SHOVEL WAS STARTED AT BEGINNING OF SHIFT TO WARMUP BUT WAS NOT MANNED. SOMETIME BETWEEN 9:00 PM & 11:15 PM, AN ELECTRICAL SHORT OR LIGHT BULB CREATED ENOUGH HEAT TO IGNITE INSULLATION ON 24V WIRING. FIRE SUBSEQUENTLY PROPAGATED TO SOUND DEAFENING INSULATION IN CAB OF SHOVEL.