B777 captain reports passenger oxygen masks drop at the gate after a power interruption.

Attributes
ACN	779972
Time
Date	200803
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : zzz.airport
State Reference	US
Altitude	agl single value : 0
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	B777-200
Operating Under FAR Part	Part 121
Flight Phase	ground : preflight
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
ASRS Report	779972
Person 2
Affiliation	company : air carrier
Function	flight attendant : on duty
Events
Anomaly	aircraft equipment problem : less severe
Independent Detector	other other : 2
Resolutory Action	none taken : detected after the fact
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Aircraft

Narrative:

Ground crew member gave me a hand signal to disconnect ground power. Both primary and secondary ground power sources were connected. The APU was operating supplying power to the left AC bus. Primary ground power was powering the right AC bus. I gave the ground crew man a hand signal to disconnect the ground power after I had pushed one of the ground power switches on the overhead panel. The request to disconnect ground power interrupted what the relief pilot and I had been doing and I returned to our previous activity immediately after pressing the switch and giving the hand signal to disconnect ground power. The power interruption occurred 3 or 4 mins later while we were starting the before starting engines checklist. One of the flight attendants came to the cockpit and told us that all oxygen masks in the cabin had dropped. It is possible that the power interruption was caused because I had inadvertently pressed the secondary ground power switch on the overhead panel which reconnected secondary ground power to the left AC bus and disconnected the APU generator. If that was the case; after ground power was disconnected there was a momentary power interruption before the bus tie relays were able to allow the APU generator to power the left and right AC busses. This event could have been avoided by taking time to verify that the correct system lights were illuminated before engaging in other cockpit duties.

Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of May 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.