TM  1-1520-240-10 2-11-2            When the switches are ON, the respective main relay operates, which energizes and connects the generator to the buses. At OFF RESET, the generator is deenergized and disconnected from the bus. This position is also used to reset a generator. The TEST position is provided to allow the generator to be energized but disconnected from the bus to determine whether the AC produced is of proper frequency and voltage, except the APU genera- tor. Figure  2-11-2.  Electrical Power Panel 2-11-4.  GEN OFF Cautions. Two  generator  caution  capsules  labeled   712    NO.  1 GEN OFF and NO. 2 GEN OFF,  714A   GEN 1 AND GEN 2 are on the master caution/advisory panel ( 712   fig. 2-14-5   714A   fig. 2-14-6).These caution capsules illumi- nate whenever the generators are in operative.The cap- sules are controlled by the main generator contactors when the generator control switches are in either ON or OFF RESET.In TEST, the capsules are controlled by the generator control switch and will extinguish if generator output has the proper frequency and voltage.Power to operate the generator capsules is supplied by the DC essential bus through the LIGHTING CAUTION PNL cir- cuit breakers on the No. 1 PDP. 2-11-5.  EXT PWR Caution. CAUTION When  external  power  is  used,  a  visual check shall be made by the crew to ensure that the external power unit has been dis- connected from the helicopter before taxi- ing. An external power caution capsule labeled EXT PWR is on   the   master   caution   panel   (fig.   2-14-5   and   fig. 2-14-6).This capsule illuminates and remains illuminated whenever external power is connected.The light is con- trolled by the AC external power contactor and the DC power relay.The capsule extinguishes when the genera- tors are supplying current to the buses.Power to operate the external power caution capsule is supplied by the DC essential bus through the LIGHTING CAUTION PNL cir- cuit breaker on the No. 1 PDP. 2-11-6.  DC System. The direct current (DC) power supply system supplies 28-volt DC from the No. 1 transformer rectifier (RECT) to No. 1 DC bus and the No. 2 RECT supplies power to the No. 2 DC bus (fig. FO-4 and FO-7).RECT convert 200 VAC power to 28-volt DC power for use in the DC distribu- tion system. Cooling air for the RECTS is obtained from within the cabin.The air inlets are located at sta. 176 on the left and right side of the cabin behind the troop seats.If the inlets are blocked, the RECTS will overheat. A bus-tie relay is between No. 1 and No. 2 DC buses.If either RECT fails, the respective RECT failure relay op- erates and the bus-tie relay closes automatically to con- nect the unpowered bus to operating RECT.In addition to No. 1 and No. 2 DC buses, the DC system includes an essential bus, a switched battery bus, and a hot battery bus. During  normal  operation,  the  essential  bus  and  the switched battery bus are energized by No. 1 DC bus.If both DC buses fail or if NO. 1 DC bus fails and does not bus-tie to No. 2 DC bus, the essential bus, the switched battery bus, and the hot battery bus will be energized by the  battery  as  long  as  the  BATT  switch  is  ON.These buses  provide  power  to  emergency,  ground  mainte- nance, and communications components.The hot bat- tery bus and switched battery bus are energized as long as the battery is connected.The hydraulic reservoir level indicators and the emergency APU control circuits and cabin and maintenance lights are on these buses. The 24-volt nickle cadmium battery is located in the left forward electrical compartment.The battery capacity is 11 ampere-hours.A battery charger is connected to the battery.The battery charger receives power from No. 1 AC bus, rectifies the AC and applies the DC to the battery to maintain a charge on the battery. Sensors in the battery charger detect battery or battery charger overtemperature, short or open circuits or cell imbalance.If any of these conditions occur, the battery charger will stop functioning and activates the BATT SYS MAL caution capsule on the master caution panel. External DC power is supplied to the DC buses of the helicopter by connecting the external DC power source to the DC external power receptacle (fig. 2-11-1).Applica- tion of external power operates the DC external power relay  which  connects  the  power  source  to  No.  1  DC bus.No. 2 bus is energized when the bus tie relay opera- tes.If  the  polarity  of  the  external  power  is  reversed,  a blocking diode in the circuit prevents the external power relay from closing.