"Low" insulation resistance

[joojar]

I recently installed a new ring main and lighting circuit for my kitchen, including a new dedicated consumer unit. Just had the BCO spark round to do final testing, all is well apart from insulation resistance, which was found to be 11 Mega-ohm for both L and N conductors of the ring main.

Sparks tells me it's still a pass because the threshold is 0.5 Mega-ohm, however for a new installation he would expect nearer 200 Mega-ohm. Mentioned that it could be because wires have got squeezed in back-boxes, or could be residual damp from plastering (was plastered nearly 2 weeks ago). We agreed he'd come back in a few weeks and re-do the test.

In the meantime: anyone seen this before? Should I be worried?

 

[oharaf]

11Mega Ohms id fine, thou your spark is right you would expect to get much higher.

The regs state it must be greater than 1 megaohm, thou anything between 0 and 2 mega ohm would suggest a latent defect potentially.

did you ensure that all appliances were disconnected/unplugged, neons, fans/motors etc?

Also did he test at the CU making sure only that circuit was undertest?

As these will effect your readings as well.

The damp plaster may well be having an effect, I'm not overly sure what he means about 'squeezed' but he will know in his mind.

Wait and see what the retest shows

 

[pompeygit]

Well it complies...........but I for one would be a little upset to get a reading of 11Mohms ON A BRAND NEW CIRCUIT. As far as I am concerned if I don't get greater than 1999MOhms I wouldn't be happy. Clearly he's done a bad test or there is something wrong cos no way should you get a raeding like that on a brand new cable.

Interesting to see if others would accept such a reading without further investigation on a brand new circuit. I would want to know more before I left that site

 

[ColJack]

did you happen to have any neon indicators in the circuit?
low voltage transformer you forgot about?

 

[comms]

Yes it's a pass but I wouldn't be happy leaving it at that, I'd be splitting the circuit & testing either way.

.5meg as a minimum...not any more afraid he's wrong, unless the circuit under test was designed to 16th I suppose.

 

[RF Lighting]

Damp from the plastering seems feasible, and retesting in 2 weeks seems reasonable. We await the results

 

[Click-Sure]

greater than 1999MOhms

Blimey Pompey, what tester you using?

Or are you talking about an ideal world, if your tester went up that far?

 

[pompeygit]

I kid you not was so surprised after my dear old Robin. Just changed my Robin for Kewtech KT 64. It measures greater than 199 at 250v, but at 500v and 1000v it measures greater than 1999. What I really like is the the L probe with integral test button........no more trying to imitate an octupus!!!!!!!!

Untitled

• pompeygit
• 17 Jul 2008
• 1

 

[oharaf]

Other way round for me!!

Always used to getting >1999 M ohms with a Robin then when I got my Megger 1502/2 highest reading is >299M ohm.

L Probe very cool!!!!

 

[joojar]

Cheers for all the comments guys - I'll be sure to post the second set of test results when they're in.

 

[ban-all-sheds]

Well it complies...........but I for one would be a little upset to get a reading of 11Mohms ON A BRAND NEW CIRCUIT. As far as I am concerned if I don't get greater than 1999MOhms I wouldn't be happy.

I was going to say the same - whenever I've tested a new circuit I've always seen ">1.999G" on the display.

Maybe the forthcoming calibration will shatter my illusions...