OneSDR is reader-supported. When you buy via links on our site, we may earn an affiliate commission at no extra cost to you. This helps support our research and writing.
In the world of electrical switches, relays, and contactors, the terms “normally open” (NO) and “normally closed” (NC) describe the default state of a circuit when no external force or input is applied.
This article explores the differences between normally open and normally closed contacts, how they work, and their applications.
Table of Contents
What is a Normally Open (NO) Contact?
A normally open contact is one where the circuit remains open (i.e., incomplete) in its default state. No current flows through the circuit unless the switch or relay is actuated. When the switch is activated, the contacts close, completing the circuit and allowing current to flow.
Key Characteristics:
- Default state: Circuit is open; no current flows.
- Activated state: Circuit closes; current flows.
- Requires an external signal (e.g., pressing a button or energizing a relay coil) to function.
Examples:
- Doorbell: A doorbell switch remains open until pressed. Pressing it closes the circuit, causing the doorbell to ring.
- Sensors: Many sensors use normally open contacts to detect specific events, such as motion or object presence.
Advantages:
- Energy-efficient since no power flows in the default state.
- Ideal for applications requiring momentary actions.
What is a Normally Closed (NC) Contact?
A normally closed contact is one where the circuit remains closed (i.e., complete) in its default state. Current flows through the circuit until the switch or relay is actuated. When activated, the contacts open, breaking the circuit and stopping current flow.
Key Characteristics:
- Default state: Circuit is closed; current flows.
- Activated state: Circuit opens; current stops.
- Interrupts current flow when an external signal is applied.
Examples:
- Emergency Stop Button: In industrial machinery, an emergency stop button uses normally closed contacts. The machine runs until the button is pressed, which opens the circuit and halts operation.
- Refrigerator Light Switch: The light remains off when the refrigerator door is closed (circuit complete). Opening the door breaks the circuit, turning on the light.
Advantages:
- Fail-safe design ensures operation unless actively interrupted.
- Ideal for safety-critical applications where power should stop during emergencies.
Comparison of Normally Open vs. Normally Closed
| Feature | Normally Open (NO) | Normally Closed (NC) |
|---|---|---|
| Default State | Circuit is open; no current flows | Circuit is closed; current flows |
| Activated State | Circuit closes; current starts flowing | Circuit opens; current stops flowing |
| Energy Usage | No energy used in default state | Energy used continuously in default state |
| Common Applications | Doorbells, motion sensors | Emergency stop buttons, safety systems |
| Safety Considerations | Requires activation to energize | Automatically deactivates during emergencies |
Applications of NO and NC Contacts
Industrial Automation:
- NO contacts are used for starting machinery or activating devices only when needed.
- NC contacts are employed in emergency stop systems to ensure safety by cutting power during faults.
Lighting Control:
- NO switches can turn lights on momentarily (e.g., motion sensors).
- NC switches ensure lights remain off unless triggered by specific conditions.
Motor Control:
- NO contacts activate motors when energized.
- NC contacts stop motors automatically in case of faults or emergencies.
HVAC Systems:
- NO switches are used to start heating or cooling elements when required.
- NC switches act as safety cutoffs to prevent overheating or overcooling.
Power Distribution:
- NO relays connect circuits only when necessary.
- NC relays isolate circuits during maintenance or faults.
How to Choose Between NO and NC Contacts
This depends on your application’s requirements:
Safety Needs:
- Use NC contacts for fail-safe operations where power should stop during emergencies.
- Use NO contacts where power should only flow when actively engaged.
Energy Efficiency:
- NO contacts are more energy-efficient as they don’t consume power in their resting state.
- NC contacts continuously draw power in their default state, which may not be ideal for battery-operated devices.
Application Type:
- For momentary actions (e.g., ringing a doorbell), use NO contacts.
- For continuous operation that stops only upon activation (e.g., emergency stops), use NC contacts.
Summary
The terms “normally open” and “normally closed” refer to the default states of electrical switches or relays when no external force or input is applied. Understanding these states helps determine how a circuit behaves under normal and activated conditions.
