Connect the external power to the external power socket EPR, and the red external power hot bus (EXT PWR HOT BUS) warning light on the P34 board will illuminate. At this point, the DC interlock circuit connected to the BPCU will engage, and the BPCU will automatically check the quality of the external power supply. If the external power voltage, frequency, and phase sequence are correct, the white AC power connected (AC PWR CONNECTED) light on the P30 panel will light up. A white external power indicator light is present on the B757's electrical system control panel; if the external power is not selected, the "PWR NOT IN USE" light on the P30 board will clearly illuminate.
② The external power (EXT PWR) switch on the external power supply control panel is triggered, activating the power system. This means that the power system is immediately engaged upon pressing this switch. The Bus Power Control Unit (BPCU) causes the auxiliary power circuit breaker and the generator circuit previously connected to open, supplying power to the external power contactor. The external power available (EXT PWR AVAIL) indicator on the control panel illuminates, as does the AC PWR CONNECTED light on the P30 panel. The PWR NOT IN USE light on the P30 panel is turned off. Pressing the EXT PWR switch again disconnects the external power. Aviation ground power 2, 36V aviation power 1, 400Hz variable frequency power supply 7, 115/200V power supply 2, 27V DC power supply 5, 400Hz power supply 12, aviation power.
If the engine starts and the speed reaches an adequate level, the voltage emitted by the generator will meet the requirements, at which point the EPC will engage. The generator circuit breaker will be activated, and power will be supplied by the main generator. Pressing the external power (EXT PWR) switch will connect the external power to the busbars.
If the external power supply current exceeds 330±70 A per phase, the BPCU will disconnect the external power supply.
