What is the purpose of a 'Master' or 'MAIN' switch in the electrical system?

• A. To regulate cabin lighting
• B. To monitor electrical system temperature
• C. To establish or remove power from the aircraft electrical system, enabling/disabling electrical systems entering service
• D. To connect the battery to the standby bus

Answer: (C)

The main function is to act as the primary gatekeeper for the aircraft’s electrical system. When this switch is ON, it connects the battery and electrical sources (like generators or alternators) to the busbar network, allowing power to flow and the aircraft’s electrical systems to enter service. When it is OFF, those connections are opened, isolating the electrical system and preventing power from reaching the buses and loads. This gives a safe, simple way to turn the entire electrical system on or off for starting, operations, or maintenance, and helps prevent unwanted battery drain or backfeeding during servicing. It’s not a control for cabin lighting or temperature monitoring, and it isn’t a switch that specifically ties the battery to the standby bus, which is a separate function in many layouts.