B737-500 WAS DISPATCHED WITH ONE ENG FIRE LOOP DEFERRED. CAPT DISCOVERED THAT OTHER LOOP WAS ALSO INOP.

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ACFT HAD #2 ENG B FIRE LOOP DEFERRED. EXCEPT FOR THE FIRST FLT OF THE DAY; IT IS NOT A REQUIREMENT TO DO A FIRE TEST; HOWEVER AS A PRECAUTIONARY MEASURE I ELECTED TO VERIFY THE REMAINING SYS WAS STILL FUNCTIONING. I DISCOVERED BOTH LOOPS TO BE INOP AND CONTACTED MAINT TO DISCOVER THIS IS A CHRONIC ITEM AND THE LOG HISTORY INDICATED 499 HRS OF PROBS. AS A CAPT AND SAFETY ADVOCATE; I FIND THIS TO BE UNACCEPTABLE AND ANOTHER EXAMPLE OF THE COMPANY'S REACTIVE MAINT PROCESS. AFTER RESETTING THE CIRCUIT BREAKER AND CYCLING THE SYS SELECTOR SWITCH SEVERAL TIMES; THE A LOOP RETURNED TO NORMAL OPERATING STATUS. MAINT ELECTED TO SIGN OFF THE ACFT AFTER WE TESTED AND RETESTED THE SYS MANY TIMES; ENSURING THAT IT WAS FUNCTIONING PROPERLY. WE OPERATED THE FLT 1.5 HRS LATE; WITH THE AGREEMENT THE AIRPLANE WOULD BE REMOVED FROM SVC AND REPAIRED ONCE AND FOR ALL. AS ANOTHER SAFETY MEASURE; I ELECTED TO PLACE OUR JUMP SEATER IN A WINDOW SEAT NEXT TO THE #2 ENG; TO ACT AS AN OBSERVER. I AM CONCERNED WITH OUR CURRENT MAINT PROCS AND THE FACT THE MEL DOES NOT REQUIRE THE REMAINING FIRE SYS TO BE TESTED PRIOR TO EACH DEP; KNOWING 1 LOOP IS ALREADY DEFERRED.