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?
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Insulation testing of cables 4
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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.
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.
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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.
I wouldn't necessarily expect the megger to damage a TVSS. They produce very low current.
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electricpete
Electrical
I would think TVSS could be damaged because it is designed for transient overvoltage, not steady DC overvoltage.
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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.
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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?
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
So a 100V may be ok to check a telephone cable (for example) but not for 300V or higher rated cables
electricpete
Electrical
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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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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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..
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..
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benlanz
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- Mar 26, 2004
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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
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
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.
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.
electricpete
Electrical
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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It does have some relevance to the question at hand but again just wanted to point out the applicability
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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.
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.
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.
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.
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.
electricpete
Electrical
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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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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