TM 55-1520-228-10
SECTION XI. ELECTRICAL POWER SUPPLY AND DISTRIBUTION SYSTEM
2-49. DIRECT CURRENT PRIMARY POWER.
The OH-58A/C helicopter is equipped with a 28-volt
direct current dual bus (essential and non-essential)
system supplied by a starter-generator and battery
(figure 2-18). Components of the direct current power
system include battery, starter-generator, voltage
regulator, relays, switches, and circuit breakers. All
circuits in the electrical system are single wire with
common ground return. The negative terminals of the
starter-generator and the battery are grounded to the
helicopter structure. In the event of generator failure,
with the NON-ESS BUS switch in the NORM position,
the non-essential bus is automatically deactivated. The
battery then supplies power to the essential bus load.
The non-essential bus may be manually reactivated by
placing the NON-ESS BUS to the MAN position. In the
event of engine failure, generator power will be lost.
a. Indicators and Controls.
(1) DC System Indicator. The ammeter is
mounted in the instrument duster on the instrument
panel and indicates the ampere load being used. The
circuit is protected by two circuit breakers, labeled
LOADMETER in the aft electrical compartment.
(2) DC Power Control. The DC power is
controlled by the BAT switch, GEN switch, NON-ESS
BUS switch, and circuit breakers labeled GEN & BUS
RESET and GEN FIELD.
(3) Battery Switch. The Battery switch is
located in the overhead console and is a two-position
toggle switch, labeled BAT and OFF. Battery electrical
power is supplied to the helicopter electrical system
when the switch is in the BAT position. When the switch
is in the BAT position, it closes the circuit to the
actuating coil of the battery relay and battery power is
then delivered from the battery to the essential bus.
When the switch is placed in the OFF position, it opens
the circuit to the actuating coil of the battery relay and no
power is delivered from the battery to the essential bus.
(4) Generator Switch. The generator switch is
located on the overhead console and is a three-position
switch. This switch is labeled GEN in the forward
position, OFF in the center position, and RESET in the
aft position. The RESET position is spring-loaded to
return to the OFF position when released. Therefore, to
reset the generator, the switch must be held in RESET
position momentarily, and then moved to the forward
position.
(5) Non-Essential Bus Switch. The non-
essential bus switch is located in the overhead console
and is a two-position switch labeled NON-ESS BUS
NORM and MAN. When the switch is in the NORM
position, power is supplied to the non-essential bus
provided the generator is operating or when external
power is applied. When the switch is in the MAN
position, power is supplied to the non-essential bus
regardless of generator operation.
b. Circuit Breakers. The DC circuit breaker panel
(figure 2-12) is located in the overhead console. Each
individual circuit breaker is dearly labeled for particular
electrical circuits protected. In the event a circuit is
overloaded, the circuit breaker protecting that circuit will
deactivate. The circuit is reactivated by pushing the
circuit breaker button.
c. External Power Receptacle. During ground
operations, external power may be connected to the
electrical system through an external power receptacle
located on the lower right side of the helicopter just aft of
the rear landing gear crosstube. The GEN and BAT
switches should be in the OFF position when external
power is connected. The external power relay closes
automatically and connects the ground unit to the
essential and non-essential buses. A GPU power of 300
to 750 amperes and 28 vdc is recommended.
d. Auxiliary Power Receptacle. The auxiliary
power receptacle is mounted on a bulkhead aft of the
copilot seat and provides power take off of 28 Vdc to
operate miscellaneous auxiliary equipment. The circuit
is powered by 28 Vdc essential bus and is controlled and
protected by AUX RECP circuit breaker switch on the
overhead console.
2-40 Change 10
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