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Discuss IR testing - results creep up! in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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pencilpusher

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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
 
P

pencilpusher

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  • #3
Yeah sure, I was just curious why sometimes this happend and other times you get >999 M ohms very quickly.

PP
 
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Engineer54

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


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...
 
I've normally found dampness to cause this.
 

spark 68

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Arms
Esteemed
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.
 
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topquark

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Mentor
Arms
Yep, the old 3 core two way lighting strikes again! :)
 
P

pencilpusher

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  • #10
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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...
So one minute it is then, unless I get >999 M ohms immediately of course!
 
1

1shortcircuit

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  • #12
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
 
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Engineer54

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  • #13
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....
 
1

1shortcircuit

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  • #14
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
 
P

pencilpusher

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  • #15
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:
 
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P

pencilpusher

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  • #16
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
My Megger doesn't beep, it just keeps counting until it stabilises OR hits >999 M ohms.
 
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Engineer54

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  • #18
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
No push test button, just an on/off switch a KV increment dial, and a timer. Bit more complicated than that, but that's the basics... lol!!!
 

spark 68

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Arms
Esteemed
My experiences of pressure testing are a bit simpler lol,

IR test @ 1000V ,Hold trigger on the flash testing gun for 30 seconds, push 2.5kV AC through the switch gear for 30s, then IR test it all again @1000V. This was for simple type testing.

Edit: should have said for LV switchgear
 
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nicsin02

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  • #20
my lecturer at college suggested it was due to heat in the circuit
 

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