Normally Open vs Normally Closed: Contacts and Ladder Logic
Normally open vs normally closed explained — what NO and NC mean for contacts and sensors, when to use each, and the fail-safe rule for E-stops and ladder logic.
Normally open (NO) means the contact is open — no continuity — in its resting, de-energized state. Normally closed (NC) means the contact is closed — passing current — in its resting state. Both terms describe what the device does before anything activates it.
That single definition answers the most common interview question in industrial automation. But applying it correctly in PLC ladder logic — especially for safety devices — requires understanding the distinction between the physical contact state in the field and the instruction type you choose in the program.
What Normally Open and Normally Closed Mean (De-energized / Resting State)
The key word in both terms is normally. "Normally" means the de-energized, at-rest condition — no coil powered, no actuator pressed, no sensor triggered.
- Normally open (NO): At rest, the contact is open. No current flows through it. When the device is energized or actuated, the contact closes and current flows.
- Normally closed (NC): At rest, the contact is closed. Current flows through it continuously. When the device is energized or actuated, the contact opens and current stops.
Think of NO as a door that is normally shut with the latch removed — nothing passes. Think of NC as a door propped open — everything passes until something pushes it closed.
This resting state is defined by the physical position of the contact mechanism before any energy is applied. It has nothing to do with what the PLC program does. It is purely a hardware characteristic.
| State | Normally Open (NO) | Normally Closed (NC) |
|---|---|---|
| At rest / de-energized | Open — no continuity | Closed — continuity |
| When activated / energized | Closes — continuity | Opens — no continuity |
| Symbol (physical schematic) | Gap between two lines | Two lines with diagonal slash |
NO vs NC in Physical Contacts and Switches
Every pushbutton, limit switch, relay, and selector switch ships with a combination of NO and NC contact blocks. The contact designation is stamped on the device or documented in its datasheet.
Pushbuttons: A standard momentary pushbutton has both NO and NC terminals. The NO contact is open while your finger is off it. Press it and the NO closes. The NC contact is closed while your finger is off it. Press it and the NC opens.
Limit switches: A limit switch has a mechanical actuator arm. In its free (un-tripped) position, the NO contact is open and the NC contact is closed. When a moving part presses the arm, the contacts flip.
Contactors and relays: A contactor coil, when energized, closes its main NO power contacts and may also actuate auxiliary NO/NC contacts used for interlocking in the control circuit.
Proximity sensors and other electronic sensors: Electronic sensors use the terms NO and NC slightly differently from mechanical contacts but the logic is the same. An NO proximity sensor outputs a signal when the target is present; an NC proximity sensor outputs a signal when the target is absent. See the related guide on sinking vs sourcing for the NPN/PNP output type distinction, which is a separate but often confused topic.
NO vs NC in Ladder Logic: XIC vs XIO — and the Key Distinction
In PLC ladder logic, contacts are instructions, not hardware. The instruction examines the state of a bit in the PLC's data table and either passes or blocks power flow on the rung based on that bit.
The two fundamental contact instructions are:
- XIC — Examine If Closed (Normally Open instruction): The rung passes power when the referenced bit is 1 (ON). If the bit is 0 (OFF), the rung does not pass power. This is what most programming software draws as two vertical parallel lines — the standard "normally open" contact symbol.
- XIO — Examine If Open (Normally Closed instruction): The rung passes power when the referenced bit is 0 (OFF). If the bit is 1 (ON), the rung does not pass power. This is drawn as two vertical parallel lines with a diagonal slash — the "normally closed" contact symbol.
Allen-Bradley Logix platforms use the terms XIC and XIO explicitly. Siemens TIA Portal uses the same visual symbols with slightly different nomenclature (NO contact / NC contact). The function is identical across platforms.
The Critical Distinction: Field Device State vs Instruction Type
This is where most beginners make expensive mistakes. The NO/NC designation of the field device and the XIC/XIO instruction type in the program are independent choices. They interact — and the combination determines the PLC's behavior.
Here is the logic:
- When an NC field device is wired to a PLC input, the PLC input bit is ON (1) while the device is at rest (not activated). Current flows through the closed NC contact into the input module.
- When an XIC instruction references that input address, the rung is true (power flows) when the input bit is 1 — meaning the rung is true while the NC device is at rest.
- When the NC device is activated (contact opens), the input bit drops to 0 — and the XIC rung goes false.
The table below shows all four combinations:
| Field Device | PLC Instruction | Rung True When |
|---|---|---|
| NO contact (open at rest) | XIC | Device is activated (contact closed, bit = 1) |
| NO contact (open at rest) | XIO | Device is at rest (contact open, bit = 0) |
| NC contact (closed at rest) | XIC | Device is at rest (contact closed, bit = 1) |
| NC contact (closed at rest) | XIO | Device is activated (contact open, bit = 0) |
Read the ladder logic symbols complete guide for the full reference diagram set and how contacts interact with output coils and function blocks.
When to Use Each Type
Choosing between NO and NC is not arbitrary. The decision is driven by the function of the device, the required failure behavior, and the direction that is easier for operators to understand.
Use NO when:
- The default machine state is "stopped" or "not doing anything" and an action is required to start.
- A pushbutton should initiate a cycle only while held, with no action as the default.
- A sensor should trigger a response only when the target object is present.
- The cost of an unintended activation is lower than the cost of a failure to respond.
Use NC when:
- The device must interrupt or stop a process when activated.
- Continuous operation depends on the contact remaining made — a fault (broken wire, loose terminal) should cause the safe state.
- You are implementing a safety interlock or guard.
A standard motor start/stop station illustrates both:
- The START button is wired NO — press it to close the circuit and start the motor.
- The STOP button is wired NC — it holds the circuit closed while idle; press it to open the circuit and stop the motor.
This is not just convention. The NC stop button provides wired fail-safe behavior that a NO stop button cannot. If a wire breaks at a NO stop button, the circuit remains closed and you lose the ability to stop the motor from that button without knowing anything is wrong. With an NC stop button, a broken wire opens the circuit and stops the motor immediately.
The Fail-Safe Rule: Why E-Stops and Safety Devices Are Wired NC
The fail-safe principle states: the failure of a component or wiring should cause the system to move to its safe state, not its dangerous state.
For a machine that must stop in an emergency, the safe state is stopped. Therefore, every device in the emergency stop circuit must be wired so that a wiring fault — open circuit, broken wire, loose terminal, corroded connection — causes the machine to stop.
An NC E-stop wired in series with the control circuit achieves this:
- At rest (button not pressed): NC contact closed → circuit complete → machine can run.
- Button pressed: NC contact opens → circuit broken → machine stops.
- Wire breaks anywhere in the loop: circuit broken → machine stops.
If you wired an E-stop NO instead, a broken wire would be indistinguishable from an unpressed button — the machine would continue running, unaware of the wiring fault. This is why every major safety standard (ISO 13849, IEC 62061, NFPA 79) mandates NC wiring for safety devices. The deeper guide on E-stop safety circuits in PLC ladder logic covers ISO 13849 performance levels and the role of safety relays versus standard PLC inputs.
The same logic applies to:
- Gate interlock switches on guarded zones
- Safety light curtain output relays (OSSD outputs are NC in their relay form)
- Safety mats and pressure-sensitive devices
- Door interlocks on enclosures covering hazardous mechanisms
Reading a Rung with an NC E-Stop Correctly
A common source of confusion: an NC E-stop is typically programmed with a XIC (normally open) instruction in the ladder program — not an XIO.
Here is why, step by step:
- The NC E-stop contact is closed at rest. Current flows into the PLC input terminal.
- The PLC input bit is therefore ON (1) under normal operating conditions.
- A XIC instruction on that address passes power when the bit is 1 — so the rung is true during normal operation. Correct.
- When the E-stop is pressed, the NC contact opens. The input bit drops to 0 (OFF).
- The XIC instruction now sees bit = 0 and blocks power flow — the rung goes false and the output (motor, drive enable, etc.) turns off.
If you mistakenly used a XIO instruction on an NC E-stop input:
- Normal operation: NC contact closed → input bit = 1 → XIO sees bit = 1 → rung is false → machine cannot run.
- E-stop pressed: NC contact open → input bit = 0 → XIO sees bit = 0 → rung is true → machine tries to start.
That is exactly backwards. The machine runs only when the E-stop is pressed and stops when it is released. Reading the rung correctly means understanding what the field wiring does to the input bit before reasoning about the instruction.
The relay logic explained article covers how this same hardwired NC logic functioned in relay panels before PLCs, which is useful context for understanding why the convention exists.
Understanding how the PLC evaluates every instruction on every scan is also relevant — the PLC scan cycle explained article covers how input image tables are updated and when rung evaluation takes effect during a scan.
Held-Open and Held-Closed
Two additional terms appear in wiring documentation and troubleshooting:
Held-open: A contact that is mechanically or electrically held in the open position continuously — it cannot close regardless of actuation. This indicates a fault (broken actuator arm, seized mechanism, coil failure) rather than normal switching behavior.
Held-closed: A contact held in the closed position continuously — it cannot open on actuation. This is a serious fault for safety devices because the protective function is defeated. A welded contactor contact is a common example: the contacts fuse together under high fault current and remain closed even when the coil is de-energized.
Safety relays with guided (positively driven) contacts address the held-closed failure mode. Guided contacts guarantee that NO and NC contacts on the same device cannot both be closed simultaneously — if an NC contact is welded closed, the safety relay detects this during its internal self-check and refuses to energize.
For standard control applications outside safety circuits, held-open and held-closed simply describe a stuck contact that requires maintenance.
Frequently Asked Questions
What is the difference between normally open and normally closed?
Normally open (NO) means the contact is open (no continuity) in its de-energized resting state and closes when actuated. Normally closed (NC) means the contact is closed (continuity) in its de-energized resting state and opens when actuated. The distinction describes the physical hardware state before activation, not the PLC instruction used to read it.
Should an E-stop be NO or NC?
An E-stop must be wired NC (normally closed). This is required by ISO 13849, IEC 62061, and NFPA 79. An NC E-stop ensures that any wiring fault — open circuit, broken wire, disconnected terminal — causes the machine to stop. A NO E-stop wired open looks identical to an unpressed button, hiding the fault and allowing continued machine operation.
What is XIC vs XIO in ladder logic?
XIC (Examine If Closed) is the normally open contact instruction in Allen-Bradley ladder logic. It passes power (rung true) when the referenced bit is 1 (ON). XIO (Examine If Open) is the normally closed contact instruction. It passes power when the referenced bit is 0 (OFF). Most PLC platforms use equivalent terminology: NO contact and NC contact instruction, or normally open contact and normally closed contact.
Why are safety contacts normally closed?
Safety contacts are NC because a broken wire or open circuit must produce the safe machine state — stopped. An NC circuit breaks (goes open) on any wiring fault, removing the signal and causing the controlled device to de-energize. An NO safety circuit, by contrast, would require the wire to break in a way that completes the circuit to stop the machine, which is not achievable with a simple open-circuit fault.
Summary
| Topic | Key Point |
|---|---|
| NO definition | Open at rest; closes when actuated |
| NC definition | Closed at rest; opens when actuated |
| XIC instruction | Rung true when bit = 1 (ON) |
| XIO instruction | Rung true when bit = 0 (OFF) |
| NC E-stop + XIC | Rung true during normal operation; breaks when E-stop pressed or wire faults |
| Fail-safe rule | Wire safety devices NC so any fault causes the safe (stopped) state |
The relationship between field device wiring and ladder instruction type is the practical core of NO/NC knowledge. An NC E-stop wired correctly to an XIC instruction gives you a rung that is true during normal operation and false the moment anything in the safety circuit fails — broken wire, pressed button, or corroded terminal. That is exactly the behavior industrial safety standards require.
For a deeper look at how contacts combine with coils to build complete control rungs, see the ladder logic tutorial.
