GENERAL
AC and DC electrical systems are both divided into to independent systems, left and right. Normally the systems operate independently of each other. The electrical system will automatically transfer power and isolates faults. System controls and indicators are located on the overhead panel and the AC panel has a built in schematic.
There are four sources of power for the AC system: two engine driven generators, an APU generator, usable on the ground or in flight, and external power. The AC crosstie relay provides automatic protection for generator failure by powering both AC busses from one generator.
DC power normally comes from four transformer rectifiers (TRs), two on each side, powered from the main AC busses. The DC crostie provides a manual backup for DC bus failure.
A rotary selector switch and meters are used to monitor voltage, frequency, and battery amperage.
POWER DISTRIBUTION
Each generator provides power to its respective AC system. Through normally isolated, both the left and right systems can be connected to a single generator through the AC crosstie relay. The AC system provides power for high load systems such as hydraulic pumps, heat exchanger cooling fans, galley power and the TR units along with other AC loads. Either AC bus can power the AC Emergency bus. The L or R AC BUS OFF annunciator light comes on if the respective generator bus looses power.
The left and right DC busses provide 28-volt DC power to aircraft equipment and associated radio buses.Either DC bus can power the DC Emergency bus.
AC GENERATING SYSTEM
The primary sources of power for the AC system are the two engine driving generators providing 115-volt, 400- cycle, 3 phase output. A constant speed drive (CSD) controls the engine generator's speed throughout the engines operating range. The APU generator is driven at a constant speed and does not have a CSD.
CSD INDICATIONS
Each CSD is monitored by a CSD OIL PRESS LOW light, and an indicator that displays oil outlet temperature. When the CSD temperature rise button is pressed, the indicator displays the difference between the CSD oil cooler inlet and oil outlet temperature.
Each CSD has a guarded disconnect switch. Placing the switch to its momentary disconnect (DISC) position disconnects the CSD form the engine. Once disconnected, the CSD cannot be reconnected in flight.
The CSDs will reset when both red CSD covers are closed and the following conditions are met:
1. Aircraft on the ground
2. Parking brake set
3. Both engines shutdown
GENERATOR CONTROL
Each generator has a generator control unit (GCU) that provides automatic control and protective functions. Each GCU and generator relay are controlled by a generator switch on the electrical control panel on the overhead. Each generator switch has three positions, ON arms the generator relay to close when generator conditions are normal. OFF disconnects the generator from its bus by opening the relay. RESET is used to reset a tripped GCU.
In normal operation, the generator switch remains in the ON position and the generator relay closes when phase, voltage, and frequency are within operating limits. The generator relay opens automatically if the fire handle is pulled, or if certain faults exist. The GCU must be reset after the fire handle has been pushed back in.
APU GENERATOR POWER
The APU can provide power to either AC bus on the ground or in flight through the APU bus switches. In flight the APU can crosstie through the AC crosstie relay. An APU GEN OFF annunciator light indicates that the APU is operating, but is not providing any electrical bus. The light is active when the APU master switch is not in the OFF position. The APU generator has RESET and NORM positions. RESET is used to reset a tripped GCU. NORM allows normal APU generator operation. Power is supplied from the APU generator to each AC bus through left and right APU bus switches.
AC CROSSTIE RELAY
The AC crosstie relay is normally in the open position to allow the left and right AC systems to operate independently. If no other source is available, the relay will close automatically to allow a single generator to power both busses. The AC crosstie is operated by a AC crosstie switch which has OPEN and AUTO (normal position). In AUTO, the relay is open and each generator powers its associated AC bus. If the generator looses power, the crosstie relay closes automatically, allowing the other generator to power both AC busses. APU power can crosstie to the other AC bus in flight. APU and external power are not able to crosstie on the ground. Placing the crosstie switch in the OPEN position prevents the automatic crosstie function and forces the relay to remain open, isolating the AC busses.
AC LOAD METERS
Ac load meters on the electrical power panel monitor the load on each generator. Indications range from 0 to 150% of the generators rated capacity. An indication of 1.0 represents full rated capacity (100%) of the generator.
PRIORITY SYSTEM
An automatic system ensures that AC power for each AC bus comes from only one source. The system automatically determines which available source to use based on a priority system:
1. Associated engine-driven generator
2. APU generator
3. External power
4. AC crosstie relay through the opposite engine-driven generaor.
For example, if all electrical power sources are available with all generator and bus switches ON, the right AC bus is powered by the right generator through the right generator relay. If the right engine shuts down , the right generator relay opens, and the right APU relay closes, powering the right generator bus. If the APU shuts down, the APU relay opens and the external power relay closes, powering the right generator bus. If neither the APU nor external power is available, the shutdown of the right engine causes the AC crosstie relay to close, allowing the left engine-driven generator to power both AC busses.
DC POWER
Left and right DC busses are powered from their respective AC busses through four TRs. Left and right DC systems normally operate independently to supply power to the various DC loads. DC load meters show each TR load. A reading of 1.0 indicates the TR output is 100% of its rated capacity. A DC BUS OFF annunciator light comes on if either the left or right DC bus is not powered.
DC CROSSTIE RELAY
The DC crosstie relay is normally open to allow the left and right DC systems to operate independently. The DC crosstie is a manual relay that is controlled by the DC crosstie switch on the electrical power panel. The switch has two positions: OPEN and CLOSE. In the normal OPEN position, it separates the two DC systems. If both TRs on either side fail, selecting CLOSE allows the TRs from the opposite side to power both DC buses.
EMERGENCY AC AND DC POWER
Normally the AC Emergency bus is powered by the Left AC bus with backup power provided by the Right AC bus. Normally the DC Emergency bus is powered by the Left DC bus with backup power provided by the Right DC bus. If all AC power fails, turning the emergency power switch ON connects the Battery to the DC emergency bus, and to the emergency inverter which powers the AC Emergency bus. With the emergency power switch on, the white EMER PWR IN USE light next to the switch comes on.
BATTERY
Two 14 volt nickel-cadmium batteries are connected in series to produce 28 volt DC power. Battery power can be used for APU starting. If all AC power is lost, the battery can provide AC and DC power for approximately 45 minutes.
BATTERY SWITCH
The battery switch is a locking switch with two positions: OFF and ON. The switch must be clicked once on the lower half to turn the battery ON and then again in the same spot to lock the switch. The switch is locked when the stripe on the switch is horizontal. To turn the battery OFF, click the upper portion of the switch to unlock the switch and then click again in the same spot to turn the battery OFF.
BATTERY CHARGING
The battery charger is powered form the Right AC Bus. Charging occurs when the Right AC Bus is powered, the battery switch in ON, and the emergency power switch is OFF. Battery charging rate varies as indicated by negative readings on the DC Volt/AMP meter. As the battery approaches full charge, the charger goes into a pulse charging mode. When fully charged, intervals between pulses become longer. Pulse intervals can vary from 5 seconds to approximately 30 minutes depending on loads supported and battery condition.
The charger disconnects from the battery if the battery switch is OFF, the emergency power switch is ON, or the Right AC Bus is not powered.
LOCATION:
Overhead Panel
APU, EXTERNAL POWER AND AC BUS CROSSTIE SWITCH
APU POWER AVAILABLE LIGHT
Indicates that APU voltage and frequency output is normal.
APU BUS SWITCH
ON - Selects APU generator to respective bus if APU PWR AVAIL light is on and bus is not powered by engine driven generator.
OFF - Disconnects the APU generator from respective bus.
EXTERNAL POWER BUS SWITCH
ON - Selects external power to respective bus if EXT PWR AVAIL light is on and bus is not powered by engine or APU generator.
OFF - Disconnects the external power from respective bus.
POWER IN USE LIGHT
Indicates selected power source is connected to respective bus.
EXTERNAL POWER AVAILABLE LIGHT
Indicates that external power voltage and frequency output is normal.
AC BUS CROSSTIE SWITCH
OPEN - Opens AC crosstie relay, isolating left and right AC busses.
AUTO - Normal position. AC crosstie relay closes if engine generator fails with no APU or external power source available. Remaining generator
powers both AC busses. In flight, permits the APU generator to power both busses. Does not close on the ground if APU or external
power is powering either bus.
GENERATOR SWITCHES AND OIL TEMPERATURE INDICATORS
CSD DISCONNECT SWITCH
Two position switch that is covered by a red cover. Click on the lower portion of the cover to open and close the cover.
NORM - Normal position of the guarded switch.
CSD DISC - Momentary. Disconnects CSD from engine drive. Click above the switch (+) to operate.
The CSDs will reset when both red CSD covers are closed and the following conditions are met:
1. Aircraft on the ground
2. Parking brake set
3. Both engines shutdown
ENGINE GENERATOR SWITCH
RESET - Momentary. Resets generator control relay.
OFF - Disconnects generator from AC bus by opening generator relay.
ON - Arms generator relay to close and power the AC bus if generator conditions are normal.
APU GENERATOR SWITCH
RESET - Momentary. Resets generator control relay.
NORM - Normal switch position. Arms APU generator to power AC bus(es).
OIL TEMPERATURE INDICATOR
Indicates CSD oil outlet temperature or oil temperature rise.
CSD TEMPERATURE RISE SWITCH
SWITCH NORMAL - Both oil temperature indicators monitor oil outlet temperature.
SWITCH IN - When pressed, both oil temperature indicators monitor difference between oil out and oil in temperature.
AC POWER INDICATORS AND METER SELECTOR SWITCH
AC VOLTMETER
Indicates AC voltage of the selected generator or external power.
METER SELECTOR SWITCH
APU - Monitors APU generator voltage and frequency.
EXT
PWR - Monitors external power voltage and frequency.
L - Monitors left generator voltage and frequency. DC meter monitors left DC bus.
R - Monitors right generator voltage and frequency. DC meter monitors right DC bus.
BATT
VOLT - DC meter monitors battery or battery charger volts.
BATT
AMP - DC meter monitors battery charge rate (minus) or discharge rate (plus).
FREQUENCY METER
Indicates frequency of the selected generator or external power.
DC VOLT/AMP METER
Indicates charge or discharge current of battery, battery voltage or DC bus voltage.
AC LOAD METER
Indicates left generator, APU generator, and right generator loads from 0 to 150% of generator capacity.
GALLEY, DC, EMERGENCY, AND BATTERY POWER
GALLEY SWITCH
OFF - Disconnects galley power
ON - Connects generator busses to power galleys
DC LOAD METER
Indicates left TR-1, TR-2 and right TR-1, TR-2 loads from 0 to 150% or rated TR capacity.
DC BUS CROSSTIE SWITCH
CLOSE - Connects left and right DC busses to allow any TR to power all DC loads.
OPEN - Normal position. Isolates left and right DC busses.
EMERGENCY POWER IN USE LIGHT
Indicates emergency power switch is on and battery is providing emergency power.
EMERGENCY POWER SWITCH
OFF - Battery disconnected as source of emergency power.
ON - Battery powers DC Emergency bus and also powers inverter for AC Emergency bus. Battery is disconnected from charger.
BATTERY SWITCH
OFF - Disconnects battery from battery bus. Switch must be vertical before it can be placed to off.
ON - Connects the battery to the battery bus and charger. Switch remains ON with line vertical. Rotate switch until line is horizontal to lock switch in position.
ELECTRICAL CAUTION AND WARNING ANNUNCIATORS
AC EMER BUS OFF - AC Emergency bus not powered.
DC EMER BUS OFF - DC Emergency bus not powered.
AC CROSSTIE LOCKOUT - Indicates AC crosstie relay is locked open due to a fault.
APU GEN OFF - APU is operating but not providing power.
DC TRANSFER BUS OFF - DC transfer bus not powered.
AC BUS OFF (L or R) - Indicates respective AC bus is not powered.
GENERATOR OFF (L or R) - Indicates generator relay is open, disconnecting generator from its AC bus.
CSD OIL PRESS LOW (L or R) - except STEAM - Indicates respective CSD oil pressure is low or oil temperature is high.
CSD OIL - STEAM - Indicates respective CSD oil pressure is low or oil temperature is high.
DC BUS OFF - Indicates either left or right DC bus is not powered.