The normally open (NO) symbol represents a switch, relay, or contactor contact that prevents current flow in its resting, unactuated state. In North American standards (NEMA/IEEE 315), it is typically drawn as two parallel horizontal lines separated by a gap, intersected by a diagonal or perpendicular actuator line. In international standards (IEC 60617), it appears as a simple gap between two horizontal lines with a strictly perpendicular actuator bar. When the device is actuated—whether by a human finger, a mechanical limit, or an electromagnetic coil—the gap closes, completing the circuit and allowing current to flow.
Normally Open Symbol Reference Chart
Use this table to identify the specific variation of the normally open symbol you are looking at on a schematic. The visual representation changes depending on the physical mechanism driving the contact.
| Component Type | NEMA / IEEE 315 Symbol Description | IEC 60617 Symbol Description | Real-World Application Example |
|---|---|---|---|
| Pushbutton (Momentary NO) | Two horizontal lines with a diagonal "bowtie" or slanted actuator line above the gap. | Two horizontal lines with a straight vertical actuator line ending in a flat horizontal bar. | Start button on a 3-phase motor starter circuit (e.g., Allen-Bradley 800F). |
| Limit Switch (NO) | Standard NO gap with a small roller or wedge symbol attached to the actuator line. | Standard NO gap with a distinct roller or lever arm drawn at the end of the vertical actuator. | Conveyor belt end-of-travel safety stop. |
| Relay / Contactor Auxiliary (NO) | Standard NO gap with the actuator line mechanically linked (dashed line) to a coil symbol. | Standard NO gap with a vertical dashed line linking it to the main contactor coil. | Sealing (holding) circuit in a standard 3-wire motor control. |
| Pressure / Temperature Switch (NO) | NO gap with a specific process symbol (e.g., a circle with 'P' or 'T') driving the actuator. | NO gap with an IEC process instrument bubble driving the actuator. | Air compressor cut-in switch triggering the motor contactor. |
Regional Standards: NEMA, IEC, and Legacy UK
Assuming a single global standard for electrical schematics is a fast track to miswiring a control panel. The visual language of the normally open symbol shifts depending on the governing standard of the region where the machinery was designed.
North America (NEMA / IEEE 315): Governed by IEEE Std 315, North American schematics favor functional clarity over strict geometric orthogonality. The actuator line for a pushbutton is frequently drawn at a 45-degree angle (the "diagonal slash"). This diagonal line is strictly an actuator indicator and does not imply a normally closed state.
International (IEC 60617): The IEC 60617 standard mandates strict orthogonal geometry. Conductors are horizontal, and actuators are strictly vertical. A pushbutton actuator will always be a vertical line dropping down to a horizontal bar. If you see a diagonal line on an IEC drawing, it usually indicates a mechanical linkage or a specific type of thermal trip, not a standard pushbutton.
Legacy UK (BS 3939): Before the UK fully harmonized with IEC standards, BS 3939 was the norm. You will still encounter these on older machinery in British facilities. BS 3939 often mixed IEC-style horizontal gaps with NEMA-style diagonal actuators.
Never assume a diagonal actuator line on a faded, pre-1990 UK schematic automatically means "momentary pushbutton." In BS 3939, diagonal lines were sometimes used to denote delayed-action contacts. Always verify the physical component with a multimeter before applying control voltage.
Rows People Get Wrong & Faded Marking Protocols
Even when you know the standard, misinterpreting the normally open symbol in practice leads to short circuits or failed safety interlocks. Here are the most common errors and how to resolve them when physical markings fail.
The "Open Means On" Terminology Trap
The most frequent mistake among beginners is assuming "open" means the circuit is active, much like an "open" water valve allows flow. In electrical theory, an open circuit means infinite resistance and zero current flow. A normally open contact is "off" in its resting state. It only closes (turns on) when actuated.
Confusing NO with Dual-Channel Safety Contacts
On modern safety relays (like the Pilz PNOZ series), you will see symbols that look like NO contacts but are mechanically linked. If a schematic shows two NO symbols with a solid line connecting their actuators, this denotes a force-guided or positively driven contact set. This means if one NO contact welds shut, the mechanically linked NC contact is physically prevented from closing, allowing the safety controller to detect the fault. Do not substitute a standard ice-cube relay for a force-guided safety relay.
Safe Interpretation When Markings are Faded or Missing
Industrial environments are harsh. The NO/NC stamping on a Siemens 3RT20 contactor or a Schneider Electric Harmony XB4 pushbutton block will eventually wear off from oil, heat, and vibration. When the schematic is missing and the physical stamping is gone, use these two protocols:
- The IEC Terminal Number Cheat: If the device follows IEC naming conventions, look at the terminal numbers. Auxiliary contacts ending in 13 and 14 are universally Normally Open. Contacts ending in 21 and 22 are Normally Closed. Main power poles are typically 1-2, 3-4, 5-6 (L1-T1, L2-T2, L3-T3).
- The De-Energized Continuity Test: Lock out and tag out (LOTO) the panel. Verify zero voltage with a CAT III/IV meter. Switch your digital multimeter (e.g., Fluke 117) to continuity mode. Place probes across the suspect terminals. If the meter beeps immediately, it is NC. If it is silent, manually push the contactor armature or pushbutton with an insulated tool. If it beeps only while depressed, it is NO.
Frequently Asked Questions
What does a normally open symbol look like on a PLC wiring diagram versus ladder logic?
This is a massive point of confusion. On the physical wiring diagram (the I/O schematic), a normally open physical switch is drawn as the standard NO gap symbol wired to a PLC input terminal (e.g., I:0/0). However, inside the PLC's ladder logic, that same physical NO switch is represented by an Examine If Closed (XIC) instruction, which looks like two vertical parallel lines: | |. It is called "Examine If Closed" because the PLC is looking for the physical circuit to close (go high) to make the logic rung true. Conversely, a physical Normally Closed switch is represented in ladder logic by an Examine If Open (XIO) instruction: |/|.
Is a normally open symbol the same as a momentary switch?
No, they describe two entirely different characteristics. "Normally Open" describes the electrical resting state of the contacts (the circuit is broken). "Momentary" describes the mechanical actuation method (a spring returns the switch to its resting state when you release it). A standard household light switch is a maintained switch, but when flipped to the OFF position, its internal contacts are in a normally open state. Conversely, a motor start button is both momentary and normally open.
How do I wire a 24V DC proximity sensor using the normally open symbol?
When a schematic calls for a NO proximity sensor, you are usually dealing with a 3-wire DC setup. The sensor acts as a solid-state NO switch. Wire the Brown wire to +24V DC and the Blue wire to 0V DC. The Black wire is the signal output. If the sensor is PNP (sourcing), the Black wire will output +24V only when the target is detected (closing the NO logic). If the sensor is NPN (sinking), the Black wire will pull the signal down to 0V when the target is detected. Always check the PLC input card's datasheet to ensure you match the sensor's PNP/NPN sourcing or sinking type to the card's requirements.
