B777 CAPT REPORTS PAX OXYGEN MASKS DROP AT THE GATE AFTER A POWER INTERRUPTION.

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GND CREW MEMBER GAVE ME A HAND SIGNAL TO DISCONNECT GND PWR. BOTH PRIMARY AND SECONDARY GND PWR SOURCES WERE CONNECTED. THE APU WAS OPERATING SUPPLYING PWR TO THE L AC BUS. PRIMARY GND PWR WAS POWERING THE R AC BUS. I GAVE THE GND CREW MAN A HAND SIGNAL TO DISCONNECT THE GND PWR AFTER I HAD PUSHED ONE OF THE GND PWR SWITCHES ON THE OVERHEAD PANEL. THE REQUEST TO DISCONNECT GND PWR INTERRUPTED WHAT THE RELIEF PLT AND I HAD BEEN DOING AND I RETURNED TO OUR PREVIOUS ACTIVITY IMMEDIATELY AFTER PRESSING THE SWITCH AND GIVING THE HAND SIGNAL TO DISCONNECT GND PWR. THE PWR INTERRUPTION OCCURRED 3 OR 4 MINS LATER WHILE WE WERE STARTING THE BEFORE STARTING ENGS CHKLIST. ONE OF THE FLT ATTENDANTS CAME TO THE COCKPIT AND TOLD US THAT ALL OXYGEN MASKS IN THE CABIN HAD DROPPED. IT IS POSSIBLE THAT THE PWR INTERRUPTION WAS CAUSED BECAUSE I HAD INADVERTENTLY PRESSED THE SECONDARY GND PWR SWITCH ON THE OVERHEAD PANEL WHICH RECONNECTED SECONDARY GND PWR TO THE L AC BUS AND DISCONNECTED THE APU GENERATOR. IF THAT WAS THE CASE; AFTER GND PWR WAS DISCONNECTED THERE WAS A MOMENTARY PWR INTERRUPTION BEFORE THE BUS TIE RELAYS WERE ABLE TO ALLOW THE APU GENERATOR TO PWR THE L AND R AC BUSSES. THIS EVENT COULD HAVE BEEN AVOIDED BY TAKING TIME TO VERIFY THAT THE CORRECT SYS LIGHTS WERE ILLUMINATED BEFORE ENGAGING IN OTHER COCKPIT DUTIES.