Sounds like an arcing ground fault. Once something starts an arcing fault in that voltage system, the MCC or switchgear can sometimes completely melt down without tripping the OCPD on the line side of the fault. The fault could have been initiated by a small animal or build up of damp dust. This arcing burn down problem on 480/277 volt systems is the reason that ground fault protection of equipment is required when the main OCPD is rated 1000 amps or more, however the problem can occur on smaller systems.
Don(resqcapt19)
