With the battery switch off and no GPU hooked up, these buses are powered these buses are powered. When any of the following doors are opened, the Ground Handling Bus is energized through the Battery Transfer Relay. GND HDLG BUS HOT EMER BAT BUS +- HOT MAIN BAT BUS +- Hot Main Battery Hot Emergency Battery 1. Cargo Door Access Panel 2. Refueling Panel 3. Main Cabin Door 4. Auxiliary Hydraulic Pump Push Button is depressed. Push Button is depressed. BTR

If you walk into the cockpit and observe a GREEN AVAIL light on the main electrical panel, these buses are now powered. EXTERNAL POWER EGHR When the GPU is plugged in, the External Ground Handling Relay closes taking priority over the Bus Transfer Relay. The BTR then opens. GND HDLG BUS HOT EMER BAT BUS +- HOT MAIN BAT BUS +- Hot Main Battery Hot Emergency Battery BTR

DC SVCE BUS If the DC SERVICE BUS push button is depressed on the flight attendant panel, then the DC SERVICE BUS is powered through the Service Bus Transfer Relay. ON GND SVCE BUS PASSLAVCAPTLAVCARGO EMER LT CALLSSMOKE GALLEYLAV LAT PASS DIM UPR PASS ENTR AFT CARGO READ LT VENT ACDC The GPU is now powering the Ground Handling Bus and the DC Service Bus. EXTERNAL POWER EGHR GND HDLG BUS HOT EMER BAT BUS +- HOT MAIN BAT BUS +- Hot Main Battery Hot Emergency Battery BTR SBTR

Now we turn on the Main Battery Toggle Switch. AVAIL SHED EXT PWRDC GEN 2DC GEN 1 AC BUS 2 INV 1INV DC CH DCH ESS BUS DC SVCE/UTLY BUS BAT SEL MAINEMERGBTC STBY BUSEMER BUSDC BUS 2 OFF ON OVRD BAT MAIN BAT CHG EMER BAT CHG DC BUS 1AC BUS 1 DC SVCE BUS EXTERNAL POWER EGHR GND HDLG BUS HOT EMER BAT BUS +- HOT MAIN BAT BUS +- Hot Main Battery Hot Emergency Battery BTR SBTR

Now we turn on the Main Battery Toggle Switch. AVAIL SHED EXT PWRDC GEN 2DC GEN 1 AC BUS 2 INV 1INV DC CH DCH ESS BUS DC SVCE/UTLY BUS BAT SEL MAINEMERGBTC STBY BUSEMER BUSDC BUS 2 OFF ON OVRD BAT MAIN BAT CHG EMER BAT CHG DC BUS 1AC BUS 1 DC SVCE BUS EXTERNAL POWER EGHR GND HDLG BUS HOT EMER BAT BUS +- HOT MAIN BAT BUS +- Hot Main Battery Hot Emergency Battery BTR SBTR

Now we turn on the Main Battery Toggle Switch. AVAIL SHED EXT PWRDC GEN 2DC GEN 1 AC BUS 2 INV 1INV DC CH DCH ESS BUS DC SVCE/UTLY BUS BAT SEL MAINEMERGBTC STBY BUSEMER BUSDC BUS 2 OFF ON OVRD BAT MAIN BAT CHG EMER BAT CHG DC BUS 1AC BUS 1 DC SVCE BUS EXTERNAL POWER EGHR GND HDLG BUS HOT EMER BAT BUS +- HOT MAIN BAT BUS +- Hot Main Battery Hot Emergency Battery BTR SBTR

Now with the main battery on, these buses are powered. FAULT AVAIL SHED DC GEN 2DC GEN DC CH DCH DC SVCE/UTLY BUS BAT SEL MAINEMERGBTC DC BUS OFF DC BUS OFF BUS OFF INV FAULT BUS OFF EXT PWR EMER BAT CHG EMER BUSESS BUSSTBY BUSAC BUS 2AC BUS 1 INV 1 DC BUS 1 OFF ON OVRD BAT MAIN BAT CHG INV 2 DC BUS 2 DC EMER BUS DC STBY BUS DC ESS BUS HMBTC HEBTC SBTC HBTC INV 1 DC SVCE BUS EXTERNAL POWER EGHR GND HDLG BUS HOT EMER BAT BUS +- HOT MAIN BAT BUS +- Hot Main Battery Hot Emergency Battery BTR SBTR HEBTC = Hot Emergency Bus Transfer Contactor HMBTC = Hot Main Bus Transfer Contactor SBTC = Standby Bus Transfer Contactor HBTC = Hot Battery Transfer Contactor 115 VAC SBY 26 VAC SBY

When the external power pushbutton is depressed, these buses are powered. FAULT AVAIL SHED DC GEN 2DC GEN DC CH DCH DC SVCE/UTLY BUS BAT SEL MAINEMERGBTC DC BUS OFF DC BUS OFF BUS OFF INV FAULT BUS OFF EXT PWR EMER BAT CHG EMER BUSESS BUSSTBY BUSAC BUS 2AC BUS 1 INV 1 DC BUS 1 OFF ON OVRD BAT MAIN BAT CHG INV 2 DC BUS 2 DC EMER BUS DC STBY BUS DC ESS BUS HMBTC HEBTC SBTC HBTC INV 1 DC SVCE BUS EXTERNAL POWER EGHR GND HDLG BUS HOT EMER BAT BUS +- HOT MAIN BAT BUS +- Hot Main Battery Hot Emergency Battery BTR SBTR 115 VAC SBY 26 VAC SBY

SDTC When the external power pushbutton is depressed, these buses are powered. FAULT AVAIL SHED DC GEN 2DC GEN DC CH DCH DC SVCE/UTLY BUS BAT SEL MAINEMERGBTC DC BUS OFF DC BUS OFF BUS OFF INV FAULT BUS OFF EXT PWR EMER BAT CHG EMER BUSESS BUSSTBY BUSAC BUS 2AC BUS 1 INV 1 DC BUS 1 OFF ON OVRD BAT MAIN BAT CHG INV 2 DC BUS 2 ON UTILITY BUS 2 UTILITY BUS 1 DC BUS 1DC BUS 2 DC EMER BUS DC STBY BUS DC ESS BUS HMBTC HEBTC SBTC HBTC INV 1 DC SVCE BUS EXTERNAL POWER EGHR GND HDLG BUS HOT EMER BAT BUS +- HOT MAIN BAT BUS +- BTR SBTR 115 VAC SBY 26 VAC SBY 115 VAC BUS 2 26 VAC BUS VAC BUS 1 26 VAC BUS 1 INV 2 ACBTR ASTR #1 INVERTER FAIL SIGNAL INVERTER FAIL SIGNAL

SDTC When DC Bus 1 is powered the Static Inverter Override Transfer Contactor (SDTC) closes and the HBTC (Hot Bat Transfer Contactor opens). FAULT AVAIL SHED DC GEN 2DC GEN DC CH DCH DC SVCE/UTLY BUS BAT SEL MAINEMERGBTC EXT PWR EMER BAT CHG EMER BUSESS BUSSTBY BUSAC BUS 2AC BUS 1 INV 1 DC BUS 1 OFF ON OVRD BAT MAIN BAT CHG INV 2 DC BUS 2 ON UTILITY BUS 2 UTILITY BUS 1 DC BUS 1DC BUS 2 DC EMER BUS DC STBY BUS DC ESS BUS HMBTC HEBTC SBTC HBTC INV 1 DC SVCE BUS EXTERNAL POWER EGHR GND HDLG BUS HOT EMER BAT BUS +- HOT MAIN BAT BUS +- BTR SBTR 115 VAC SBY 26 VAC SBY 115 VAC BUS 2 26 VAC BUS VAC BUS 1 26 VAC BUS 1 INV 2 ACBTR ASTR #1 INVERTER FAIL SIGNAL INVERTER FAIL SIGNAL Once the DC Buses are powered, the associated lights are extinguished on the main electrical panel.