SYSTEM DESCRIPTION (cont)
(4) External Power and Ground Service Utility Receptacle.
The purpose of the external power and
ground service utility receptacle system is to provide a means of connecting a 3-phase, 115/200 VAC, 400 Hz
ground power source to the helicopter.
(a) When the external power access door (fig. 980) is opened, a switch is activated which lights the
EXT PWR indicator on the pilot caution/warning panel. An external 3-phase, 115/200 VAC, 400 Hz power source
is applied through the external power receptacle to an external power monitor and an external power contactor.
The external power monitor checks the incoming power for proper frequency, voltage, and phasing. If the
incoming power is correct, the external power monitor develops a 28 VDC interlock voltage. The interlock voltage
is applied through two pins in the external power receptacle to generator 2 contactor. Voltage flows through the
two pins to ensure that the external power connector is properly seated in the external power receptacle.
(b) If generator 2 contactor is operating, the interlock voltage is interrupted and external power cannot
be applied. If generator 2 is not operating, power is applied through generator 2 contactor to the BATT/EXT PWR
switch on the pilot ELEC PWR panel. When the BATT/EXT PWR switch is placed in the EXT PWR position the
interlock path is completed to the generator 1 contactor. If generator 1 contactor is operating, the interlock voltage
is interrupted and external power cannot be applied. If generator 1 is not operating, power is applied through
generator 1 contactor to the external power contactor. When the external power contactor is energized, 3-phase
external voltage is connected to ac essential bus 1. The 28 VDC interlock signal is routed back to energize the
bus tie relay in the generator 1 contactor. When the bus tie relay is energized, external power from ac essential
bus 1 is supplied to ac essential bus 2.
(c) If the power monitor detects an incoming power failure, interlock voltage is removed. This
deenergizes the interlock voltage, the external power contactor, and removes external ac power.
(d) The external power monitor (fig. 981) monitors the incoming external ac power for proper voltage
level, frequency and phase sequence. If the input power is within tolerance, the external power monitor develops
a 28 VDC interlock signal which energizes the external power contactor. When the external power contactor is
energized, external power is applied to the helicopter ac essential buses.
(e) If out-of-tolerance external power parameters exist, the power monitor does not develop the 28 VDC
interlock signal. The external power contactor either cannot be energized or if energized, is deenergized. The
deenergized external power contactor prevents application of external power to the helicopters ac essential
(f) External power is locked out until the external power monitor is reset by the EXT PWR RESET
switch on the pilot ELEC PWR panel. The external power monitor checks the input voltage and if still
out-of-tolerance, the system remains locked out. If the external power is within tolerance, the external power
operation sequence begins.
(g) The external power contactor K7 (fig. 980) connects the incoming external power to the helicopters
ac essential buses. K7 is deenergized without external power applied. Contacts B2 and B3 provide the ground
path to energize ac contactor 2 during normal operation. Contacts L1, L2, and L3 are open, preventing application
of external power. Contacts A2 and A3 route bus tie voltage to generator 1 contactor bus tie relay if generator 1
fails. If external power is applied and checks good, the 28 VDC external power interlock signal developed by the
external power monitor energizes K7. When K7 energizes, contacts B2 and B3 deenergize ac contactor 2.
Contact T3 mates with L3, T2 mates with L2, and T1 mates with L1, applying external ac power to ac essential
bus 1. Contacts A1 and A2 provide 28 VDC to bus 1 tie relay which connects ac essential buses 1 and 2 together.