Testing at 250v

keh7486 · Feb 6, 2013

Hi just a quick question. I've seen a few times people say test insulation resistance 250v first to make sure there isn't any points missed in loft or where ever. This seem to me a sensible thing to do. Is it 100% safe to test all ways l-n l-e n-e. I thought it was as its not far of the running voltage but someone at work today disagreed.
Thanks

Octopus · Feb 6, 2013

How anybody could disagree with the very sensible first step of IR testing is beyond me.

somersetsparks · Feb 6, 2013

You could still zap some very sensitive electronic equipment with 250V DC from a megger, you'd be pretty unlucky but it can happen, I always try it on the kOhms setting first, If you can't be sure use test method 2 and note it on your paperwork.

keh7486 · Feb 6, 2013

Thanks lads. I'm not a tester yet just about to start my course but wasn't sure

ifindoubtdont · Feb 6, 2013

OSG section 10 page 84..... 612.3.2

SirKit Breaker · Feb 6, 2013

keh7486 said:
Hi just a quick question. I've seen a few times people say test insulation resistance 250v first to make sure there isn't any points missed in loft or where ever. This seem to me a sensible thing to do. Is it 100% safe to test all ways l-n l-e n-e. I thought it was as its not far of the running voltage but someone at work today disagreed.
Thanks

Nothing wrong with testing at 250v as a precaution before testing at 500v. If you dont test at 500v you are not testing it properly, the reason it is 500v is to put the cable under a bit of pressure.

You could connect L and N together and test them to E, this way you wont do any harm, but if you do a lighting circuit you will still need to remove dimmers as they are in the line only, and you will damage them.

If you are new to testing, then you are doing the right thing by asking, but i advise all who post similar questions to go and buy IET Guidance note 3, this is the testing bible, and is very easy to understand, and is priceless in you position.

Cheers............Howard