DHC 8 -100

Chapter

24
ELECTRICAL
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 deHavilland Dash 8 100/300 24 - Electrical Power
Table of Contents Normal Operation . . . . . . . . . . . . . . 30
Test − AC Power Monitor System . . . . . . 30
ELECTRICAL POWER . . . . . . . . . . . . . . . . 6 Inverter Failure. . . . . . . . . . . . . . . . 30
Description. . . . . . . . . . . . . . . . . . . . . 6 26 Volt AC Bus Failure. . . . . . . . . . . . 32
Bus System. . . . . . . . . . . . . . . . . . 6
Control Panels . . . . . . . . . . . . . . . . . 6 DC POWER SYSTEM . . . . . . . . . . . . . . . 34
Circuit Breaker Panels . . . . . . . . . . . . . 6 Description. . . . . . . . . . . . . . . . . . . . 34
General. . . . . . . . . . . . . . . . . . . . 34
AC VARIABLE FREQUENCY SYSTEM . . . . . . 10 Bus System. . . . . . . . . . . . . . . . . .34
General . . . . . . . . . . . . . . . . . . . . . 10
Generator. . . . . . . . . . . . . . . . . . .10 DC GENERATION SYSTEM . . . . . . . . . . . . 36
Generator Control Unit (GCU) . . . . . . . . 12 Description. . . . . . . . . . . . . . . . . . . . 36
Contactors . . . . . . . . . . . . . . . . . . 14 General. . . . . . . . . . . . . . . . . . . . 36
Current Transformers . . . . . . . . . . . . 14 Starter−Generator. . . . . . . . . . . . . . 36
AC Control . . . . . . . . . . . . . . . . . . 16 Bus Bar Protection Unit . . . . . . . . . . . 38
AC Power Monitoring System . . . . . . . . 16 Current Transformers. . . . . . . . . . . . .40
Operation . . . . . . . . . . . . . . . . . . . . 16 DC Contactor Box. . . . . . . . . . . . . . 40
General. . . . . . . . . . . . . . . . . . . . 16 DC Control . . . . . . . . . . . . . . . . . . 40
Generator Operation. . . . . . . . . . . . . 18 DC Power Monitor System. . . . . . . . . . 40
Test − AC Power Monitor System . . . . . . 20 Transformer−Rectifier Units. . . . . . . . . 44
Abnormal Operation. . . . . . . . . . . . . 20 Operation . . . . . . . . . . . . . . . . . . . . 46
External Power Operation. . . . . . . . . . 22 General. . . . . . . . . . . . . . . . . . . . 46
Generator Operation. . . . . . . . . . . . . 46
AC 400 HZ SYSTEM . . . . . . . . . . . . . . . . 26 TRU Operation. . . . . . . . . . . . . . . . 54
Description. . . . . . . . . . . . . . . . . . . . 26 External DC Power. . . . . . . . . . . . . . 54
General. . . . . . . . . . . . . . . . . . . . 26 Test − DC Power Monitor System. . . . . . 54
Static Inverters. . . . . . . . . . . . . . . . 26 Failure Conditions. . . . . . . . . . . . . . 54
Paralleling Control Box. . . . . . . . . . . . 26
Inverter Warning Control Box . . . . . . . . 28 BATTERY SYSTEM . . . . . . . . . . . . . . . . .60
Autotransformers. . . . . . . . . . . . . . . 28 Description. . . . . . . . . . . . . . . . . . . . 60
Caution Lights. . . . . . . . . . . . . . . . 28 General. . . . . . . . . . . . . . . . . . . . 60
Inverter Control. . . . . . . . . . . . . . . . 28 Battery Control Circuit. . . . . . . . . . . . 60
Power Monitoring. . . . . . . . . . . . . . . 28 Operation . . . . . . . . . . . . . . . . . . . . 62
Operation . . . . . . . . . . . . . . . . . . . . 30 Battery Venting System. . . . . . . . . . . . . 64

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General. . . . . . . . . . . . . . . . . . . . 64
Battery Temp. Monitor System. . . . . . . . . . 64
Description. . . . . . . . . . . . . . . . . . 64
Battery Temperature Monitor. . . . . . . . . 64
Temperature Sensors . . . . . . . . . . . . 66
Electrical Power . . . . . . . . . . . . . . . 68
Operation. . . . . . . . . . . . . . . . . . . 68
Self−Test. . . . . . . . . . . . . . . . . . . 68
EXTERNAL POWER . . . . . . . . . . . . . . . . 72
General . . . . . . . . . . . . . . . . . . . . . 72
External DC Power. . . . . . . . . . . . . . . .72
Description. . . . . . . . . . . . . . . . . . 72
Operation. . . . . . . . . . . . . . . . . . . 72
External AC Power. . . . . . . . . . . . . . . . 78
Description. . . . . . . . . . . . . . . . . . 78
Operation. . . . . . . . . . . . . . . . . . . 80
ELECTRICAL LOAD DISTRIBUTION . . . . . . . 82
General . . . . . . . . . . . . . . . . . . . . . 82
Circuit Breaker Panels. . . . . . . . . . . . . . 82
Left DC Circuit Breaker Panel. . . . . . . . 82
Right DC Circuit Breaker Panel . . . . . . . 84
115V AC VAR. FREQUENCY C/B Panel. . . 86
Avionics Circuit Breaker Panel. . . . . . . . 88

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 deHavilland Dash 8 100/300 24 - Electrical Power
List of Figures Figure 24 – DC Schematic (3 of 5). . . . . . . . 53
Figure 25 – DC Schematic (4 of 5). . . . . . . . 55
ELECTRICAL POWER Figure 26 – DC Schematic (5 of 5). . . . . . . . 57
Figure 1 – Electrical Component Locations . . . . 7 Figure 27 – Bus Bar Protection Unit (BBPU) . . 59
Figure 2 – Control Panels . . . . . . . . . . . . . 9
BATTERY SYSTEM
AC VARIABLE FREQUENCY SYSTEM Figure 28 – Battery System. . . . . . . . . . . 61
Figure 3 – AC Power Schematic. . . . . . . . . 11 Figure 29 – Battery System Schematic. . . . . 63
Figure 4 – AC Generator. . . . . . . . . . . . . 13 Figure 30 – Battery Vent System . . . . . . . . 65
Figure 5 – AC Contactor Box . . . . . . . . . . 15 Figure 31 – Battery Temperature Monitor . . . . 67
Figure 6 – AC Power Monitor & Control Panel. . 17 Figure 32 – Battery Temp. Monitor Operation . . 69
Figure 7 – AC Generators - Operation. . . . . . 19 Figure 33 – Battery Temperature Monitor . . . . 71
Figure 8 – AC Gen Operation. . . . . . . . . . 21
Figure 9 – Generator Failure. . . . . . . . . . . 23 EXTERNAL POWER
Figure 10 – A.C Ground Power . . . . . . . . . 25 Figure 34 – External Power. . . . . . . . . . . 73
Figure 35 – External Power Schematic. . . . . 75
AC 400 HZ SYSTEM Figure 36 – External Power - TRU. . . . . . . . 77
Figure 11 – Static Inverters - Locations. . . . . 27 Figure 37 – AC External Power. . . . . . . . . 79
Figure 12 – Static Inverter -- Schematic. . . . . 29 Figure 38 – AC External Power - Operation. . . 81
Figure 13 – 400 Hz Schematic. . . . . . . . . . 31
Figure 14 – 26 VAC 400HZ Failure . . . . . . . 33 ELECTRICAL LOAD DISTRIBUTION
Figure 39 – Left DC C/B Panel. . . . . . . . . . 83
DC POWER SYSTEM Figure 40 – Right DC C/B Panel. . . . . . . . . 85
Figure 15 – DC System Schematic . . . . . . . 35 Figure 41 – 115V Var. Freq. AC C/B Panel . . . 87
DC GENERATION SYSTEM Figure 42 – Fixed Frequency AC C/B Panel. . . 89
Figure 16 – DC Starter Generator. . . . . . . . 37
Figure 17 – DC GCU and BBPU Locations. . . 39
Figure 18 – DC Contactor Box and Current
Transformer. . . . . . . . . . . . . . . . . . . 41
Figure 19 – DC Monitor and Control Panels. . . 43
Figure 20 – Transformer Rectifer Units . . . . . 45
Figure 21 – DC Operation . . . . . . . . . . . . 47
Figure 22 – DC Schematic (1 of 5). . . . . . . . 49
Figure 23 – DC Schematic (2 of 5). . . . . . . . 51

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 deHavilland Dash 8 100/300 24 - Electrical Power
ELECTRICAL POWER ■■L and R DC Essential buses.
■■L and R DC Main buses.
Description
■■L and R DC Secondary buses.
Electrical Power is provided by the following systems: ■■L and R AC 115V, 400 Hz buses.
■■A direct−current (dc) generation system ■■L and R AC 26V, 400 Hz buses.
which provides 28 volts, generated by two
engine−driven dc starter−generators and from ■■L and R AC 115/200V Variable Frequency buses.
a variable frequency ac system through two For emergency purposes, certain equipment is connected
transformer−rectifier units. These power sources directly to the main battery which is one of the power
are supplemented by two nickel−cadmium sources for the dc bus systems.
(Nicad) batteries, one of 40 ampere−hour
capacity, the other of 15 ampere−hour capacity. Control Panels
(Two 40 ampere−hour batteries with SOO 8070 The electrical power systems are controlled by switches
incorporated). Provisions are included to power on the DC CONTROL and AC CONTROL panels on the
the system from an external (ground) power overhead console. Adjacent to the two control panels are
source. two power monitoring panels labeled DC SYSTEM and
■■The ac variable frequency power system provides AC SYSTEM respectively, which provide digital voltage
115/200 volts, 3−phase alternating current power and load readouts for the systems.
from two engine−driven ac generators. Provisions Circuit Breaker Panels
are also included to power this system from an
external (ground) power source. Circuit breaker consoles are installed beside the pilot and
■■An ac 400 Hz system provides 115 volts single copilot seats, outboard side The circuit breaker panels for
phase power, generated by three solid−state the dc main, essential and secondary buses are mounted
inverters using 28 volts dc input. Two stepdown on the top surfaces; left buses on the pilot side, right
autotransformers with 115 volt input from the buses on the copilot side.
inverter system provide 26 volts 400 Hz single The consoles each contain three shelves which mount
phase power. the 400 Hz inverters and other power−related equipment.
Bus System The avionics circuit breaker panel is mounted above and
Electrical power is distributed by the following bus behind the pilot’s circuit breaker console, on the rear
system: flight compartment bulkhead at Station X 182.00.

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 1 – Electrical Component Locations

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 deHavilland Dash 8 100/300 24 - Electrical Power
The 115 V ac variable frequency circuit breaker panel is
mounted on the copilot’s side in a similar position on the
rear flight compartment bulkhead.

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 2 – Control Panels

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 deHavilland Dash 8 100/300 24 - Electrical Power
AC VARIABLE FREQUENCY SYSTEM Each ac generator is driven through its propeller gear
box. The speed, and hence the output frequency of
General each generator, varies directly with propeller speed, but
The ac variable frequency power generating system the output voltage remains constant over a wide range
provides 115/220 volt three− phase power for ac of propeller speed, being controlled by its individual
systems that are not frequency−sensitive, such as generator control unit (GCU).
deicing, fuel auxiliary pumps, and electric motor−driven Each GCU automatically controls operation of the
standby hydraulic pumps. It also supplies power to associated generator by monitoring output voltage,
the transformer−rectifier units, which are part of the dc current, and speed. These monitored signals are in turn
generation system. used by internal circuits of the GCU to vary the excitation
Two generators are installed, one on each engine, field current of the generator and to control operation of
supplying power independently to left and right bus the bus contactor. In the event of a generator failure, logic
systems. An ac external power receptacle and control switching in the GCU operates the bus contactor to a
circuitry are provided to enable the system to be powered cross−tie position, switching the feeder lines to the other
from an external source while the aircraft is on the generator.
ground. Facilities are provided to automatically tie the bus Current information is supplied to the GCU from current
systems in event of either generator failure, or the aircraft transformers within the generators as well as externally
being operated on external ac power. on each output phase. Separate transformers supply
The system consists of two generators, two current information to the power monitor system.
generator−control units (GCU), ac contactors, current When external power is applied and selected, each
transformers, control circuits and caution lights. Digital bus contactor is automatically selected to the cross−tie
voltage and load monitoring systems are installed for position by its associated GCU, and external power
monitoring system conditions and operation. contactors connect the external power to both left and
The generators are connected to the variable ac buses right bus systems. Generator operation is inhibited while
by bus contactors in the ac contactor boxes located in the external power is selected.
engine nacelles. The bus loads are distributed from the Generator
ac contactor boxes to the left and right variable frequency
ac buses circuit breaker panel located in the flight The generator is fitted to, and driven by the propeller gear
compartment. box on each engine. The output power is 115/200 volts,
three−phase, variable frequency, with normal capacity
of 20 KVA and overload capacity of 30 KVA. It is a

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 3 – AC Power Schematic

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 deHavilland Dash 8 100/300 24 - Electrical Power
two−stage brushless, two−bearing design, with stationary Generator Control Unit (GCU)
and rotary excitation.
The generator control unit is all solid state, with the
On the rotary shaft is the first stage generator, excited by exception of two control relays which are hermetically
the stationary field. This generates an ac current which sealed. The components and printed circuit boards are
is rectified by diodes mounted on the shaft to supply encased in a single−piece aluminum cover on which is
current to the main field for excitation of the second, or mounted a multi−pin connector. One unit is located in an
main stage stator output windings. A magnetic speed ac contactor box in each engine nacelle.
sensor and current transformers are also mounted in the
The GCU contains the voltage regulator and control
generator housing.
circuits for the generator, protection circuits, and
An overtemperature switch is installed in the generator. bus contactor control. Contactor control includes the
The switch contacts close when the stator windings reach automatic switchover function of the feeders and buses in
a temperature of 210 degrees C, to operate the AC GEN the event of generator failure.
HOT lights on the caution panel. It will automatically reset
With both generators selected ON, each GCU monitors
when the temperature returns to a safe level.
the following functions for control of the generator and
The output frequency of the generator varies with contactor:
propeller speed, and ranges from 333 Hz at 10,000 ■■Generator output voltage on each phase
rpm, to 528 Hz at 15,850 rpm, but the output voltage is measured at the bus contactors, and supplied to
controlled by the GCU. pins F, E and D through circuit breakers CB3, 5
Lubrication and cooling of the generator is provided by and 7 (left) and CB4, 6 and 8 (right) located in the
internal oil circulation. Oil supplied from the engine enters ac contactor boxes.
an inlet port at the flange (mounting) end under pressure. ■■Feeder current in each phase, developed in
The oil is ducted to and through the hollow rotor shaft of transformers T1 and T4, supplied to pins M, N and
the generator, exiting through eight spray nozzles which P for differential current sensing.
direct the flow onto the rotor and stator windings. Bearing ■■Internal generator current, developed from current
lubrication is provided by a controlled leakage device transformers within the generators, supplied to
within the rotor shaft. The oil flows through a screen to pins R, S and T.
a collector sump in the bottom of the generator and is
■■Generator speed (rpm) developed from the
returned to the engine through the outlet port by action of
magnetic speed sensor within the generator, and
the oil scavenge pump.
supplied to pins U and V.

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 4 – AC Generator
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 deHavilland Dash 8 100/300 24 - Electrical Power
■■Operational status information from the other frequency buses circuit breaker panel in the flight
GCU.This information is provided by n, b, s, i, j compartment.
and r.
Contactors
The GCU circuitry uses the above information to control
Two bus contactors (left and right) switch the
the generator output by adjusting the amount of current
three−phase feeders between their own generator or the
flowing in the exciter field winding, or by switching it off.
opposite generator (cross−tie position). The cross−tie
The GCU circuitry uses the above information to control
position is also used when external power is selected.
the generator output by adjusting the amount of current
flowing in the exciter field winding, or by switching it off as The left and right bus contactors are hermetically sealed,
required. three position center−off type, with a three−pole double
throw (3PDT) contact arrangement. Separate coil
Time delay circuits prevent nuisance tripping due to
windings, designated X and Y, are used to energize the
normal switching transients or line noise, and enable the
contact arm to either of the two closed positions. Heavy
GCU to verify a fault before taking corrective action.
threaded studs provide electrical connections to external
AC Contactor Box circuits.
There are two ac contactor boxes, one mounted in each The external power contactors are conventional
engine nacelle, containing most of the components for two−position contactors, hermetically sealed, with a
the left and right ac systems. Each box contains a bus 3PDT contact arrangement actuated by a single coil. The
contactor, an external power contactor, a generator de−energized position provides the cross−tie connection
control unit, current transformers, output feeder fuses, for the bus contactors. When energized, external power
and control circuit breakers for the associated side (left or is supplied to the bus contactors.
right).
Current Transformers
Also contained in each box is the remote control circuit
Six current transformers are installed in the system,
breaker for the left or right standby hydraulic pump. The
labeled T1 thru T6. Transformers T5 and T6 measure
right ac contactor box holds the external power protection
total generator output current for the ac power monitor
unit and fuses. All electrical connections are via multi−pin
system. Transformers T1 and T4 measure total current
connectors mounted on two sides of the box assemblies.
flow in the main and cross−tie feeders. The sensed
The output feeders consist of three No. 12 wires, routed output is used in the GCU differential current sensing
from each contactor box to the left and right ac variable circuits. Transformers T2 and T3 measure cross−tie
current. They sense current flow only when the bus

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 5 – AC Contactor Box

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 deHavilland Dash 8 100/300 24 - Electrical Power
contactors are in the cross−tie position. Their output The LOAD display window monitors load currents
is summed with the output of the opposite T1 or T4 for expressed as a percentage factor of the rated circuit
differential current sensing within the GCU. load. For example, a readout of 1.00 indicates full load
(87 amps per phase), .50 indicates half load; and a
Each current transformer assembly consists of three
reading of 1.20 indicates a twenty percent overload. An
single phase toroidal current transformers, installed in a
overload condition is advised by a flashing positive sign
light weight aluminum housing. Each line of the 3−phase
(+) preceding the readout. The TEST pushbutton tests
system to be monitored is passed through its toroid.
the monitor display and circuit operation when pressed.
Electrical connections to the toroids are via a multi−pin
connector. The mounting holes for the assembly are Voltage readings are from individual circuit breakers on
asymmetrical to avoid improper installation. each phase of the left and right power buses; current
indications are obtained from transformer T5 on the left
AC Control
generator output and T6 on the right generator output.
Manual control of the ac variable frequency system
is provided by switches on the AC CONTROL panel, Operation
located on the overhead console. The panel contains two General
toggle switches for generators on−off control, labeled
GEN 1 and GEN 2 with the OFF position centrally With the aircraft engines running with no external ac
marked. These also serve to reset the GCU trip circuits. power connected, and the AC CONTROL panel switches
A toggle switch for selection of external power, marked GEN 1 and GEN 2 selected on, the generators are
EXT POWER/OFF, and an external power indicator brought on line for normal operation and disconnected as
lamp are also located on the control panel. There are required automatically by the associated GCU.
three additional toggle switches for control of the 400 Hz The two systems are essentially identical and normally
inverters. operate independently. They are however, cross−tied
AC Power Monitoring System during single−generator operation by the logic switching
function of the GCU.
The ac power monitor panel labeled AC SYSTEM is
located adjacent to the ac control panel. The right half of The generator output lines are connected to one side of
the panel is labeled VARIABLE FREQUENCY, with two the two−position bus contactors K1 (left) and K2 (right).
digital readout windows labeled VOLTS and LOAD, and The other side of the contactors are connected to the
a 6−position rotary selector switch. The digital readouts normally−closed contacts of the cross−tie/external power
indicate voltage and current readings on the selected left contactors K3 and K4 for opposite generator feed or for
or right phase.

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 6 – AC Power Monitor & Control Panel

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 deHavilland Dash 8 100/300 24 - Electrical Power
external power feed, when they are in the energized which varies the current flow through the exciter field
position. winding. Within the normal operating speed range of the
generator the output frequency varies between 333 and
The variable frequency buses are connected through
528 Hz.
limiters F7, F9 and F11 (left) or F8, F10 and F12 (right)
to the arm of the contactors, which select one of the Operation of the opposite generator is identical. When
two power sources. Operation of the bus contactors is it reaches its “on line” speed, the GCU operates the
controlled automatically by the GCU. contacts of K1 by removing the ground return from coil
Y2 and applying it to coil X2, connecting the left feeders
The GCU continuously monitors several different
to the left generator. The left and right buses are then fed
parameters for its control function. Voltage and load
independently as the second generator increases to its
information is supplied to the AC SYSTEM panel for
normal operating speed.
monitoring by the flight crew.
The GCU voltage regulator senses the average of the
Generator Operation
three phase voltages applied to pins D, E and F for
Assuming initial engine startup (the right engine is primary regulation, and also senses phase overvoltage or
normally started first), with the ac generators selected on, undervoltage conditions. Overvoltage protection is preset
the GCUs hold all contactors in the open condition. As to shut down the generator if output voltage exceeds
right generator speed increases to approximately 5000 125 volts, and automatically transfers the feeder buses
rpm, the right GCU energizes contactor K2 by applying a to the opposite generator by switching the bus contactor.
ground to terminal X2. The undervoltage circuit operates on sensing a low bus
voltage (below 90 volts), providing bus fault protection.
This action connects the generator output to the right
It operates a latching relay (BCCR−1) within the GCU to
variable frequency buses. Simultaneously, an “on line”
remove energizing voltage from pin B, preventing either
signal is sent from the right GCU pin b to the left GCU pin
coil of the bus contactor from operating, thus isolating the
j, causing it to energize its contactor K1 in the cross−tie
faulty bus from any power source until the fault has been
position by energizing coil terminals Y1/Y2. The right
cleared.
generator is then supplying both variable frequency
buses through normally−closed contacts, A, B and C of NOTE: If the latching relay is actuated, the GCU must
K3. At this speed, output voltage is low,approximately 41 be manually reset by selecting the appropriate
to 42 volts per phase, with a frequency of approximately GEN switch on the AC CONTROL panel to Off,
150 Hz. When the generator reaches its normal operating then on. The OFF position provides a reset to
speed of 10,000 to 15,850 rpm, the output voltage is the GCU circuits and the latched relay.
regulated to 115 volts + or − 2.8V per phase by the GCU,

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 7 – AC Generators - Operation

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 deHavilland Dash 8 100/300 24 - Electrical Power
Currents flowing through the generator output windings ensures that 28 volts control power is available for bus
and the external current transformers are normally contactor operation, independent of the other system.
identical. These currents are monitored by the GCU The second (b to j) provides a logic signal indicating
differential current sensing circuits through pins R, S, T that the generator is “on line” supplying its own bus. The
connected to the generator internal current transformers, undervoltage signal (t to i) and bus fault time delay (n
and M, N, P connected to T1 (left) or T4 (right). A to r) enable logic circuits to determine if, and when, a
change within the main or cross−tie feeders producing load bus should be transferred to the other generator or
a difference in these values, triggers the GCU circuits to locked out due to a fault.
transfer the load to the other generator. The output of the
Test − AC Power Monitor System
generator current transformer is also used by the GCU
for overcurrent protection of the generator. To operate the internal test circuits of the AC SYSTEM
power monitor panel, the TESTpushbutton is pressed
During cross−tie operation, the transferred bus supply is
and held. Pressing the button causes all segments of
routed through a separate current transformer (T2 on the
both VOLTS and LOAD digital display characters to light.
left, and T3 on the right) the output windings of which are
When held longer than approximately two seconds, the
paralleled and summed with the bus supply current from
internal test circuits simulate VOLTS of 150 + or − 3 and
the operational system.
LOADS of 1.05 + or − .03, which is displayed on both
Separate current transformers T5 (left) and T6 (right) sections of the panel.
provide generator output current indications to the ac
Abnormal Operation
power monitor panel.
Generator Hot Indicator
The speed sensor input to pins U and V of the GCU
provides additional control under high and low speed The overtemperature sensor in each generator closes
operation of the generator, and disconnects the a circuit if the temperature exceeds a preset limit. The
load bus(de−energizes the bus contactor) below a sensor is connected to pins A (grounded) and B of
predetermined frequency of 150 Hz. On high frequency generator connector P2. Upon closing, a ground path
output the GCU maintains voltage regulation, but on low is provided to pin B, lighting the AC GEN HOT (NO. 1
frequency output below 325 Hz the output voltage is or NO. 2) lamp on the caution panel.
permitted to sag to a voltage/frequency ratio limit, until
An overheat condition will not automatically take
the preset limit of 42V/150 Hz (approximately) is reached.
the generator off the line. The generator must be
There are four interconnecting circuits between the manually shut off by selection of the GEN OFF. If the
GCU that provide added functions. One (m to s)

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 8 – AC Gen Operation

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 deHavilland Dash 8 100/300 24 - Electrical Power
generator cools to a safe level, the caution light goes ■■Currents flowing within the generator and the
off. feeder current transformers are compared in
the GCU differential current sensing circuits.
Generator Failure
Any difference in these values causes the GCU
Loss of output voltage is sensed by the GCU pins to transfer the load to the other generator by
F, E and D, connected to the generator output at operation of the bus contactor. Time delay circuits
terminals A1, B1 and C1 of the bus contactor. The within the GCU provide allowance for normal
GCU control circuits remove the ground from bus transients caused by on−off switching of various
contactor terminal X2, and apply a ground to terminal loads, and these will not cause any GCU switching
Y2, transferring the contactor to the cross−tie position. action.
The GCU also shuts down the generator by removing
External Power Operation
the exciter field current from the generator pins K
and J. Simultaneously, a voltage is supplied by the With ac external power connected to the aircraft, voltage
GCU from pin e, lighting the AC GEN (No. 1 or No. 2) is supplied to the external power bus through fuses
caution light. F1, F3, F5 (left side) and F2, F4, F6 (right side). The
external power control switch on the AC CONTROL panel
Bus Bar/Feeder Failure
is interlocked with the external power protection relay
An overload or short circuit in the bus or feeder 2442−K1, which monitors the external power source for
system is detected by the GCU by voltage or current proper voltage, frequency and phase rotation.
monitoring.
With the external power protection relay energized,
■■If the voltage at the point of regulation falls below selection of the EXT POWER switch to on energizes left
90 volts, the GCU will remove the energizing and right external power contactors K3 and K4, powered
voltage supply to the bus contactor coil (pin B from the 28 V dc essential buses (or the ground power
on GCU) by operating an internal latching relay unit if equipped with dc output). The green EXT PWR
(BCCR−1). The appropriate bus fault caution light advisory light on the ac control panel is also operated by
(L AC BUS or R AC BUS) is switched on by GCU the external power switch.
relay contacts BCCR−2 providing a ground return
on pin d. When the fault is cleared, the latching Simultaneously, 28 V dc is supplied to the GCUs through
relay must be manually reset by selecting the pin f applying power for the left (K1) and right (K2)
appropriate GEN switch on the AC CONTROL bus contactors terminals Y1/Y2, which operate to the
panel to OFF, then on to return the generator to cross−tie position. The left and right ac power buses are
the line. then parallel−connected to the external power source.

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 9 – Generator Failure

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 deHavilland Dash 8 100/300 24 - Electrical Power
NOTE: Both GCUs must be installed and operational
for external power to be applied.

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 10 – A.C Ground Power

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 deHavilland Dash 8 100/300 24 - Electrical Power
AC 400 HZ SYSTEM V at 400 Hz + or − 1%. The outputs from the three
inverters are supplied to the paralleling control box. The
Description primary and secondary inverters are separately supplied
General from the left and right essential dc buses through the PRI
INV PWR and SEC INV PWR circuit breakers, and the
The AC 400 Hz system supplies electrical power auxiliary inverter from the left main dc bus through the
for instruments, powered flight control surfaces AUX INV PWR circuit breaker. The inverters are mounted
position indicators, hydraulic quantity, navigation and on the top, center and bottom shelves of the left hand
communication equipment. The system consists of circuit breaker console (pilot’s side).
three static inverters (primary, secondary and auxiliary),
a paralleling control box and two 26 volt 400 Hz Paralleling Control Box
autotransformers, supplying power to left and right 115V The paralleling control box incorporates frequency
400 Hz buses and left and right 26V 400 Hz buses. The synchronization and load sharing circuits to control the
system also includes an inverter warning control box operation of the inverters, thus permitting the three
which operates inverter and bus failure caution lights and outputs to be applied in parallel to the tied left and right
a 400 Hz control box which contains the primary power 115V ac buses. The paralleling control box applies the
contactors K1, K2 and K3, output current transformer primary inverter output directly to the left bus and the
T3, and voltage monitoring circuit breakers CB7, CB8 secondary inverter output to the right bus.
and CB9 for the inverters. Monitoring provisions for the
400 Hz system are included on the AC SYSTEM power A switching circuit in the box connects the auxiliary
monitor panel. The left and right 115V 400 Hz buses are inverter output to the left or right bus through either one
interconnected through a 115V BUS TIE circuit breaker of two AUX INV INPUT circuit breakers. The switching
located on the avionics circuit breaker panel. In normal circuit is controlled by the AUXILIARY INVERTER
operation the primary inverter supplies the left bus and selector switch located on the AC CONTROL panel.
the secondary inverter the right bus. The auxiliary inverter The paralleling control box also contains circuits to
is normally operativeand can be selected to supply either detect an inverter fault and provide a fault signal to the
bus by a switch located on the AC CONTROL panel. inverter warning control box. The paralleling control box
is mounted on the center shelf of the left hand circuit
Static Inverters breaker console, adjacent to the secondary inverter.
The three solid state static inverters each generate a
115V 400 Hz, single phase, sine wave output from 28 volt
dc input. They contain frequency and voltage regulator
circuits which regulate the output to 115V ac + or − 5.75

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 11 – Static Inverters - Locations

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 deHavilland Dash 8 100/300 24 - Electrical Power
Inverter Warning Control Box applied to the warning control box which processes the
signal to switch on the related caution light.
The inverter warning control box processes inverter
fault signals from the parallelingcontrol box and controls Two caution lights labeled L 26 AC and R 26 AC provide
the inverter failure caution lights. Power to the warning indication for a left or right 26V bus failure. Logic circuits
control box is supplied from the left essential dc bus within the caution lights panel are connected to the left
through an INV WRN CONT circuit breaker. The inverter and right buses through the L 26V FAIL and R 26V FAIL
warning control box is mounted on top of the 400 Hz circuit breakers on the avionics circuit breaker panel. The
control box, located on the bottom shelf of the left hand logic circuits are designed to put on the left or right 26
circuit breaker console. volts caution light in the absence of voltage on the 26V ac
buses or if the voltage on either bus falls below 20 volts.
Autotransformers
Inverter Control
Two 26V 400 Hz autotransformers and power factor
correction capacitors are installed on the rear face of the Inverter control and selection is by three switches on
bulkhead at Sta X182.00, behind the pilot circuit breaker the AC CONTROL panel on the overhead console. The
console. The transformers are supplied from the left and three INVERTERS switches are labeled PRIMARY,
right 115V 400 Hz buses through the 115/26V AC XFMR SECONDARY and AUXILIARY, with the OFF positions
LT circuit breaker (CB13) and the 115/26V AC XFMR RT marked on the panel.
circuit breaker (CB12). The outputs are connected to the
The PRIMARY and SECONDARY switches are
related 26V 400 Hz bus.
two−position toggle switches which operate power
Caution Lights contactors K1 and K2 for the two inverters. The
AUXILIARY switch is a three−position, center−off toggle
Caution lights are incorporated in the ac 400 Hz system
switch, marked L, OFF and R. It operates contactor K3
to provide a visual indication of an inverter failure or a
supplying power to the auxiliary inverter and, through the
26V ac bus failure. The lights are located on the caution
paralleling control box, connects its output to the left (L)
lights panel.
or right (R) bus as desired.
Three inverter failure lights, PRI, SEC and AUX INV,
Power Monitoring
powered by the left essential dc bus through the INV
WRN CONT circuit breaker, are controlled by the inverter Power monitoring for the 400 Hz system is provided on
warning control box. If an inverter failure occurs, a fault the left side of the AC SYSTEM panel on the overhead
signal is generated in the paralleling control box and console. Two digital readout windows indicate voltage
and load on the inverter outputs as selected by the

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 12 – Static Inverter -- Schematic

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 deHavilland Dash 8 100/300 24 - Electrical Power
three−position rotary switch labeled INVERTERS PRIM/ by the AUXILIARY INVERTER selector switch on the
AUX/SEC. AC CONTROL panel. The left and right buses are
interconnected through the 115V BUS TIE circuit breaker
The LOAD display window monitors load currents
on the avionics circuit breaker panel.
expressed as a percentage factor of the rated circuit
load. For example, a readout of 1.00 indicates full load With power applied to the left and right 115V 400 Hz
(3.5 amps per phase), 0.50 indicates half load; and a buses, the autotransformers connected to each bus step
reading of 1.20 indicates a twenty percent overload. An down the applied voltage and provide a 26V 400 Hz
overload condition is advised by a flashing positive sign output to the left and right 26V 400 Hz buses.
(+) preceding the readout. The TEST pushbutton tests
Test − AC Power Monitor System
the monitor display and circuit operation when pressed.
The TEST pushbutton on the AC SYSTEM power monitor
The voltage indicator is connected to the inverter outputs
panel energizes built−in test equipment (BITE) circuits to
through circuit breakers CB7, CB8 and CB9 located
produce pre−assigned display indications, and verifies
in the 400 Hz control box. The load indicator receives
proper operation.
its information from the monitor outputs of current
transformer T3, also located in the 400 Hz control box. Inverter Failure

Operation In the event of a single inverter failure, the paralleling

control box senses the fault, disconnects the inverter
Normal Operation from the associated load, and applies a fault signal to the
All three inverters are normally operated simultaneously. warning control box. The warning control box processes
With the dc buses powered and the primary and the fault signals and the related inverter caution light is
secondary inverter switches on, and the auxiliary inverter switched on. The two remaining inverters continue to
switch selected to either L or R, each inverter generates operate to power the buses. The auxiliary inverter may be
a 115V ac 400 Hz output which is applied to the inverter manually switched if necessary, to the side on which the
paralleling control box. The paralleling control box failure occurred.
provides the necessary feedback signals to the inverters If a second inverter subsequently fails leaving one
to obtain frequency synchronization and load sharing. operational, the paralleling control box disconnects that
This being achieved, the paralleling control box inverter from the load and applies a second fault signal
connects the primary inverter output to the left bus, to the inverter warning control box, which switches on the
the secondary inverter output to the right bus and the related inverter caution light.
auxiliary inverter output to the left or right bus as selected

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 13 – 400 Hz Schematic

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 deHavilland Dash 8 100/300 24 - Electrical Power
The ON/OFF switch for a failed inverter must be manually
switched off.
26 Volt AC Bus Failure
Loss of voltage on a 26 volt 400 Hz bus will be indicated
by the L 26 AC or R 26 AC caution lights coming on. As
these buses are dependent upon the autotransformers
being supplied power from the main 115V ac buses,
loss of power from the left or right 115V ac bus will result
in failure of the 26V ac buses, and illumination of the
associated 26V AC caution light.

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 14 – 26 VAC 400HZ Failure

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 deHavilland Dash 8 100/300 24 - Electrical Power
DC POWER SYSTEM from an external dc source. When the aircraft is on
external dc power, all buses are automatically tied and
Description powered.
General The dc bus system may also be powered when the
The dc power system provides 28−volt dc power for aircraft is connected to external ac power by using the
operation of the aircraft dc electrical services. It also TRUs, powered by the external ac power source, to
provides the starting facility for the engines. The sources develop the 28 volts dc.
of dc power are as follows: Bus System
■■Left and right dc starter−generators, one installed The dc power system powers a bus distribution system
on the accessory gearbox of each engine. consisting of left and right main feeder buses, and left
■■Left and right transformer rectifier units (TRU), and right secondary feeder buses located in the contactor
powered from the left and right variable−frequency box; left and right essential buses, along with the left and
ac generators. right main and secondary distribution buses located in the
■■A main (right) battery of 40 ampere−hour capacity. flight compartment of the aircraft.
■■An auxiliary (left) battery with 15 ampere−hour The left and right main feeder buses are normally
capacity. (SOO 8070 provides a 40 ampere−hour separated, and powered by the respective left and right
battery in the auxiliary position). The batteries starter−generators. The left and right main feeder buses
are placed in the aircraft as supplementary dc power the main distribution buses.
power, but are on charge when the generators
are operating. The right battery supplies power to The essential buses are powered from either the main
the main buses, and both batteries supply power distribution buses or the batteries.
to the essential buses when power is not being The secondary feeder buses are powered by the TRUs
supplied by the generators or TRUs. and are connected to the respective left and right
The starter−generators and TRUs normally operate secondary distribution buses.
independently: the left and right generators powering the The main feeder buses are automatically tied when only
corresponding left and right main feeder buses, and the one generator is operating, or when the aircraft is on
left and right TRUs powering the corresponding left and external power. The secondary feeder buses are tied
right secondary feeder buses. when only one TRU is supplying power. The essential
An external dc power receptacle and control circuits are buses are tied by a circuit breaker.
incorporated to enable the dc bus system to be powered

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 15 – DC System Schematic

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 deHavilland Dash 8 100/300 24 - Electrical Power
DC GENERATION SYSTEM to display voltages available on the various buses, and
current from the generators, TRUs and battery circuits.
Description
Starter−Generator
General
The dc starter−generators are used to motor the engines
The dc generation system consists of two engine for engine start, as well asgenerate dc power in the
driven starter−generators, one on each engine, and generation system.
two Transformer Rectifier Units (TRUs) powered by the
variable frequency ac system. The four power sources The starter−generators are mounted on the upper right
normally operate independently: the left and right side of each engine accessory gearbox, through which
generators powering the corresponding left and right they are driven. They are electrically connected to the dc
main feeder buses, the left and right TRUs powering the generator system by a multi−pin electrical connector for
corresponding left and right secondary feeder buses. control and monitoring functions, and by two threaded
stud terminals for power output (positive and negative).
Two batteries supply supplementary dc power, but are on The generators are rated to supply 300 amperes at 30
charge when the generators are operating. volts, and are self cooled.
The generators are controlled by their individual Each generator is controlled by its individual GCU so
Generator Control Units (GCUs) to produce a steady that the output voltage remains constant over the speed
output of 28 volts dc under varying conditions of engine range of approximately 5600 rpm (min.) to 12,000 rpm
speed and electrical loads. (max.).
Common to both generators and their associated GCUs The starter−generator is a four−pole, shunt connected,
is a Bus Bar Protection Unit (BBPU) which is designed to fully compensated dc design with interpole windings,
isolate a defective bus from the associated generator, as brush commutation, and an internal 4−blade aluminum
a protection against overloading the generator system. die−cast cooling fan. Four brush blocks, spaced 90
The functions of the GCU and BBPU are achieved degrees apart around the commutator, are employed. A
with the aid of current transformers, placed around the pair of brushes with leads joined to a common terminal
generator’s positive and negative output leads, and a are retained in each brush block by springs. A removable
number of contactors, control relays and circuit breakers window strap provides access to the brushes.
in the dc contactor box. An air inlet duct is secured to the non−driven end of the
Manual control is provided by control switches in the starter−generator and an exhaust duct is incorporated
flight compartment. A dc monitoring system is provided in the mounting flange to provide cooling. A thermostatic

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 16 – DC Starter Generator

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 deHavilland Dash 8 100/300 24 - Electrical Power
switch is installed in the starter−generator to close a ■■Field weakening, used during engine start to
circuit if temperature goes above a predetermined limit, reduce starter field current as engine speed
providing an appropriate DC GEN HOT indication on the increases.
caution lights panel.
If necessary, the GCU will shut down a generator and
Generator Control Unit isolate it from the main feeder bus.
The GCUs are mounted in the RH circuit breaker The above functions are achieved by GCU circuits
console on the center and bottom shelves. They provide which monitor external signals. The principal circuits
automatic control, monitoring and voltage/current are differential voltage sensing, reverse current and
regulation for the dc generators, and control the operation overcurrent protection, overexcitation and overvoltage
of the main feeder bus contactors. An overcurrent sense control, feeder fault detection and field relay trip.
signal is also supplied to the bus bar protection unit.
Bus Bar Protection Unit
When the generators are used in the starter mode, the
GCU controls the start cycle. Operation of both GCUs is The bus bar protection unit (BBPU) is located on the
identical. top shelf of the RH circuit breaker console. It operates
in conjunction with the two GCUs providing overcurrent
The GCUs are hard−mounted by four mounting
protection against a bus fault in the generating system
bolts, in the electrical equipment rack under the right
by isolating the faulty bus from the power source and by
dc circuit breaker panel beside the copilot’s seat. A
signaling the GCU to shut down the affected generator
formed aluminum case houses the circuitry, which is all
and disconnect the associated battery.
solid−state with the exception of control relays. Electrical
connections are via a multi−pin connector on the side of The BBPU receives an overcurrent signal from the GCU
the unit. if the generator interpole current exceeds 400 amperes.
The BBPU immediately opens the coil energizing source
The GCU provides the following control functions for the
for contactors K21, K5 and K6, and turns on a DC BUS
DC generators:
fault light on the caution panel. Simultaneously a 7–to–10
■■Voltage regulation, by varying the generator field second timer is started within the BBPU. If isolation of the
current to maintain a preset voltage output. faulty bus removes the overcurrent condition, no further
■■Paralleling control, to equalize the load on both action is taken by the BBPU.
generators when operating with the main feeder
If the excessive current continues to flow, at the end of
bus tie closed.
the timer cycle the BBPU disconnects the associated
■■Current limiting, preventing excessive current from battery and applies a trip voltage to the GCU, which in
flowing during starting of the opposite engine.

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 17 – DC GCU and BBPU Locations

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 deHavilland Dash 8 100/300 24 - Electrical Power
turn opens the primary contactor and shuts down the on opposite sides of the contactor box assembly, and
affected generator by removing its field current. are connected by bus bars mounted directly on the
contactors.
Current Transformers
External connections to the generators and batteries
Two current transformers are installed in each generator
are routed through the ends of the contactor box to their
system. One is installed in the positive feeder cable
respective contactors with the generator leads passing
adjacent to the generator contactor in the DC contactor
through current transformers within the contactor box.
box, and the second transformer is mounted on the
negative feeder cable connecting the generator E The side end panels mount the various circuit breakers
terminal to system ground. for battery connections, main and secondary distribution
buses, and control circuits.
Each transformer consists of a core that provides a feed
through for the primary feeder. The secondary winding The contactor box assembly also contains the
is wound on this core, and the assembly is installed in a auxiliary battery charging diode and relay, TRU output
formed aluminum case. undervoltage and external power overvoltage protection
relays.
A terminal block is secured to the top of the case
to provide terminal connections for the transformer DC Control
secondary, leading to the GCU. A removable plastic cover
The dc generating system is manually controlled through
plate provides protection over the terminals.
the DC CONTROL panel located in the overhead
Normal currents sensed by both transformers produce console. The panel contains eight control switches,
canceling signals in the GCU differential circuits. An namely: BATTERY MASTER, MAIN BATT, AUX
instantaneous differential fault current produces a signal BATT, GEN 1, GEN 2, (generators 1 and 2 energizing
in one transformer that is used by the GCU to trigger a controls), MAIN BUS TIE, BUS FAULT RESET, and
trip of the field relay of the affected system. EXTERNAL POWER. The BUS FAULT RESET is a
momentary−action toggle switch; the remainder are
DC Contactor Box
normal two−position ON/OFF toggle switches.
The dc contactor box, located in the aircraft nose,
DC Power Monitor System
contains all primary contactors, the main and secondary
feeder buses, bus tie contactors, protective fuses, and A digital−readout power monitor panel is located in the
circuit breakers for the main and secondary distribution overhead console, adjacent to the dc control panel. The
buses and various control and interlocking circuits. panel contains four digital readout display windows, a
The left and right contactors are physically located voltage selector switch, a load selector switch, and a

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 18 – DC Contactor Box and Current Transformer

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Figure 19 – DC Monitor and Control Panels

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 deHavilland Dash 8 100/300 24 - Electrical Power
test pushbutton. The TEST pushbutton tests the monitor is advised by a flashing positive sign (+) preceding the
display and circuit operation when pressed. readout. A negative condition (example battery discharge)
is indicated by a flashing minus sign (−) preceding
The display windows present standard digital readouts,
the readout. The rated loads are 100 amperes for the
one reading voltage and the other three currents.
batteries, 250 amperes for the main generators, and 200
The left digital display window, labeled BUS VOLTS, amperes for the TRUs.
monitors the voltages on the various buses as selected
Transformer−Rectifier Units
by the adjacent switch. The switch selects left secondary
bus (L SEC), left main (L MAIN), left essential (L ESS), Two identical left and right transformer−rectifier units
right essential (R ESS), right main (R MAIN) and right (TRUs) are located in the nose compartment on a shelf
secondary (RSEC) buses. The sources for these at Z112.00 LHS, and supply 28 volt dc power separately
indications are as the titles read, through circuit breakers to the left and right secondary feeder buses. They consist
CB14, CB1, CB7, CB19, CB16 and CB27, on the basically of transformers and diode rectifiers. The dc
respective buses. output capacity of each TRU is 200 amperes at 29.5
volts maximum and 25 volts minimum at 100 percent
The center two display windows are labeled LOAD/
load. Power is converted from the variable frequency ac
MAIN BATT/AUX BATT, and continuously monitor the
3 phase input. The output voltage is dependent on input
current flowing through the batteries, being permanently
voltage and transformer regulation.
connected to the battery shunts R3 (auxiliary) and R4
(main). The TRUs receive input power to their wye connected
primary windings independently from the left and right
The right digital display window is labeled LOAD, and its
variable frequency ac generators.
readout is selectable by the adjacent 4−position switch
to monitor output currents of the left TRU, left generator In each TRU, dual secondary winding outputs are
(GEN 1), right generator (GEN 2) and right TRU. The interconnected by an interphase transformer and six
sources for these indications are shunts R5, R1, R2, and diodes to provide full−wave, 12−phase rectification.
R6. An internal fan provides cooling air for the diodes and
transformer. A thermostatic switch in each TRU provides
The three current display windows monitor load currents
indication of excessive temperature to the L TRU HOT or
expressed as a percentage factor of the rated circuit
R TRU HOT warning light on the caution lights panel.
loads. For example, a readout of 1.00 indicates full
load, .50 indicates half load; a reading of 1.20 indicates The negative outputs of the TRUs are connected to
a twenty percent overload. Maximum readout is + or − airframe ground through shunts R5 (left) and R6 (right).
1.99. An overload condition on generator or TRU outputs The left shunt is mounted immediately forward of the

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 20 – Transformer Rectifer Units

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 deHavilland Dash 8 100/300 24 - Electrical Power
left TRU and grounded through a short bus bar to the on sensing a generator output voltage in excess of the
adjacent ground stud GS9. The right shunt is mounted on main feeder bus voltage sensing reference, will supply
the right side of the shelf, grounded through a short bus contactor control power to contactor K2, connecting the
bar to the ground stud at GS10. generator output to the right main feeder bus. Auxiliary
contacts on K2 connect the right essential bus to the
Operation GCU to complete the GCU logic circuits requirement.
General Simultaneously, the GCU sends a "power ready" voltage
to energize the generator logic relay K12, which will
Normal operation of the dc generation system includes cause No. 2 GEN caution light to go off, provided K2
energizing the battery system to provide power for control remains energized to open contacts 13 and 14 that also
circuits and engine start. At the DC CONTROL panel, control the GEN caution light.
the BATTERY MASTER, MAIN BATT and AUX BATT
switches are selected on. The batteries supply power Having generator logic relay K12 energized, and K11
to the essential buses and right main bus, and receive de−energized (left generator off, and right generator
charging current when the generators are operational. on), their interconnecting contacts provide a power path
to energize the main feeder bus tie K21. The power is
Generator Operation supplied by CB26 from the right essential bus, through
With the aircraft engines running, and the GEN 1 and the Bus Bar Protection Unit internal relay RL3. Power is
GEN 2 switches (DC Control Panel) selected on, the then available on the left feeder bus.
starter−generators assume their generator function. Each Action of the left generator and GCU is identical to the
starter−generator is controlled by a separate generator right side; the GCU in this case operating contactor K1,
control unit (GCU) which automatically switches from and supplying power ready voltage to logic relay K11.
starter to generator function upon release of the starter Activation of K11 while K12 is also energized (both
circuits. generators functioning), interrupts the voltage path to bus
When switched to generator operation, the GCU’s tie K21, and the left and right generating systems operate
receive current sensing information (output and ground independently.
return), speed sensing, interpole current, and output The main battery receives charging current from the right
voltage signals from the generators. Output voltage and feeder bus when battery relay K8 is energized. The relay
current is regulated by the GCU by varying the current is energized when MAIN BATT switch S4 is selected on.
flowing in the shunt field winding of the generator. The auxiliary battery is charged from the left feeder bus
Assuming the right generator (No. 2) is selected on through diode CR1 and battery relay K7 in the energized
and the left generator (No. 1) is OFF, the right GCU

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 21 – DC Operation
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Figure 22 – DC Schematic (1 of 5)
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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 23 – DC Schematic (2 of 5)
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 deHavilland Dash 8 100/300 24 - Electrical Power
THIS PAGE INTENTIONALLY LEFT BLANK

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 24 – DC Schematic (3 of 5)
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 deHavilland Dash 8 100/300 24 - Electrical Power
position. It is energized when the AUX BATT switch S3 is External DC Power
selected on.
The connector for external dc power application is on
When K7 and K8 are deenergized (batteries the nose of the aircraft, lower left side. Selection of the
disconnected from the buses) their auxiliary contacts EXT POWER (switch S8) on the DC CONTROL panel
13/14 provide a ground connection to the caution lights to on position applies power simultaneously to external
panel to turn on the AUX BATT or MAIN BATT caution power contactors K9 and K10, and interlock relays K15
lights. and K16. Application of external dc power inhibits dc
generator operation
TRU Operation
Test − DC Power Monitor System
The TRU s operate directly from the primary 3−phase
variable−frequency buses when the ac generators To operate the internal test circuits of the DC SYSTEM
are on, or when external ac power is applied. Their power monitor panel, the TEST pushbutton is pressed
nominal 28−volt dc output is applied to the left and right and held. Pressing the button causes all segments of
secondary buses through TRU contactors K17 and the VOLTS and LOAD digital display characters to light.
K18. Auxiliary contacts on these contactors complete When held longer than approximately two seconds, the
the ground return circuits for the TRU logic relays K13 internal test circuits simulate VOLTS of 30.5 + or − 0.3
and K14 when energized, and the associated L TRU and LOADS of 1.05 + or − 0.03 which is displayed on
fail and R TRU fail indicators on the caution panel when applicable sections of the panel.
deenergized. Power for the logic relays is from CB6 on
Failure Conditions
the essential bus.
Generator Failure
The TRU contactors receive operating voltage from
either the essential bus (CB21 and diodes CR7 or CR8) The generator logic relays K11 and K12, being
or from the TRU outputs, through diodes CR9 or CR10, controlled by the GCU power ready voltage, provide
ensuring adequate operating voltage during any voltage automatic closing of the main bus tie in event of a
fluctuations from the TRU. The contactors are controlled generator failure. Assuming a No. 1 (left) generator
however by undervoltage protection relays K19 (left) or failure, loss of generator output voltage is sensed by
K20 (right). These relays close on receiving a voltage of the No. 1 GCU which operates to open the left feeder
18 volts or more from the TRU outputs at their voltage contactor K1, disconnecting the generator from the
sensing control terminals C2. left feeder bus and applying a ground to the No.1 DC
GEN caution light. Simultaneously, the GCU power
ready voltage disappears, de−energizing the left

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 25 – DC Schematic (4 of 5)
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 deHavilland Dash 8 100/300 24 - Electrical Power
generator logic relay K11. This action applies bus tie contactor K22 and restoring power through the bus tie
operating voltage from the bus bar protection unit to the affected bus from the functioning TRU.
(pin E) through K11−A2/A3 to K12−A1/A2 to the main
Failure of both TRUs will result in both logic relays
feeder bus tie K21, restoring power through the bus
K13 and K14 being deenergized, removing operating
tie to the left main feeder bus from No. 2 generator.
voltage from bus tie contactor K22. Also the
Deenergized contacts D2/D3 of K11 maintain the
secondary/main feeder bus contactors K5 and K6 will
caution light ground. Failure of the No. 2 generator
be energized from the BBPU bus tie voltage through
operates in a similar manner through No. 2 GCU,
K11−B2/B1, to contacts B2/B3 of K13 and K14, and
opening right contactor K2, and de−energizing right
K12−B1/B2. The secondary feeder buses will then be
generator logic relay K12 which applies operating
supplied power by the left and right main feeder buses
voltage to K21.
and their respective generators.
In event of failure of both generators, bus tie K21
Bus Failure
is opened through the interruption of the bus tie
operating voltage by both deenergized generator logic The Bus Bar Protection Unit (BBPU) operates in
relays. Simultaneously, contact B2 or K12 applies conjunction with the two GCUs. When a GCU senses
the BBPU bus tie operating voltage to secondary/ an overcurrent condition (in excess of 400 amperes),
main feeder bus contactors K5 and K6, connecting a signal is sent to the BBPU, which in turn inhibits
the secondary feeder buses supplied by the TRUs, to possible operation of bus tie contactor K21 and
the main feeder buses. This bus tie operating voltage secondary/main feeder contactors K5 and K6 by
path is from K11−B2/B3 through energized K13−C2/ removal of the bus tie voltage output (BBPU pin E)
C1. K14−C1/C2, and deenergized K12−B3/B2. and triggers on the DC BUS light on the caution light
panel. Closure of these contactors is inhibited until
TRU Failure
the fault has been cleared, and the BBPU has been
Loss of TRU output voltage is sensed by undervoltage manually reset by the dc BUS FAULT RESET switch
relays K19 (left) or K20 (right) which de−energizes (S6) on the DC CONTROL panel.
the associated TRU contactor K17 or K18 and TRU
If the fault remains for longer than 7 to 10 seconds,
logic relays K13 or K14. The associated caution
the BBPU sends a trip signal to the GCU, which shuts
light is turned on by contacts 13 and 14 of K17 or
down the affected generator, opens its main feeder
K18. Opening of either logic relay while the other
bus contactor K1 or K2, and disconnects the battery
is energized causes the essential bus voltage from
connected to its bus, locking out until manually reset
CB26/CR13 at terminal A2 of K13, to be applied to
terminal A2 of K14, energizing secondary bus tie

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 deHavilland Dash 8 100/300 24 - Electrical Power

Two Stages of Protection:

Stage 1: Isolate buses

Stage 2: Isolate power sources

Figure 26 – DC Schematic (5 of 5)
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 deHavilland Dash 8 100/300 24 - Electrical Power
by S6. The time delay is developed electronically by
timer circuits within the BBPU.

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 27 – Bus Bar Protection Unit (BBPU)

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 deHavilland Dash 8 100/300 24 - Electrical Power
BATTERY SYSTEM breakers are located in the dc contactor box which is
mounted on the forward side of the upper pressure
Description bulkhead.
General Battery Control Circuit
The battery system consists of two nickel−cadmium The main battery and the auxiliary battery are connected
batteries, with venting and temperature monitoring directly to their respective buses. They are also
equipment, current metering shunts, power contactors connected to the essential bus through contactors which
and their control circuits. are manually controlled by battery control switches. The
The batteries are mounted in the lower left section main battery is connected to the main feeder through a
of the nose compartment on shelf Z97.00, parallel to contactor which is controlled by a main battery switch.
the aircraft centerline forward of the lower pressure The contactors are also controlled by the dc generation
bulkhead. The main, or right battery, has a capacity system and the dc external power control circuits
of 40 ampere−hours, and is mounted in the forward The left (auxiliary) battery is connected to the left
position. The auxiliary (left) battery has a capacity of 15 essential bus through left essential bus contactor K3 and
ampere−hours and is installed at the rear of the main isolation diode CR5. Charging current for the left battery
battery. (SOO 8070 replaces the 15 ampere−hour battery is provided from the left main feeder bus through isolation
with a 40 ampere−hour battery in the auxiliary position). diode CR1 and left battery bus contactor K7.
Both batteries are secured to their mounting trays by
tie−down bolts. The right (main) battery is connected to the right essential
bus through right essential bus contactor K4 and isolation
The two battery cases are commonly vented to diode CR4. It is also directly connected to the right main
atmosphere through a sump jar. The jar is mounted aft feeder bus through the right battery bus contactor K8.
of the frame at X68.44, left hand side, and is connected The right main battery also powers the BATTERY PWR
to the battery cases and outside air by hoses and a tee bus through a circuit breaker (CB74) located in the DC
union. contactor box.
The negative side of each battery is connected through The left and right essential buses are connected by the
separate battery metering shunts to airframe (ground) on BUS TIE circuit breaker located on the pilot’s left dc
the underside of shelf Z112.00 above the batteries. circuit breaker panel.
Each battery is equipped with a temperature sensor All control switches for the battery system are located
which is connected to a temperature monitoring system. on the DC CONTROL panel located in the overhead
The battery contactors, control relays, and circuit

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 28 – Battery System

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 deHavilland Dash 8 100/300 24 - Electrical Power
console. The BATTERY MASTER switch operates four When K7 or K8 are de−energized (batteries disconnected
separate circuits, providing a ground return to essential from the buses) their auxiliary contacts 13/14 provide a
bus contactors K3 and K4 (powered from the left and ground connection to the caution lights panel to turn on
right battery buses), and power to the auxiliary and main the AUX BATT or MAIN BATT warning lights.
battery switches; this power is supplied from the left and
With the battery switches on and the contactors
right essential buses through the bus bar protection unit
energized, the right main feeder bus and right main
and its reset switch.
distribution buses are powered for the operation of any
The AUX BATT and MAIN BATT switches operate battery system connected to them, including right engine start
bus contactors K7 and K8 through normally−closed operation.
contacts of the external power interlock relay K15
To power the left main feeders and left distribution
The MAIN BUS TIE switch is normally lever−locked in buses from the right battery the MAIN BUS TIE switch
the open or OFF position. In emergency situations it is is set to on to energize the main bus tie contactor K21.
selected ON to energize the main bus tie contactor K21. The contactor is energized from the right essential bus,
through the bus bar protection unit.
Operation
NOTE: Bus tie contactor K21 is normally operated
Actuation of the BATTERY MASTER switch on the DC automatically by the Generator ControlUnit.
CONTROL panel energizes essential bus contactors K3 Manual selection of the bus tie switch is
and K4, connecting the auxiliary (left) battery to the left necessary for battery operation only. battery
essential bus through diode CR5, and the main (right) switches turned on, the external dc power
battery to the right essential bus through diode CR4. source (when applied to the buses) supplies
Simultaneously, power becomes available at the MAIN charging current to the batteries.
BATT and AUX BATT switches, through the Bus Bar
Protection Unit (BBPU) and the BUS RESET switch. CAUTION: IF THE BATTERIES ARE SELECTED
ON WITH EXTERNAL POWER
Operation of the AUX BATT and MAIN BATT switches APPLIED, THE BATTERY CHARGE
energizes battery contactors K7 and K8. RATE AND TEMPERATURE MUST
Contactor K7 provides a diode−protected charging path BE CONTINUALLY MONITORED
for the auxiliary battery from the left main feeder bus. TO ENSURE THAT BATTERY
TEMPERATURE LIMITS ARE NOT
Contactor K8 connects the main battery directly to the EXCEEDED.
right main feeder bus.

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 29 – Battery System Schematic

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 deHavilland Dash 8 100/300 24 - Electrical Power
Current flow in each battery (charge or discharge) is the batteries are plastic tubes joined at a tee connector,
continuously monitored on the DC SYSTEM panel in the which in turn is connected to a fitting on top of the sump
overhead console. The two battery shunts are connected jar. The tubes are clamped at various places to prevent
to the MAIN BATT LOAD and AUX BATT LOAD on the movement and/or damage.
DC SYSTEM power monitor panel, which indicate current
The second vent fitting on the sump jar is connected
flow expressed as a percentage factor of the rated load.
through a plastic tube, the end of which forms the
The rated load for the batteries is 100 amperes. A overflow vent through the aircraft skin on the bottom of
readout of 1.00 indicates full load, 0.50 indicates half the nose section. It is clamped to prevent movement and
load; a reading of 1.20 indicates a twenty percent the end is cut flush with the aircraft skin.
overload. Max. readout is + or − 1.99. A discharge
condition is indicated by a flashing minus sign (−) Battery Temp. Monitor System
preceding the readout. Description
Battery Venting System The battery temperature monitor system provides
continuous temperature indication and overheat warning
General
indication for each battery. The system consists of
The battery venting system consists of the battery a battery temperature monitor and four temperature
sump jar, breather tubes from the two batteries, and the sensors. Two sensors provide temperature information
overflow vent through the aircraft skin to atmosphere. to the monitor, and two operate indicator lights on the
warning lights panel. The lights are labelled MAIN
The sump jar is made of styrene plastic and consists of
BAT HOT and AUX BAT HOT. If Mod 8/0235 is not
two separate parts. The top is mounted to the left wheel
incorporated in the aircraft, these lights are located on the
well web forward of the main battery and contains two
caution lights panel.
vent pipe fittings, one connected to the battery breather
tube, the other to the overflow vent. The bottom part of Battery Temperature Monitor
the sump jar screws into the top and is held in place by
The battery temperature monitor is identified BATTERY
two half−clamps, also attached to the wheel well web.
TEMPERATURE, and is located on the overhead
The jar contains a pad soaked with a boric acid solution
console. Panel lighting is provided by integral lamps. The
which neutralizes any fumes or electrolyte spillovers from
monitor contains two temperature indicators, indicator
the batteries.
driver circuits, overheat warning circuits and built−in test
The batteries are each equipped with two vents, one of equipment (BITE) which includes sensor fail indicator
which is capped and unused. The breather tubes from lamps and a test switch.

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 30 – Battery Vent System

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 deHavilland Dash 8 100/300 24 - Electrical Power
One of two different monitors are installed, depending on overheat light in event of failure of the overheat warning
the production date and Mod status of the aircraft. Mod temperature sensor.
8/1584 introduced a monitor from a new manufacturer.
Both are similar in physical appearance and general The built−in test equipment (BITE) provides a continuous
operation, but with slightly different self−test indications. self−check of the temperature monitor circuits and visual
indication of open or shorted overheat light sensor
The temperature indicators on the monitor are labeled circuits by turning on the applicable yellow SENSOR FAIL
MAIN and AUX, and separately monitor the temperature light. An open or shorted display sensor is indicated by
of the two batteries. The displays consist of colored only the affected yellow 60° C segment turning on.
incandescent light bar segments or oval−shaped LED
s, arranged above and below a horizontal temperature The test switch is a combined pushbutton/2−position
scale. The scale is graduated in Celsius from 15° to 80°. momentary toggle switch labeled TEST, located on the
The displays are colored green from 15° to 50° , yellow face of the monitor panel. The switch energizes the
from 50° to 65° , and red from 65° to 80°, corresponding BITE circuits and simulates predetermined conditions
to normal, caution and danger operating ranges. and failures for manual check of selected circuits and
displays.
The input of each indicator driver circuit is connected to
the display temperature sensor installed in each battery. Temperature Sensors
The circuit responds to temperature−related changes in There are two independent temperature sensors
resistance of the thermistor to generate a drive signal (thermistors) in each battery. Each thermistor is mounted
proportional to the sensed temperature. The driver circuit on an intercell connector link of the battery, with external
output is connected to the related indicator. connections provided through a six−pin connector on
The input of each overheat warning circuit which drives the battery case. One sensor (display sensor) provides
the warning lights (or Pre−Mod 8/0235 caution lights), is temperature input to the related indicator driver circuit in
similarly connected to the overheat temperature sensor the monitor, and the other (overheat temperature sensor)
thermistor installed on the related battery. The circuit is to the overheat warning circuit. The sensor thermistors
essentially a comparator with a set point equal to 65° C. If vary their internal resistance non−linearly with changes in
the thermistor resistance exceeds a value corresponding temperature, ranging from 31,439 ohms at 24° C to 3251
to 65° C, the circuit applies a positive voltage to the ohms at 80° C, with a tolerance of ±5%.
related MAIN BAT HOT or AUX BAT HOT warning light. Overheat Lights
A second input is applied to the overheat warning circuit The battery overheat warning/caution lights AUX BAT
from the related indicator driver circuit if the indicated HOT and MAIN BAT HOT are located on the warning
temperature exceeds 65° C, ensuring operation of the

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 31 – Battery Temperature Monitor

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 deHavilland Dash 8 100/300 24 - Electrical Power
or caution lights panel. The lights are controlled by Self−Test
the related overheat warning circuit in the BATTERY
Self test is accomplished by the operation of the TEST
TEMPERATURE monitor.
switch. This switch energizes the built−in test equipment
Electrical Power (BITE) and when operated provides three different test
modes:
Power to the system is supplied from the right essential
dc bus. The indicator driver circuits are supplied through Mode 1. When pressed in and held, simulates a
the BATT TEMP IND circuit breaker (location L8), and the temperature of 70° C.
overheat warning circuits are supplied through the BATT
Mode 2. When moved to the left and held, simulates
TEMP CAUT LTS circuit breaker (location M8).
shorted display/overheat sensors.
Panel lighting is supplied from the 5 volt dc panel lighting
Mode 3. When moved to the right and held, simulates
system to the overhead console.
open display/overheat sensors.
Operation
On Pre−mod 8/1584 aircraft, the test indications are as
Operation of the battery temperature monitor is automatic follows:
and continuous when power is applied to the right
When the TEST switch is pressed in and held, the BITE
essential bus and the applicable circuit breakers are
turns on all green and yellow segment lights, and the red
closed. Valid temperatures below 15° C are indicated by
segment lights to the 70 degrees point. In addition, the
the lowest−reading green segment being on. Increases
MAIN BAT HOT and AUX BAT HOT overheat warning
in battery temperature turn on additional segments,
lights flash on and off (master warning light will also
the value being read on the adjacent scale. Excessive
flash). Upon release, all indications return to normal.
battery temperature is indicated by the display lighting in
the yellow zone (50° − 65° C) and eventually red (over When the TEST switch is selected and held to the left,
65° C). At 65° C the related AUX BAT HOT or MAIN BAT only the yellow 60−degree temperature segment lights
HOT warning light is also turned on. come on, along with both SENSOR FAIL lights. Upon
release, all indications return to normal.
NOTE: The monitor normally indicates internal battery
temperature. If the aircraft has beenoperating Holding the TEST switch to the right produces the same
or sitting in hot sun, temperature indications indications on the monitor as the second mode, however
may be higher than atmospheric. the overheat warning lights do not turn on. Upon release,
all indications return to normal.

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 32 – Battery Temp. Monitor Operation

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 deHavilland Dash 8 100/300 24 - Electrical Power
On Post−mod 8/1584 aircraft, the test indications are as
follows:
When the TEST switch is pressed in and held, the BITE
sequentially turns on all green and yellow segment lights,
and the red segment lights to the 70 degrees point.
In addition, the MAIN BAT HOT and AUX BAT HOT
overheat warning lights flash on and off (master warning
light will also flash). Upon release, the light indicators go
out, and the segment lights sequentially reduce to normal
battery temperature.
When the TEST switch is selected and held to the left,
the display sequentially turns on all green, yellow and red
segment lights to full scale, then all extinguish; followed
by lighting of only the yellow 60 degree segments and
both SENSOR FAIL lights. The MAIN BAT HOT and
AUX BAT HOT overheat warning lights flash on and off
(master warning light will also flash).
Upon release, the yellow 60 degree segments, both
SENSOR FAIL lights and the overheat warning lights
go out. All temperature light segments come on and
sequentially reduce to normal battery temperature.
When the TEST switch is selected and held to the
right, the normal temperature light segments extinguish
sequentially, followed by lighting of only the yellow
60−degree segments and both SENSOR FAIL lights. The
overheat warning lights do not flash.
Upon release, the 60−degree segments and SENSOR
FAIL lights extinguish, and the temperature segment
lights return sequentially to normal battery temperature.

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 33 – Battery Temperature Monitor

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 deHavilland Dash 8 100/300 24 - Electrical Power
EXTERNAL POWER left and right feeders buses. The limiters, contactors
and feeder buses are contained in the dc contactor
General box mounted on the forward side of the upper pressure
Two external power system receptacles are provided bulkhead in the nose compartment.
in the aircraft; one for 28−volts dc, and the other The control circuits consist of an external power advisory
for 115/200−volts ac. Each system consists of the light and control switch S8, an overvoltage protection
receptacle, control circuits and an advisory light. The dc relay K23, and overvoltage latching relay K26. Relays
receptacle is on the nose of the aircraft, lower left side, K23 and K26 are mounted in the dc contactor box; relay
at Sta. X101. The ac receptacle is located in the right K15 is mounted on relay panel No. 2, situated on the left
engine nacelle, lower inboard side at Sta. Xn 182.70, side of the aircraft behind the wardrobe compartment.
to the rear of the main shock strut hinge point. Both are The control switch (EXT PWR) and green advisory light
enclosed by hinged access doors. are in the lower right corner of the DC CONTROL panel
on the overhead console.
External DC Power
Operation
Description
External DC Power Application
The external dc power source takes precedence over
aircraft electrical power to supply the dc bus system. When connected to an external 28V dc power
When external dc power is applied and selected on, the source, control power is fed from the receptacle
control circuits provide isolation of the generators from through CB44, CR6 and normally closed contacts of
the external power source. Overvoltage protection from overvoltage protection relay K23 in the dc contactor
the external power source is also provided, as well as an box to the EXT PWR control switch S8.
advisory light for visual indication that external dc power
Selection of the EXT POWER switch S8 (DC
is selected.
CONTROL panel) to on supplies energizing power
The dc receptacle incorporates a large positive pin, a to contactors K9 and K10, and simultaneously to
large ground pin, and a small positive control signal input relay K15, and control power to Y1 of K16. When
pin. On Pre−Mode 8/1890 aircraft, the large positive input energized, contactors K9 and K10 apply external
pin is connected through a 275 ampere limiter to external power to the main feeders and essential buses.
power advisory light K9 and K10; with Mod 8/1890
Auxiliary contacts 11 and 12 on K10 close to provide a
incorporated through a 275 ampere limiter to K9, and
ground return for K15 and the GCU inhibit control line.
through a 225 ampere limiter to K10. When energized,
K9 and K10 supply external dc power to the associated

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 34 – External Power

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 deHavilland Dash 8 100/300 24 - Electrical Power
Grounding the GCU inhibit line prevents actuation of the energizing coils of K9 and K10, and controlling
the generators while on external dc power. voltage to K16 on terminal Y1.
Relay K16 (terminal X1) is powered from the right When the external power switch is selected
essential bus through CB25, and since control OFF, external power contactors K9 and K10 and
power is applied by external power switch S8, K16 is time−delay−off relay K16 remain operated by latched
immediately energized.from Y1, and energizing power K25, maintaining external power on the buses.
is retained for that interval. Simultaneously, K15 de−energizes, applying voltage
through contacts A2/A3 to main battery contactor
When K16 operates, contacts A1/A2 apply essential
K8, and through contacts B2/B3 to auxiliary battery
bus voltage to the secondary/main feeder bus
contactor K7, connecting battery power to the
contactors K5 and K6, and turn on the EXT PWR
buses with external power still connected. When
advisory light through auxiliary contacts 11 and 12
K8 operates, its auxiliary contacts cause K24 to
of K9. Contacts B1/B2 apply essential bus voltage
energize, removing the latching voltage to K25,
to main bus tie contactor K21, and all dc buses are
which in turn removes external power from K9, K10
then powered by the external source. Voltage applied
and K16 control. Relay K16 remains energized for
to the buses may be measured on the DC SYSTEM
approximately .5 second after K25 releases, holding
power monitoring panel.
the bus ties for that period, maintaining battery power
Switching from External Power to Aircraft Power on all buses during the power changeover and until
the generators come on line.
When switching from external to aircraft power the
first procedure is to select the BATTERY MASTER, Selection of the BATTERY MASTER, MAIN BATT
MAIN BATT and AUX BATT switches to the on and AUX BATT switches to on position energizes K4,
position. Contactor K4 is then energized, connecting K7 and K8, connecting the batteries to the feeders
the main battery to the right essential bus. immediately. Selection of the EXT PWR switch to OFF
removes power from K9, K10 and K16, which in turn
The open contacts B2/B3 and A2/A3 of energized
releases K5, K6 and K21.
relay K15 prevent operation of battery contactors K7
and K8. The MAIN BATT switch however supplies Switching from Aircraft Power to External Power
essential bus voltage through K15−A2/A1 to
During normal aircraft power operation, battery
operate relay K25 (X1). Relay K25 latches on from
contactors K7 and K8 operate via normally closed
the essential bus voltage through relay K24−A2/
contacts A2/A3 and B2/B3 of K15. When K8 operates,
A3 to K25−A1, and K25 contacts B1/B2 provide
its auxiliary contacts 11 and 12 allow operation of
a direct patch from the external power source to

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 35 – External Power Schematic

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 deHavilland Dash 8 100/300 24 - Electrical Power
K24. When external power is applied and EXT PWR exceeds 31.5 vdc, K23 energizes and removes the
switch (S8) is switched on, contactors K9, K10 and path of voltage to the external power switch.
K16 close, energizing bus tie contactors K5, K6 and
When K23 energizes, its contacts B1/B2 close to
K21, and connecting external power to the buses with
energize K26, the overvoltage latching relay. While
battery power still connected.
K26 is energized, the ground to the coil of K9 and
After K10 closes its auxiliary contacts 11 and K10 is removed causing them to de−energize
12 allow K15 to operate. Relay K15 opens the disconnecting the external power from the main
energizing circuits to battery contactors K7 and K8, buses. Contacts 2/5 of K26 are also closed, putting
de−energizing K24, also energizing relay K25 through 28 Vdc from the external power logic breaker CB25
its contacts A1/A2. K25 latches itself until theMAIN through contacts 5/6 of S8 and onto its own coil
BATT switch is selected OFF. through de−energized contacts 13/14 of K10, keeping
K26 latched (energized). K26 will remain energized
Selection of the EXT PWR switch to ON immediately
even if the overvoltage condition no longer exists
energizes K9, K10 and K16, which in turn energizes
and overvoltage relay K23 is de−energized. Under
K5, K6 and K21, connecting the external power to all
this condition, the main buses cannot be powered
buses. The battery switches may be selected OFF or
from the external supply until K26 is de−energized by
left on to provide charging current to the batteries.
selecting external power switch S8 off and then on
CAUTION: IF EXTERNAL GROUND POWER IS again.
CONNECTED, AND THE BATTERIES
Alternate DC Power (TRU System)
ARE SELECTED ON WITH EXTERNAL
POWER APPLIED, THE BATTERY The dc bus system may be energized by the TRUs
CHARGE RATE AND TEMPERATURE when the aircraft is connected to an external ac
MUST BE CONTINUALLY MONITORED power source. When external ac power is applied
TO ENSURE TEMPERATURE LIMITS to the aircraft the energized buses power the TRUs,
ARE NOT EXCEEDED. producing 28 volts dc which is applied to the left and
right secondary buses by normal action of contactors
Overvoltage Protection
K17, K18 and their associated control relays K19 and
Relay K23 energizes at an input voltage of 31.5V dc K20.
(+ 0 − 0.6 vdc). When externalpower voltage is less
When the BATTERY MASTER switch (S7) is selected
than 31.5 vdc, K23 is not energized, allowing power
on, logic relays K13 and K14 will be energized from
to be supplied to S8 through the normally closed
the left essential bus (CB6). Through these relays
contacts of K23 from CB44. If external power voltage

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 36 – External Power - TRU

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 deHavilland Dash 8 100/300 24 - Electrical Power
power from the BBPU bus tie output will energize External AC Power
secondary/main feeder contactors K5 and K6, which
then connect the TRU output voltage to the main Description
feeder and essential buses. The control voltage which The external ac power source takes precedence over
energizes K5 and K6 is supplied by the BBPU (pin aircraft electrical power to supply the variable frequency
E), and routed through K13−C2/C1, K14−C1/C2, and ac bus system. The control circuit incorporates an
K12−B3/B2 (de−energized contacts). external power protection unit which ensures that the
After the essential buses are powered from the TRUs external power source phase rotation is correct and
the BATTERY MASTER switch could be selected within frequency and voltage limits. The external power
OFF, as the logic relays will remain energized by the protection unit is mounted on the bottom side of the right
essential bus power from the TRUs, and all dc buses ac contactor box which is located in the right engine
will remain energized until the ac external power is nacelle at Sta. Xn 197 32, adjacent to the external
switched off. power connector. A control switch and green advisory
light marked EXT POWER, are located on the lower
When the aircraft is operated with both AC and DC left corner of the AC CONTROL panel on the overhead
External Power. Both the DC External Power Source console.
and TRU’s supply the DC Bus System in parallel.
If AC External power is selected off, the TRU fail The ac external power receptacle provides connection
caution light remain off. The secondary DC buses of the three phase external power source and control
remain powered by the DC External Power source. voltage to the system. Pins A, B and C are connected
Secondary bus is power by 28 VDC through closed through limiters F1, F3 and F5 (left) and F2, F4 and
relays K5 & K6 and contactors K17 & K18 −A1/ F6 (right), mounted on the bottom side of the right ac
A2 to control relays K19 & K20−C2. This prevents contactor box. They are protected by a fibre epoxy cover
contactors K17 & K18 from de−energing and the TRU held in place by two quick−release camloc fasteners.
FAIL caution light from illuminating. TRU load meter The limiters are connected to normally open contacts A1,
reading will show zero load on DC Power Monitor B1 and C1 of external power contactors K3 (left) and K4
Panel. (right). The arms of these contactors A2, B2 and C2 are
connected to terminals A3, B3 and C3 of left and right
bus contactors K1 and K2 which connect the aircraft
buses to the external power contactors.
Pins A, B and C of the ac receptacle are also connected
to the external power protection unit 2442−K1 pins C,

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 37 – AC External Power

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 deHavilland Dash 8 100/300 24 - Electrical Power
D and E. The protection unit contains a sensing circuit and control power will be applied through the jumper to
which controls an integral normally−open relay connected pins H and K of the protection unit.
in series with the control circuit. If the external source
When the essential dc buses are energized by the
three phase output is correctly connected, the frequency
TRUs as described in Para. 2.B. (5), control power is
is within 370 to 450 Hz + or − 2% and the voltage within
maintained from the right essential bus through the dc
106 to 124 volts + or − 2% the integral relay energizes,
EXT PWR LOGIC circuit breaker, and the BATTERY
supplying power to the external power control circuit.
MASTER switch may be selected OFF.
The external power control circuit consists of the EXT
Operation
POWER switch 2442−S1, external power contactors K3/
K4 operated by S1, and bus contactors 2421−K1/K2 With the application of proper ac three−phase input
operated by the left and right GCUs. The GCUs receive within frequency and voltage limits, the external power
28 volts on pin f when S1 is selected on, to operate protection unit integral relay energizes connecting
K1 and K2. Switch S1 also operates the EXT POWER control power to the ac external power control switch.
advisory light to provide a visual indication that external When the control switch is set to on, terminals 3 and 6
ac power is applied to the variable frequency bus system. simultaneously apply voltage to the energizing coils of
contactors K3 and K4, the EXT POWER advisory light
The control circuit can be supplied from the external ac
(through the dim and test control box) and to pin f of the
power equipment through the aircraft wiring from the EXT
GCUs. Contactors K3 and K4 energize to the external
PWR LOGIC circuit breaker on the right essential bus,
power position. The GCUs, through internal circuitry,
when powered.
operate bus contactors K1 and K2 to the crosstie position
If the external ac power equipment incorporates an through coils Y1/Y2, connecting the buses to external
integral 28−volt dc supply, the control circuit is supplied power and isolating the generators. Voltage applied to
from the equipment through pin E of the external power the buses may be measured on the AC SYSTEM power
receptacle and pins H and K of the external power monitor panel
protection unit to the control switch. If the equipment
NOTE: Both GCUs must be installed to complete the
does not incorporate an integral 28−volt dc supply, the
contactor circuitry for external power to be
power source for the control circuits is provided from the
applied to the buses.
right essential bus (when energized) through the EXT
PWR LOGIC circuit breaker 2431−CB25 to Pin B of the
protection unit. The external ac power connector will, in
this case, be wired with a jumper between pins E and F,

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 38 – AC External Power - Operation

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 deHavilland Dash 8 100/300 24 - Electrical Power
ELECTRICAL LOAD DISTRIBUTION Circuit Breaker Panels
General Left DC Circuit Breaker Panel

Electrical load distribution from the three basic power The left dc circuit breaker panel is located on the top
systems (28 volts dc, 115/200 volts 3−phase ac variable surface of the left circuit breaker console beside and
frequency, and 115 volts 400 Hz) is provided through the outboard of the pilot’s seat. It contains the left main, left
following four circuit breaker panels located in the flight essential and left secondary dc distribution buses. Panel
compartment: markings indicate the division of the buses and system
designation of the circuit breakers (CBs). Power feed to
■■Left dc circuit breaker panel,
the left main distribution bus is supplied by four separate
■■Right dc circuit breaker panel, lines from the left main feeder bus in the dc contactor
■■115V AC VARIABLE FREQUENCY circuit breaker box, through the L MAIN INPUT circuit breakers A10,
panel, and B10, C10 and D10.
■■Avionics circuit breaker panel (distributes 28 volts, In addition to supplying power directly to various
dc, 115 volts and 26 volts ac, 400 Hz). equipment, the main distribution bus supplies power to
Thermal push−pull circuit breakers are used for the the avionics panel left buses through CBs (A9, B9, C9)
protection of all ac and dc wiring. The circuit breakers and to the left essential bus CB (E10).
(CBs) and associated buses are mounted on the four The left essential bus is powered through isolation diodes
panels. from the left main distribution bus CB L MAIN INPUT,
With the exception of the AC VARIABLE FREQUENCY (L10) and the left battery bus CB AUX BATT INPUT,
panel, alpha/numeric panel markings are placed along (M10). The left essential bus is normally tied by the BUS
the bottom and left edges of the panels to identify the TIE circuit breaker (K10) to the right essential bus in the
rows and line in which CBs are located, for the benefit of copilot’s panel.
location description. The left secondary buses are powered from the left
The electrical buses in the circuit breaker panels are the TRU. The secondary feeder bus in the contactor box
distribution buses, but the word "distribution" is omitted in is connected to the L SECONDARY distribution bus by
the titles on the panels. three separate lines applied through the L SEC INPUT
circuit breakers (P10, Q10 and R10). The secondary bus
A small number of control circuit breakers are mounted in operates independently and is electrically separate from
the dc contactor box. the main and essential buses.

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 39 – Left DC C/B Panel

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 deHavilland Dash 8 100/300 24 - Electrical Power
Right DC Circuit Breaker Panel NOTE: The generator and TRU outputs are normally
separate. Interconnection of these sources
The right circuit breaker panel is located on the top
of power and the various buses is through
surface of the right circuit breaker console, beside and
contactors and circuit breakers in the dc
outboard of the copilot’s seat. It is similar in design and
contactor box which provide control and
function to the left dc circuit breaker panel, with the
isolation when required, according to the
addition of a separate bus and circuit breakers labeled
conditions of operation.
BATTERY PWR, at the bottom of the R ESSENTIAL
section. The BATTERY PWR circuit breakers are
connected directly to the right battery bus through a
10−amp circuit breaker located in the dc contactor box.
If CSI 82062 is installed, ground service lights are also
powered from the right battery.
Power to the right main distribution bus is fed by four
separate lines from the right main feeder bus, and
through the R MAIN circuit breakers (P10, Q10, R10,
S10). The main distribution bus supplies power to the
avionics panel right dc buses through the AVIONIC
FEEDERS circuit breakers Q9, R9, S9.
The right essential bus is powered by the right main
distribution bus and the right battery bus through isolation
diodes (CR2 and CR4) and circuit breakers R MAIN
INPUT (M10), R ESS BUS (N10), and MN BATT INPUT
(L10).
The right secondary distribution bus is powered from the
right TRU. Power input is connected through the three R
SEC INPUT circuit breakers (C10, D10, E10), supplied
by three separate lines from the right secondary feeder
bus in the dc contactor box. The right secondary bus also
operates independently and is electrically separate from
the main and essential buses.

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 40 – Right DC C/B Panel

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 deHavilland Dash 8 100/300 24 - Electrical Power
115V AC VAR. FREQUENCY C/B Panel
The 115 V AC VARIABLE FREQUENCY circuit breaker
panel is mounted on the rear flight compartment
bulkhead above and behind the copilot’s circuit breaker
console, on the rear flight compartment bulkhead
at Station X182.00. Variable frequency ac power is
distributed by independent left and right 115/200 volt
three−phase variable frequency buses identified by
appropriate panel markings. Power from the ac contactor
boxes (one located in each nacelle) enters the panel
through connector 2421−P26, and is wired to each circuit
breaker on the panel.

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 41 – 115V Var. Freq. AC C/B Panel

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 deHavilland Dash 8 100/300 24 - Electrical Power
Avionics Circuit Breaker Panel
The avionics circuit breaker panel is mounted above and
behind the pilot’s circuit breaker console, on the rear
flight compartment bulkhead. It distributes 28 volts dc,
115 and 26 volts 400 Hz to avionics equipment and any
other frequency sensitive ac systems. Panel markings
identify the appropriate buses and circuit breakers.
The 28 volt input from the left and right main distribution
buses enters the avionics panel through 2431−P1
(left) and 2431−P2 (right) and is applied to their
respectivedistribution buses through separate 20−amp
circuit breakers (J8, K7, L6, right) and M5, N4, P3, left).
The three 400 Hz inverters supply the left and right
115V buses through the paralleling control box, applied
to the avionics panel through connector 2422−P8.
The paralleling control box directs the output from the
auxiliary inverter to either the left or right bus by selection
of a switch located on the AC CONTROL panel in the
flight compartment. The BUS TIE circuit breaker located
at the top of the panel connects the two buses.
Each 115 volt bus supplies power to a 26 volt
autotransformer through the 115/26 VAC XFMR RT and
115/26 VAC XFMR LT circuit breakers located at G10
and H9. The output of the transformers directly supplies
26−volt 400 Hz power separately to the left and right 26−
volt buses. These buses are monitored by the master
caution light system to provide indication of power loss to
the buses.

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 deHavilland Dash 8 100/300 24 - Electrical Power

Figure 42 – Fixed Frequency AC C/B Panel

Oct. 1/2009 - © Jazz Air LP Page 89

FOR TRAINING PURPOSES ONLY