DC Switch Gear Ground Fault

[metroelectric]

We are trying to troubleshoot a frame to ground test on a DC switch gear cubicle. (Background-Installing temporary DC switch gear for a transit project. The switch gear rests on 1/2" thick insulation sheets, anchored to ground using insulated anchors and joined together using insulated bolts. The switch gear is rated for 8000A). Performed a frame-to-ground test on each cubicle at 2500VDC. All but one cubicle passes at that voltage. (criteria to pass is to exhibit a resistance of 100Mohms). The failing cubicle gives a resistance value of 2.5GOhms at 1500VDC but fails as we increase the voltage. we tried all possible trouble shooting methods but could not identify the fault. Any help to solve this issue would be appreciated.

 

[petersonra]

Have you tried disconnecting the incoming and outgoing wires while you do the test? Maybe the insulation on one of them has started to degrade.