Class one luminaires

One MegOhm. 643.3.3 at 250 Volt D.C. testing Voltage.

Z.

It's equipment subject to other standards so not a BS 7671 issue - some (with electronic ballasts for instance) might give quite low readings or not be suitable for being exposed to an IR test at all. So check with the relevant product standard or the manufacturer.

   - Andy.

I think that a new fitting should be infinity Lyle, whatever that is on your meter. There is no reason at all why this should not be the case, and although the usual megohm would not be dangerous, why would it be that low. Also with several of these fittings the circuit would also immediately be unsatisfactory.

Regards David

Andy, take care you are not quite right. The IR to Earth, which is the one Lyle is discussing has no reason at all not to be infinite, whether electronic or not. Remember that if you test an installation with these lights installed the reading is reflected in BS7671, it would be mad to disconnect fixed lighting, and they still need the Ze measurement anyway to check the CPC is connected to the exposed conductive parts. Disconnection is potentially a dangerous procedure, and dismantling similarly like to cause as many problems as it cures. A L-N insulation test is becoming a waste of time, and it is very rare it tells one much nowadays as insulation is only in 2 states, good or shorted, PVC cables do not give poor insulation readings, and even a 500V test will only detect spacings of perhaps 20 microns, so throw out the old textbooks with stuff about sparks, it was never true. This is the realm of the HIPOT test for appliances only, particularly Class 2. The IR test is good at finding water ingress, but inside this is extremely unlikely and shows between lives and Earth anyway. Actually I am extremely dubious as to whether a 500V test will damage anything that is already not faulty, after all the peak normal L-N voltage is about 360V plus any spikes etc, so 500V is actually common, although not for much time. The days of using 400V Triacs (that really would fail at 401V) are long gone and any decent kit has surge protection at somewhat over 500V anyway. IGBTs for mains are always rated at 600V or above simply because this is now easy to do and costs nothing. If you read some of the data sheets you will see that this is true.

Exactly David! The requirement to carry out an insulation resistance test prior to the connection of equipment will require careful planning on site. A further test between live conductors and earth is required following the connection of the equipment. This test, whilst perhaps conducted at 250v still requires a minimum of 1Mohm. As I understand it, the standard for luminaires allows a fitting to be placed on the market at a minimum of 1Mohm so with two such luminaires connected we are beat! 
I doubt it will matter much as the IR column is magically populated with +999Mohms no matter what!