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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
    Sparkingchip:


    For testing a domestic socket circuit they are both more than adequate, as the minimum requirement according to the wiring regs is 1 Mohm and that is not set in stone, the HSE recommend 2 Mohm, but it may be considered that less than 1 Mohm is acceptable depending on the circumstances.


     Andy Betteridge 


     


    The minimum requirement in 7671 is set for new installations and is required to be 1Mohm minimum when the test is applied to the distribution circuit with all the final circuits on but connected equipment removed. Thus two final circuits at 1Mohm would mean failure. For periodic inspections, there used to be a recommendation in GN3 that a final circuit should not fall below 2Mohms. That was removed for further editions but I note that in the most recent version there    is reference to using the 1Mohm merely as a reference for taking a view. 

    mike would maybe know how these IR testers work but I think that the applied voltage is increased incrementally. 

    edit to ask Andy for the HSE reference for the 2Mohms.  


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  • 0
    Sparkingchip:


    For testing a domestic socket circuit they are both more than adequate, as the minimum requirement according to the wiring regs is 1 Mohm and that is not set in stone, the HSE recommend 2 Mohm, but it may be considered that less than 1 Mohm is acceptable depending on the circumstances.


     Andy Betteridge 


     


    The minimum requirement in 7671 is set for new installations and is required to be 1Mohm minimum when the test is applied to the distribution circuit with all the final circuits on but connected equipment removed. Thus two final circuits at 1Mohm would mean failure. For periodic inspections, there used to be a recommendation in GN3 that a final circuit should not fall below 2Mohms. That was removed for further editions but I note that in the most recent version there    is reference to using the 1Mohm merely as a reference for taking a view. 

    mike would maybe know how these IR testers work but I think that the applied voltage is increased incrementally. 

    edit to ask Andy for the HSE reference for the 2Mohms.  


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