Insulation readings rising

[ohmeelectrical]

Hi everybody
I Have come across this a number of times on installations with old white pvc cable, testing with a Fluke 1652. Elecsa told me that this was due to capacitance in the wire and that it was charging up during the test. Does anybody have a more detailed explanation? The readings start at around 350meg and just keep on rising.
I am wary of holding the test button down for too long in case there is VIR hidden somewhere, but the reading is still rising after a couple of seconds. What figure should I put on my paperwork if it keeps on rising? I am currently putting the initial reading as this would be worst case scenario but not sure if this is correct.
Cheers

 

[xraytek]

Just put >300M down as the reading.
Why waste your batteries

 

[lilygee]

I've got a fluke 1653 and it does the same.Ipress the button, it beeps, the IR reading rises, then it beeps again. The reading to take is the second beep.

 

[Steve1412]

You know a capacitor is two conductors with insulation between. Then think of T&E.

Think back to ohms law ( the R=V/I part. ) as you charge the cable the current begins to fall...which in turn means the Res will begin to increase thus your creeping IR readings.

 

[ohmeelectrical]

Hi and thanks for that, although I thought the material separating the plates of a capacitor is a dielectric, which is a type of insulation with dielectric polarisation properties. An ideal insulator is a material that resists the flow of current, and that is what one would hope the pvc to do in the cable. It appears that something is happening to the pvc causing it to become dielectric and this is the question I was originally asking. Could it be an early indication of the formation of diisooctyl phtalate perhaps or some other additive reaction or some other process. Just wondered if anybody knew the chemistry.

 

[Steve1412]

eerrmmmm of course that old cookie. Sorry mate. Just know the basic facts.

All insulators can breakdown at some point.
And by the way an insulator is a dielectric

 

[ohmeelectrical]

Nearly right- all dielectrics are insulators, some insulators are dielectrics (according to everything I studied and wikipedia, check it out here)
Dielectric - Wikipedia, the free encyclopedia