Polycarbonate Stress Shields

[BritEng]

Hi,

We have had an insulation breakdown in an 11kV VCB. We have sent this away for investigation and the culprit was a stress shield mounted on the end of the vacuum bottles. The clear shields had black traces on them. We have been informed that Polycarbonate will sooner or later give up it's carbon when surrounded by an electrical field. Has anyone else experienced this or have any further information?

Many Thanks

 

[safy2004]

Hi
I have seen many cases like this.
In the circuit breaker you have four main active parts the mechanism, the vacuum bottles, the (shields & insulators) and the (wiring & control).
I guess you have no trouble with the mechanism and the wiring & control.
If you have tested the vacuum bottles and they are OK, you have only one part remaining.. the shields.
The decision for keeping or replacing the shields depends on the cracks, if there are cracks in the shields you have to change them because any cleaning or repairing will be useless, but if the carbon is just covering the shields and not effected their material, you can just clean them and use the circuit breaker again without any troubles. Most of the times the shields are not destroyed in one breakdown.