I am using a 350A DC, normally open vacuum contactor that experiences a failure while the contactor is closed (coil energized by 24VDC) for an industrial 1000VDC application. The manifests as the contact set being "stuck" in the closed state even after the 24VDC is removed from the coil's terminals. It is unclear to me why the spring mechanism would fail to release after removing the control voltage.
After some failure analysis, it seems that if one taps the solenoid casing with a soft hammer, the spring will release and the contactor will return to its normally open state. If I reapply 24VDC to the coil, the contactor will close and, if I remove the 24VDC from the coil, the contactor will open.
Has anyone seen this phenomenon before? Any insight into what can be happening electrically to cause the coil to get "stuck" closed would be appreciated.
Edit 1; I've added a photo of an example failed contact that was opened up to view the damage. Please note the pitting, but no obvious signs of melting on the copper conductor used to switch the voltage. The pitting was smooth to the touch and doesn't seem like it would have kept the contactor together against the force of the spring trying to release it.