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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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    Alasdair Anderson:

    Sounds to me as if there could be capacitance in the circuit. The reading rises as the capacitance charges up until it reaches a stable reading when the capacitance is fully charged. This is most often seen on very long cable runs due to the cable capacitance.

    If you have a long circuit that is ramping up during the test and you then split the circuit into two or more sections to retest individually they may not ramp up, because you no longer have a long circuit!


    Put the tester away and do some reading up on the subject and watch a few videos online, but be selective about whose videos you watch on YouTube.


    Ideally you need to actually get some tutoring getting hands on experience.
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    @Sparkingchip‍ 


    The test rig on the video you posted has one of my personal irritations ? (I know we all have them) ...


    Helix coils on the bonding cables. There are two reasons why this is poor practice:
    1. Bonding conductors should be as short as possible.

    • Increases inductance, so bonding is ineffective for EMC.


    At present, I don't think anyone would worry in many domestic / small commercial installs ... but it's potentially a big issue in industrial controls, transport infrastructure, some highways installs, etc.


    In the future, as we have more "smart home" devices etc., such practice will start to become problematic in domestic / small commercial installs.



    Anyway, it's not an immediate safety issue, just getting it out there.
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    Lol. I was looking for a YouTube video by someone who does not twaddle on.


    Geoff Blackwell used to have a mutter on this forum about electricians making coiled springs with bonding conductors next to the pipe clamps and earth terminals, the reasoning for doing so appeared to be that it left a bit of slack in case the pipework was altered and the clamp needed to be reinstalled, but actually it is bad workmanship (obviously the ladies know better, so we would not have bad workladyship,).


     Andy Betteridge.


    edited!
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    If you pay attention whilst watching that YouTube video, you will see the Megger tester he is using DOES NOT EVEN HAVE A 1000V INSULATION TEST available.


    Stop misusing the 1000 volt test on your meter!


    Andy Betteridge.
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    Note the the absolute lack of concern about the insulation test result and the ramping up.
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    Before rushing off in panic about curly wurly wire ends, some thoughts.

    Some of us who work with more 'bitey' electronics often wish to only have mains CPC for 50Hz ADS purposes, and prefer  RF and fast transients not to be injected into the mains wiring, but to be persuaded to take an alternate route. In that case we may very deliberately put a micro henry or two in series with the mains earth, and we may even tune it to resonance with a small capacitor to make an RF trap.

    If you are working on a facility with a MW or shortwave transmitter, or a shielded room, or any kind of HV accelerator,  and find a coil in an earth lead, please do not try and tidy it up by making the coil straight again !

    As shown in that video the effects are not especially significant,  be honest if the EMC perforamance in a domestic system is upended by 6 turns around a 3/4" former ( i.e. about 500nanoHenries, or a similar inductance to 2 feet of 15mm dia. straight copper rod) it probably needs a review anyway.

    The only place I'd really worry if I saw it would be a lightning protection system  (they tend to straighten out, rather as if the electrons want to keep going straight) or in the cables to any kind of SPD or RFI filter.
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    Behave yourself Mike.


    We are trying to get to grips with the basics of interpreting insulation test results and the tester ramping up, not the finer points of associated electronics.


    My first insulation tester, a Robin multifunction tester that I still have, only goes up to 200 megohms, the test results that Rusty recorded of 280 Mohms and rising would really would be off the scale with that tester, however I also have a Megger tester that measures in Gigaohms which is completely over the top for domestic work and offers more ways of testing than I will ever use.


    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.


    My Portable Appliance Testers go up to 20 Mohm.


     Andy Betteridge 


  • Former Community Member
    0 Former Community Member
    Thanks for the answers, guys
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    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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