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Further investigation IR below 2 Megohms or 20 Megohms??

RobW

Has the guidance on minimum insulation resistance changed in the new OSG?? I subscribe to the IET electronic versions, the current online IET OSG says that further investigation is required below 20Mohms rather than the previous versions advising further investigation below 2 Megohms? Is this just an electronic typo or has thid guidance changed.

  • 0 in reply to lyledunn

    A three phase existing installation with several 16 way TPN boards - Id never expect a global IR test to pass......possibly with a LOT of lengths of run involved - even if you did each individual DB as a whole global IR test. Hundreds of items of connected (electronic) equipment.....DC rectifiers etc.....

    Would it not be reasonable in this type of situation to use a milliamp earth leakage clamp meter on the supply to each DB to get a better determination of the possible state of the cabling IR? I ask because I do do this, and I'd like to check with others on here if this is generally accepted as reasonable? 

    Shutting stuff down in a large GP complex must have been difficult/complex to determine the effects of. Not just of the electrical but the impact to the business too.. 

  • 0 in reply to gkenyon

    Mind you what happens if the chaps had used a 2.5 inch screw... everyone has then followed 'best practice'  Who gets prosecuted for what exactly?

    It may be better to recommend that folk consider cables with foil or braid  in metal lined voids and partitions etc, where the use of SWA is unreasonable but something to operate ADS of some sort (be that RCD or a fuse) is desired. I think with T/ E an 'instruments only' 'test' can still miss  screw damage and that damage may of course occur sometime after testing anyway. I also cannot see a conventional cable finder working well to spot things within a metal partition.

    In many ways surface wiring or other methods (trunk or whatever) where you can see it is safer if not very aesthetically satisfying.

    M.

  • 0 in reply to broadgage

    I'd generally moderate that with a bound about the size of installation (well total cable lengths) and cable type.  It is not sensible with MICC if you have  hundreds of metres of it. I know it is not often used, but it still does appear, and is mandated in some types of listed building. But yes, a new run of up to half a km of PVC, 20megs is entirely credible., and will probably spot that back box that got squeezed full of plaster...

    Mike

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