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Minimum IR values

OCB99

Hi everyone, 

First time poster so try and take it easy on me Grin

been looking at threads on this forum for quite a while now and it’s been a great help during my training so thanks everyone.

Just wanted to know your opinions on IR testing and how to apply the minimum value stated in BS7671. The regs state that the values in table 64 applies to a distribution circuit with all final circuits connected. Would that mean if on an EICR for example there were 10 circuits each measuring 9Mohms between two conductors (Very unlikely I know) it would be a fail as the total resistance for 10 circuit would be theoretically 0.9Mohm?

Also would this apply to simple installations where the consumer unit is directly connected to the origin, essentially classing the DNO cable as a ‘distribution circuit’.

Thanks in advance.

Parents

  • the GN3 recommendation of 20Mohm.

    I'm afraid I don't have a copy of the latest GN3 - is that 20MΩ recommended for each final circuit, or just as a 'replacement' for 1MΩ below which you really should be investigating even though it nominally complies? (i.e. acknowledging that 1MΩ is a pretty weak test for modern plastic wiring systems). If the latter then presumably it should be applies in the same context as 1MΩ - i.e. distribution circuit + DB + outgoing final circuits.

       - Andy.

Reply

  • the GN3 recommendation of 20Mohm.

    I'm afraid I don't have a copy of the latest GN3 - is that 20MΩ recommended for each final circuit, or just as a 'replacement' for 1MΩ below which you really should be investigating even though it nominally complies? (i.e. acknowledging that 1MΩ is a pretty weak test for modern plastic wiring systems). If the latter then presumably it should be applies in the same context as 1MΩ - i.e. distribution circuit + DB + outgoing final circuits.

       - Andy.

Children
  • in reply to AJJewsbury

    Yes 20Mohms global. It frustrates me sometimes that we have a national standard that is almost usurped by guidance from the very people that feed into it. 

  • in reply to lyledunn
    lyledunn said:
    Yes 20Mohms global

    Except for complex installations ... but I agree there is no definition of complex.

    At the end of the day, the standard (BS 7671) has a different precedence in a court of law to the guidance.

    The issue at hand, is that the industry wants a "rule of thumb" to apply across the board, and "hang their hat on", to remove the need for anyone to think about what they are seeing in a test result out there in the real world, but with the wide variety of installations served by the standard and the guidance, sadly the mutually exclusive nature of the "desire" vs reality shows through.

    The wording in GN 3 can be read two ways, and one way is that subdivided (to a suitable level) each division having insulation resistance exceeding 20 MΩ is OK is one of those ways.

    My view, is that, to understand the arbitrary limits of insulation resistance testing isn't something limited to a classroom, or book, using succinct words, but best learned by getting out there and doing the work in a variety of real installations.

    When you consider that the limit in BS 7671 (at various times 1 MΩ and other close values) is only an arbitrary "industry rule of thumb" and doesn't relate directly to BS IEC 60479-1 Effect of current on human beings and livestock, my explanation above will hopefully seem clear enough

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