A B737-700 WAS DISPATCHED WITH AN 'ELEC' ANNUNCIATOR LIGHT DEFERRED PER THE MEL. THE ACFT WAS UNKNOWINGLY OPERATED WITH AN AC STANDBY BUS THAT COULD NOT BE POWERED FROM THE BATTERY. RPTR RECOMMENDS MEL SPECIAL PROC REQUIRE FLT CREW TO CONDUCT STANDBY POWER CHK PRIOR TO DAILY FLT.

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WE RECEIVED OUR ACFT IN ZZZ APPROX 4 HRS LATE. THE ACFT HAD MEL 24-16 INOP ELEC LIGHT DATED SEVERAL DAYS PRIOR. THE MEL HAD TWO PROVISOS; 1ST; A STANDBY POWERED CHK DONE AT THE TIME OF DEFERRAL AND 2ND A CHK OF THE BATTERY CHARGER PRIOR TO FLT USING THE PROC SPELLED OUT IN THE MEL. WE COMPLETED OUR PREFLT AND ALSO DID THE BATTERY CHARGER CHECK. IT WENT AS PUBLISHED. ENRTE DURING MY CRUISE FLOW I WAS CHKING THE ELEC PANEL AND NOTICED THAT THE INV ON THE AC SIDE WAS UN-POWERED. WHEN WE ARRIVED AT OUR DEST; I CALLED MAINT CTL TO TALK TO THEM ABOUT THE WRITE-UP IN THE LOGBOOK CONCERNING THE MEL AND THE DEFERRAL. THE DEFERRAL STATED NOTHING ABOUT THE STANDBY POWERED CHK BEING COMPLETED WHEN THE ELEC LIGHT WAS DEFERRED. THIS WAS NOT A SPECIFIC MAINT PROC BUT JUST A PROVISO IN THE MEL. THE FO AND I DECIDED TO DO A STANDBY POWERED CHK JUST FOR OUR OWN COMFORT. WE USED THE STANDBY POWERED CHK FOR -700 ACFT THAT WAS IN THE MEL. THE TEST FAILED. THE AC STANDBY BUS WAS UNPOWERED. I THEN CALLED MAINT CTL BACK AND WE RAN THRU ANOTHER CHK AND DECIDED THAT THE AC STANDBY BUS WAS NOT POWERED. I WROTE THE ACFT UP AND WE WERE ASSIGNED A NEW ACFT. THE ORIGINAL MEL DEFERRAL SAID THAT AT STARTUP; THE ELEC LIGHT ILLUMINATED. THE SIGNOFF WAS A VERY SIMPLE 'OK TO CONTINUE PER MEL 24-16.' I WAS SIGNED OFF BY A MECH. IT IS MY THINKING THAT AT THE TIME OF THE DEFERRAL; THE STANDBY POWERED CHK WAS NOT DONE BUT THE BATTERY CHARGER CHK WAS. IF THIS WERE THE CASE THEN THIS ACFT FLEW FOR SEVERAL DAYS WITHOUT AN OPERATING AC STANDBY BUS. I WOULD RECOMMEND THAT AS A PART OF THIS MEL; A PROC BE PUT IN PLACE TO REQUIRE A STANDBY POWERED CHK PRIOR TO THE FIRST FLT OF EACH DAY.CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE 'ELEC' ANNUNCIATOR LIGHT WAS DEFERRED PER THE MINIMUM EQUIPMENT LIST AND IT WAS OBVIOUS THE REQUIRED SPECIAL PROC CHK OF THE STANDBY POWERED TEST WAS NOT ACCOMPLISHED AT THE TIME OF DEFERRAL. THE RPTR INDICATED THE ONLY MINIMUM EQUIPMENT LIST SPECIAL PROC REQUIRED BY THE FLT CREW WAS A CHK OF THE BATTERY CHARGER AND NOT A STANDBY POWERED CHECK. RPTR SUGGESTED THIS BE CHANGED TO ADD A BATTERY CHARGER AND NOT A STANDBY POWERED CHK PRIOR TO FLT WITH THIS MINIMUM EQUIPMENT LIST DEFERRAL. REPORTEDLY THIS AIRPLANE WAS OPERATED FOR SEVERAL DAYS WITH THE AC STANDBY INVERTER INOP.