Electrics - ETOPS Show Schematic only

DC Electrical System. Airplanes with 90-minute standby power - The main and APU batteries are paralleled if power is lost at the left 28 volt DC bus. APU Battery main Battery Battery Switch Battery Discharge Lights Static Inverter Standby Power Selector Switch Standby Power OFF Light Hot Battery Buss Battery Bus Standby DC Bus Standby AC Bus Left Main DC Bus Right Main DC Bus Centre Buses HDG - As Installed AC Transfer Relays Power will transfer providing at least 2 generators are operating and 3 autopilots armed for approach If there is a loss of power on one of the two DC buses, the DC BTB will close automatically. Left AC Transfer Bus Right AC Transfer Bus AC Electrical System. Start APU Connect External Power Shutdown Left Engine Shutdown Right Engine AC Buses and Load Shed System Generator Drive Disconnect Generator Drive Disconnect AC Bus Tie System AC Bus Tie System Generator Control Switch Generator Control Switch Ground Service Switch Ground Service Bus Ground Handling Bus - The Ground Handling Bus can only be powered on the ground by the APU or External Power. It cannot be powered from the engine-driven generators Left AC Bus OFF Light Right AC Bus OFF Light Left Flight Instrument Transfer Relay Right Flight Instrument Transfer Relay Left AC Bus Right AC Bus Captain's Flight Instruments Transfer Bus First Officer's Flight Instruments Transfer Bus

A Hydraulic Driven Generator (HDG), supplied from the Left Hydraulic System, is installed on some airplanes as a fourth AC generating source. It operates automatically, after a short delay, providing a backup power source with the loss of both left and right AC buses in flight.

Because the DC voltage from an operating HDG may be less than that from a fully charged Battery, the Battery discharge lights may illuminate for a period time when the HDG first begins to operate.

The HDG powers the:

The First Officer's Flight Instrument Transfer Bus is not powered by the Hydraulic Driven Generator.