Electric System


click on diagram to enlarge

Three Electrical Systems: 

I. The DC system-


  • 2 NiCad 24v batteries (in L & R wing roots)
  •  1 lead acid 24v (emergency) battery (in avionics rack behind cockpit)

Ground Power

  • 28v DC, 1200-1600 amps
  • auto disconnect <10v/>31v

Generators (left & right, 28v engine driven)

  • A starter at <55% Ng, uses BAT PWR (in parallel or series) or ground power for engine start
  • A generator at >55% Ng, supplies 28v to DC elec system (when reset & GPU not applied)
  • Max load 600AMP, nominal load400AMP

II.  AC Inverter System 115v/26v

  • #1 & #2 AC inverters (one operates while the other is in STBY), no auto switching if an inverter fails
  • powered by DC generators
  • MOD 2544: inverter runs off BAT BUS
  • 26v output failure will not trigger amber INVERTER light, only 115v failure 

III.  AC wild frequency system115v

  • supplies power to all electrical ice protection systems but STBY PITOT (24v) when props are >1,000 RPM
  • 2 generators, one per engine on prop gearbox 460-600 Hz
  • A, B, & C phases produced by each AC wild generator a frequency that varies with prop RPM

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


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
• 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

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 be
tery voltage decreases below 24 volts. Master caution alerting is triggered with flashing amber ELEC on the

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) may 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.




AC Inverters 

AC Inverter System
Static inverters convert DC electrical power to 115v and 26v AC at 400 Hz constant frequency. All aircraft are
equipped with two inverters. However, only one inverter operates at any time while the other remains in standby.
The inverters are controlled via the INVERTER switch located on the overhead AC ELEC panel. There is no automatic switching after inverter failures The remaining inverter must be manually selected to recover AC power. Failure of the 115v output of the inverter in use causes amber INVERTER illumination on the AC ELEC panel with master caution alerting and flashing amber ELEC on the CWP. Failure of the 26v output of the inverter does not cause a master caution alerting or CWP alerts. However, it will cause a failure of the RMl and VOR CDI indications.


Inverters are powered from the respective Battery Bus. The inverter currently in use supplies all AC buses. On older aircraft, this is indicated by the installation of "Mod 2544". In aircraft with this mod installed, it may be necessary to reset the inverter after engine start.



AC Wild Frequency System

AC Wild Frequency System
Each propeller accessory gearbox drives a 115v AC wild frequency generator. These generators supply power to
all electrically heated.ice protection systems, except the. standby pitot mast Three phases (A, B, C) are produced by each generator at a frequency which varies with propeller speed. Each phase output is normally directed to the respective (A, B or C) 115v generator bus network. A generator control unit (GCU) associated with each AC generator provides generator control and fault detection in conjunction with the AC GEN switches located on the
AC HEAT panel. The AC generator relays control distribution of the respective AC generator to one or both 115v Generator. 

Bus networks 

If an AC generator fails, the affected side is automatically switched to the opposite side generator. If necessary, the affected AC wild network can be manually switched to the operating AC generator via the related AC GEN switch. The loss of one or two phases does not result in automatic switching. AC wild generator failures are indicated by illumination of the corresponding amber AC GEN light which triggers master caution alerting with flashing amber ICE PROT on the CWP. Generator reset is automatic when normal propeller rpm resumes if failure is due to generator underspeed (propeller rpm <1000). AC wild phase failures are indicated by master caution
alerting with flashing amber ICE PROT on the CWP and random lights on the ICE PROTECTION panel butthe related AC GEN light does not illuminate.



The GEN relay will open for malfunctions other than overheating, when the GPU is applied and if the fire handle is pulled.

An overheating generator will not automatically drop offline. It's still producing power and of use. It must be manually disconnected.

An overheating battery will automatically open its relay and disconnect to prevent thermal runaway.

Bad EFIS indications on cockpit screens may indicate something is wrong with the GEN output.

Anything downstream from a MAIN BUS is 28v.

Applying EXT PWR opens the GEN & BAT relays

31 elec buses on the Saab!

The BUS TIE CONN indicates relay position.

Anytime a BAT switch is on, the ESS bus will be powered.

During engine start the ESSENTIAL and MAIN buses shed or lose power.

The utility bus is only powered with both L&R GENs online OR with ground power.

During battery starts, the batteries go into series to produce 48V

The L AC GEN heats the coffee coil while the UTILITY BUS only controls the hot jug on/off switch 

Anytime you have less than 2 GENs online, you should see the BUS TIE CONN illuminated

BUS TIE CONN auto function control is powered from the LEFT BAT BUS. If you turn the RIGHT BAT switch on first, it won't close.

BUS TIE CONN conect function is powered by the emergency bus.

The only time the BUS TIE should be open (green light not illuminated) is when both generators are online. 

If an engine driven generator fails, you will get a GEN light and theBUS TIE light. You will lose the utility bus.

Only two attempts to reset a GEN are allowed.

Emergency floor lighting in the cabin is powered by 5 NiCad battery packs under the floor. They provide emergency lighting for 10 miniutes.