Reading a relay schematic symbol correctly is the difference between a fast troubleshooting session and chasing ghosts through a control panel. Whether you are wiring a 24VDC PLC output or diagnosing a 480VAC motor starter, the symbols on the page dictate your physical wiring. Below is the definitive reference for relay coils, contacts, and timing variations across global standards.
Master Relay Schematic Symbol & Pinout Table
The table below maps the most common relay schematic symbols to their physical pinouts. Pin mappings reflect the standard 8-pin (octal) or 11-pin plug-in formats (like the Omron MY2/MY3 or Finder 60.13 series), as well as standard PCB relay numbering.
| Component / Function | IEC 60617 Symbol Description | ANSI/IEEE 315 Symbol Description | Standard Pin Mapping (IEC 2-Digit) |
|---|---|---|---|
| Relay Coil | Rectangle (often labeled K, KA, or KM) | Circle or Rectangle (often labeled CR or M) | A1 (+/L), A2 (-/N) |
| SPST-NO (Form A) | Angled line resting below a horizontal gap | Angled line resting below a horizontal gap | 13 (COM), 14 (NO) |
| SPST-NC (Form B) | Angled line crossing over a horizontal gap | Angled line crossing over a horizontal gap | 11 (COM), 12 (NC) |
| SPDT (Form C / Changeover) | Break-before-make switch (COM connected to NC, angled toward NO) | Similar break-before-make representation | 11 (COM), 12 (NC), 14 (NO) |
| Time-Delay (On-Delay) | Coil rectangle with an upward-pointing arrow inside or beside it | Coil circle/rectangle with a specific timer hash mark | A1, A2 (Contacts follow standard 11/12/14) |
| Time-Delay (Off-Delay) | Coil rectangle with a downward-pointing arrow | Coil circle/rectangle with alternate timer hash mark | A1, A2 (Contacts follow standard 11/12/14) |
| Latching / Magnetic | Rectangle with two separate coils or a mechanical latch hash mark | Circle with a mechanical latch indicator | Set Coil (A1/A2), Reset Coil (A3/A4) |
Regional Standard Variants: Which One Applies to You?
A schematic symbol is only useful if you know which drafting standard the engineer used. Mixing up IEC and ANSI conventions can lead to miswiring a normally-open contact as normally-closed.
IEC 60617 (International, Europe, UK, Australia)
The IEC 60617 standard dominates global industrial automation. It uses a rectangle for the relay coil and a strict two-digit numbering system for contacts. The first digit indicates the contact sequence (1, 2, 3 for multiple poles), and the second digit indicates the function (1/2 for NC, 3/4 for NO, 1/2/4 for changeover). If you are working on modern Siemens, ABB, or Schneider panels, you will use IEC.
ANSI/IEEE 315 & NEMA (North America)
In the US and Canada, ANSI/IEEE 315 and NEMA standards are prevalent, especially in older motor control centers (MCCs) and HVAC systems. ANSI typically represents the relay coil as a circle (labeled 'CR' for Control Relay or 'M' for Motor starter). Contact numbering is often sequential (1 through 8) rather than functional, meaning you must rely on the physical relay's datasheet rather than the schematic numbers to determine NO/NC states.
Legacy UK BS 3939 (Pre-1980s)
If you are retrofitting a facility in the UK built before the 1980s, you may encounter the withdrawn BS 3939 standard. It used a distinctive 'semi-circle' for coils and different cross-hatching for contacts. Treat these diagrams with extreme caution; trace every wire physically, as the symbols do not map cleanly to modern IEC conventions.
Rows and Symbols People Get Wrong
Even experienced technicians misread specific schematic variations. Here are the most common pitfalls on the bench:
In IEC schematics, an arrow pointing up (away from the contact line) indicates an On-Delay (contacts change state after the coil is energized). An arrow pointing down (toward the contact line) indicates an Off-Delay (contacts change state after the coil is de-energized). Swapping these in a safety interlock circuit can cause machinery to start unexpectedly during power-loss events.
The 'Form C' Break-Before-Make Assumption: A standard SPDT (Changeover) relay schematic symbol implies a 'break-before-make' sequence. The moving contact breaks from the NC pin before touching the NO pin. If your circuit requires a 'make-before-break' transition to avoid dropping a signal or shorting two power supplies, the standard schematic symbol will not warn you. You must check the manufacturer datasheet (e.g., specifying a specialized telecom relay) to confirm the physical wiping action.
Confusing the Coil with the Contact: Beginners often trace the line from the load directly into the coil rectangle. Remember: the coil (A1/A2) is the input (the electromagnet). The contacts (11/12/14) are the output (the switch). They are electrically isolated from each other. If a schematic shows line voltage entering a rectangle and exiting to a motor, it is a contactor, not a relay coil.
Safe Interpretation When Markings Are Faded or Missing
In harsh industrial environments, relay stickers peel off, plastic housings become brittle, and schematic binders go missing. When you are staring at an unmarked 8-pin octal relay (like a generic 24VDC ice-cube relay), do not guess the pinout based on the physical orientation of the pins.
Step 1: Identify the Coil Pins via Resistance
Set your multimeter (e.g., Fluke 87V) to the Ohms (Ω) setting. Probe pairs of pins until you find a resistance reading that makes sense for a coil.
- 12VDC Coil: Typically reads between 150Ω and 400Ω.
- 24VDC Coil: Typically reads between 600Ω and 1,500Ω.
- 120VAC / 240VAC Coil: Reads much higher, often 3,000Ω to 15,000Ω.
If the meter reads 'OL' (Open Loop) across all combinations, the internal coil wire is broken, and the relay is dead. If it reads near 0Ω, it is internally shorted.
Step 2: Map the Contacts via Continuity
Switch your meter to Continuity mode (the diode/beep setting). With the relay unpowered (de-energized), find the pin that shows continuity to two other pins. That common pin is your COM (e.g., Pin 11). The pin that beeps is your NC (Pin 12). The pin that does not beep is your NO (Pin 14). Press the relay's manual test button (if equipped) to verify the COM switches continuity to the NO pin.
Relay Schematic Symbol FAQ
What does the diagonal line or box in a relay schematic symbol mean?
A rectangle with a diagonal line through it, or a box containing a diode/transistor symbol, represents a Solid State Relay (SSR). Unlike electromechanical relays (EMRs) which use a physical coil and moving contacts, SSRs use an optical isolator (optocoupler) and a semiconductor switch (like a TRIAC for AC loads or a MOSFET for DC loads). On a schematic, the input side (pins 3/4 or A1/A2) will often show an LED symbol, while the output side (pins 1/2) will show a TRIAC or transistor symbol. SSRs have no moving parts, meaning they do not suffer from contact bounce or mechanical wear, but they do require heat sinking for loads above 5A.
How do I identify a double-throw (Form C) relay symbol on a diagram?
A Form C (SPDT) relay symbol is drawn as a single-pole switch that rests against one contact and points toward another. In the IEC standard, you will see a continuous line (the pole/COM) that connects to a downward-angled line (NC), with a gap to an upward-angled line (NO). The key identifier is the three connection points on the contact side. If you see only two connection points, it is a Form A (NO) or Form B (NC) relay. Always verify if the symbol includes a 'break-before-make' mechanical link line connecting the two stationary contacts.
Why do some relay schematic symbols show two separate coils?
If a schematic shows a single contact block but two distinct coil rectangles (often labeled 'Set' and 'Reset', or 'CW' and 'CCW'), you are looking at a Latching Relay (also known as a bistable or magnetic latching relay). Examples include the Finder 26-series or heavy-duty latching contactors used in power metering. These relays require only a momentary pulse of voltage to change state, and they remain in that state even when power is completely removed. One coil pulses to pull the mechanical latch closed; the other coil pulses to release it. This is highly common in battery-powered systems or fail-safe lighting circuits where continuous coil power would drain the battery or generate excess heat.
