Problem with IR test result on RFC's

[bobodhi]

Hi, I've been testing my house and few others in the area as a way of gaining experience.
I'm mostly getting satisfactory results expect for insulation resistance testing on the ring circuits. I follow procedure and remove all plugs from sockets and turn them on but when I do the test I get a zero reading both on live/neutral and live/earth.
This problem is appearing in several houses not just mine so I'm obviously missing something, probably 'bleedin obvious', and I'll kick myself.
Any ideas?
Regards, Kevin

[fernway]

Are you removing all fuses and switching off Circuit Breakers, or just isolating at main switch?

[bobodhi]

The method I'm using is to turn off the supply, remove the wires from their positions on the board, remove all the plugs from sockets, turn all socket switches on, then test between live/neutral and live/earth ends of the ring using Megger 1552 MFT @ 500V. What fuses are you referring to, fused spurs perhaps? What if there are fused spurs that have a permanent connection at the end, could that be a problem, also the premises have electrically supplied heating controls on the same RFC's, another possibility? If so, what's the remedy?
Cheers, Kevin

[fernway]

The fuses I was refering to were those in the Consumer Unit. It sounds like you've done everything to isolate the rings from the rest of the installation. The comments you make about Heating controls etc. could be the cause. You should also bear in mind that if doing a periodic Inspection it is acceptable to link line and neutral and test both simiutaneously to earth with all wires in position and Circuit Breakers/Fuses in place and just the main switch locked off. The items you mentioned above such as Heating controls may well be the cause of zero readings across line and neutral, but to earth is a bit more concerning. Might I suggest first doing a low voltage continuity test on both live to neutral then live to earth and finally neutral to earth. If you get unexpectedly low readings the comments you make above are probably correct. You should then try a 250volt test from line/Neutral to earth as this is acceptable where electronic devices, such as surge protectors etc, may give anomalous readings or be damaged by the 500v test.
The only other thing that springs to mind is a borrowed neutral giving a path back to earth through one of the other circuits, but as it seems to be on several properties, this seems less likely.