Insulation resistance, odd results!

[elecypwn]

Hey guys! first real post on the forum so be gentle!

Basically i have installed a new installation feeding a new cinema room, it has 8 mains powered down lighters ( no transformer ) and 2 wall lights, When i tested the installation for IR i got the following results:

L-N - 1000 M ohms
L-E - varies from 300 -500 M ohms
N-E - Varies from 300-500 M ohms

This is a brand new installation, that has been installed behind plasterboard so at least to my knowledge there can be no moisture, there is no screws/nail penetration and all my terminals are sound and tight with in Junction boxes and light fittings, there not hooked up to the consumer unit yet, testing has been done from the cable that will be entering the board.

Another strange thing is, i disconnected the wall lights from the circuit and tested that individually, that was sound, then i tested the 8 down lighters again and still had the persisting problem! now the weird bit! i disconnected the cable entering the choc box that been fed from the switch live to feed the first light so the daisy chain cables feeding the lights were separate, and the cables from the consumer unit to the switch were separate, now when i tested both them separatly the readings where spot on all 1000+ but when i connected them back into the choc box i got the same weird results, so took them back out, split them tested the cable feeding the lights, sound, tested the incoming cable to first light, through the switch back to the consumer unit, them results were sound, connected them back up again, Weird reading! can somebody tell me what the hell could be going on please xD

also iv never had this problem before and relied on my >1000MOhm readings, if i interpret the regs correctly would a 300-500Mohm reading be a pass anyway? would 1.0M ohm be just 1.0M ohm on my tester ( metrel easi multi tester )? but id ideally like it all to be >1000, could my 300-500Mohm readings be acceptable on a test cert? cheers

 

[Risteard]

300 million ohms is well, well, well above the minimum 1 million ohms required for an installation, and also 150 times higher than the 2 Megohms for the installation requiring further investigation.

So not sure why you wouldn't be happy with it.

300 Megohms is not indicative of poor insulation - more likely the circuit lengths in parallel etc.

 

[Knobhead]

What voltage are you testing at? What is the voltage rating of the cable?

At 300MΩ I’d be quite happy, I’ve put 3.3KV on to lower than that.

 

[elecypwn]

Thank you for the replies guys, Basically its 1.5mm lighting cable twin+earth with a test voltage of 500V been applied to it, since when i disconnect the cable feeding the first light and test that i get a >1000 M ohm result on each conductor, and same when i test the cables daisy chaining each other, but asoon as i reconnect the feed giving power to the first light which enables all lights to work i get half the reading of N-E and L-E which would be around 300-500 M ohms and it stumped me and fellow other experienced electricians, i even had to ask a guy who tests off shore installations and he said he had a similar scenario where when 2 earths shared the same sleeving it gave him a weird readings, when it was put back into 2 separate sleeves it was back up to the full reading! crazy stuff today.

So do you think it could be the length of the run and its parallel paths causing this problem roughly 20 meter run? or are there any other options?

 

[MarkieSparkie]

If your installation is supplied at 230V[SUB]AC[/SUB] and you are testing at 500V[SUB]DC[/SUB] then you appear to have outstanding IR results. A reading >=1M Ohms complies with BS7671, but <=2M Ohms requires further investigation. Your results are >150x better than the investigation level and your still not happy! Either you suffer from OCD or have to much spare time to waste trying to achieve perfection.:joker: Smile, be happy, no worries, ... maybe.
A note of caution however, you seem to like to see a maximum reading of >1000M Ohms on your meter, this being your goal of "perfect insulation" on a newly installed circuit presumably. How do you distinguish this reading from that of an open-circuit? If I saw similar on my meter, alarm bells would be ringing in my head saying "Is this just to good to be true?", "Are my test leads faulty or poorly connected?" or "Do I actually have an open-circuit?"
The variation in results between testing parts of the circuit and the full circuit are due to cumulative leakage paths, as this is a new circuit it is most likely caused by inter-conductor and conductor-earth leakage capacitances of the cables and accessories.

 

[elecypwn]

So to conclude after briefly reading regs! anything above 1M ohm is a pass, but below 2M ohm needs to be inspected further, and if im getting 500M ohms thats 500x higher than what the regs are requiring? if so, sorted cheers chaps =P

 

[Risteard]

So to conclude after briefly reading regs! anything above 1M ohm is a pass, but below 2M ohm needs to be inspected further, and if im getting 500M ohms thats 500x higher than what the regs are requiring? if so, sorted cheers chaps =P

Yes, but just to be clear the Regulations are referring to the insulation resistance of the installation, and not an individual circuit. So if less than the 2 Megohms requiring further investigation then you would need to look at individual circuits.