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APPENDIX: Electrical System

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← The T-1 electrical system consists of 3 major systems: DC, AC and standby power systems. The 3 DC power sources are: Battery, Starter generators and GPU. AC power is provided by two inverters that convert DC-AC.

← DC Power

← External DC power (GPU) is used for ground engine starts, battery charging and maintenance requiring electrical power. The source of external power is a GPU that connects to the aircraft through a receptacle on the aft left lower fuselage. Before using GPU must ensure power output meets 28-28.5 VDC and 1000 – 1500 (1000 Amps max New FCIF) amps. 3 requirements to be able to use external power: generators off-line, battery switch must be ON and 28 VDC must be at the plug. If external power is connected to the aircraft while the aircraft battery switch is ON, the external power unit will charge the battery. Charging a low state battery (less than 22 volts) using external power will significantly reduce battery life due to the high charge rate.

← Battery. A sealed lead-acid 24 VDC 40 amp battery is located in the aft fuselage. Used for: engine start, preflight and emergency power if the generators fail. When we turn the battery switch to on, the battery powers the entire electrical system and can be rapidly depleted. Minimum of 22 VDC must exist in the battery in order to charge it with a GPU or generators otherwise reduces battery life. Minimum also 22 VDC for a battery start. Highly desirable 24 VDC. When the battery switch is turned to On, the following relays are closed: External power, Battery, Battery tie-in and emergency bus power relay.

← A Battery Feeder Protection Unit provides ground fault protection for the battery and external power feeder and ground lines. The battery relay, battery tie-in relay and emergency bus relay are energized by the battery feeder protection unit when the BATTERY switch is in the on position. If a ground fault occurs, the BFPU illuminates the BATT FDR FAIL annunciator, de-energizes the battery relay and battery tie in relay and external power relay removing power from the DC system. It will also illuminate the BATT CUT OFF annunciator. In addition, the BATT FDR FAIL annunciator will illuminate if the Battery feeder protection circuit breaker is out.

← The Start Bus enables the battery and external power to provide DC power to the Starter/Generators during engine start.

← The Battery Charge bus allows the battery to be charged by the generators once they are running and provide an electrical connection to enable the battery or external power to power the entire electrical system. The battery should be able to power the entire electrical system for about 30 minutes depending on load. When the battery switch is set to OFF or EMER, the battery is isolated from the main DC electrical system.

← One starter-generator is installed on each engine’s accessories drive gearbox. The generators supply primary 28 VDC power for the dc power system and function as starter motors. They provide engine-starting torque and produce DC power for the aircraft. Starter limited to 30 sec starts followed by cooling periods of 5, 15 and 60 minutes.

← Each generator contains a generator control unit (GCU) that controls starting and generating modes of operation. The GCU’s incorporate an equalizing circuit, which allows both generators to equally share the system’s load. The GCU also provides reverse-current protection and fault protection from over-voltage conditions and ground faults. The GCU’s monitor each generator feeder line. Should a generator feeder line ground fault occur, the GCU disconnects the generator’s field excitation, which drops the output voltage to zero. This disconnects the generator from the main bus. This fault is simulated by placing the master test to L or R GEN GND FAIL.

← During engine start, the GCU removes power from the starter when engine reaches 35-40% N2. During a battery start, do not attempt to start the second engine until the generator load is below 150 amps. Operating engine must be at idle.

← If one of the generators fail, the load is limited to 400 amps from sea level to 32,500 MSL and to 280 amps from 32,500 to 41000 ft. Minimum RPM on the operating generator is 58% N1.

← After starting the engine and a generator is on line, the external power relay is automatically de-energized, which isolates the ground power unit from the aircraft start bus.

DC Power Distribution

← The L and R MAIN BUSES serve as a source and selector of main DC power between the battery and the generators and provide a way to charge the battery from the generators through the Battery Charge Bus.

← The main buses supply the main DC power to the LH and RH Load buses for distribution through the 3 bus feeder lines. Normally only two of these high amp dc lines are in use at any time. Due to their high DC current, the circuit breakers are controlled at a distance by RCCB or remote controlled circuit breakers from the cockpit. If one of these transmission lines fail, the 3rd one is activated and the L/R BUS FDR fail annunciator illuminates. Do not reset these RCCB ckt breakers. If the third feeder RCCB trips, the annunciator will extinguish. Additionally, 100 amp circuit breakers are used for circuit protection against high current flow from the load bus to the main bus. These circuit breakers are on the main circuit breaker panel, located on the flight deck

← The LH/RH Load buses distribute DC power to equipment and isolate them from the source of DC power. The DC is shared through a Load Bus Tie that connects the two buses through the LOAD BUS TIE circuit breaker located on the main circuit breaker panel.

Standby Power

← The dc standby power system consists of: Standby battery power subsystem, AHRS battery power subsystem and Emergency supply subsystem

← The dc power standby battery and the AHRS battery are identical but separate systems. The systems supply 24 VDC to selected systems in case of aircraft electric power failure. In the event of total electrical failure, including aircraft battery, the standby bus supplies electrical power for approximately 30 minutes.

← The standby power system provides a backup power source that supplies DC power to the standby bus in the event that both generators and the battery fail.

← The standby battery is normally charged through the emergency bus. The standby bus is normally powered by the LH load bus. If we lose the generators, the stand by bus can be powered /charged by the main battery through the emergency bus. We test the standby bus during preflight when we do the battery feeder fail test. This isolates the battery from the standby bus without the generators operating. Diodes placed between the standby bus /battery and the LH load bus/Battery prevent the standby battery from being drained by other electrical equipment if no other power source exists.

← The standby battery is located in the nose electronics compartment and supplies power to the: Copilot instrument light power supply, RTU #1, UHF radio, ITT indicator No. 1 (left), Standby attitude indicator, EFC – start, Standby altimeter vibrator. CRUISSE.

AC Power

← Two static inverters, NO. 1 and NO. 2, supply 115 and 26 VAC, 400 Hz, single phase electrical power at an apparent power rating of 250 volt-amperes each. The voltages and frequency of each inverter is regulated internally by each inverter. AC inverters convert DC power from the DC electrical system into AC power.

← The NO. 1 inverter is supplied by 28 VDC electrical power from the left load bus and is remotely controlled by the NO. 1 inverter control switch. The NO. 2 inverter is supplied 28 VDC electrical power from the emergency bus or the right load bus. It is remotely controlled by the NO. 2 inverter control switch. When the NO. 2 inverter switch is on, the NO. 2 inverter relay is energized if and only if the right load bus is powered. With the relay energized, the NO. 2 inverter is powered by the right load bus. If power on the right load bus fails, the NO. 2 inverter power relay is automatically de-energized and the NO. 2 inverter is powered by the emergency bus.

AC Power Distribution

← The primary AC buses power essential AC equipment. The primary AC Shed buses power non-essential AC equipment. The secondary AC buses power general aircraft non-essential AC equipment.

← Normally the No. 1 inverter powers the primary AC and Primary AC shed buses while the No. 2 inverter powers the secondary. If Inverter No. 1 fails, an Inverter control printed circuit forces the No 2 inverter to power the primary buses and leave the secondary buses un-powered. If generator power is lost and the No 2 inverter was powering the primary buses then the primary AC bus shed buses are un powered and the AC BUS SHED annunciator illuminates

Circuit Protection

← Circuit breakers provide circuit protection against over-current conditions. There are three types of circuit breakers used on the aircraft: Push/Pull type, which indicates an overload condition when it pops out, Rocker actuated snap on switch type and toggle switch type

← Fuses are safety devices consisting of a metal strip that melts and opens the circuit when the rated current is exceeded. Blown fuses must be replaced. Fuses are not accessible to the flight crew while in flight.

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APPENDIX

Electrical System

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