cable aging test (insulation resistance vs time)

[alwaink]

Hi,

I'm working on a long term testing (26 weeks) on cable insulation. The readings that I've taken are capacitance, AC/DC resistance, and dissipation factor at constant 75 deg C.

As expected, the AC insulation resistance for different samples has either stayed the same or dropped after 26 weeks due to aging. What I don't get is that the DC insulation resistance readings have actually gone up quite a bit for all samples. Any logical explanations?

Thanks

 

[DanDel]

What rate of increase are you seeing? Are the increases constant over the 26 weeks, or just during the beginning?

 

[PHovnanian]

Just a guess on the DC resistance: you are 'cooking' the moisture out of the sample under test. I would anticipate that this effect will reach a plateau after some time.

 

[Zogzog]

What is the duration of each individual test?

 

[alwaink]

Sorry that I didn't clarify in the first post...it's a waterbath immersion test where the samples are submerged in water for 26 weeks with constant voltage and temperature applied at all time except when taking measurements (once a week).

For each individual DC measurement, I charge up the megger at 500V for 15 sec and then wait 1 minute to record the reading.

I've got 4 different insulation materials and 4 samples each. 2 of the 4 insulation materials have stayed around 20 Tera ohms through out. The other two have gone up gradually each week from 20 T ohms in the beginning to over 80 T ohms around 20th week. So I'm not sure what's going on here with the DC resistance readings.

Thanks.

 

[thewellguy]

I have seen results vary greatly based on the humidity or the air at the test site.

 

[benlanz]

alwaink,

In order to comment in a meaningful way, I need to know a few parameters of your cable.


-voltage class of cable
-insulation type
-insulation thickness

Are you testing a shielded cable? If so, what is the shield type and construction? Does the cable have a jacket?

Thanks,


Benjamin Lanz
Vice Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants