Insulation Reesistance results.

[SSE]

I installed a new ring for a utility room today, I tested the two legs before anything was connected and everything was all clear. Then once I connected all the fittings but never hitched anything in at the CU side my results seemed to change. Live-Neutral was fine >299 mega ohms, but Live-Earth was now giving me about >220-240 mega ohms & Neutral-Earth >250 megaohms. I then went and split the live, earth & neutral at a fitting, tested each leg and they were clear again, >299. Am I missing something glaringly obvious here or what??????

 

[GLENNSPARK]

There are plenty of 1KV IR testers out there that have a 1000 megohm range. No problem at all, shoving 1KV up a domestic circuit. ..lol!!

yep....and i`ll certainly go up to 1KV if i suspect it needs it...

 

[Lucien Nunes]

just hadn't come across this before. Any time I've done it it's been >299 on new wiring.

I'm surprised. Small circuits on dry days that's fair enough, but large stuff and most anything when it's really humid you cannot always get up to hundreds of megs. With humidity, it's not so much the cable but the creepage across the accessories, i.e. the insulation itself is OK but it's being shunted by conductive moisture paths. The more of these you have in parallel the more likely the reading is to fall below the range limit on your tester.

The other thing I noticed was if I held the lead on, very slowly the value started increasing sometimes going the full way to >299. This just my tester, leads or what?

Quite common - usually a combination of dielectric absorption in the insulation and driving off the moisture at the terminations. It's fun watching it climb towards the range limit but for normal work serves little purpose.

Lucien

 

[Rockingit]

I've been seeing a pattern lately of new cable not giving quite the IR that maybe the previous generation did, but I only start looking for obvious things if it's under 100M, and then not very hard.

Unlike today, when I found myself chasing an IR fault so obvious I could actually use my continuity tester to chase it! (Screw through cable).

 

[GLENNSPARK]

I'm surprised. Small circuits on dry days that's fair enough, but large stuff and most anything when it's really humid you cannot always get up to hundreds of megs. With humidity, it's not so much the cable but the creepage across the accessories, i.e. the insulation itself is OK but it's being shunted by conductive moisture paths. The more of these you have in parallel the more likely the reading is to fall below the range limit on your tester.


Quite common - usually a combination of dielectric absorption in the insulation and driving off the moisture at the terminations. It's fun watching it climb towards the range limit but for normal work serves little purpose.

Lucien

Lucien....come on.....you cant proffer statements like this one .....

for a start dont try lecturing the more experienced of us in here about parrallel paths and the effects it has on IR readings....you embarrass yourself and insult us...

creapage as you call it across accessories is however a perfectly fair statement....as is poor IR readings that are purely down to accessories/outlets alone...

 

[telectrix]

glenn, he did not say parallel paths as such, just that the more sockets you have ( which are in effect, in parallel across the circuit, ) each with some moisture involved, will reduce the reading accordingly.the more sockets, the more the leakage.

 

[GLENNSPARK]

glenn, he did not say parallel paths as such, just that the more sockets you have ( which are in effect, in parallel across the circuit, ) each with some moisture involved, will reduce the reading accordingly.the more sockets, the more the leakage.

hmm....

 

[GLENNSPARK]

i`m tired and i`m off to bed....
i gets ratty....
good night all...

 

[Engineer54]

My 1000V Megger only goes to 999MΏ.

The Megger I had at work went in to 100’s of TΏ. Rotten sods wouldn’t let me take it with me when I retired. All the other test gear I was allowed to take but not that!

I wonder how many know the 1/10,000[SUP]th[/SUP] FLC calculations for IR readings. A must with old / large systems.

If your works Megger was a 5 KV unit, it would be more like 10 Tohms A 10 KV Megger around 20 Tohms.... Accuracy tends to start getting low too, like around + or - 20% or more...

 

[Lucien Nunes]

I'm inclined to agree with Rockingit also, that some cables are turning up with a slightly lower insulation resistance. We get quite a good test of this on theatre lighting installs in conduit cable where there might be say 96 identical circuits 25m long commoned at a neutral bar, all drawn together from one batch of cable and made off at DIN rails. We sometimes used to see the full 2.5km of cable come in at 200MΩ on a good day - off the scale whether you were testing one circuit or all 96, but the average seems to be a bit lower these days. Or perhaps it's just the dingy weather.

PS not 'lecturing' YOU Glenn, I know you know, just trying to flesh out the explanation a bit and answering a bit of the question that hadn't been answered. Sheesh.