1. Citation II Electrical Power System
AST 473
Dr. Barnhart

2. Electrical System Introduction
The electrical system on the citation is divided into two separate systems:
1. DC- storage, generation, distribution and monitoring.
2. AC- generation, distribution, monitoring
- There are also provisions for:
-Emergency
-GPU connection

3. DC- Principal Electric Power
2 Engine driven DC Generators- primary power sources.
Normal distribution- 3 left and 3 right buses connected by a TIE bus.
Allows either generator to power entire system (working in parallel sharing load)
*Hot battery and emergency buses- normally tied to main system but can be isolated to battery or External sources (ie. GPU for gnd. Ops).

4. DC Power- Battery
19 cell Ni-cad battery provides 24-volt power rated to 39 amp-hours.
Located in tail-cone and has quick-disconnect terminals
Always connected to hot battery bus.
Ni-cads.- susceptible to excessive charging and overheat; as such, batt. Is limited to 3 eng. Starts/hour
Battery isolated during GPU start to prevent discharge

5. Battery Life- In good condition
10 minutes to all buses
30 minutes if only hot battery bus and emergency bus used

6. Starter-Generators 2 each. 1 per engine mounted on accessory gearbox.
Air Cooled, rated to 30 volts, DC regulated to 28.5 volts, 400 amps.
Capable of 50% overload to 600 amps. For 5 min. (starting)
Used as starters until 50% N2
Distribution systems are in parallel- gens. Share load equally (+/- 10%) via an equalizer connection between generator control units (GCU’s).

7. External Power Receptacle- below left engine nacelle
Power routed to hot battery bus
Battery charges from GPU regardless of battery switch position.
Ensure GPU regulated to 28.5v and btn. 800 & 1,000 amps. Prior to connecting to A/C.

8. External Power Operation
Connecting GPU:
Energizes external power relay which connects the external power unit to hot batt. Bus.
Placing battery switch to BATT closes battery relay (on UN’s 0002 – 0626 emergency relay also closes) allowing DC external power to be connected to the battery bus, emergency bus, and left and right main buses.

9. External Power Operation- ctn.
If either generator power relay is closed- external power relay deenergizes which removes external power from hot battery bus (prevents simultaneous airplane and GPU power).
GPU unit should have reverse current protection. If not and unit is turned off while connected to A/C, rapid discharge and battery damage can occur. Disconnect GPU from airplane when not in use

10. Electrical Distribution
DC- distributed by 9 buses (fig. 2-6)
Main junction box (tail-cone compt.)- 2 main DC buses, battery bus, and hot battery bus.
Cockpit (pilot’s CB panels)- 2 main extension buses, two crossover buses and emergency bus.

11. Electrical Distribution ctn.
2 main DC buses are normally powered by rt. And left generators tied by the batt. Bus (can also be powered from batt. Or GPU).
Hot batt. Bus- always directly connected to battery- normally powered by generators but may be by GPU.
Battery bus- sole function is a tie bus connecting hot battery , emergency bus, and two main DC buses (4 elements into 1 integral system).

12. Electrical Distribution ctn.
DC power from engine-driven generators- distributed to two main DC buses. Two main DC buses are paralleled through the battery bus by two 225-amp. Current limiters (connects each main bus to batt. Bus)
Generator power routed through battery bus to hot batt. Bus by a batt. Relay and to emerg. Bus via the batt. Bus or hot batt. Bus depending on position of BATT switch.

13. Electrical Distribution ctn.
From each DC bus in tailcone- ext. bus provides power dist. To components via switches and CB’s in cockpit
L & R main ext. buses- located behind pilot’s and co-pilot’s CB panels (respectively)
Left and right cross-over buses enable logical grouping of circuit breaker (i.e. L & R. fire detection) providing power from opposite main extension bus.

14. Electrical Distribution ctn.
Emerg. Bus- located at copilot’s CB panel
Hot Battery bus powers:
Lights (cabin entrance, both baggage compartments and emerg. Exit)
Ignition- start only
Voltmeter (batt. Switch in BATT. Or EMERG.), LH/RH generator voltage regardless of BATT switch position)
Emerg. Nicad battery pack (generators/GPU online)

15. Emergency Bus Distribution
The following are powered by the emerg. Bus thus power is avail with batt. Switch in the EMER. position:
COMM 1
NAV 2
Copilot’s HIS (DG 2)
Cockpit floodlights
ON UN’s 0627 and on, also: cockpit speakers, copilot’s ADI, and rt. Pitot/static heat [682 and on]

16. Electrical Control Battery switch and 2 generator switches
Batt. Switch is 3 positions; BATT, OFF, and EMER.
Battery OFF- hot battery bus isolated from all other buses
Battery BATT- closes battery relay connecting batt. To system
Battery EMER- only emergency energized closed- connects emerg. Bus to hot batt. Bus- powered by battery or GPU.

17. Electrical Control
When GPU not connected and gens. Are online, placing battery sw. to OFF or EMER isolates battery from any charging source (30 min. max.)
Gen. switch has 3 positions: GEN, OFF, and RESET.
In GEN position- GCU closes power relay & connects generator to main DC bus

18. Electrical Control ctn.
Generator switch:
In the OFF position- power relay will NOT close and generator assumes no load
RESET- close the generator field relay if opened.
Engine Start buttons: activate a circuit to close the start relay- allows current to the hot batt. Bus then to the starter/generator.
Starter disengage button opens start relay- manually terminates the start sequence.

19. Electrical Monitoring
DC system- monitored by voltmeter, 2 ammeters, 2 GEN OFF annunciator warning lights a BATT O’TEMP warning light & batt. Temp gauge (optional)
L/R GEN OFF annunciator- indicates an open power relay, if both lights are on the MASTER WARNING lights also flash.

20. Electrical Monitoring
Voltmeter selector switch- permits monitoring of voltage on the hot batt. Bus or from a point btn. Each generator and its power relay.- spring loaded to BATT pos which indicates batt. Bus voltage.
Indicates highest of the source connected to point being monitored (either gen. or batt voltage)
Will NOT read hot batt. Bus when batt. Switch in OFF.

21. Elec. Monitoring ctn.
Ammeters- read current flow from their individual generators. During normal ops. Indication should be = (+/- 10% of total load).
BATT O’TEMP light on annunciator (and flashing MW light) comes on when batt. Temp = 145 deg. F. At 160 deg. F light flashes. Optional temp. guage goes to 180 deg. F.
Engine start buttons- illuminate when respective start relay is closed

22. Electrical Protection
2 GCU’s Regulate, Protect, and Parallel generators. Each unit controls the power relay connecting the gen. to its main DC bus.
GCU permits relay to close when cockpit generator switch is in GEN AND gen. output equals (within .3 v) or exceeds system voltage.

23. Electrical Protection ctn.
Field relay- in the GCU either allows or prevents field excitation within the generator.- If open the field relay shuts off power relay solenoid (by opening gnd.) and opens the power relay.
If a short circuit or over-voltage is detected, the field relay opens; also opens when engine fire switch is activated. Also a reverse current (10% or >) or undervoltage opens the power relay.

24. Electrical Protection ctn.
Circuits btn. Each main DC bus and its ext. bus are protected by three 75 amp CB’s in parallel on the CB panel.
A 35 amp. CB on each ext. bus provides protection btn. The ext. bus and the crossover bus on the opp. CB panel.
A 225 amp. Current limiter (CL or fuse) protects against system overload. When open, the elec. System splits into 2 halves.

25. Electrical Protection- CL’s
When one gen. pwr. Relay is closed the operator must protect the CL from the high amps. During eng. Start.
The respective batt. relay automatically opens during eng. Start and closes after start seq.
When starting w/ GPU- batt. Disconnect relay opens and removes batt from gnd- ensures batt is not used for start (starting limits do not apply)

26. Electrical Protection ctn.
If GPU output becomes excessive- overvoltage/overcurrent sensor opens the ext. power relay & opens circuit to hot batt. Bus. Ext. pwr. Disable relays also disconnect the GPU from the hot batt. Bus whenever a pwr. Relay closes, bringing a gen. online.

27. System Operation
During preflight place gen. switches to GEN for battery start or OFF for GPU start
Place batt. Switch to BATT and check for 24 – 25 min. voltage. Check lights and pitot heat then batt. Sw. to OFF.
During exterior walk-around check for battery corrision/deterioriation, or other signs of overtemp.

28. System Operation ctn.
Prior to eng. Start verify switch positions & voltage.- batt. Sw. to BATT (allows pwr. From main ext. bus to close the start relay when START depressed)
Depressing START also activates the elec. Fuel boost pump. Arms the ignition, and activates the engine instrument floodlight (pwr. From emerg. Lighting nicad batt.)

29. System Operation ctn.
When start relay closes (light comes on in START button) hot batt. Bus pwr. Is connected to starter.
BTN. 8 & 10% N2, move throttle from idle cutoff, ignition activates via throttle pos. switch- grn. Lt abv. Ign. Sw = pwr. To ign.
Combustion should occur w/I 10 sec.- evidenced by ITT rise. At approx. 38% (N2), start seq. terminates automatically:
Elec. Boost & ign. Deactivate, start relay opens, floodlights extinguish all via a speed sensor on starter

30. System Operation ctn.
Starter reverts to a gen. and GCU allows it to come online (pilot initiated) after start termination and when gen. output = or > system. Voltage.
For next eng. Start- operating gen. assists battery. Operating eng. Must be steady at 49 – 50% N2.
Start sequence is repeated (pump, light, etc.) but this time both start relays close allowing generator assist (both start lights on). No generator assist capability when airborne via squat switch.

31. System Operation ctn.
When one gen pwr. Relay is closed and the other is working as a starter- batt. Relay opens & interrupts circuit btn. Batt. bus and hot batt. Bus to protect the 225 amp. CL on the side of the op. engine.
All airborne starts are batt. Starts- this is required by certification regulations.

32. System Operation- GPU starts
Check GPU for 28V and 800 – 1,000 amps.
Place gen. sw. in OFF until GPU removed. Otherwise when the 1st gen. comes online the GPU is automatically disconnected from the hot batt. Bus & second start becomes a generator assisted start.

33. Abnormal Operation
Battery Overheat can result from excessive amount and charge rate, discharge, or internal batt. Damage.
If batt overheats it can “run away”. Batteries have actually fallen through the bottom of some aircraft due to run away heating.
When overheat occurs- place batt. Sw. to EMER to open batt. Relay removing it from charging- consult checklist. Ensure relay has opened by monitoring ammeter for a drop. & a min. voltage drop of 1 volt in 30s to 2 min.

34. Abnormal Operation
If start sequence fails to terminate (starter lights and flood lights remain on etc.) depress STARTER DISENGAGE switch.
Monitor ammeters periodically for proper gen. operation.
If GEN OFF light comes on check voltmeter to see if field relay or only power relay opened.
Field relay trip indicated by near 0 voltage (can be caused by short, temp. over-voltage, or actuation of eng. Fire switch). Potential exists for reset.

35. Abnormal Operation- ctn.
A tripped power relay can be caused by an under-voltage or reverse current situation. If selected generator shows normal voltage; reset is not likley.
Pulling 3 (75) amp. CB’s on pilot’s CB panel (left main ext. bus) along with the 35 amp LH CB PANEL breaker on the copilot’s CB panel (crossover bus from rt. Main extension bus) disables the pilot’s CB panel if necessary.

36. Abnormal Operation ctn.
NOTE: when the three 75 amp. Main bus breakers are pulled, the crossover bus to the opposite CB panel is also disabled.
Failure of a 225 amp CL after start can be detected during the gen. check after eng start.
When one gen. sw. is in OFF the other gen. should pick up the entire load (as indicated on the ammeter). If this does not happen it could mean a failed CL.

37. Abnormal Operation ctn.
If the previous occurs and the gen. on the side with the failed limiter is selected to OFF the buses on that side lose pwr and the opposite-side MW light illuminates steady and cannot be reset.
This is most easily detected by looking for off flags in the instruments. DO NOT fly in this condition.

38. AC power
AC power provided via two static inverters powered by the main DC system.
AD is distributed via two buses: 115v and 26v.

39. AC power protection
Power source to inverters protected by the CB’s on the left and rt. CB panels.
Inver switching relay and inverter fail warning light are both protected by an AC sw. CB. In the rt. CB panel. This CB provides pwr. For: AC FAIL light, autopilot disengage light/horn, go-around button, and inverter switch.

40. AC power protection ctn.
AC not available from the No. 2 inverter when the AC switch CB is disengaged. If the No. 2 inverter is on line when the CB is tripped, the inverter sw. must be moved to inv. 1 to restore AC.

41. AC power- Control
Control sw. is grouped with the avionics master power sw. It has 3 positions: INV 1, OFF, and INV 2.
System is monitored by one red AC FAIL light- illuminates for a loss of AC power on the 115-VAC bus due to any number of reasons: loss of DC, voltage fluctuations, etc. Since this light is a red annunciator it trips the MW lights

42. AC power Operation
One inverter is sufficient to provide all AC needs on board. It is recommended that the inverters be used equally.
Place switch in either the INV 1 or INV 2 positions which supplies a ground path for the inverter (#1 or #2) circuit; power is supplied to the switching relay, energizing it and thus AC pwr. From the inverter is directed to the AC pwr. Buses.

43. AC power- Abnormal ops.
If one inverter should fail, simply select the other inverter and continue normally.
If both inverters fail (unlikely) the battery switch must be placed in EMER to allow an integral inverter in the copilot’s vertical gyro to provide emergency AC power to the copilot’s CDI needle on VOR mode.

44. THE END