Electrical System SGR
Electrical System Overview

  • Generators ON
  • Bus Transfer switch in AUTO

AC Power

  • AC power is supplied to two AC Transfer
    Busses by either the Integrated Drive
    Generators (IDG), the APU generator or, on
    the ground, external AC power.

  • Automatic switching through a Bus Tie
    System insures uninterrupted power to the
    AC Transfer Busses. When a power source
    is selected, the previously selected power
    source is disconnected.

  • A single IDG or the APU generator can
    power both transfer busses.

  • If external power is connected, selecting an
    IDG or APU generator switch ON disconnects
    external power from the Bus Tie.

  • Two Ground Service Busses may be
    powered by either their respective Transfer
    Busses or External Power when selected by
    the Ground Service switch at the Forward
    Attendant Panel.

  • An automatic "Load Shedding" feature is
    activated when powering both AC Transfer
    Busses from a single IDG or the APU
    generator if electrical loads exceed limits.

  • AC power is converted to 28V DC power by
    three Transformer Rectifiers to supply DC
    power to DC Bus 1 and DC Bus 2.

DC Power

  • DC Bus 1 is powered by TR 1 and backed up
    by TR 2 and TR 3.

  • DC Bus 2 is powered by TR 2 and backed up
    by TR 1 and TR 3.

  • If power to TR 2 is lost, the TR 3 Transfer
    Relay shifts and TR 3 receives power from
    Transfer Bus 1.

  • The Battery Bus is powered by TR 3 and
    backed up by the Battery. TR 3 is normally
    backed up by TR 2 or alternately by TR 1
    through the TR 3 Transfer Relay.

  • The  Switched Hot Battery Bus and Hot
    Battery Bus are powered by the
    Battery/Battery Charger.

  • A DC Cross Bus Tie Relay isolates DC Bus 1
    from DC Bus 2 at Glide Slope capture during
    an Autopilot ILS or Flight Director approach.
Standby Power

  • The AC and DC Standby Busses supply "essential"
    AC and DC power with a loss of all generators.

  • The DC Standby Bus is normally powered by TR 1,
    TR 2 and TR 3.
  • The AC Standby Bus is normally powered by AC Transfer Bus 1.

  • The alternate sources of AC and DC power to the AC and DC
    Standby Busses are the Static Inverter, Battery Bus, Hot Battery Bus,
    Switched Hot Battery Bus and Battery.