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Regulation 643.3.3 (ii) clarification

DR Darren Hughes

I want to ask if someone can help clarify the requirements on the regulation 643.3.3, notably the second para  (or ii in GN3)

Whilst it is understood that an initial insulation resistance test should be carried out with all Sensitive loads disconnected, once they are reconnected the regulation reads "Following connection of the equipment, a test at 250 V DC shall be applied between live conductors and the protective conductor connected to the earthing arrangement."

Does this mean we are conducting two separate tests, i.e. line to CPC and Neutral to CPC? Or a single test where we link line and neutral, then test that to CPC (i.e. like previous guidance). I suspect it is the letter, but would be grateful for your thoughts.

  • 0 in reply to Sparkingchip
    Sparkingchip said:
    The mininium test result value of one megohm is for the entire installation and the entire installation should be tested all at the same time to earth.

    I think that we are all agreed concerning the difference between the procedures for EIC and EICR.

    643.3.2 The insulation resistance ... each distribution circuit tested separately ..., so not necessarily the entire installation. That said, in a typical dwelling, there is no distribution circuit.

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    The one megohm minimum value is longstanding, it is discussed in my copy of The Pocket Manual of “Megger” Practice published in America by the James G. Biddle Company in 1935, though during the last ninety years we went through a period when two Megohms was recommended.

    A little Friday evening light reading:

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    However, my copy of of the Megger “A pocket book on insulation testing” published June 1946 gives a different method of determining the insulation test value requirements for house installations to meet the IET Wiring Regulations requirements.

    The current expectation given in the IET GN3 is that a new installation will have a test result value of not less than 20 megohms, bearing in mind we have better insulation on conductors and in equipment, plus modern testers can measure higher values, my Megger MIT420/2 provides measurements up to 200GΩ where as those old hand cranked Meggers  measured to 10000 Megohms, with higher values being described as “Infinity”.

    We were told when training we could not record infinity as a test result, we had to record the highest stated value on the meter scale, because we had to record a definite value and infinity is an unknown value.

    Time to hang my anorak up and have a cup of tea!

  • 0 in reply to Sparkingchip

    Thank you for those interesting documents.

    I suspect that 1MΩ was the limit of reliable measurement with your Megger in those days. My Avometer 8 runs out of scale at 2 MΩ, though granted not at 250+ V.

    Sparkingchip said:
    those old hand cranked Meggers  measured to 10000 Megohms, with higher values being described as “Infinity”.

    Or nowadays ">999 MΩ".

    Let's not forget the movement of the needle, or digital equivalent. If it whizzes up to the top of the scale, happy days! If it stays rooted at the bottom, go and find the screw or nail with which some wally has impaled a cable.

    If it steadily goes up the scale, but runs out of steam well above 1 MΩ, you have just charged up a capacitor.

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