Electrical Bus (None)

What Is an Electrical Bus?

An electrical bus is a conductor or set of conductors forming a common connection point that distributes electrical power from one or more sources to multiple consumers. In aviation, buses are organized hierarchically to ensure that the most safety-critical systems — flight instruments, navigation, engine controls — receive power even when generation capacity is severely degraded.

How It Works

Modern commercial aircraft operate at 115V AC (400 Hz) for primary distribution and 28V DC for essential and battery-backed systems, though the Boeing 787 uses a 235V AC architecture to reduce conductor weight. Power sources — typically two or more engine-driven generators rated at 90–150 kVA each, plus the APU generator — feed into AC transfer buses. Bus Tie Breakers (BTBs) connect or isolate these buses to prevent a fault on one from cascading to others.

The distribution hierarchy on a typical airliner flows as: Main AC Bus → AC Essential Bus → DC Essential Bus → Hot Battery Bus. The Hot Battery Bus receives power directly from the main battery without any intervening contactors, ensuring instruments and warning systems remain powered even after a complete electrical failure.

On fly-by-wire aircraft, electrical bus integrity is especially critical since flight control computers rely on stable power. The Airbus A320 maintains three independent electrical channels, each supplied from separate buses. Glass cockpit displays draw from dedicated avionics buses with conditioned power to prevent voltage fluctuation from affecting display accuracy.

In total electrical failure scenarios, the ram air turbine (RAT) deploys to power the essential bus directly, maintaining minimum flight control and instrument capability.

Key Components

• Generator Control Units (GCUs): Regulate generator output voltage and frequency; protect buses from over/under voltage.
• Bus Tie Breakers (BTBs): Motorized contactors connecting or isolating bus sections.
• Transformer Rectifier Units (TRUs): Convert AC to 28V DC for secondary distribution.
• Static Inverters: Convert DC battery power back to AC for essential avionics during emergency.
• Load Control Centers: Circuit breaker panels enabling crew to shed non-essential loads.

Aircraft Examples

• Airbus A320: AC Bus 1, AC Bus 2, AC Essential Bus, DC Bus 1, DC Bus 2, DC Essential Bus, and Hot Battery Bus — seven distinct buses.
• Boeing 787-9: 235V AC four-channel architecture; more-electric design eliminates pneumatic loads, increasing generator demand to 1 MVA total.
• Boeing 777: Four main generators (two per engine) producing 120 kVA each; APU provides a fifth source.
• Airbus A380: Four 150 kVA generators plus two APU generators; first airliner to use 115V Variable Frequency AC, eliminating constant-speed drives.