A limit switch is open if what voltage reading is observed across its terminals?

The correct voltage reading across the terminals of an open limit switch is often expected to be 24 volts. When the limit switch is open, it indicates that the circuit is incomplete, preventing current from flowing. In an HVAC system, limit switches are commonly used for safety purposes, such as ensuring that certain conditions are met before allowing the system to operate.

When the limit switch is operating properly, and its contacts are open, no current flows through the switch itself; however, the voltage across the terminals can still be present, reflecting the system’s supply voltage, which is usually 24 volts in many HVAC applications. This means that if you measure the voltage and see 24 volts, it confirms that the switch is open and not allowing current to travel, which in turn helps to maintain system safety and proper function.

In the context of the other choices, no voltage would indicate a complete disconnection or failure in the circuit, while 12 volts or 48 volts does not commonly reflect standard operational voltages found in typical HVAC limit switches, leading to an inaccurate assessment of the switch's status. Therefore, a 24-volt reading aligns with how limit switches are designed to function within HVAC systems.