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
Hi pot test is usally referred to for medium voltage cables..rated 2kV and above..
RE: Insulation testing of cables
The normal conductors for power circuits are rated for 600V minimim or even 1000V.
RE: Insulation testing of 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
I wouldn't necessarily expect the megger to damage a TVSS. They produce very low current.
RE: Insulation testing of cables
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RE: Insulation testing of cables
RE: Insulation testing of cables
RE: Insulation testing of cables
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
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
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
RE: Insulation testing of cables
RE: Insulation testing of cables
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
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
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
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
RE: Insulation testing of cables
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
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
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
RE: Insulation testing of cables
A cable marked with 600/1000V. Is it rated for 600 or 1000?
RE: Insulation testing of cables
RE: Insulation testing of cables
RE: Insulation testing of cables
For the 6K/10KV rating cable, can I use it to carry 10KV?
RE: Insulation testing of cables
RE: Insulation testing of cables
My question is: which voltage is the rate figure, Uo or U?
RE: Insulation testing of cables
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
Please feel free to ask questions and comment.
Benjamin Lanz
Vice Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability