The electric relay symbol on a schematic tells you exactly how an electromagnetic coil controls a set of mechanical contacts. In modern IEC 60617 schematics, the coil is drawn as a simple rectangle, while legacy ANSI/IEEE 315 uses a circle. Contacts are universally shown in their de-energized (shelf) state. If you are reading a 24V DC control schematic, the coil rectangle will contain a solid line or a minus sign, whereas a 24V AC coil will display a sine wave. Understanding these distinctions prevents catastrophic miswiring, such as applying AC voltage to a DC coil, which causes immediate armature chatter and coil burnout.
The Complete Electric Relay Symbol Reference Table
The table below maps the most common relay symbols to their physical counterparts. Note that all contact symbols represent the de-energized state. When the coil is powered, Normally Open (NO) contacts close, and Normally Closed (NC) contacts open.
| Component / Function | IEC 60617 Symbol Description | ANSI / IEEE 315 Symbol Description | Physical Terminal Markings |
|---|---|---|---|
| Standard Coil (DC) | Rectangle with a solid straight line inside | Circle with a solid straight line inside | A1 (+), A2 (-) |
| Standard Coil (AC) | Rectangle with a sine wave (~) inside | Circle with a sine wave (~) inside | A1, A2 (Polarity insensitive) |
| Normally Open (NO / Form A) | Two parallel lines with a diagonal slash bridging them | Two parallel lines with a diagonal slash bridging them | 11-12 (or 13-14 depending on mfr) |
| Normally Closed (NC / Form B) | Two parallel lines bridged by a diagonal slash, crossed by a perpendicular line | Same as IEC, sometimes drawn with the armature resting on the contact | 21-22 (or 11-12 depending on mfr) |
| Changeover (SPDT / Form C) | Three parallel lines; center line is COM, outer lines are NO/NC with respective slashes | Same as IEC, often drawn as a single pole throwing between two stationary contacts | COM, NO, NC (or 11, 14, 12) |
| On-Delay Timer (NO) | NO contact symbol with an arrow pointing towards the contact gap | Circle coil with a delayed-contact slash and inward arrow | Usually solid-state or specialized timer module pins |
| Off-Delay Timer (NC) | NC contact symbol with an arrow pointing away from the contact gap | Circle coil with a delayed-contact slash and outward arrow | Usually solid-state or specialized timer module pins |
| Latching / Polarized Coil | Rectangle with a small mechanical latch box attached to the side | Circle with a latch box or dual opposing coils | Set (S), Reset (R), or polarity-dependent A1/A2 |
Regional Standards: IEC vs. ANSI/NEMA vs. Old UK
Before tracing wires, you must identify which drafting standard the original engineer used. Mixing up standards leads to misidentifying coils and contacts.
- IEC 60617 (International / Modern Global): The dominant standard worldwide and in modern PLC software (TIA Portal, Studio 5000). Coils are rectangles. Contacts are drawn separately from the coil and linked only by an alphanumeric tag (e.g., coil K1 controls contacts K1). This modular drawing style keeps complex schematics readable.
- ANSI / IEEE 315 (Legacy US Industrial): Coils are circles. In older schematics, contacts were sometimes drawn physically attached to the coil's armature line. You will still see this in US power generation, water treatment, and older industrial panels. Reference the IEEE 315 standard documentation for legacy vector symbols.
- NEMA (US Motor Control): Highly specific to motor control centers (MCCs). NEMA uses specific letter designations rather than generic numbers. A control relay is designated CR, a motor starter is M, and a timing relay is TR.
- Old UK (BS 3939): Obsolete since the 1990s but frequently encountered when retrofitting British manufacturing plants. It used circles for coils like ANSI, but contact slashes were drawn at distinct angles to denote specific switching sequences. Treat these panels with extreme caution and verify every pin with a meter.
Rows People Get Wrong: Common Symbol Misinterpretations
Even experienced technicians misread specific symbol variations. Here are the most common traps:
- Time-Delay Arrow Direction: The arrow on a time-delay contact indicates the direction of the delay, not the movement. An arrow pointing towards the contact gap means it is slow to close (On-Delay). An arrow pointing away means it is slow to open (Off-Delay). Reversing these in a star-delta motor starter schematic will cause a dead short across the line.
- Form C (Break-Before-Make) vs. Make-Before-Break: A standard SPDT (Form C) symbol implies break-before-make. If the circuit requires make-before-break (to prevent an open-circuit spike during switching), the symbol will show the moving contact overlapping both stationary contacts. Standard off-the-shelf relays like the Omron G2R series are strictly break-before-make.
- AC vs. DC Coil Indicators: Failing to notice the sine wave (~) or solid line (—) inside the coil rectangle. Applying 24V AC to a 24V DC coil causes the armature to vibrate at 50/60Hz, generating excessive heat and destroying the coil insulation in minutes. Conversely, applying 24V DC to a 24V AC coil will result in a weak magnetic pull because the DC resistance of an AC coil is intentionally kept very low.
Faded or Missing Markings: How to Safely Identify Relay Pins
When working on legacy equipment, the silk-screen pinout on the relay socket is often burned off, and the OEM schematic is lost. You can safely reverse-engineer the pinout using a digital multimeter (DMM) and basic circuit theory.
Step 1: Identify and Verify the Coil
Set your DMM to the Ohms (Ω) range. Probe the suspected coil pins (usually A1/A2 or 13/14 on older sockets). A 12V DC coil typically reads between 100Ω and 200Ω. A 24V DC coil reads roughly 400Ω to 800Ω. If you are dealing with an AC coil, the DC resistance will read surprisingly low (often 20Ω to 50Ω for a 24V AC coil) because AC impedance relies heavily on inductive reactance, which a standard DMM cannot measure. If the meter reads OL (Open Line), the coil is burnt out.
Step 2: Map the Contacts (COM, NO, NC)
Set your DMM to Continuity mode. Probe the remaining pins. The pin that shows continuity to one pin at rest, and continuity to a different pin when you manually depress the relay armature with a non-conductive tool (like a plastic spudger), is your Common (COM) terminal. The pin showing continuity at rest is NC; the pin showing continuity when pressed is NO.
Decision Path: Which Relay and Symbol Standard to Use
Use this decision matrix to select the correct physical relay and ensure your schematic symbols match your hardware.
| Application Scenario | Required Symbol Standard | Concrete Hardware Pick |
|---|---|---|
| High-density PCB mounting (IoT, Arduino, custom boards) | IEC 60617 (Standard schematic capture) | Finder 40.52 (DPDT, 8A, PCB mount) or Omron G5V-2 |
| DIN-rail industrial control panels (PLC I/O isolation) | IEC 60617 (Tag-linked coils and contacts) | Omron G2R-14 (4PDT, 6A) with PYF14A-E socket |
| US Legacy Motor Control Centers (MCCs) | NEMA (CR designations) | Eaton C30CN or Square D Class 8502 control relays |
| Switching loads >10A or 3-phase motors | IEC 60617 (Use Contactor symbol KM, not relay K) | Schneider TeSys D (LC1D) or Eaton XT series contactors |
Default Recommendation: If you are designing a new general-purpose control panel or upgrading an existing one without strict legacy constraints, default to the IEC 60617 standard and use Omron G2R series relays on PYF sockets. They offer the best balance of global availability, clear physical pin markings, and reliable 6A to 10A switching capacity for standard PLC output modules.
