Single wire conductor insulation testing

[solardoc]

I test for ground faults on a lot of single wire conductors that are run along metallic racking. When testing conductor insulation integrity, we normally isolate all condutors and test with a megger set at 1000V. We attach the red lead to the conductor and the black to the ground which is bonded to the racking. This test works well mot of the time, but sometimes the ground fault is caused by rain and by the time we are able to get to the site and test, the rain has dried up and the fault has cleared, all megger results are good. Will testing the conductor with one lead on the copper and one lead on the insulation show all damaged conductors? Is there another test that will reveal all compromised conductors?

Thanks,

Paul

 

[Smart $]

Testing the conductor with one lead on the copper and one lead on the insulation WILL likely NOT show damaged conductors... but it may show one on a rare occasion or two.

A hi-pot test may reveal the insulation fault(s) noted, but there is no certainty that it will after the fact, so to speak. I was told a few years back that a hi-pot test is not recommended unless you must find a fault, for it is a destructive test... by subjecting insulation to a substantially higher voltage than which it is rated. I'm uncertain of the validity of that statement.

 

[solardoc]

Thanks for the input. I have considered doing the hi-pot tests as well, but heard that it is a destructive test and not recommended as the voltage is 5000 and up. The conductors I'm testing are rated at 600v, so I think it may be too much.

 

[ptonsparky]

What happens when these faults occur? Does a breaker trip? Flash and bang?

 

[solardoc]

A GFI fuse in the unit blows. We are never onsite to hear or see anything, but once the fault is located there is evidence of a flash. The GFI is a 4amp trip so not much energy is transferred.

 

[don_resqcapt19]

You can have bare wire and the megger will not tell you that unless there is some contact with a metallic surface or some type conductive contamination that makes contact with the metallic surface. In your case, the water makes the circuit and unless you are testing in wet conditions, your tests many not show the problem.

 

[solardoc]

Yes, that is the problem I'm running into. We have thought about using a hose to wet the exposed conductor in an effort to replicate the fault, but I was just wonerding if there was a test method other than what we are using that would be more effective. I was wondering if a step voltage test would be better?

 

[SG-1]

Thanks for the input. I have considered doing the hi-pot tests as well, but heard that it is a destructive test and not recommended as the voltage is 5000 and up. The conductors I'm testing are rated at 600v, so I think it may be too much.

A hi-pot or dielectric test for new 600V conductors would be about 1500 Volts AC RMS for one minute. As stated the test is distructive in that it degrades the insulation, even if no insulation failure occurs. We use this value to test all the secondary wiring in switchgear. The wire type is SIS. The particular dielectric test set is variable between 0 & 3000 volts.

Can the conductor be visually inspected along it's run ?

I would suspect the flash would be at the fault.

 

[mivey]

Will testing the conductor with one lead on the copper and one lead on the insulation show all damaged conductors?

Not unless the insulation is compromised right where you are clamping.

 

[mivey]

Yes, that is the problem I'm running into. We have thought about using a hose to wet the exposed conductor in an effort to replicate the fault, but I was just wonerding if there was a test method other than what we are using that would be more effective. I was wondering if a step voltage test would be better?

Simulating rain would be my choice if the dry tests were not helping. With your setup, it sounds like the good thing about this test is you can hydrate in sections until you get a failure.