Low megger readings on a cable reel

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philly

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We have a cable reel on which a 16/c #10 cable is wrapped around a cable reel which rotates to unspool cables as a piece of equipment moves.

We noticed that we were getting low megger readings on a particualr motor when megged from the starter bucket, so we started isolating parts of the cable run. Finially we isolated it down to the cable reel.

When we disconnected (5) of the cables in the cable reel at both ends and read them to ground at 1000V with a megger, we saw the the readings bounced around between 12meg and 18meg. We did this on a few more cables disconnected at both ends and saw the same thing. The weird thing however was that when checking L-L readings between any two of these cables our readings were over 4000meg. I would suspect that if each cable had a low L-G reading that two of then would have a low L-L reading. This does not seem to make sense. Does anyone agree?

This cable is several years old and has been in use for a while and was just recently re-located to a different piece of equipment. I have told management that with these low megger readings between 12-18meg the motor and equipment will still run o.k. but it is a sign that the insulation is breaking down and we should be prepared to get a new cable sometime in the near future. Do you guys agree with this assesment?

Is there any explanaion as to why L-G readings would be low, however L-L readings between cables would be high?

Some of the cables are operated at 480V and some at 120V.
 
I agree with your assesment, ANSI/NETA spec for this is at least 100M but depending on the length of your cable 12-18M may be serviceable, although degraded.
 
zog said:
I agree with your assesment, ANSI/NETA spec for this is at least 100M but depending on the length of your cable 12-18M may be serviceable, although degraded.
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The total cable length is about 300ft or so wound on a reel.

zbang said:
Just to make sure, the cable is off any slip-rings involved during the test? I could see slip-rings covered with dirt lowering the readings. If you're really just on the cable, then I'd definitely be watching this.
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The cables being tested are totally removed from the slip rings.


I still dont understand however why the readings between two of these cables would be bad.
 
Have you tried meggering one conductor at a time with the others short circuited and grounded?
Also, does your megger have a guard terminal? If it does, try guarding the end of the cable to eliminate any leakage current where the conductors leave the cable.
 
when megging 3 hase motors i meg at the starter by disconnecting the 3 t leggs and meg one at a time to ground @ 1000v for 1 minute, and i always seem to max out the megger at 2.2 g . i use a fluke insulation tester and never get any readings lower then 2.2 gigs. sounds similar.
 
puckman said:
when megging 3 hase motors i meg at the starter by disconnecting the 3 t leggs and meg one at a time to ground @ 1000v for 1 minute, and i always seem to max out the megger at 2.2 g . i use a fluke insulation tester and never get any readings lower then 2.2 gigs. sounds similar.
Click to expand...
When meggering a motor from the bucket, you only have to megger one leg, if the motor has only three conductors coming from the motor. These conductors are all tied in either a wye or delta configuration. A megger treats the coils as a straight wire, so a good reading on one leg indicates a good reading on the other conductors and the motor. If you get a low reading then you need to disconnect the motor and check each one independently.
 
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