Fuse block megger reading low

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Davebones

Senior Member
Location
North Of Tampa Fl
I replaced a bad contactor for a 150 hp , 480v motor . Reconnected everything and was doing a quick megger to verify everything was safe to power back up . Noticed that I was reading .8 meg ohms at 500 v to ground each leg . Couldn't find any issue with anything I did , so I isolated the fuse block . The fuse block is a Littlefuse ( LR60400-3CR 400 amp ) fuse block . The fuse block is reading .8 meg to ground . Fuse block is in a clean cabinet not sure why I'm seeing this low of reading . Any thoughts on this ...
 
Yes I removed the fuse's and load side cable's too . Wiped down the fuse block . There was dust but not bad . Have got it up to 1.5 meg . Never saw this with 480 volt fuse block before .
 
Davebones said:
Yes I removed the fuse's and load side cable's too . Wiped down the fuse block . There was dust but not bad . Have got it up to 1.5 meg . Never saw this with 480 volt fuse block before .
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Sounds failed to me. Is this a new or used fuse block?
 
I have found tracking on the back of the block from clip retaining screws to enclosure. I assumed it was from lightning.
 
Davebones said:
Used fuse block . About 15 years old .
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Failed. Get a new one.

I’ve seen tracking through to the back side from the terminals/studs. If you remove the fuse block and re-test without anything conductive on the back side you will probably see the reading jump back up to normal, but again it’s indicative of tracking to the back plane.
 
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