The push button switch symbol represents a momentary or latching mechanical actuator in electrical schematics. Because a single physical button can control multiple isolated contact blocks (Normally Open and Normally Closed), misreading the schematic can lead to dead shorts, unintended machine starts, or bypassed safety interlocks. Below is the master reference for identifying, interpreting, and troubleshooting these symbols across global standards.
Complete Push Button Switch Symbol Reference
The table below maps the most common push button switch symbols to their physical behavior and standard schematic representations. Reference designations (like -S1 or -PB1) are used to link the schematic symbol to the physical component on the panel layout.
| Switch Type | IEC 60617 Symbol Description | NEMA / IEEE 315 Symbol Description | Contact State (Resting) | Standard Terminal Numbers |
|---|---|---|---|---|
| Normally Open (NO) Momentary | Two parallel vertical lines (contacts) with a horizontal actuator line above, connected by a dashed drop-line to the button head. | Similar parallel lines, but the actuator is often drawn as a distinct mechanical linkage or a solid triangle pointing at the gap. | Open (No continuity) | 13 and 14 |
| Normally Closed (NC) Momentary | Parallel vertical lines intersected by a diagonal crossbar. Actuator line above with a drop-line. | Parallel lines with a crossbar, mechanical linkage actuator above. | Closed (Continuity) | 21 and 22 |
| NO Mechanical Latch (Push-On/Push-Off) | NO symbol with a small mechanical latch hash-mark (often a small hook or square) on the actuator stem. | NO symbol with a distinct 'latch' box or interlocking line on the stem. | Open (Changes state on press) | 13 and 14 |
| Emergency Stop (E-Stop) Mushroom | NC symbol with a solid red mushroom-head actuator drawn above it. Often includes a twist-to-release or pull-to-release indicator arrow. | NC symbol with a mushroom head and a mechanical latch indicator (since E-stops lock when pressed). | Closed (Opens on press) | 21 and 22 (Safety rated) |
| Illuminated Push Button (NO) | NO momentary symbol with a small circle (lamp indicator) drawn adjacent to or integrated into the button head. | NO symbol with an integrated lamp circle and separate lamp terminal designations. | Open (Lamp circuit independent) | 13/14 (Contacts), X1/X2 (Lamp) |
| Double Contact (1 NO + 1 NC) | Two sets of contacts (one NO, one NC) linked by a horizontal dashed line, sharing a single actuator drop-line. | Two sets of contacts linked by a dashed mechanical tie line, single actuator. | One Open, One Closed | 13/14 (NO), 21/22 (NC) |
Note: Terminal numbers listed above follow IEC 60947-5-1 conventions for control circuit auxiliary contacts. Power contacts (if applicable on heavy-duty contactors triggered by the button) use 1/2 (NC) and 3/4 (NO).
Regional Standards: IEC vs NEMA vs Legacy UK
When reading a wiring diagram, your first task is identifying which standard the drafter used. Applying NEMA logic to an IEC drawing (or vice versa) rarely causes catastrophic failure, but it does cause confusion regarding mechanical interlocks and latch states.
| Standard Body | Primary Region | Key Visual Characteristics | Common Reference Prefix |
|---|---|---|---|
| IEC 60617 | EU, UK (Modern), AU, Global | Minimalist geometry. Actuators are simple straight lines. Mechanical links are always dashed lines. Emphasizes function over physical construction. | -S (e.g., -S1, -S2) |
| NEMA / IEEE 315 | USA, Canada, Mexico | More pictorial. Actuators often show mechanical linkage details (springs, pivots). Latching mechanisms are drawn with explicit interlocking boxes. | -PB (Push Button) or -S |
| BS 3939 (Legacy) | UK (Pre-1990s panels) | Highly abstract. Push buttons were sometimes represented simply by a circle with a line through it, requiring the tech to read the adjacent text label to determine NO/NC status. | PB or S |
The 'Rows People Get Wrong' Field Guide
Even experienced journeyman electricians and control technicians misinterpret specific nuances of push button symbols. Here are the most common field errors and how to avoid them.
1. The Dashed Line Means 'Mechanical', Not 'Electrical'
When you see a dashed horizontal line connecting a NO contact symbol and a NC contact symbol, it means they are mechanically linked—they are operated by the exact same physical push button. It does not mean they are electrically bridged. A common beginner mistake is wiring a jumper between terminal 14 and 21 because the schematic shows them connected by a dashed line. That dashed line represents the physical plastic plunger inside a switch stack (like a Schneider Electric Harmony XB4), not a copper wire.
2. E-Stop Twist vs. Pull vs. Key Release
The standard E-stop mushroom symbol tells you it is an NC contact that locks when pressed. However, the schematic often includes a tiny annotation next to the actuator stem:
- Curved Arrow: Twist-to-release (Most common, e.g., Siemens 3SU1).
- Straight Arrow pointing up: Pull-to-release.
- Small Keyhole icon: Key-release (Used in high-security or multi-zone safety PLC setups).
3. Time-Delay Attachments (Dashpots)
Sometimes a push button symbol features a small box with a diagonal line or a 'U' shape attached to the stem. This indicates a pneumatic or electronic time-delay attachment. The contact will not change state the exact millisecond the button is pressed; it will delay (e.g., 3 seconds on-press, 0 seconds on-release). Failing to notice this symbol leads technicians to falsely diagnose the switch as 'broken' when it is simply timing out.
Safe Interpretation When Markings Are Faded or Missing
In harsh industrial environments (washdown areas, metalworking shops with conductive dust, or UV-heavy outdoor enclosures), the physical NO/NC markings on the contact blocks fade away. If the schematic is also missing or illegible, you must verify the switch state electrically. Never guess based on the color of the button (Red is not always NC; Green is not always NO).
Step-by-Step Verification Procedure:
- Set your multimeter to Continuity: Use the diode/sound-wave setting. Touch the probes together to verify the meter beeps and reads near 0.0 ohms.
- Identify the Terminals: Look for stamped numbers on the plastic contact block. If faded, look for the physical layout: standard IEC blocks usually place the NO block on the left and NC on the right when facing the front of the panel, but this is not guaranteed.
- Test Resting State (Button NOT pressed):
- Place probes across the two terminals of the first block. If it beeps (reads < 1 ohm), it is Normally Closed (NC).
- If it reads OL (Open Loop), it is Normally Open (NO).
- Test Actuated State (Button Pressed and Held):
- Have a helper press and hold the button (or use a zip-tie/tape to hold an E-stop in the depressed state).
- Re-test the terminals. The NO block should now beep; the NC block should now read OL.
- Verify Safety E-Stop Forced-Opening: If testing an E-stop, you must verify positive-guided (force-guided) contacts. When the E-stop is pressed, the NC contacts must open. If the internal spring is broken, a standard multimeter might still show continuity due to a welded contact. For critical safety circuits, refer to OSHA machine guarding guidelines and perform a functional drop-test of the safety relay circuit rather than relying solely on a bench continuity test.
By cross-referencing the schematic symbol with physical terminal numbers (13/14 vs 21/22) and verifying with a meter, you eliminate the guesswork that leads to 24VDC short circuits and tripped control transformers.
