Insulation test of MV cables

[1000V]

Hi all, I have tried searching without much luck so here is my question:

I was recently talking to somebody about a group of motor cables at their place of work that have been tested and showed dead shorts to earth. They are connected to 3.3kV motors that have been run without problem, they have since been inactive for several months. The cables are run underground. As a matter of precaution the cables have been meggered to earth and between phases (disconnected both ends). Three of the twelve have shown dead shorts to earth on a phase. However he mentioned that there is sometimes a problem with DC testing MV/HV cables and suggested that this could not be a true reading and/or the actual test could have caused damage to weak points in the cable.
Am I showing my ignorance/inexperience by being slightly confused with this? Is a short not a short? Also isn't the low current in the test the reason why damage should not be caused?

Thankyou for your time in reading this question.

Regards, Tommy

 

[Zogzog]

A megger test should not damage you cables, but some types of MV cables can be damaged by a DC hipot test, which is why IEE, NETA, ANSI, and the ICEA recommend not doing a DC hipot test on service aged MV cables.

This topic has been discussed in this forum many times in great detail, do a quick serach and you ill find lots of good info.

The test may have damaged the cables, but if it did they were either already damaged or near end of life anyways, but if you have a "Dead short" forma megger reading, and the test was done properly (Was this a certified testing company?) it looks like you will be replacing some or all of these cables.

 

[1000V]

Hi thanks for your reply. Now that you have posted the reply I have found the info I was previously searching for but could not find!! Aint that a pip!

Sorry for creating a superfluous thread. Keep on trucking.

 

[Zogzog]

Let us know what happens with this.