Citation II Electrical Power
System
AST 473
Dr. Barnhart
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
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).
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
Battery Life- In good condition
 10 minutes to all buses
 30 minutes if only hot battery bus and
emergency bus used
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).
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.
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.
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
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.
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).
Electrical Distribution ctn.
 DC power from engine-driven generatorsdistributed 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.
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.
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)
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]
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. Buspowered by battery or GPU.
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
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 relaymanually terminates the start sequence.
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.
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.
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
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.
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.
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.
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)
Electrical Protection ctn.
 If GPU output becomes excessiveovervoltage/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.
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.
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.)
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
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.
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.
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.
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.
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.
Abnormal Operation- ctn.
 A tripped power relay can be caused by an undervoltage 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.
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.
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.
AC power
 AC power provided via two static inverters
powered by the main DC system.
 AD is distributed via two buses: 115v and
26v.
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.
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.
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
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.
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.
THE END