Insulation testing of cables 4

[a10jp]

For insulation resistance testing on low voltage system (120/208), typically, our specification requires the test voltage to be 500Vdc applied for one minute between each conductor and grounds and with other conductors. However, the contractor stating that since 500V is dangerous, he used 100V instead for the test. Is it an acceptable practice to use 100V? If we are allowed to use 100V, then why do we call this hi-pot test?

 

[Warpspeed]

Warpspeed takes of his philosophers hat, and puts his engineers hat back on.

I agree electricpete, for any sort of standard mains circuit testing, a 500v megger test is the normal industry practice.

 

[a10jp]

I am taking that there are cases where people choose to use 100V as test voltage, based on engineering judgement as mentioned above. Yes, the spec stated clearly this is test is only insulation resistance test, and further it specifies that 500Vdc for 1 minute be used. I have checked the local reference here in Japan, and they do use 500Vdc for "low voltage system", and 1000V ~ 2000V for higher voltage ratings (not comprehensive, at this point I am only concerned 120/208 systems). Further, there are 3 ranges tobe used: 250V test (50MOhm range), 500V (100MOhm) and 1000V (2000MOhm). So our contractor says he used 100V test and get 100MOhm on all test reading I feel funny, and did more research. However, even though I had believe that our contractor may have used 500V DC test without checking all medical equipment were disconnected, and subsequently damaged them, but then again, if the equipment were on, and connected, then we would not get 100Mohm as a result. So despite all these seemingly detective work to back track what the contractor has done, nonetheless, there is still other distribution problem elsewhere and the casue is still not conclusive.

 

[Xuewei]

Another question:
A cable marked with 600/1000V. Is it rated for 600 or 1000?

 

[davidbeach]

A 600/1000V rating probably means that for use at 1000V it would have to be on solidly grounded wye system. That would keep the maximum phase to ground voltage below 600V.

 

[rbulsara]

I agree with davidbeach on last post

 

[Xuewei]

Normally for HV cable, there is a grounded screen outside the cord insulation. Does it have any phase to phase rating?
For the 6K/10KV rating cable, can I use it to carry 10KV?

 

[davidbeach]

Is it a solidly grounded wye system? Do you not read previous answers and extend the logic?

 

[Xuewei]

For HV cable, there is a thin layer of semiconductor (black)outside the insulation, and outside the semiconductor is another lay of screen(normally copper). This screen is always grounded in any IT or TN system. (It may not have this screen for UL cable). The cable is marked Uo/U. (U=1.732*Uo).
My question is: which voltage is the rate figure, Uo or U?

 

[benlanz]

Okay, if we are starting to talk about shielded power cable with extruded insulation for 5kV class systems and higher, this is where the discussion of insulation resistance should end. I realize that it is easy to think of low voltage and high voltage cables in the same way but they are vastly different in terms of failure modes and therefore the tests used must be different.

IEEE 400 clearly indicates that the old DC tests no longer fulfill the definition of an acceptance test or a maintenance test. An effective HIPOT for shield power cable with extruded insulation is a low frequency AC HIPOT (or VLF). (IEEE 400.2) However, no HIPOT can predict future performance. If the sample fails, one can clearly argue the cable was defective but, you don't know if you have harmed any other weak point during the HIPOT process. If the cable passes the HIPOT test (does not fail) you don’t know if you have harmed anything (grown a defect) without detecting (failing) a defect.

I you want to use the manufactures’ performance standards you must compare apples to apples. IEEE 400 recommends repeating the manufactures’ test in the field. A power frequency, off-line PD test calibrated to 5pC sensitivity is the only test which can fulfill this requirement.

If you would like more information please refer to my post on 17 Jun 05 at 10:50 at the following link

http://www.eng-tips.com/viewthread.cfm?qid=125873&page=1

Please feel free to ask questions and comment.



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