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Insulation testing of cables
4

Insulation testing of cables

Insulation testing of cables

(OP)
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?

RE: Insulation testing of cables

500VDC should be used.. its not really called hi-pot test


Hi pot test is usally referred to for medium voltage cables..rated 2kV and above..

RE: Insulation testing of cables

100v is not acceptbale..

The normal  conductors for power circuits are rated for 600V minimim or even 1000V.

RE: Insulation testing of cables

(OP)
Thank you for clearing the terminology.  I just read the spec again, It should be insulation resisitance test.  The DC hi-pot test is an additonal test required for medium voltage cables.  

Here is another question:  If 100V is not acceptable, then why would the instrument allows 100V as DC test voltage?  Does that make sense?

I fear the contractor is covering himself by lying, since earlier we had several medical equipment, nurse call system, and TVSS damaged.  While it is not sufficiently conclusive that this test has damaged the medical equipment, if he had applied the test correctly, he would have dmaaged the TVSS since it was not disconnected.

RE: Insulation testing of cables

Insulation resistance test is the proper terminology. 500VDC is standard for 300V rated insulation. 1000VDC is standard for 600V insulation. Some testers have lower voltage selections available. 250V is common. Perhaps those are used for lower rated insulation, like 150V control/electronics wiring.

I wouldn't necessarily expect the megger to damage a TVSS. They produce very low current.

RE: Insulation testing of cables

I would think TVSS could be damaged because it is designed for transient overvoltage, not steady DC overvoltage.

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RE: Insulation testing of cables

For any insulation test (megger test) all condutors must be disconnected from loads and supply, including pulling off control fuses to avoid false readings and the type damage you saw. It also possible that test voltage used was higher..since the damage did occur.

RE: Insulation testing of cables

(OP)
I have another question.  I have just been reading some older articles from EC&M website.  It talks about insulation test with 100V to 10,000V being used as test voltage, even though in practice they commonly use 500V and 1000V.  And the purpose of the insulation test is to study the effect of electrical insulation under high potential (or like the exmaple in EC&M, under high pressure as in piping systems and observe leakage), isn't it?  But at 100V, which is lower than the system rated voltage 120/208V, would we expect to see any desirable effect, as the contractor has calimed that's what he used, instead of 500V?  (The test results all said 100Mohm.  But what meaning do they have?) I guess the most I could say is that the test was done incorrectly?

RE: Insulation testing of cables

Test voltage depends on the rated voltage of conductor insulation being tested. Test voltage should be higher than the rated voltage. NETA, ATS and MTS specifies some.

So a 100V may be ok to check a telephone cable (for example) but not for 300V or higher rated cables

RE: Insulation testing of cables

To add to the confusion:

NETA Maintenance Testing Spec 2001 Section 7.3.2 "Cables, Low-Voltage, 600 Volt Maximum"

"Perform insulation-resistance test on each conductor with respect to ground and adjacent conductors. Applied potential shall be 500 volts dc for 300 volt rated cable and 1000 volts dc for 600 volt rated cable. Test duration shall be one minute."

To the best of my knowledge, all power system wiring 120vac/220vac is 600vac rms rated.  That is 848vac peak.  I'm not sure how we get to 1000vdc without considering it an overvoltage test of the cable.  Maybe 600vac cable is intended to apply on system up to 600vac with some tolerance for voltage variation above 600?  I'm not sure.

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RE: Insulation testing of cables

electricpete:

If I may render my view on your last post:

It certainly is not intended to be overvoltage test, but if a cable is good to withstand 600V indefinietly, it inhenrently will have some capability to withstand higher volages for shorter (definite) period of time without damage. In fact cables with stand several times the rated voltage for a few minutes. Hence 1000VDC for "one minute" is appropriate voltaeg for the insulatino testing of 600V rated cables.

Again all these test standards and accepted practice have the backing of the historical experiences, many test procedures are based on past expereiences and not necessarily on some mathematical calculations.

This is no different than testing a strucrural member or assembly by exerting forces much higher than desinged (in normal uses) strength per say..or testing of the pipes under pressrue for leaks..

RE: Insulation testing of cables

(OP)
Thanks.  I really appreciate the insights. Is insulation testing something of common knowledge a licensed electrican would know how to do?  Does the argument "500VDc is dangerous and therefore 100VDC was used" hold any merit?  

RE: Insulation testing of cables

Licensed electrians should know how to do it. It is routine practice. 500VDC is the standard and required voltage for 208/120VAC systems. It is not exceptionally dangerous with basic precautions.

RE: Insulation testing of cables

I support many of the previous statements. A standardized HIPOT test will show that you have done due diligence should your low voltage cable fail but please consider the following caveat.

Just remember, a WITHSTAND test by definition can only find a failure in the insulation and can not predict future performance. Once the HIPOT is ended there is no guarantee the cable insulation will continue to perform.  Since the insulation is not uniformly shielded with a ground plain, the HIPOT is not stressing the cable insulation uniformly. Therefore, faults and defects can go undetected with a HIPOT.  Although the HIPOT is the excepted standard for unshielded cable, there is, unfortunately, no to sure way to test unshielded cable.

Benjamin Lanz
Sr. Application Engineer
IMCORP

RE: Insulation testing of cables

IEEE Std 43-2000 Table 1 gives the guidelines for DC voltages to be applied during insulation resistance test.
Rated voltage of <1000 V the test voltage is 500V DC.

There are cases where alternator bearing insulation can be tested at lower voltage at 250V/100VDC and this is by engineering judgement.The bearing insulation is there to protect the bearing against shaft currents.

RE: Insulation testing of cables

I just wanted to point out that IEEE43-2000 applies specifically to rotating electrical machines.  

It does have some relevance to the question at hand but again just wanted to point out the applicability

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RE: Insulation testing of cables

Yes, agree with you. The IEEE states for electrical machines.

The best practice would be to find the design voltage of cable. I am not sure of the design voltage in the US. Like in the UK, Australia area the design voltage of cable is 600/1000V. The minimum test voltage would be 500V. The manufacturers in the US should provide details of voltage rating of cables.

Like said earlier the test voltage differs for application because it can puncture the insulation. E.g is the bearing insulation where letheriod paper insulation is used.

RE: Insulation testing of cables

Having worked for a utility, frequently supervising tests of power cable upto 115kV, the 1000VDC is appropriate for 600V rated cables for megger testing.  The minimum acceptable results from NETA start at 100MegaOhms, although new cables should easily be in the Giga Ohm range.  And yes all end use equipment must be disconnected, as this test is for cables only.

RE: Insulation testing of cables

What are we really trying to do here ??

There are two completely different measurements. One is a circuit resistance measurement, or megger of the circuit. That requires a dc voltage to do the test, the voltage needed can be safely chosen to suit the type of circuit and resistance range being measured.

The thing here is we don't want the circuit to break down, we just want to measure some high ohmic resistance without damaging anything. 100v may be more than sufficient to do that, or 500v or even 1Kv may be required to get suitable sensitivity. The important feature is the resistance reading obtained, the test voltage being incidental.

Another totally different test is a high voltage insulation test where we need to know if a circuit will successfully withstand or break down at a specified dc voltage. In this case the test voltage will be specified and is a very important feature of the test.

If the purpose of the test is as stated in the first post INSULATION RESISTANCE, in a low voltage circuit, 100v will do the job.

Insulation resistance is definitely not the same as high voltage withstanding ability.


 

RE: Insulation testing of cables

I agree with your conclusion based on strict interpretation of the word resistance which implies we are within the linear range current vs voltage.  In addition, 100v test should be same as 500v test for most cases.

However, there will be cases where 500v is enough to cause partially breakdown the insulation and 100v is not and so 500v will be more probing (will show a lower resistance).  I think what is important is the standard practice which is 500v, not 100v.

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RE: Insulation testing of cables

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.

RE: Insulation testing of cables

(OP)
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.  

RE: Insulation testing of cables

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

RE: Insulation testing of cables

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.

RE: Insulation testing of cables

I agree with davidbeach on last post

RE: Insulation testing of cables

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?

RE: Insulation testing of cables

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

RE: Insulation testing of cables

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?

RE: Insulation testing of cables

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&amp;page=1

Please feel free to ask questions and comment.

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

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