250V / 500V IR differences

[JohnW2]

My plan was to use 250V, or actually 350 since that's about the peak voltage for 240V AC. Working on the basis that if anything can't tolerate that, it shouldn't be connected to the mains anyway. And as I think we've agreed, it's going to be a very special case for the insulation to withstand that but not 500V.

Oh, I see. In the case I described at the start of this thread, there was, of course, no problem at 250V. Although I didn't try it, I imagine that would have remained the case had I reversed the polarity of the test voltage. As you imply, the same should also be the case with any test voltage no greater than the peak of the supply voltage - although you'd probably have to improvise to do IR tests at ~350V, since I don't know that you'd find meters designed to do that.

As you say, I think that we are generally agreed that it would be very unlikely that there would be a true IR problem (e.g. an insulation problem) which showed itself at 500V but not at 250V (or ~350V), so if you were undertaking the tests just for your own satisfaction, testing at the lower voltages would probably be OK. However, if you wanted/need to test per BS7671, you would presumably be stuck with 500V in most cases.

Kind Regards, John

 

[Spark123]

If there is an arc between two conductors it may show itself at 500v but not at 250v.

 

[JohnW2]

If there is an arc between two conductors it may show itself at 500v but not at 250v.

I suppose in theory, but I find it fairly hard to imagine how, in practice, an arc (obviously not a 'drawn arc') would get initiated during IR testing. If it ever did happen, the very low resistance of the arc would presumably cause the meter to drop its test voltage to close to zero, and any indicated IR (it might 'refuse'!) would be extremely low.

Kind Regards, John