Megging Insulation

[ptonsparky]

Fire Job.

I have talked about a Forensics Engineer taking samples of smoke deposits on/in a transformer. While he was there we megged the 208 3ph panels with all the breakers off. He was gone getting some supplies for his test so for the hell of it I put the megger leads on the insulation of a larger set of wires leaving this enclosure then repeated this "test" on his return. Clamped to the insulation about 8" apart. While the readings are still high, it is pretty obvious they don't match up to a similar length of new that we compared it with.

Have any of you tried this on older installations that just had an accumulation of dust? Do you remember what you had? I know this is comparing apples to oranges.

A set of paralled feeders that also "failed" totals about 2000' of 300MCM CU.

Any way to clean the carbon from them that would make you feel good about hooking them back up?

 

[Cow]

So what did you test at and what were the results? It seems like most of the older feeders I test usually come in around 1500-2500 megohms at 1000V.

I wouldn't have a problem wiping them down(at the termination points) with simple green and a rag to get the majority of carbon off. Then follow up with some sort of contact cleaner to make sure they were spotless. Re-meg and see if the results are any different. Cleaning the wire is really the easy part. It's all the carbon that builds up in the enclosures, switchboards, breakers, etc that you have to figure out how to deal with...

Bottom line though, I wouldn't worry if they didn't test the same as new as long as they tested high.

 

[ptonsparky]

My help did the tests on (6 cables) about 18 inches of insulation. Ranged from 1680M (1052vdc) to 6.4G(1053vdc).

Last 4" before entering conduit ranged from 840M (1052vdc) to 175.3M (1052vdc). Oddly enough these readings are respective to the cables above.

New cable tests full range of 11.G.

 

[brian john]

I have done it on carbon covered conductors, THHN and THW, got from a dead shoot "0" megohms and infinity (full scale of the meter)

 

[T.M.Haja Sahib]

My help did the tests on (6 cables) about 18 inches of insulation. Ranged from 1680M (1052vdc) to 6.4G(1053vdc).

Last 4" before entering conduit ranged from 840M (1052vdc) to 175.3M (1052vdc). Oddly enough these readings are respective to the cables above.

New cable tests full range of 11.G.

Did you meggar conductor to ground or insulation to insulation?

 

[ptonsparky]

Insulation to insulation.

 

[T.M.Haja Sahib]

Insulation to insulation.

To ascertain the condition of the insulation,you may do time resistance test using your meggar.Keep your meggar on for ten minutes or so and note down insulation resistance at regular intervals.Make a graph of it:insulation resistance verses time.If the graph rises with time,your installation insulation is sound.But if the graph does not rise but flattens out after an initial rise as time progresses,the insulation resistance of your installation is poor.Without drawing a graph,the condition of insulation may also be ascertained by 'polarization index'or 'dielectric absorption ratio' as given in
http://ecmweb.com/mag/electric_insulation_testing/