ELECTRICAL
Description
Aircraft Operations Manual
5.1
PAGE
3
Jun 01/19
−
External power available.
−
External power overvoltage relay not energized.
The EXT PWR switch is provided with an electro-
magnetic hold function which will hold the switch
lever in ON position when external power is
switched ON. The EXT PWR switch will also auto-
matically be de
−
energized and retarded to OFF
position if the external power voltage drops below
approximately 10 V. The EXT PWR relay de
−
ener-
gizes and the batteries will be connected if the
BAT
−
switches are ON.
Applicable to a/c without Mod. 2617
During the first part of the engine start sequence
while the start switch is held in start position, the
EXT PWR switch will retard to OFF if the external
power voltage should drop below approximately 7V.
The start sequence will continue on batteries. Dur-
ing the remaining part of the start sequence when
the start switch is released, the drop out voltage is
approximately 10 V. If now dropping below that val-
ue the start sequence will be interrupted.
Applicable to a/c with Mod. 2617
If EXT PWR should drop off line while the Start
Switch is being held in the START position or any-
time during a start
−
up, there will be loss of electrical
power. The only busses that will be powered are LH
and RH Hot Battery Busses and Emergency Battery
Bus until the Start Switch is released (during motor-
ing or motoring start) or the Condition Lever is
moved to Fuel Off (during a start
−
up).
L/R Battery Relay
When the battery relays are closed, the left battery
is connected to the START BUS and the right bat-
tery is connected to the R GEN BUS.
The following conditions must be met for a battery
relay to close:
−
L/R BAT switch on overhead panel in ON posi-
tion.
−
EXT PWR switch off.
−
Battery temperature below 71
C
or
−
Battery switch in OVRD positions (for emergency
use only).
The batteries will be charged from respective GEN
BUS when at least one generator is on line.
L/R Start Relay
When the start relay is closed, the starter/generator
receives power from the START BUS.
During a battery start, the start relay is controlled by
the series/parallel relay connecting the two batteries
in series.
The start relay closes when the start switch is in ON
position. The relay has a hold function and will re-
main closed when the start switch is released. The
relay will open at 55% Ng controlled by the GCU.
Series/Parallel Relay
When the series/parallel relay is in de
−
energized
position, the START BUS is connected to the L
GEN BUS. When energized, the L GEN BUS is
separated from the START BUS.
This relay is slaved to the battery bus relay and is
energized during a battery engine start which will
connect the batteries in series giving 48 V for start.
L HOT BAT BUS Relay
When the HOT BAT BUS relay is de
−
energized, it
connects the left battery to the L HOT BAT BUS.
This relay is energized during engine battery start,
and connects the L HOT BAT BUS to the R GEN
BUS (right battery).
Bus Tie Relay (left PDU)
When the bus tie relay is closed, it connects the two
generator buses. This is done for three reasons:
−
To supply both L and R GEN BUS when external
power is on.
−
To enable power supply to both L and R GEN
BUS from only one generator.,
−
To connect the two main batteries.
The bus tie relay function is controlled by a BUS
TIE switch and monitored by a BUS TIE CONN
light, both located on the overhead panel.
The bus tie relay is closed when the BUS TIE
switch is in CONN position or in AUTO position with
one or both generators failed or with only batteries
ON. It is open if the BUS TIE switch is in SPLIT