A secondary circuit used to control a device that allows current to flow in the main circuit is known as what?

The correct answer is a control relay. A control relay functions as a switch that is activated by a lower voltage secondary circuit. When the relay is energized by the secondary circuit, it closes or opens contacts in the main circuit, allowing or interrupting the flow of current. This setup is crucial in various refrigeration and HVAC systems where a low voltage control signal must manage higher voltage equipment safely and effectively.

Understanding the role of a control relay is essential in ensuring devices operate efficiently and safely. The use of a secondary circuit in this context highlights how control systems are designed to minimize risks while providing functionality, underscoring the importance of proper circuit design in HVAC applications.