TM 1-1520-238-10 2-66 Change 2 Section XI. ELECTRICAL POWER SUPPLY AND DISTRIBUTION SYSTEMS 2.47 ELECTRICAL POWER SYSTEM. All the helicopter electrical power requirements are sup- plied by two ac generators, two transformer/rectifiers (T/Rs) and in the case of complete failure, a 24-volt bat- tery will supply flight critical systems. For ground opera- tions, 115 vac external power can be supplied to the heli- copter through the external power receptacle. The electrical power distribution system is shown in figure 2-38. The system caution lights are listed in tables 2-4 and 2-5. CAUTION In the event any circuit breaker opens for unknown reasons, do not attempt to re- set the breaker more than one time. Re- peated tripping of a circuit breaker is an indication of a possible problem with equipment or electrical wiring. Multiple attempts to reset the circuit breaker may result in equipment damage and/or an electrical fire. 2.47.1 DC Power Supply System. Two essential ac busses supply power to the two 350-ampere T/Rs through the XFMR RECT 1 and XFMR RECT 2 circuit breakers on the pilot overhead circuit breaker panel (fig 2-39). The No.1 and No. 2 T/Rs convert the ac input to 28 vdc. The 28 vdc power is applied to a dc contactor, which routes 28 vdc from T/R No. 1 to the No. 1 essential dc bus and from T/R No.2 to the No. 2 essential dc bus. The No. 1 and No. 2 essential dc buses each apply 28 vdc, in parallel, through isolation diodes to power the emergency dc bus during normal operation. For emergency operation, the battery powers the emergency dc bus, and the emergen- cy dc bus diodes isolate the battery from the noncritical loads. The two essential dc buses also supply 28 vdc power to the No. 3 essential dc bus. The T/Rs are self- monitoring for overtemperature and will illuminate the HOT RECT 1 and HOT RECT 2 caution lights on the pilot caution/warning panel when an overheat condition exists. The dc contactor monitors the T/Rs for output loss or drop in voltage. If the dc contactor senses a fault in T/R No. 1, it disconnects T/R No. 1 from the No. 1 essential dc bus and illuminates the RECT 1 caution light on the pilot cau- tion/warning panel. At the same time, it connects No. 1 es- sential dc bus to No. 2 essential dc bus through relay ac- tion inside the contactor. The same action takes place if T/R No. 2 fails. If both T/Rs fail, the ELEC SYS FAIL cau- tion light on the CPG caution/warning panel will illuminate. The caution/warning lights receive 28 vdc from the emer- gency dc bus through the CAUT circuit breaker on the pi- lot overhead circuit breaker panel and the EMERG BATT CAUT circuit breaker on the CPG main circuit breaker panel (fig 2-40). a. Battery. A 24-volt, 13 ampere-hour, 19 cell, nick- el-cadmium battery provides emergency power. The bat- tery is located in the aft avionics bay. The battery is charged by the battery charger which receives 28 vdc power from the No. 1 essential dc bus through the BATT CHGR DC circuit breaker and 115 vac from the No. 2 es- sential ac bus through the BATT CHGR AC circuit break- er. Both circuit breakers are on the pilot overhead circuit breaker panel. The battery charger will completely charge the battery and then maintain a trickle charge. The battery charger also contains fault sensing that monitors battery temperature, cell balance, and charger operation. If a fault occurs, either the HOT BAT or CHARGER caution light will illuminate on the pilot caution/warning panel. The heli- copter is equipped with a battery heater which operates automatically when the battery is on. b. CPG Battery Override Switch. A two-position guarded toggle BAT OVRD switch is located on the CPG PWR lever quadrant (fig 2-12). With the guard down, the switch is in the NRML position and enables the pilot EXT PWR/BATT switch on the pilot ELEC PWR control panel (fig 2-37) located on the left console (fig 2-11). When the guard is positioned up and the switch is set to OVRD, the pilot EXT PWR/BATT switch is inoperative, and the bat- tery is disconnected from the emergency dc bus. 2.47.2 AC Power Supply System. The ac power sup- ply system is the primary source of electrical power. It sup- plies 115 vac from two 35 kilovolt-ampere generators. Each generator and its associated components comprise an independent ac generating system that supplies about one-half of the total electrical requirements to the ac buses. The generators are mounted on, and driven by, the main transmission accessory gear box. The 115 vac pow- er is monitored and regulated by the generator control unit (GCU). If the generator output is normal, the GCU applies voltage to the ac contactor which energizes and connects the generator output