What was found
(CAN) DURING PREFLIGHT INBD ENG FIRE EXTINGUISHER WAS DISCHARGED. A NEW FIRE BOTTLE WAS INSTALLED & FIRE HANDLES WERE TESTED SERVICEABLE, ACFT WAS RELEASED FOR FLIGHT. UPON ARRIVAL BACK, INSPECTED FIRE BOTTLES & FOUND SAME BOTTLE HAD FIRED AGAIN. ACFT WAS GNDED TO DETERMINE CAUSE OF FAILURE. FURTHER TROUBLESHOOTING REVEALED WIRING ON SWITCH 1WB2, FIRE BOTTLE SELECT SWITCH NR 1 ENG, WAS COMING INTO CONTACT WITH FEATHER HANDLE MECHANISM IN CTR PEDESTAL. ON REMOVAL OF SWITCH HOT WIRES ON SWITCH WERE FOUND ABRADED APPROX 1" FROM SWITCH. WIRING ALSO MIS-ROUTED WHICH EXACERBATED THE PROBLEM. IT WAS NOT EVIDENT WHAT WAS CAUSING THE ABRASION. ON CAREFUL REASSEMBLY OF THE SWITCH & WIRING IT WAS FOUND TO ABRADED BY A GUIDE PIN WHICH WAS SITTING PROUD FROM THE FEATHERING HANDLE. PIN WAS PROPERLY INSTALLED & WIRING REPAIRED, PROPERLY ROUTED & NEW SWITCH INSTALLED. SYS CHECKED SERVICEABLE WITH METER & ACFT DISPATCHED WITH NO FURTHER FAULTS. ON PREFLIGHT OPERATION OF THE FEATHERING HANDLE IT WAS CONTACTING THE BARE WIRES & SENDING POWER TO THE SQUIB CAUSING THE SQUIB TO FIRE.