N
NickD
Hi guys,
Fairly recently qualified sparky here starting to do small solo jobs. I'm interested in views on insulation resistance testing on works to be signed off with a Minor Works Certificate. (Sorry this is a long post but I wanted to cover everything.)
One view on this is that you only test the work you've done - if you add a spur or new light, you just IR test your own new wiring before wiring it into the installation (probably with the CPC wired in). But say you want to be thorough. Or say all you've done is replaced a broken socket or light fitting - if you're going to meaningfully IR test at all, you have to test the existing wiring.
I guess that whatever testing you're going to do, it's main DB switched off (separates E & N if TN) and the fuse pulled/MCB off for the circuit in question.
If it's lighting, then with all bulbs out of the light fitting you have worked on and the relevant light switch off you can test L-N and L-E at the fitting assuming there's no transformer involved. But testing E-N is problematic because the test voltage will go E-N throughout the whole installation (risk of blowing up electronics? unless test at 250V). PLUS any connected equipment anywhere in the installation (bound to be some) may pass the test voltage from N up to L on that circuit (and if circuit not isolated, through MCB/fuse to DB busbar and thus L on the whole installation). So, nonsense reading plus risk of damage.
With the relevant light switch on, or if it's a socket ring/radial or whatever, then now you're testing the whole circuit. Obviously the E-N testing problem above is the same and unchanged. But if there is connected equipment on the circuit under test then there's now damage risk from any test involving L (unless test at 250V, or test (L+N)-E - but see below). PLUS the L-N test is impossible with the connected equipment. PLUS the L-E test will cause test voltage to be passed from L through the connected equipment to N and now you've got test voltage going across E-N throughout the whole installation again.
Some people suggest the magic cure-all answer is to test (L+N) combined to E. Yes this potentially helps with any connected equipment on the circuit under test. BUT as you are still putting test voltage E-N you still cannot avoid the problem of the test voltage going across E & N through the installation, and so on as per above.
**** So is the answer in fact to NOT isolate the circuit under test at the MCB/fuse, and always just test (L+N) combined to E on the whole installation?!? ****
(I'm conscious the reality is that on a low-cost minor works job you're not going to want to waste time running round disconnecting absolutely all appliances, taking all bulbs out of everything. Often it will be totally impractical anyway. I know that in a correct design all equipment should be isolatable, but sometimes it just ain't so. And the last thing you want is to blow up something expensive on a job you only stood to make a few tens of quids out of anyway.)
So it all seems to be a bit of a can of worms (mix of harmonised and non-harmonised ones no doubt). I'd be really grateful for people's views especially as I have a competent person scheme assessment coming up. Cheers, Nick.
Fairly recently qualified sparky here starting to do small solo jobs. I'm interested in views on insulation resistance testing on works to be signed off with a Minor Works Certificate. (Sorry this is a long post but I wanted to cover everything.)
One view on this is that you only test the work you've done - if you add a spur or new light, you just IR test your own new wiring before wiring it into the installation (probably with the CPC wired in). But say you want to be thorough. Or say all you've done is replaced a broken socket or light fitting - if you're going to meaningfully IR test at all, you have to test the existing wiring.
I guess that whatever testing you're going to do, it's main DB switched off (separates E & N if TN) and the fuse pulled/MCB off for the circuit in question.
If it's lighting, then with all bulbs out of the light fitting you have worked on and the relevant light switch off you can test L-N and L-E at the fitting assuming there's no transformer involved. But testing E-N is problematic because the test voltage will go E-N throughout the whole installation (risk of blowing up electronics? unless test at 250V). PLUS any connected equipment anywhere in the installation (bound to be some) may pass the test voltage from N up to L on that circuit (and if circuit not isolated, through MCB/fuse to DB busbar and thus L on the whole installation). So, nonsense reading plus risk of damage.
With the relevant light switch on, or if it's a socket ring/radial or whatever, then now you're testing the whole circuit. Obviously the E-N testing problem above is the same and unchanged. But if there is connected equipment on the circuit under test then there's now damage risk from any test involving L (unless test at 250V, or test (L+N)-E - but see below). PLUS the L-N test is impossible with the connected equipment. PLUS the L-E test will cause test voltage to be passed from L through the connected equipment to N and now you've got test voltage going across E-N throughout the whole installation again.
Some people suggest the magic cure-all answer is to test (L+N) combined to E. Yes this potentially helps with any connected equipment on the circuit under test. BUT as you are still putting test voltage E-N you still cannot avoid the problem of the test voltage going across E & N through the installation, and so on as per above.
**** So is the answer in fact to NOT isolate the circuit under test at the MCB/fuse, and always just test (L+N) combined to E on the whole installation?!? ****
(I'm conscious the reality is that on a low-cost minor works job you're not going to want to waste time running round disconnecting absolutely all appliances, taking all bulbs out of everything. Often it will be totally impractical anyway. I know that in a correct design all equipment should be isolatable, but sometimes it just ain't so. And the last thing you want is to blow up something expensive on a job you only stood to make a few tens of quids out of anyway.)
So it all seems to be a bit of a can of worms (mix of harmonised and non-harmonised ones no doubt). I'd be really grateful for people's views especially as I have a competent person scheme assessment coming up. Cheers, Nick.
