Electrical Systems: Generation, Distribution and Protection
DC generation and batteries
Know the DC sources: battery, generator or alternator, and transformer-rectifier units. Understand that a generator produces DC directly while an alternator produces AC that is rectified. The battery supplies essential loads and starts the engine on many light aircraft. Traps include the difference between a generator control unit and a voltage regulator, and the fact that a battery contactor connects the battery to the bus.
AC generation
AC systems use an alternator, often with a constant-speed drive or integrated drive generator on turbine aircraft. Know the difference between 115 V AC and 28 V DC buses, and why AC is used for high-power loads. Traps include confusing the constant-speed drive with the generator itself, and forgetting that a CSD disconnect is a one-shot action on the ground in many types.
Distribution and protection
Distribution uses buses, contactors and circuit breakers. Know that a circuit breaker protects wiring from overcurrent and that a fuse is a one-time device. Understand the role of the master switch, the avionics master and the external power contactor. Traps include the question of resetting a circuit breaker in flight: only once, and only if the relevant procedure permits.
Worked example
If the ammeter shows a high charge after start and then falls to near zero, that is normal: the battery is recharging and then the generator carries the load. Rule of thumb: a discharge indication with the generator offline means the battery is carrying the bus and time is limited.