Pull the cover off a 2-button control station. Two pushbuttons sit in the same housing. Both are spring-return momentary devices with the same form factor, the same mounting hardware, the same contact blocks bolted to the back. Press either one and release it. The spring returns it to the same resting position. The difference is invisible from the front of the enclosure. It lives inside the contact block: one button's contact is open at rest, the other's is closed at rest. That difference in rest state is the entire subject of NO and NC contact designations, and it determines whether a motor control circuit fails safe or fails dangerous.
What "Normal" Means
"Normal" is a mechanical term. It refers to the rest state of a contact when no external force is applied. No finger on the button, no coil energized, no cam depressed, no float raised. The spring has returned the mechanism to its resting position. Whatever the contact does in that state is its normal condition.
A normally open (NO) contact has an air gap between its contact surfaces at rest. No current can flow through it. An external force must close the gap before the contact conducts.
A normally closed (NC) contact has its contact surfaces touching at rest. Current flows through it whenever the circuit is energized. An external force must separate the surfaces to break the path.
The word "normal" carries no judgment about which state is correct or preferred. It is purely a description of the spring-return position. A STOP button's NC contact is not "normally stopping the motor." It is normally closed, meaning the contact surfaces are touching when nobody is pressing the button. The electrical consequence of that mechanical fact is what matters to circuit design.
Why STOP Is NC and START Is NO
The choice of contact type for each pushbutton in a motor control circuit is not arbitrary. It follows directly from fail-safe design.
Consider the STOP button first. STOP is wired as an NC contact in series with the contactor coil. At rest, the contact is closed, completing the series path. Current can flow through the stop circuit to the coil. When the operator presses STOP, the contact opens, the series path breaks, the coil de-energizes, and the motor stops.
The fail-safe reasoning: a broken wire has the same electrical effect as an open contact. If a wire in the stop circuit breaks, cracks at a terminal, or vibrates loose, the series path opens. The coil loses power. The motor stops. The circuit has failed to a safe state without anyone pressing a button. A broken wire and a pressed STOP button produce the same result.
Now reverse the thought experiment. If STOP were wired as an NO contact, the series path would be open at rest. The circuit could never complete the coil path in its resting state. The motor would never run in normal operation. That is not a stop button. That is a permanently open circuit.
The key principle: the STOP contact is in series with the coil, so any break in that series path de-energizes the coil. A broken wire, a loose terminal, a corroded connection all produce the same result as pressing the button. The circuit cannot distinguish between a deliberate stop and a wiring failure, and it does not need to. Both result in a stopped motor.
START is wired as an NO contact in parallel with the seal-in auxiliary. At rest, the contact is open. No current flows through the start path. Pressing START closes the contact momentarily, energizing the coil. The contactor pulls in, the seal-in auxiliary contact closes, and the coil holds itself energized through the auxiliary path. The operator releases START, the NO contact returns to its open state, and the motor keeps running on the seal-in.
If a wire to the START button breaks, the motor simply cannot be started from that station. The motor does not start on its own. That is a safe failure mode. The worst outcome is a service call, not an uncontrolled start.
Common NO Devices
NO contacts appear throughout motor control circuits wherever the design intent is "inactive until actuated."
START pushbuttons. Momentary NO. Pressed to initiate a motor start, released immediately. The seal-in contact takes over.
Seal-in auxiliary contacts. NO contacts mechanically linked to the contactor armature. When the coil energizes and the armature pulls in, the auxiliary closes. When the coil de-energizes, the auxiliary opens. This is the memory of the 3-wire control circuit.
Main power contacts. The three NO contacts inside the contactor that connect line power to the motor. They close when the coil energizes and open when it de-energizes. All three phases switch simultaneously.
Float switches (NO type). Close when liquid level rises to the set point. A sump pump float switch closes when water rises, starting the pump to drain the pit. Open at rest when the float hangs at the low position.
Pressure switches (NO type). Close when system pressure reaches the set point. Used in compressor control and hydraulic circuits. Open at rest when the system is depressurized.
The pattern across all NO devices: the contact is inactive at rest and activates when its operating condition is met.
Common NC Devices
NC contacts appear wherever the design intent is "active until a protective condition intervenes."
STOP pushbuttons. Momentary NC, held closed by the spring at rest. Pressing the button opens the contact and breaks the coil circuit. Releasing the button returns the contact to its closed state.
Overload relay auxiliary contacts. The 95-96 terminals on an overload relay are NC. Under normal motor operation, the OL contact is closed and current flows through it to the coil. When the overload relay trips on sustained overcurrent, the 95-96 contact opens. The coil de-energizes and the motor stops. The contact stays open until the overload relay is manually or automatically reset.
Limit switches (NC type). Open when a machine element reaches the end of its travel. A conveyor limit switch opens to stop the drive motor before the carriage overruns the track. At rest, with the carriage clear of the limit, the contact is closed and the circuit can operate normally.
Emergency stop (E-stop) buttons. NC contacts, red mushroom-head, latching. Pressing the E-stop opens the NC contact and de-energizes the machine. Unlike a standard STOP button, the E-stop latches in the pressed position and must be manually pulled or twisted to reset. The fail-safe logic is identical: a broken wire to the E-stop opens the series path, stopping the machine without anyone pressing the button.
Safety interlock switches. NC contacts on guard doors and safety gates. Closed when the guard is in place. Opening the guard opens the contact and de-energizes the machine. A broken wire, a disconnected plug, or a missing guard all produce an open circuit, which stops the machine.
The pattern across all NC devices: the contact is active at rest, permitting circuit operation, and deactivates when a protective condition requires the circuit to stop.
Reading NO and NC on a Ladder Diagram
On a NEMA/JIC ladder diagram, NO and NC contacts have distinct symbols.
An NO contact is drawn as two short vertical bars with a gap between them. The gap represents the open air space between contact surfaces at rest. When the contact closes (actuated state), a horizontal bridge line appears between the bars on animated diagrams.
An NC contact is drawn as two short vertical bars with a diagonal slash from lower-left to upper-right. The diagonal represents the closed path through the contact at rest. When the contact opens (actuated state), the diagonal disappears, leaving two bare bars with a visible gap.
Tracing current through a ladder rung starts at L1 on the left rail and follows the series path to L2 on the right rail. Every NC contact in that path is passing current at rest. Every NO contact in that path is blocking current at rest. The coil energizes only when every series element in the rung is simultaneously closed.
In the standard motor starter control rung, the path from L1 to L2 passes through the STOP contact (NC), through either the START contact (NO) or the seal-in auxiliary (NO), through the contactor coil, and through the OL auxiliary contact (NC). At rest, both NC contacts are closed and both NO contacts are open. The circuit is incomplete. Pressing START closes one NO path, completing the rung, and the coil energizes. The seal-in auxiliary then closes to hold the circuit after START is released.
Every contact on the diagram tells you its rest state at a glance. The symbol is the circuit's default condition. The actuated state is the opposite of whatever the symbol shows.
Reading is one thing — wiring it yourself is another. Open the interactive trainer and build this circuit from scratch.
Wire NO and NC contacts in a motor control circuit in the trainer →Frequently asked questions
What does "normally open" mean?
The contact is open (no current path) when no external force is applied. 'Normal' refers to the rest state of the contact — the position the spring returns it to when no one is pressing the button, no coil is energized, and no mechanical actuator is engaged. An NO contact must be actuated to close and allow current to flow.
What does "normally closed" mean?
The contact is closed (current can flow) when no external force is applied. The spring holds the contact surfaces together in the resting position. An NC contact must be actuated to open and break the current path. STOP buttons and overload relay auxiliary contacts (95-96 terminals) are common NC devices in motor control circuits.
How do you identify NO vs NC contacts on a ladder diagram?
NO contacts are drawn as two short vertical bars with a gap between them. The gap represents the open contact at rest. NC contacts are drawn as two short vertical bars with a diagonal line (slash) between them. The diagonal represents the closed path at rest. When tracing a ladder rung from L1 to L2, every NC contact is passing current at rest and every NO contact is blocking current at rest.
Can a device have both NO and NC contacts?
Yes. Many devices include both NO and NC contact blocks. A motor contactor has main NO power contacts and may have auxiliary contacts in both NO and NC configurations. A pushbutton station can be fitted with NO contact blocks, NC contact blocks, or both — the mechanical actuator operates all attached contact blocks simultaneously. Overload relays include NC auxiliary contacts (95-96) and some models also provide NO contacts (97-98) for alarm or status indication.