An electrical contact symbol dictates the default logical state of a switch, relay, or contactor when its actuator (coil or mechanical lever) is unpowered and at rest. The two foundational states are Normally Open (NO) and Normally Closed (NC). Below is the complete reference mapping physical terminals to their schematic symbols across global standards, allowing you to translate a 2D schematic into physical wiring on an Omron MY2N or Schneider TeSys contactor.
Master Electrical Contact Symbol Reference Table
The following table maps the schematic symbols to the physical terminal numbers you will find stamped on modern industrial relays (following EN 50013 / DIN 46199 numbering standards). Read the IEC column for modern global PLCs and European panels, and the NEMA column for legacy North American heavy industry.
| Contact Type | IEC 60617 Symbol Description | NEMA / ANSI Y32.2 Description | Standard DIN Terminal Numbers | Practical Application |
|---|---|---|---|---|
| SPST-NO (Form A) | Hinge line open; gap between moving arm and stationary contact. | Limit switch symbol with open gap; actuator line clear of the contact. | 13 and 14 (Single) 3 and 4 (Legacy) |
Start buttons, enable circuits, coil energization paths. |
| SPST-NC (Form B) | Hinge line closed; diagonal slash crosses the hinge to indicate "held closed". | Limit switch symbol with contacts touching; actuator line resting on contact. | 11 and 12 (Single) 1 and 2 (Legacy) |
Stop buttons, safety interlocks, emergency stop chains. |
| SPDT (Form C / Changeover) | Single moving arm pivoting between two stationary contacts (one NO, one NC). | Three-terminal limit switch symbol; common arm rests on one, gaps the other. | 11 (Common), 12 (NC), 14 (NO) | Directional control, alarm vs. normal status indication. |
| DPST-NO | Two parallel NO hinges linked by a dashed mechanical line. | Two parallel open limit switches linked by a mechanical bar. | 13-14 and 23-24 | Switching both Line and Neutral in single-phase AC control circuits. |
| Time-Delay ON (NOT) | NO hinge with an "umbrella" (arc) on top of the moving arm. | NO limit switch with a pneumatic timer bellows symbol. | 15 and 18 (Typical timer relay) | Motor star-delta transition delays, alarm horn silencing. |
| Time-Delay OFF (NCTO) | NC hinge (with slash) and an "umbrella" (arc) on the bottom of the arm. | NC limit switch with pneumatic bellows symbol. | 16 and 18 (Typical timer relay) | Cooling fan run-on after main motor contactor drops out. |
Regional Standards: IEC 60617 vs. NEMA/ANSI vs. Old UK
When reading a schematic or tracing a panel, your first task is identifying which drafting standard the engineer used. Misinterpreting the standard is the leading cause of wiring errors in cross-border machinery imports.
IEC 60617 (Global / EU / Modern North America)
The International Electrotechnical Commission standard uses a "hinge and slash" methodology. It is the dominant standard for modern PLC programming (Siemens TIA Portal, Rockwell Studio 5000) and European machinery. If you are working on equipment built after 2005, even in the US, it likely uses IEC symbols due to global supply chains.
NEMA / ANSI Y32.2 (North American Legacy)
The National Electrical Manufacturers Association standard relies heavily on "limit switch" style symbols, often utilizing circles to denote connection points and specific geometric shapes for actuators (like a mushroom head for E-stops). You will predominantly see this in North American heavy industry, older MCCs (Motor Control Centers), and legacy Allen-Bradley ladder logic printouts.
Old UK BS 3939 (Deprecated)
Before harmonizing with Europe, the UK used BS 3939. You will only encounter this in pre-1990s British infrastructure. It used distinct rectangular blocks for relays rather than the IEC hinge or NEMA limit switch styles. Safe interpretation: If you see rectangular block symbols with internal diagonal lines, treat them as legacy BS 3939 and verify every contact physically with a multimeter, as modern IEC/NEMA conventions do not apply.
The "Rows People Get Wrong" Guide
Even experienced bench technicians misread specific symbol variations. Here are the most common traps and how to avoid them.
1. The IEC "Slash" on NC Symbols
In IEC 60617, a Normally Closed contact is drawn as a closed hinge with a diagonal slash crossing it. Beginners frequently interpret the slash as a "break" or "open" indicator, wiring it as an NO contact. Correction: The slash represents the mechanical spring force holding the contact closed at rest. No slash = NO. Slash = NC.
2. Time-Delay "Umbrellas" (Make vs. Break)
Time-delay relays use a semi-circle (umbrella) to denote the pneumatic or electronic delay. The position of the umbrella dictates the behavior:
- Umbrella on top (NOT): The umbrella "catches" the arm as it moves up. Delay is on energization (Normally Open, Time Delay).
- Umbrella on bottom (NCTO): The umbrella "catches" the arm as it falls down. Delay is on de-energization (Normally Closed, Time Delay Open).
3. Form A / B / C vs. NO / NC
Datasheets for PCB relays (like the ubiquitous Songle SRD-05VDC-SL-C or Omron G5V-2) often use US telephone terminology instead of NO/NC.
- Form A = SPST-NO
- Form B = SPST-NC
- Form C = SPDT (Changeover)
Safe Interpretation When Markings Are Faded or Missing
Industrial environments are harsh. Heat, ozone from contact arcing, and chemical exposure frequently burn the terminal labels off contactors and relays. When you cannot read the physical stamping, you must deduce the contact state safely.
Step-by-Step Deduction Protocol
- Identify the Coil: Set your multimeter to resistance (Ohms). Probe pairs of terminals. The coil (A1/A2 on IEC, L1/L2 on some NEMA) will typically read between 15Ω and 500Ω depending on the voltage rating (24VDC coils are lower resistance than 120VAC coils). Contacts will read <0.5Ω (closed) or infinite/OL (open).
- Test for NC (Form B): With the relay completely unpowered and removed from the circuit (or isolated), probe the suspected contact pairs. A reading of <0.5Ω with a continuous audible beep indicates a Normally Closed contact. Mark these with a piece of labeling tape.
- Test for NO (Form A): Pairs that read "OL" (Over Limit) or infinite resistance at rest are your Normally Open contacts.
- Verify Changeover (Form C): If you find a common terminal that reads <0.5Ω to one pin, and "OL" to another adjacent pin, you have identified the Common, NC, and NO pins of an SPDT block.
- The Manual Actuator Test: Most industrial relays (like the Schneider RXM or Omron MY series) feature a small plastic test button on the front. Press and hold this button with a non-conductive tool while monitoring continuity. The contacts that "beep" only when the button is pressed are your NO contacts.
For authoritative reference on schematic drafting standards, consult the NEMA ICS 19-2002 standard documentation for North American conventions, or review the IEC graphical symbols database for international harmonization. When testing components in the field, always adhere to the testing methodologies outlined in the Fluke guide on relay testing to ensure both personal safety and diagnostic accuracy.
