Did I find the short?

[Ken9876]

I must admit that I?m no expert when it comes to the use of a megger, however today I was using one to find a short that a DMM would not pick up. I just want to verify my findings, we had 6 suspect conductors all were isolated at both ends. We used the megger, all but one when right to 55 M ohms at the 50 volt test level. The last one slowly climbed up in value, so I figured that must be the bad conductor, am I correct in this assumption?

 

[LEO2854]

I must admit that I?m no expert when it comes to the use of a megger, however today I was using one to find a short that a DMM would not pick up. I just want to verify my findings, we had 6 suspect conductors all were isolated at both ends. We used the megger, all but one when right to 55 M ohms at the 50 volt test level. The last one slowly climbed up in value, so I figured that must be the bad conductor, am I correct in this assumption?

Did it show that it had continuity to ground or on of the other conductors?

 

[dereckbc]

Likely but what you descried by the resistance starting low, and then climbing is just what one would expect testing a good large value capacitor. Resistance starts low at the very first then builds up as the capacitor charges.

Try this preform the same test on the suspect wire, then immediately reverse the polarity. If it kicks negative direction (less than 0 Ohm's), then reverses and climbs tells you there is a capacitor somewhere on the conductor.

 

[stew]

op said that the conductors were isolated at both ends. hence where would a cap be? my opinion is that if you got a low reading (And I assume you meant 500 v not 50) you have the bad conductor. was the reading taken to ground? If it was then it seems like not a direct ground but one that has failed to ground at some time. The megger will then show a very low reading to start but will look like a capacitor because the carbon track will conduct readily at first and the degrade over a short span.This phenomena occurs with electric motor windings as well. You pull the defective coils out of the slot and you find a nice little burn hole thru the slot insulation. Happened to me many times in the motor business when I had to analyze a winding failure for warranty purposes and confirm my intial diagnosis.

 

[ptonsparky]

Dereck means the wires are acting as a cap when they are being charged with the megger. I hope the OP is using the 500v setting.

BM I used a Greenlee tester to charge 1/4 mile runs of wire to pivots/wells. Good wires would hold that charge for a time. The bad ones would show fault to grnd right away or charge slowly and discharge very quickly. Compared to the others.

 

[stew]

yes i thought thats what dereck meant after I reread his reply.

 

[K8MHZ]

Being kind of a troubleshooter at heart, I have to ask what the reason was to check the conductors? Were they in service and presenting a problem, or is this pre-service testing to make sure there are no problems?

Also, what size and type are the conductors?

As Dereck alluded to, something appears to be acting as a capacitor. The reading you are getting from the megger may be well within the specs for that particular conductor at that particular length in that particular environment.

What get's my spidey senses tingling is the difference between the readings of each conductor. If all conductors are the same in every respect, they should read the same.

Are you sure that there is not moisture or tape goo on the ends of the suspect conductor? I like to clean, clean and re-clean the ends of a failed conductor and re-test. You would be amazed at how often the bad readings are due to little things like water or glue.

 

[Ken9876]

Being kind of a troubleshooter at heart, I have to ask what the reason was to check the conductors? Were they in service and presenting a problem, or is this pre-service testing to make sure there are no problems?

Also, what size and type are the conductors?

As Dereck alluded to, something appears to be acting as a capacitor. The reading you are getting from the megger may be well within the specs for that particular conductor at that particular length in that particular environment.

What get's my spidey senses tingling is the difference between the readings of each conductor. If all conductors are the same in every respect, they should read the same.

Are you sure that there is not moisture or tape goo on the ends of the suspect conductor? I like to clean, clean and re-clean the ends of a failed conductor and re-test. You would be amazed at how often the bad readings are due to little things like water or glue.

Yes they were in service, however now a low amp fuse is blowing. The conductors are #10 XHHW run of 500? . I used a fluke megger set on the 50v range to check the conductors because there are at least 50 other conductors in the same conduit all for control and PLC wiring. Which leads me to another question if there is an induced voltage on the conductors would a DMM not measure the resistance due to the presence of that voltage? We did find a set of splices that were wet inside of the wire nuts. We connected another set of spare conductors and so far the short has gone away.

 

[Cow]

I would probably have tested a little higher than 50v, what voltage do the particular wires you were testing run at? Sometimes wires look good at 50v, okay at 250v, and junk at 500v and 1000v....

Also, moisture can really screw with your readings. Don't get too excited and start condemning wires until you're sure the ends of the wires are dry.

 

[Ken9876]

I would probably have tested a little higher than 50v, what voltage do the particular wires you were testing run at? Sometimes wires look good at 50v, okay at 250v, and junk at 500v and 1000v....

Also, moisture can really screw with your readings. Don't get too excited and start condemning wires until you're sure the ends of the wires are dry.

At 120, I started low because I was a little gun shy with all the other circuits in the conduit. Five of the six wires when tested shot right up, but the last one took some time for the value to go up. So I figured there must be something wrong with that wire. I did find a wet splice under the wire nut of that conductor, it was laying in a JB in the ground, I don't remember what it was resting on however, could of been one of the bonding jumpers.

 

[kwired]

At 120, I started low because I was a little gun shy with all the other circuits in the conduit. Five of the six wires when tested shot right up, but the last one took some time for the value to go up. So I figured there must be something wrong with that wire. I did find a wet splice under the wire nut of that conductor, it was laying in a JB in the ground, I don't remember what it was resting on however, could of been one of the bonding jumpers.

If you are testing for ground faults, you should not have the meter 'shoot right up'. It could shoot up and then rapidly fall as it charges (the capacitor that is naturally made here). If they are all good and one slowly decreases amount of test current it has less capacitance for some reason (maybe open circuit resulting in a shorter conductor being tested)

I agree that higher test voltage is needed. Isn't general rule to test with at least twice normal operating voltage when looking for ground fault? Make sure there are no devices, splices, and especially no loads to grounded conductors present in conductor being tested. Simple continuity test before applying higher test voltage will confirm there is no connected loads to grounded conductors.

 

[Cow]

If you are testing for ground faults, you should not have the meter 'shoot right up'. It could shoot up and then rapidly fall as it charges (the capacitor that is naturally made here). If they are all good and one slowly decreases amount of test current it has less capacitance for some reason (maybe open circuit resulting in a shorter conductor being tested)

I agree that higher test voltage is needed. Isn't general rule to test with at least twice normal operating voltage when looking for ground fault? Make sure there are no devices, splices, and especially no loads to grounded conductors present in conductor being tested. Simple continuity test before applying higher test voltage will confirm there is no connected loads to grounded conductors.

I believe so, Zog and Brian John are the real gurus with the meggars though....

To be totally honest, 9 times out of 10 I'm testing THHN and it's being tested at 1000VDC. I'll throttle the voltage back when I'm testing Romex, etc but most of the stuff I test is usually 480v motors and T leads.

 

[kwired]

I believe so, Zog and Brian John are the real gurus with the meggars though....

To be totally honest, 9 times out of 10 I'm testing THHN and it's being tested at 1000VDC. I'll throttle the voltage back when I'm testing Romex, etc but most of the stuff I test is usually 480v motors and T leads.

Same here, if it is 600 volt insulation it usually gets a 1000 volt test no matter what it normally operates at.