We often get calls to check switchboards that have had arcing faults and have insulators covered in carbon. Quite often electricians will tell us that they checked the bus with their ohm meter and all was clear. We try to tell them not to re-energize the board till the switchboard has been properly meggered either by us or I will supply a number for a firm that can perform the test (other than us) as sometimes electrical contractors think this HOO DOO VOO DOO we do is all about trying to separate them from their money and in these cases while money is an issue, my concern is their safety.
The test shown below was completed at 1000 VDC utilizing a megger, the carbon is pencil lead. The paper is a good insulator (meggered in the Gigaohms range) till it was contaminated with the pencil lead. The same test completed with an ohm meter showed no problems reading were acceptable. The difference is a 9 VDC ohm meter VS a 1000VDC megger.
Exactly what you show there is what happens in the real world.
I deal a lot with intermittend HT faults on 11 kV transformers. Dirty bushings cause a flasover, blow HT fuse. I always insist that the line crews disconnect the 11 kV cables prior to do an insulation test on the cable because of that reason.
I will do a similar test with a 5 kV megger, to show some of our fault staff why cables and bushings have to be clean.
Thanks for those great photo's
The product of rotation, excitation and flux produces electricty.