In relay terms, what does "normally closed" refer to?

In relay terminology, "normally closed" refers to contacts that are held closed by a spring. This means that in the relay's default state, the contacts allow electrical current to pass through, completing the circuit. When the relay coil is energized, or activated, the contacts will open—interrupting the flow of current.

This characteristic is crucial in various applications, as it allows for a fail-safe operation; in the absence of power or during a system failure, the normally closed contacts ensure that a circuit remains active. This is commonly employed in safety systems, where it is essential for a machine or process to automatically revert to a safe state when power is lost. Understanding this function is vital for troubleshooters and technicians who work with electrical systems and need to determine the behavior of relay components in different states.