Electrical Power

AC/DC generation (IDG/CSD, TRUs, batteries, RAT), distribution, load management and protection.

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Main ideas

Large aircraft use AC as the primary supply. To hold a CONSTANT FREQUENCY (400 Hz) despite varying engine speed, the generator is driven through a constant-speed drive — the CSD/IDG (integrated drive generator); some modern aircraft use variable-frequency systems.
DC is produced from AC by TRANSFORMER-RECTIFIER UNITS (TRUs); batteries give start and emergency power; other sources are the APU generator, external/ground power and the ram air turbine (RAT).
DISTRIBUTION uses busbars and bus-tie breakers with load prioritisation: if generation is lost, non-essential loads are shed and the ESSENTIAL/BATTERY bus keeps critical systems and standby instruments alive.
PROTECTION comes from generator control units (voltage regulation, over-voltage/under-frequency/differential protection) and circuit breakers protecting the wiring.
For the B2 technician the wiring, busbars, control units and the sensing/indication of the electrical system are central.
⚠ Exam trap: the CSD/IDG keeps the AC generator output at a CONSTANT FREQUENCY (400 Hz) regardless of engine RPM; TRUs convert AC to DC; the battery/essential bus feeds the critical loads if generation fails.

Solved examples

After a double generator failure, what keeps the essential flight instruments and radios powered?
The battery, feeding the DC essential/battery bus (and, via a static inverter, an AC essential bus for any AC-only critical loads). Load-shedding sheds galleys and other non-essential loads so the limited battery capacity powers only the essential instruments, radios and standby systems — and a RAT may deploy to restore generation for extended flight.

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