In a relay, the terms NO (Normally Open) and NC (Normally Closed) describe how the relay’s internal contacts behave when no electrical power is applied to its coil. These two contact types determine whether the circuit is open or closed in the relay’s normal, unenergized state.
When a relay is in its normal state — meaning the coil is not energized — a Normally Open (NO) contact stays open. This means that no electrical connection exists between the input and output terminals, and current cannot flow through the circuit. When the relay coil receives an electrical signal, it becomes energized, and the NO contacts close, allowing current to pass and turning the connected device ON. This type of contact is commonly used in applications where a device should remain off until a signal activates it, such as in lighting circuits, motor starters, and control systems.
A Normally Closed (NC) contact works the opposite way. In the unenergized state, the NC contact remains closed, allowing current to flow through the circuit. When the relay coil is energized, the NC contact opens, breaking the circuit and turning the connected device OFF. NC contacts are used in situations where the circuit should stay active until a control signal interrupts it — for example, in safety interlocks or emergency stop systems.
To put it simply, NO contacts turn ON when the relay is energized, and NC contacts turn OFF when the relay is energized. Many relays, including Omron Relays, are designed with both NO and NC contacts in one unit, allowing them to control multiple functions within a single device.
Understanding NO and NC contacts is important when wiring a relay, as it helps determine how your circuit will behave under different conditions. In automation and electrical systems, using the correct contact type ensures the equipment operates safely and exactly as intended.