What am I doing wrong?

[HotConductor]

I meggered two #8 thhn conductors in an underground conduit. I tested them(Fluke 1587)against each other @500v for a one minute test period and they tested fine(550 meggohms). I then tested them each to a 277v good source with my Knopp K60 and they were both grounded. I've had the same thing happen in the past. I tested one conductor at a time with every other conductor connected together and they insulation tested fine. They weren't though.

Am I doing something wrong with the insulation test?

 

[knoppdude]

I meggered two #8 thhn conductors in an underground conduit. I tested them(Fluke 1587)against each other @500v for a one minute test period and they tested fine(550 meggohms). I then tested them each to a 277v good source with my Knopp K60 and they were both grounded. I've had the same thing happen in the past. I tested one conductor at a time with every other conductor connected together and they insulation tested fine. They weren't though.

Am I doing something wrong with the insulation test?

Hi HotConductor. Can you go into more detail about testing using the 277v applied? I don't think you are doing anything wrong with the megger, but I would need more information about the other test before I could conclude anything.

 

[ptonsparky]

Are you saying that your Knopp gave a voltage indication even though the wire under test was not connected to anything on either end? What is the length of these wires? I am not familiar with a Knopp.

Isolate the wires on each end. Test each to earth or the conduit if it is metallic. Make sure you allow it to discharge after the test. Check the next, etc. Then check each against the other.

 

[HotConductor]

The Knopp did it's job, the conductors were burned up in the conduit. The conduit was full of water too. The conductors were isolated at both ends during tests. The conduit was pvc.

I don't understand, though, why the insulation tester didn't show this.

Should I have tested them each with the egc?

 

[tx2step]

Yes, you should also meg each to the egc (when there is one in the conduit) &/or to metalic conduit (when used) to check insulation integrity to ground.

 

[ptonsparky]

We quite often can locate a fault because the pvc is usually cracked very close to the damaged area. (Buried pvc with insulated EG) Really no reason you should not be able to meg the wires to prove pass/fail. Make sure you are using it correctly. Have the leads in the correct jacks, been there, practice on something else. Maybe move your left foot to the West a bit?

 

[Cow]

You need to isolate all the wires in the conduit at both ends. Then test EVERY wire against EVERY other wire in the conduit.

If they're nicked/rubbed through in a dry spot in the conduit your megger may not show it. I usually have the opposite problem though, the wires/peckerhead are wet and the test fails. Then I dry the ends with a heatgun and they test great.

You can accidentally condemn a lot of wire and equipment if you're not paying attention.

 

[HotConductor]

Cow - when you test against every other conductor, do you test them separately or test one against every other conductor tied together?

 

[ptonsparky]

Separate each one from the other. Both ends. That will you a comparison value as well. If two check at 4G and one at 5meg time to look closer.

 

[HotConductor]

Would it ever make sense in a case like this to drive a short piece of ground rod and test each conductor against that?

Is it safe to say that insulation testing underground conductors is useful but not always 100% conclusive.

 

[ptonsparky]

All the wires would have to be broken clean and separated with the ends dry. If anything we have found problems more easily with the megging than without.

 

[cadpoint]

Frankly I thought your number was low as posted! (maybe I'm lost on that thought) Having said that, I don't recall drying the wires, from the Meggar (book) but that seperation is required for safety during testing. Frankly drying sounds like cheating,,, I've seen water dribble out of wires...

It's not that water is conductive (pure water) it's the particles that's in the water that makes water conductive.