What are Normally Closed vs Normally Open Switch Contacts?
Every electrical switch has a default state — its position when no force or signal is applied. This default is described as Normally Closed (NC) or Normally Open (NO). Choosing between NC and NO contacts is fundamental to designing safe, reliable circuits.
Quick Answer
A Normally Open (NO) contact allows no current when the switch is at rest; it closes only when actuated. A Normally Closed (NC) contact allows current when at rest; it opens only when actuated. Most switches — push-buttons, limit switches, relays, contactors — are available in both NC and NO versions, and many switches include changeover contacts that combine both.
What Are NO and NC Contacts?
The terms describe the resting state of a switch — the position when no external force, button press, or electrical signal is applied.
Normally Open (NO): contacts are separated when at rest. The circuit is broken until something actuates the switch.
Normally Closed (NC): contacts are touching when at rest. Current flows until something actuates the switch to open the contacts.
A switch's default state is what it does when nothing is happening. Once actuated, the behavior reverses.
Schematic Symbols
NO and NC Contact Symbols
The NO symbol shows an open lever; the NC symbol shows a closed lever with a strike-through bar. Standard IEC 60617 symbols.
NO vs NC Comparison
Normally Open (NO)
Open when at rest
Closes when actuated
Used for "turn on" actions
Circuit default: OFF
Common in start buttons, push-to-make
Fail-safe: turns off when power is lost
Normally Closed (NC)
Closed when at rest
Opens when actuated
Used for "stop" or "safety" actions
Circuit default: ON
Common in stop buttons, push-to-break
Fail-safe: requires NC for emergency stops
Changeover (CO / SPDT) Contacts
A changeover or SPDT contact combines both: one common terminal, one NC terminal, and one NO terminal. Actuation moves the common from the NC contact to the NO contact. Changeover contacts are extremely common in relays and limit switches because they give the designer maximum flexibility.
Changeover Contact Symbol
Real-World Switch Examples
Push-Buttons
NO push-button: press to close (start buttons, doorbells).
NC push-button: press to open (stop buttons, emergency stops).
Limit Switches
NO limit switch: closes when actuator reaches limit position (e.g., machine overtravel detection).
NC limit switch: opens when limit reached (used in safety circuits).
Relays and Contactors
Most relays have both NO and NC contacts, often combined as SPDT or DPDT.
Motor contactors typically use large NO power contacts for switching the load.
Safety relays use NC contacts for fault detection.
Door and Safety Switches
Door interlock: NC contact that opens when the door is opened.
Limit switch: NO contact that closes when the door reaches its end position.
Pressure and Temperature Switches
NC: opens when threshold exceeded (overpressure or overtemperature trip).
NO: closes when threshold reached (e.g., low-pressure alarm).
Failure Modes and Safety
The choice of NC or NO has critical implications for safety circuits.
Fail-Safe Design
In an emergency-stop or safety interlock, an NC contact is the standard choice. If the wire breaks or the contact fails open, the circuit trips and stops the machine. With an NO contact, a broken wire leaves the safety circuit unable to stop the machine — a dangerous condition.
Positive Opening
Safety contacts must be positively opening: the actuator physically pulls the contacts apart. A contact that relies on a spring alone may fail to open if the spring breaks or corrodes.
Safety Rule: Always use NC contacts in emergency-stop circuits and safety interlocks. This way, a broken wire trips the safety, not disables it.
Where Each Is Used
Application
Preferred Contact
Start button
NO
Stop button
NC
Emergency stop
NC
Door interlock
NC
Limit switch (detection)
NO
Limit switch (safety)
NC
Motor contactor
NO (power contacts)
Overpressure alarm
NC
Low-pressure alarm
NO
Push-to-test indicator
NO
Mechanical vs Electronic Implementation
Mechanical
Mechanical Switch
Physical contacts; clearly open or closed. Mechanical NC/NO contacts can switch AC and DC.
Solid-State
Solid-State Relay
Transistor-based; NO by default (off until energized). Some have NC equivalents using depletion-mode devices.
Reed Switch
Reed Switch
Magnetic actuation; typically SPST-NO or SPST-NC depending on contact arrangement.
Sensor
Proximity Sensor
NO or NC versions available. NC sensors pass current when no target; NO passes current only when a target is detected.
Choosing Between NC and NO
Determine the default state needed: ON or OFF.
For safety-critical functions, prefer NC (fail-safe principle).
For convenience functions (start, indicator), NO is standard.
Consider system behavior on power loss.
Match the contact rating to voltage and current.
For complex logic, use changeover contacts to cover both states.
Common Mistakes
Using NO for emergency stop: a broken wire cannot stop the machine.
Confusing "normal" with "idle": "normal" means "unactuated," not "off" or "on."
Mixing NC and NO logic in PLC programs without proper mapping: verify the I/O assignment matches the physical contact.
Forgetting positive-opening requirement: spring-only NC contacts can fail to open.
Underrating contact current: arcing destroys NC/NO contacts faster than expected.
Verification & Testing
Use a multimeter in continuity mode to verify contact state at rest.
Apply the actuator and recheck — NO should close, NC should open.
Measure contact resistance under load (should be < 100 mΩ when new).
Verify insulation resistance between contacts (> 100 MΩ).
Test under rated voltage and current for the expected cycle count.
Key Takeaways
NO contacts are open at rest; NC contacts are closed at rest.
"Normal" means unactuated — not "off" or "on."
NC is the standard for safety and emergency-stop circuits.
Changeover (SPDT) contacts combine both for maximum flexibility.
Always verify the default state matches the safety intent.
Frequently Asked Questions
What is the difference between NC and NO contacts?
NC (Normally Closed) contacts allow current flow at rest; NO (Normally Open) contacts block current at rest. Actuation reverses the state.
What does "normal" mean in NC and NO?
"Normal" means the unactuated, resting state. It does not imply "off" or "on" — it just describes the default position.
Which contact type should I use for emergency stop?
Always use NC contacts in emergency-stop circuits. If the wire breaks, the NC contact opens and stops the machine — fail-safe behavior.
Can a switch have both NC and NO contacts?
Yes. Changeover or SPDT switches have one common, one NC, and one NO terminal. Actuation moves the common between the two.
What is positive opening?
Positive opening is a mechanical design where the actuator physically forces the contacts apart, independent of any spring. Required for safety-certified NC contacts.
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Aug 18 ,26
About the Author — MeiXun Team
Wang
Chief Engineer Wang
High-tech Enterprise, Feifeng Talent
Chief Engineer Wang graduated with a master's degree in high-power microwave from the Institute of Electronics, University of Chinese Academy of Sciences.
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Wang
Chief Engineer Wang
High-tech Enterprise, Feifeng Talent
Chief Engineer Wang graduated with a master's degree in high-power microwave from the Institute of Electronics, University of Chinese Academy of Sciences.
In the same year, he joined CETC 40/41 for work and study. He has been committed to the design and development of microwave switches for a long time.
He has applied for 27 patents as the first inventor in the microwave switch field, with 6 authorized invention patents and 14 utility model patents.
The products he developed cover various application platforms such as civilian testing, vehicle-mounted, shipborne, airborne, and missile-borne.
