How many positions are required for the selector switch in a Hand-Off-Automatic (H-O-A) circuit?

In a Hand-Off-Automatic (H-O-A) circuit, three distinct operating positions are necessary. The "Hand" position allows the operator to manually control the device, providing direct input for immediate operational control. The "Off" position disengages any operational activity, effectively stopping the device from functioning, which is crucial for maintenance or safety reasons. The "Automatic" position permits the system to operate autonomously, often controlled by a timer, sensor, or other automated systems.

This three-position arrangement ensures flexibility in operation while maintaining safety and control. Each position serves a specific purpose that contributes to the overall functionality of the circuit, making it essential for the design of H-O-A switches.