Heak Shrink Tubing With Low IR Reading After Heating

[waynegu]

Hi,

I am working on a 33kV cable project and using the heat shrink joint kits for the In-Line Joints.

I meggered the cable before jointing every time and the 5kV DC IR (core - sheath) reading was always around 400GOMHS/1min, and I also meggered the cable with the new In-Line joint after 3 days, and the IR reading varied, 5 of 10 were 300GOHMS/1min, 3 of 10 were close to 100GOHMS/1min, and the other 2 were less than 20GOHMS/1min.

I decided to strip one low IR reading joint open to analyse the cause of the problem. Finally, I found out one heat shrink tubing was only 20GOHMS/1min, however, before heating it was 500GOHMS/1min. I also checked the tubing and found there was no obvious over heating signs on it.

Have anyone had the same experience before? Will this tubing cause the cable failure during normal operation?

Thanks for the help in advance.

Wayne

 

[7anoter4]

In my opinion, heating and cooling the tubing a thin layer of condensed water could be produced. The water dielectric constant [relative permittivity -eps.r] is 80 times the air dielectric constant and so the capacity between conductor and shield increases very much.
The measured resistance is ratio Volt/i i=V/R*[1-exp(-t/(r*C)]
where R is the insulation resistance parallel with the capacity C and r is the series resistance of the measuring circuit.
The more capacity the longer the time to reach the same resistance and will be V/R after a long time.
That does not means the insulation resistance is low but means is a gap between insulation and sheath filled with water.
The prisoner air or water in the electric field could beak down and damage the insulation.

 

[waynegu]

Thanks a lot for your reply.

However, the situation does not match your description.

After the testing, I striped the joint one layer by one layer and meggered each layer as well. Then I found out there is one heat shrink stress control tubing with very low IR reading. When I did the testing, the test object didn't touch anything.

Afterwards, I tested this tubing before shrink under same condition, the megger reading was very good.

Do you have any ideas for the reasons?

 

[dpmac]

I would imagine the "stress control" tubing would contain some semi-conducting material.

 

[waynegu]

THanks dpmac.

Actually I have some feeling, do you know how to test the material contents?

 

[oldfieldguy]

There are possibly two things at work here.

First, it is not unusual for materials to exhibit a markedly lower insulation resistance when heated.

Second, as previously pointed out, a semi-conducting layer is part of many stress relief terminations. I have seen cases where the kit was incorrectly applied, giving some very poor IR readings. Fortunately the client accepted the need to investigate (I was third-party testing agent) and the misapplication was discovered and corrected before the cable was placed into service.

old field guy