IR testing - results creep up!

[pencilpusher]

Hi all

Why is it that on some circuit IR testing, the results shoot up to say 250 M ohms but then creep up towards a much higher value? I know that the results are OK, I just thought it may be some damp somewhere causing the problem possibly. Do you wait for them to stop creeping up or take a result after say 5-10 seconds.

Any views would be welcome.

PP

 

[Jimmy Boy]

I would have thought they would have crept down if there was a leak to another conductor or Earth ?
J

 

[pencilpusher]

Yeah sure, I was just curious why sometimes this happend and other times you get >999 M ohms very quickly.

PP

 

[Des 56]

I found this,it seems to give a full explanation of your querie
I had an idea but not enough for a reply

Here is Flukes information on the test,it gives a full run down on the conditions and values that are attained for all types of insulation tests
http://support.fluke.com/find-sales/Download/Asset/1579160_6115_ENG_C_W.PDF

You can read it all or skip to page 3

 

[Engineer54]

Do you wait for them to stop creeping up or take a result after say 5-10 seconds.

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Not that many electricians do, but the correct way to record a IR value is after 1 minute, (in most cases). Which allows the test voltage to charge the complete circuit and find any weak spots in the insulation. A quick 3 to 5 second push of the test button, is not and never has been the correct procedure...

 

[IQ Electrical]

I've normally found dampness to cause this.

 

[spark 68]

With otherwise good high readings I agree dampness can cause fluctuating readings, but a slow creep up I always thought this was due to capacitance effects, and thought this was the circuit just charging up.

 

[IQ Electrical]

With otherwise good high readings I agree dampness can cause flucuating readings, but a slow creep up I always thought this was due to capacitance effects, and thought this was the circuit just charging up.

Absolutely, a bit of capacitive coupling on long runs!

 

[topquark]

Yep, the old 3 core two way lighting strikes again!

 

[pencilpusher]

[/COLOR][/LEFT]

Not that many electricians do, but the correct way to record a IR value is after 1 minute, (in most cases). Which allows the test voltage to charge the complete circuit and find any weak spots in the insulation. A quick 3 to 5 second push of the test button, is not and never has been the correct procedure...

So one minute it is then, unless I get >999 M ohms immediately of course!

 

[Engineer54]

So one minute it is then, unless I get >999 M ohms immediately of course!

Well there is nothing to say it can't also slowly reduce during that minute!! lol!!!

 

[1shortcircuit]

If I notice this, which I have in the past, I continue to keep the test button pressed in on my Fluke tester until it bleeps and enter the final figure displayed:thumbsup

 

[Engineer54]

If I notice this, which I have in the past, I continue to keep the test button pressed in on my Fluke tester until it bleeps and enter the final figure displayed:thumbsup

Be thankful your not Hi Pot testing MV/HV cables (basically the same thing but at much higher voltages), that can be anything from 15 to 30 minutes, depending on if it's initial verification or for subsequent verification after an incident for arguments sake. As Hi-Pot tests are classified as a destructive test, often both test KV and times are reduced....

 

[1shortcircuit]

anything from 15 to 30 minutes

lol

I imagine holding the test button for that length of time would be similar to holding the lever on a hand-grenade with the pin removed lol

 

[pencilpusher]

Well there is nothing to say it can't also slowly reduce during that minute!! lol!!!

Maybe, but if it slowly reduces to 300 M ohms, I am still quids in aren't I? !! :drummer: