply ground to become equipotential. Your 'Earth' is the one the supplier gives you.
Also, you do have new batteries in the tester?
More than likely. If you can hear it I'll bet it not surrounded by plaster!