Electric

NiCad batteries

The two NiCad batteries in the wing roots are ventilated by ram air in flight and via fans when on the ground. Anytime the nose gear is down and locked the ventilation fans run, powered via their respective Main Buses. Battery temperatures are displayed on the TEMP panel on the overhead. Excessive battery temperature is alerted via two thermal sensors inside the battery. One sensor causes the amber NO BAT START to illumination (at 57°C) while the other triggers the illumination of the respective amber BAT HOT light (at 71 °C) which results in the respective battery relay. If the relay opens, the battery can be manually reconnected to the Generator Bus by selecting OVRD on the respective BAT switch. The BAT HOT light also triggers a master caution alert and the associated amber ELEC on the CWP. The normal battery configuration is in parallel, producing 24 volts. During engine battery starts, the main batteries are automatically connected to the Start Bus in series to produce 48 volts.

Emergency Lead Acid Battery

The emergency battery is in the the avionics rack behind the cockpit. It is 24v lead acid and supplies power to the Emergency and Emergency Avionic Buses if power to the Left Battery Bus is lost. It is charged via the Left Battery Bus when the aircraft is powered by a generator or when on external power. The amber EMER PWR light illuminates and master caution alerting is triggered with amber ELEC on the CWP when the Emergency Bus is not supplied or when emergency battery voltage decreases below 24 volts.

External Power

When starting engines using an external ground power cart, the aircraft requires a 28v GPU that supplies 1200 to 1600 amps. External power is applied directly to the Start Bus which then powers the left DC distribution network. The Emergency Bus is powered when either BAT switch has been selected to ON or OVRD. Supply to the opposite DC network requires the bus tie relay be closed when only one engine is running. The blue EXT PWR AVAIL light on the overhead panel illuminates when the GPU supplies at least seven volts. (GPU

voltage can be checked on the VOLT meter before applying it to the aircraft.) The white EXT PWR ON light on the overhead panel illuminates when the EXT PWR switch is selected ON and external power is available. When the EXT PWR switch is ON, losure of both battery relays is inhibited as is the operation of the DC generators .

External power application to the aircraft electrical system is controlled by the EXT PWR switch which is magnetically held in the ON position when selected. External power is automatically kicked OFF if:

• an overvoltage (>31v)
• undervoltage (<10v)
• during engine starting if voltage decreases below 7v upon initial START switch actuation

Generators

Normal 28v power is supplied from two 400amp generators, located on their respective engine accessory gearboxes. The generators also function as starters by rotating the gas generator compressor-turbine spool for engine start When functioning as a generator, it is driven by the gas generator turbine. During ground operations, the starter/generators are cooled via integral fans and via ram air while inflight. Each generator also has a generator control unit/ GCU that controls generator and starter operation, regulates and provides fault detection. Modes of operation for the starter/generator are selected with the GEN or START switches. During engine start (0-55% Ng), starter power is supplied from the Start Bus.

Power to the Start Bus is supplied by one of the following sources:

• main NiCad batteries

1) Connected in series to produce 48 volts during first engine starting OR

2) Connected in parallel to produce 24 volts assisted by the output of the operating generator during cross generator starts

• DC external power

During cross-generator engine strats, the generator on the operating engine assists the batteries during the start cycle. The GCU for the assisting generator is allowed a significantly higher amperage output from the generator. This increased cranking power, along with the main batteries results in faster and cooler engine starts.

Generator Mode

Each generator develops 28 volts and normally supplies the respective DC distribution network via closure of the associated generator relay. Amperage and voltage indications are monitored at the output of generator and displayed on the DC AMP VOLT meter. After the engine accelerates through 55% Ng, the starter/generator is

available as a generator after a 10 second delay for self-testing. Generator overheating (>150°C) triggers illumination of the respective amber GEN ov TEMP light.

The GCU automatically opens the generator relay when:

• Malfunctions are detected (except overheating)

• External power is used

• The FIRE handle is pulled

Gen relay opening triggers illumination of the respective amber GEN light with master caution alerting and flashing amber ELEC on the CWP.

DC Distribution

The DC distribution networks are isolated from each other during normal operation. If a generator fails, the bus tie relay automatically closes so that all buses except the Utility Bus remain supplied from the operating generator. The utility bus requires two generators online.

DC Buses and Components

Power Distribution Unit (PDU)

Two power distribution units contain numerous relays for engine starting and primary distribution to the electrical buses. The PDUs (designated left and right) are located in the respective wing-fuselage fairing.

Hot Battery Busses

Normally, the Hot Battery Buses are supplied at all times by the respective main battery. However, the right main battery supplies both Hot Battery Buses during battery starting of the first engine (series configuration).

Start Bus

The Start Bus supplies power to the starter during engine starting. It is supplied by one of the following sources:

• External power

• Both main batteries connected:

a. In series to produce 48v during first engine starting

b. In parallel to produce 24v plus the output of the operating generator (cross-generator starting) during second engine starting

Generator Buses

Generator Buses facilitate distribution to the corresponding network or to the opposite network via the bus tie relay.

Battery Buses — The Battery Buses convey electrical power to the respective network. When selected, the voltage on each Battery is displayed on the VOLT meter.

Main Buses

Except during engine starting when the mains load shed, the Main Buses are powered when their corresponding Battery Bus is supplied via external power or when the associated gen is online. Power supply is controlled by the respective main bus relay. Voltage for each Main Bus can be displayed on the DC VOLT meter. Loss of main bus power causes the related amber MAIN BUS light to illuminate and a master caution alert sounds along with a with flashing amber ELEC on the CWP.

Main Start Buses

The Main Start Buses are powered when the corresponding Battery Bus is supplied by external power or generators. The Main Buses remain powered by the related Battery Bus for 10 seconds after both generators fail (no external power).

Essential Buses

Except during engine starting when the Essential Buses are load shed, they are powered when their corresponding Battery Bus is powered. A loss of power causes the illumination of the amber ESS BUS light, a master caution chime with the flashing amber ELEC on the CWP.

Emergency Bus

The Emergency Bus is normally supplied by the Left Battery Bus when either BAT Switch is selected ON or OVRD. The green bus tie relay must be closed to supply the Emergency Bus from the right main battery. The amber EMER PWR light illuminates when the Emergency Bus is not powered or when emergency battery voltage decreases below 24 volts. Master caution alerting is triggered with flashing amber ELEC on the CWP.

Avionics Buses

The Left and Right Avionic and Avionic Start Buses are controlled via the respective AVION switches. The Essential and Emergency Avionic Buses are both controlled by the ESS AVION switch. Only the Emergency Avionic Bus is supplied during engine starting (if the ESS AVION switch is ON). When selected, voltage for each Avionic Bus is displayed on the DC VOLT meter.

Utility Bus

The Utility Bus is powered through an RCCB if both generators are on line or external power is used. The Utility Bus is loadshed automatically if either generator is lost.

Bus Tie Relay

The normally-open bus tie relay joins the DC distribution networks by connecting the two Generator Buses. Closure is accomplished manually or automatically via the BUS TIE switch when:

• Only one generator is operating

• When external power is used

• When the left or both BAT switches are selected ON

Bus tie relay closure is indicated by illumination of the green BUS TIE CONN light. If the bus tie relay fails closed while both generators are operating, then the GCUs command identical voltage and load outputs (load-sharing). The bus tie relay opens automatically when overloads (>800A) flows across the bus tie relay. In such cases, the BUS TIE switch is inhibited for closing the bus tie. manual bus tie reset (closure) ay be attempted by pushing the ELEC PWR RESET BUS TIE (K1) pushbutton located on the forward left circuit breaker panel. The bus tie overload protection is inhibited when starting either engine.