The DC electrical power distribution system has four subsystems. Besides power source distribution and primary power distribution, which are the other two?

• A. The battery charging distribution and avionics distribution
• B. The lighting distribution and environmental control distribution
• C. The control distribution and protection distribution
• D. The secondary power (load) distribution, and emergency power distribution systems

Answer: (D)

In a DC power distribution system, power is organized in stages: where power is generated or stored, how it is carried to the aircraft systems, and then dedicated paths for the remaining needs. After power source distribution and primary power distribution, the two other subsystems are secondary power (load) distribution and emergency power distribution.

Secondary power distribution handles delivering power to the various loads downstream of the primary distribution—essentially the feeders and circuits that supply avionics, lighting, environmental control, and other systems with proper protection and fault isolation. It ensures each load gets stable power from the DC bus through its dedicated feeders and circuit protection.

Emergency power distribution provides power to critical or essential systems when normal power is unavailable, typically via an independent emergency bus backed by a separate source like a dedicated battery. This separation preserves essential operation (such as flight controls or critical avionics) during power faults or failures.

The other options describe specific loads or different functional areas rather than the two defined DC distribution subsystems.