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| Megger tests: breakdown voltage puzzle |
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| duak:
Two layers of insulation does not guarantee that the breakdown voltage is the sum of the individual layers breakdown voltages. For a number of reasons such as material type, dye type, manufacturing differences, voltage dependancy and contamination, the actual resistances of the layers will be different. Since this is a series circuit, the greatest voltage is developed across the highest resistance. eg., let's say one layer is 100 Gohm and the other is 200 Gohm and both are rated for 1 KV. If we apply 1 KV across the stack one layer sees 333 V and the other 667 V. Doubling the applied voltage to 2 KV nets 667 V across one and 1333 V across the other. If it's only rated for 1 KV, then it's being operated outside of its rating. I don't know what the resistance spread is for typical jackets, but I could easily envisage one batch of insulation being so much better than the average and ultimately failing because it took most of the voltage. I don't know if there's a name for this effect, but it must have something to do with Murphy. Many wiring standards require that all wires in a cable or raceway have to be rated for the highest voltage experienced by any one of them - unless there are special considerations such as grounded shields around the HV wires. |
| bob91343:
Confusing the issue is that these resistances are not linear. 100 G ohms in series with 200 G ohms will not measure 300 G ohms due to leakage, polarization, and other stuff. |
| SG-1:
Using old (10-15 year) 24 gauge telephone line conductors. My DC insulation tester is a Fluke 1507 & only goes to 1000VDC. Each scale would just max out. With 1000VDC all samples read 11 Gig Ohms. Moving on to a 50KVAC Dielectric Test Set. Single conductors wrapped in aluminum tape. The rate of rise was about 1KV per second. The first sample broke down at 13KV AC RMS. The second sample at 15KV SC RMS. Leads were longer past the aluminum tape in case the first sample may have failed because there was not enough creep distance. The third sample failed at 18KV AC RMS. Semi-conduction tape was added at the aluminum tape ends to weaken the e-field at the boundary. Two conductors twisted together. First sample failed at 25KV AC RMS. On this sample the loose ends flashed over once & had to be spread apart more. Second sample failed at 35KV AC RMS. |
| SG-1:
Same tests above were repeated with CAT 5. Single conductor wrapped in aluminum tape failed at 10KV AC RMS. Twisted pair failed at 25KV AC RMS. |
| taydin:
Thanks a lot for the detailed tests, that was very educational! In fact, I was so worked up when the piss poor cable I had was resisting 5 kV that I thought "would be so nice to have a 15 kV megger now to destroy this thing". But apparently I would have been much more raged to see that even 15 kV doesn't do the business :o The blue cable I tested that resisted 5 kV has a rating of 250V! This has gotta be assuming AC voltage at worst conditions (high heat, high humidity etc). |
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