A B777-200 GENERATOR DID NOT COME ON LINE AFTER A DELAYED TAKEOFF START. UPON RETURN TO THE GATE FOR MAINTENANCE; AN INCORRECT MEL SIGN OFF ALLOWED DISPATCH. IN FLIGHT THE CREW DISCOVERED THE MAINTENANCE SIGN OFF ERROR.

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OUR FLT TAXIED OUT FOR TKOF TO RWY 14L. AFTER WAITING FOR AN EXTENSIVE PERIOD OF TIME; THE DECISION WAS MADE TO START THE APU AND SHUT DOWN THE L ENG. SHORTLY AFTER THAT; WE WERE INFORMED TO START ALL ENGS FOR TKOF. UPON RESTART; WE NOTED A L GENERATOR CONTROLLER OFF LIGHT ILLUMINATED; WITH NO CORRESPONDING EICAS MESSAGE. A RESET WAS ATTEMPTED WITH NO RESULTING HELP. A SHUTDOWN AND RESTART OF THE L ENG WAS ALSO ATTEMPTED. THIS DID NOT EXTINGUISH THE LIGHT. WE TAXIED BACK TO THE GATE FOR FUEL AND MAINT. MAINT SIGNED OFF THE LOGBOOK UNDER MEL ITEM #24-8 WITH NO RESTRS INDICATED IN THE ACTION TO BE TAKEN COLUMN. AFTER TKOF; NOTED THAT THE ELECTRICAL SYNOPTIC INDICATED THE L GENERATOR DID NOT APPEAR TO BE PWRING THE BUSSES. AGAIN; THERE WAS NO CORRESPONDING EICAS MESSAGE ASSOCIATED WITH THIS CONDITION. CONTACTED DISPATCH AND WITH A PHONE PATCH TO MAINT AND AT THEIR DIRECTION; THE L BUS TIE WAS OPENED AND IT WAS ESTABLISHED THAT THE LOSS OF L MAIN AC BUS HAD OCCURRED. WITH THE FUEL ON BOARD; DISPATCH RELEASED THE ACFT AIRBORNE TO ZZZ WITH THE APU RUNNING ENRTE. AN ADDITIONAL BURN OF 3000 LBS OF GAS WAS CALCULATED BY DISPATCH. DISPATCH AND MAINT INFORMED US THAT ZZZ MAINT WOULD BE INFORMED. INADVERTENTLY; NO LOGBOOK ENTRY WAS MADE. SUPPLEMENTAL INFO FROM ACN 812824: I THINK OUR MECHS LOOKED AT OUR PROB WHEN WE RETURNED TO THE GATE AND SIGNED OFF A PROB AND PLACARDED A PROB THAT WE DIDN'T HAVE. NOT KNOWING; OR BEING REQUIRED TO KNOW; MORE ABOUT THE MAINT FUNCTIONS OF OUR ELECTRICAL SYSTEM; LEFT US IN A POOR POSITION TO TROUBLESHOOT THE PROB AND ALLOWED US TO HAVE SOMETHING SIGNED OFF THAT WE DIDN'T UNDERSTAND. IT WAS A SAFE FLT AS THE B777 HAS MULTIPLE ELECTRIC SOURCES AND THE MEL ALLOWS FOR SEVERAL FAILURES TO DISPATCH SAFELY.