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Failure on IR

RustyH
Evening All,


I’ve been running over some pre-existing circuits, unfortunately for a mate so I’m not even getting paid!!, and all circuits have passed accept one that I just cannot figure out.

It’s a standard 2.5 ring circuit for the lower sockets (only feeding a lounge area), supply comes out the CU, up through the first floor rafters, and drop to the first and last sockets. All other joining wires between sockets run under the floor boards (House has concrete floor in kitchen and hall, but wooden floor in the lounge).

Running IR tests, no issues at 250v, but then at 500v it ramps from 280 m ohms till it reaches 500, then at 1000 same scenario, starts at 280 m ohms and ramps up but never really makes it to the full 1000 m ohms. It seems worse on the L-E, but does also fail on Lives.

I’ve tried to fault find this by removing the sockets, testing individual cables (which all seem to pass), and then shortening the ring one socket at a time by use of a plug in link and disconnecting the sockets beyond the link. The conclusion I believe I have come to is that the two feed lines from the CU (by linking the first and last socket and removing the rest of the circuit out of the ring) are passing IR without any ramp up. The minute I start adding in the rest of the circuit, it begins to fail, so I’m sure there are issues with all the circuit under the lounge floor.


I am assuming it’s all 2.5 T+E under there, there is a spur which doesn’t seem to link to a socket (feed a light switch to an outside light), so hazard a guess is linked to a junction box under the floor.

With it being a ramp up on IR values, could this be a moisture issue? Maybe a cable is laying on the floor under the house and drawing moisture?

Or could it being a rodent issue maybe?


Any experience on this one and advice would be appreciated


Thanks

Rusty

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  • 0
    This subject has come up before here.

    The general opinion is that there is a tiny amount of water somewhere in the circuit, zap it with 500 volts, and a tiny part of it evaporates, giving you a perfect >500M ohms reading. If you go back a hour later, you again get the slightly low reading, which gradually rises up to >500M again.

    Something like a newly plastered wall can give a small amount of condensation in a socket or back box.
Reply
  • 0
    This subject has come up before here.

    The general opinion is that there is a tiny amount of water somewhere in the circuit, zap it with 500 volts, and a tiny part of it evaporates, giving you a perfect >500M ohms reading. If you go back a hour later, you again get the slightly low reading, which gradually rises up to >500M again.

    Something like a newly plastered wall can give a small amount of condensation in a socket or back box.
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