What was found
PRIOR TO FLIGHT, BATTERY SWITCH WOULD NOT WORK. T/S MX FOUND THE MAIN EMERG PWR RELAY HAD PREVIOUSLY CAUGHT FIRE. PORTIONS OF RELAY WERE MELTED AND EVIDENCE OF FLAMES APPROX 18 INCHES HIGH ABOVE RELAY. ACFT CONFIGURED WITH STC SH498NE 'AFT BATTERY INSTALLATION' WHICH RELOCATES RELAY FROM THE NOSE AREA TO RT SIDE SHELL OF CABIN AREA. NEVER ANY REPORT OF FIRE OR SMELL OF SMOKE REPORTED. INVEST BY MECH REVEALED ONE OF THE WIRES WHICH ENERGIZES COIL OF MAIN EMERG POWER RELAY HAD BROKEN OFF AT TERMINAL END WHERE IT CONNECTED TO ANOTHER COMPONENT. BROKEN WIRE HAD CONTACTED GROUND.