What characteristic must all relays possess for effective operation?

For a relay to operate effectively, it must possess the ability to control inductive loads. Relays are electromagnetic switches used to control circuits by a relatively low-power signal or to control several circuits with one signal. Inductive loads, such as motors and transformers, are common in HVAC systems, where relays facilitate the control of these devices.

When a relay is designed to handle inductive loads, it ensures that it can manage the inrush current that occurs when these devices start, as well as the back EMF generated when the load is turned off. This characteristic is vital for preventing damage to the relay and ensuring reliable operation in systems that involve inductive components.

While high-speed operation, low power consumption, and compatibility with multiple voltage ranges are desirable attributes for a relay, they are not as critical as the ability to handle inductive loads, especially in applications commonly found in HVAC systems.