Differences Between Normally Open and Normally Closed Contactors and How to Choose

In electrical control and new energy applications, contactors are essential components widely used in EV charging systems, energy storage equipment, and industrial automation. During the selection process, engineers often encounter Normally Open (NO) and Normally Closed (NC) contactor structures. These two types differ significantly in functionality and application scenarios. A correct understanding and choice can greatly improve system safety and reliability.

1. Normally Open Contactors (NO)

Definition:
A Normally Open (NO) contactor keeps its contacts open when the coil is de-energized. The contacts close, and the circuit becomes conductive only when the coil is energized.

Features:

Default safe state: the circuit remains off when not energized

Commonly used for controlling high-power loads

Matches the power supply on/off status directly

Typical Applications:

EV charging stations: Keep the circuit open before charging begins to prevent accidental discharge or electric shock.

Energy storage systems: Stay open during standby and close only when charging or discharging is required.

Motor control: Used for start/stop circuits, ensuring equipment operates only when needed.

2. Normally Closed Contactors (NC)

Definition:
A Normally Closed (NC) contactor keeps its contacts closed when the coil is de-energized. Once the coil is energized, the contacts open and the circuit is cut off.

Features:

Default conductive state; the circuit is cut off when the coil is energized

Often used for protection and emergency cutoff circuits

Suitable for “fail-safe” applications where power loss should stop the system

Typical Applications:

Safety circuits: Automatically cut off critical circuits in the event of power failure or system fault to prevent further damage.

Overload protection: Often paired with overload relays to disconnect circuits under abnormal current conditions.

Emergency stop systems: Ensure equipment shuts down immediately during power loss.

3. Application Differences in AC and DC Contactors

Although the concepts of “Normally Open” and “Normally Closed” are universal, their application focus differs between AC contactors and DC contactors:

AC Contactors:

NO contacts are mainly used for motor, lighting, and heating equipment control.

NC contacts are often applied in interlocking control and safety circuits, ensuring loads are disconnected during abnormal or power-off conditions.

Widely used in factory automation and industrial control systems where NO and NC contacts are combined for logic control.

DC Contactors:

NO contacts are commonly used as main circuit control in EV charging stations, energy storage inverters, and photovoltaic systems.

NC contacts are often used for fault protection, disconnecting the circuit automatically when the system loses power or detects anomalies.

Since DC arcs are more difficult to extinguish than AC, DC contactors require advanced contact design and arc suppression technology.

4. Selection Logic: NO vs NC

In real-world engineering, the choice depends on application needs:

Safety-first approach:

If the system must stop or enter protection mode when power is lost → use a Normally Closed contactor.

Energy-saving and control efficiency:

If the system should remain idle without power consumption and operate only when required → use a Normally Open contactor.

Trends in new energy applications:

In EV charging and energy storage systems, NO contactors are more widely used to keep circuits safely open before activation.

In safety protection and emergency shutdown, NC contactors remain indispensable.

5. Conclusion

Normally Open and Normally Closed contactors are not substitutes but complementary solutions designed for different scenarios. In system design, engineers often combine both types:

ØNO contactors handle routine switching and power control.

ØNC contactors ensure fault protection and emergency shutdown.

By selecting and combining them properly, systems can achieve higher levels of safety, efficiency, and reliability. With the rapid growth of new energy and electric vehicles, understanding these differences has become especially important for system integrators and equipment manufacturers.