Electrical Generation, Distribution and Protection
DC and AC generation
Light aircraft typically use a 14 V or 28 V DC system with an engine-driven alternator or generator, a voltage regulator, a battery and a bus bar. Transports use 115 V 400 Hz AC from integrated drive generators (IDGs) or variable-frequency generators, plus transformer-rectifier units (TRUs) for DC, and a battery for essential DC and emergency supply. The battery provides starting power, emergency power and bus stability; it is charged from the DC bus.
Distribution and protection
Know bus architecture: main buses, essential/emergency buses, battery bus, and the split between generator-fed and battery-fed services. Circuit protection uses fuses (one-shot) and circuit breakers (resettable); a popped breaker should not be reset unless the cause is known. Reverse-current protection prevents the battery discharging through a failed generator; overvoltage and undervoltage protection trip a generator offline.
Traps and worked example
Traps: assuming the ammeter shows battery charge when it may show generator output; forgetting that with a generator failure the battery supplies only battery-bus items; and confusing a generator (needs a regulator and reverse-current cut-out) with an alternator (rectified internally).
Worked example: in flight the ammeter shows a continuous high charge and the voltmeter is low. Reasoning: a low bus voltage with high charge current suggests the generator is not carrying the load and the battery is discharging - or the regulator has failed. The correct response is to reduce electrical load, check the breaker, and land as soon as practicable if generation cannot be restored.