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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.

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    Mike said “In a new and as yet un-energised circuit it does make more sense to test L-N and L_L as well this avoids the embarrassment of the main fuse failing on first power up, but at that point there should be no connected loads as it is under your control.”.

    Been there, done that, a completely new installation failing the insulation test, because whilst the job was awaiting completion a TV aerial installer connected a TV aerial amplifier into the back of the front bedroom light ceiling rose, I found the same issue on a few installations as well when altering lighting circuits and preparing EICRs, it doesn’t blow the main fuse, but it does leave you standing in a completely empty house or flat trying to work out what could be the issue.

    The L-L and L-N at 500 vDC tests are the first set of tests, the second test is the global 500 vDC of the entire installation to check the plasterers or other trades have not damaged cables, and the third test is the 250 vDC global test of the entire installation after completion to ensure you have not pinched any conductors putting covers back on or the like your self. 

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    Mike said “In a new and as yet un-energised circuit it does make more sense to test L-N and L_L as well this avoids the embarrassment of the main fuse failing on first power up, but at that point there should be no connected loads as it is under your control.”.

    Been there, done that, a completely new installation failing the insulation test, because whilst the job was awaiting completion a TV aerial installer connected a TV aerial amplifier into the back of the front bedroom light ceiling rose, I found the same issue on a few installations as well when altering lighting circuits and preparing EICRs, it doesn’t blow the main fuse, but it does leave you standing in a completely empty house or flat trying to work out what could be the issue.

    The L-L and L-N at 500 vDC tests are the first set of tests, the second test is the global 500 vDC of the entire installation to check the plasterers or other trades have not damaged cables, and the third test is the 250 vDC global test of the entire installation after completion to ensure you have not pinched any conductors putting covers back on or the like your self. 

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