An electromagnetic relay symbol on a schematic dictates the coil voltage, contact arrangement (SPST, SPDT, DPDT), and pinout mapping. Misreading these symbols or assuming a universal pinout standard is the fastest way to short a control board or wire a safety interlock backward. Below is the definitive bench reference for identifying, interpreting, and testing relay symbols across global standards.
The Complete Electromagnetic Relay Symbol Reference Table
The following table maps the standard schematic symbols to their physical contact configurations. According to All About Circuits, the 'Form' letter designation (A, B, C) is the most critical shorthand for ordering replacements.
| Symbol / Designation | Schematic Representation | Contact Type & Behavior | Typical Pin Count | Common Part Example |
|---|---|---|---|---|
| SPST-NO (Form A) | Switch blade open, angled away from contact point | Normally Open. Closes when coil is energized. | 4 or 5 pins | Omron G5LE-1 (SPST-NO) |
| SPST-NC (Form B) | Switch blade closed, resting on contact point | Normally Closed. Opens when coil is energized. | 4 or 5 pins | Finder 40.51 (SPST-NC) |
| SPDT (Form C) | Single blade centered between two contact points | Changeover. Common pin switches between NO and NC. | 5 pins | Omron G5LE-14 (SPDT) |
| DPDT (Form C, Dual) | Two SPDT symbols linked by a dashed mechanical line | Two isolated changeover circuits operated by one coil. | 8 pins (Octal/PCB) | Omron LY2 / MY2 |
| Standard Coil | Rectangle (IEC) or Circle (Legacy ANSI) labeled 'CR' or 'K' | The electromagnetic winding. Polarity insensitive (DC). | 2 pins | N/A (Part of relay) |
| Latching Coil (Bistable) | Rectangle with a diagonal arrow or two opposing coils | Requires a pulse to set, and a reverse pulse to reset. | 3 to 6 pins | Omron G6CK (Latching) |
Regional Standard Variants: IEC 60617 vs. ANSI/IEEE 315
Relay symbols are not globally uniform. The schematic you are reading depends heavily on the region and the era of the drafting standard. Always check the title block of the schematic to confirm which standard applies before tracing pins.
IEC 60617 (Europe, Global, Modern US)
The International Electrotechnical Commission (IEC) standard is the modern global baseline. Under IEC 60617, the relay coil is drawn as a simple rectangle. The contacts are drawn entirely separately from the coil, linked only by a dashed line or a shared alphanumeric tag (e.g., coil 'K1' operates contacts 'K1.1' and 'K1.2'). This modular approach keeps complex schematics clean but requires the reader to cross-reference tags.
ANSI/IEEE 315 (North America, Industrial)
The IEEE 315 standard (which absorbed older ANSI Y32.2 standards) historically used a circle for the relay coil, though modern revisions accept the IEC rectangle. ANSI schematics often group the coil and its associated contacts physically closer together on the page. The switch blades in ANSI use specific hatch marks: a perpendicular line on the blade indicates a Normally Open (NO) contact, while a lack of a hatch or a specific dot indicates Normally Closed (NC).
Old UK BS 3939 (Legacy British Panels)
If you are troubleshooting equipment built in the UK before the late 1980s, you will encounter BS 3939. The coil was represented by a circle with a radial line extending from the center to the edge. While officially withdrawn in favor of IEC standards, these symbols still live on in the schematics of legacy manufacturing machinery and marine switchgear.
The "Rows People Get Wrong" and Faded Marking Protocols
Even with a perfect schematic, physical relays on a bench or in a panel present unique hazards. Here are the most common misinterpretations and the protocol for handling unmarked hardware.
1. Confusing DPDT with Two SPST Relays
A DPDT symbol shows two switch blades mechanically linked. Novices often misread this as two independent SPST-NO relays. In practice, a DPDT relay (like an 8-pin octal) has a shared magnetic armature. If you wire one pole for 120VAC and the other for 24VDC, you must verify the dielectric isolation rating between the two contact sets in the datasheet. Many standard 8-pin relays only offer 1.5kV isolation between poles; exceeding this can cause arc flashovers between the low-voltage control side and the mains side.
2. Assuming Universal Coil Pinouts
Never assume the coil pins based on physical pin count alone.
- 8-Pin Octal (e.g., Omron LY2): Coil is almost always pins 2 and 7.
- 14-Pin (e.g., Omron LY4): Coil is pins 7 and 14.
- 5-Pin PCB (e.g., Omron G5LE): Coil is pins 2 and 5.
3. Safe Interpretation of Faded or Missing Markings
When a relay's silkscreen is burned off or faded, never apply mains voltage to guess the pinout. Use this bench-safe multimeter protocol:
- Set your meter to Ohms (Ω), not continuity. A 12VDC relay coil typically reads between 120Ω and 400Ω. The continuity beeper often will not trigger at these resistances, leading you to falsely conclude the coil is open.
- Locate the Coil. Probe pairs of pins until you find the two that show a stable resistance in the 50Ω–10kΩ range. All other pins should read 'OL' (Open Loop) against these coil pins.
- Map the Contacts. Set the meter to continuity (or low-ohms). Probe the remaining pins to find the Common (C) pin. The pin that shows ~0.0Ω to the Common pin when the relay is at rest is your Normally Closed (NC). The pin that reads 'OL' to the Common pin is your Normally Open (NO).
- Verify Mechanically. If safe to do so, apply the rated DC voltage to the coil pins via a current-limited bench supply. You should hear the mechanical 'click', and the NO pin should now show ~0.0Ω to the Common pin.
Frequently Asked Questions
What does the dashed line mean in an electromagnetic relay symbol?
The dashed line (or dotted line) connecting the coil symbol to the switch contacts represents the mechanical linkage. It indicates that the magnetic field generated by the coil physically moves the armature to actuate those specific contacts. In IEC schematics, if the dashed line is omitted, the coil and contacts will share the same alphanumeric designator (e.g., K1) to denote the mechanical relationship.
How do I identify the coil pins on an unmarked 8-pin octal relay?
On a standard 8-pin octal relay (like the widely used Omron MY2 or Finder 55.34 series), the coil pins are almost universally Pin 2 and Pin 7. The remaining pins are split into two isolated Form C (SPDT) contact sets: Set 1 uses pins 1 (NC), 4 (Common), and 8 (NO); Set 2 uses pins 3 (NC), 5 (Common), and 6 (NO). Always verify with a multimeter if the markings are completely illegible.
Why are there two coil symbols for some electromagnetic relays?
If a schematic shows two distinct coil symbols associated with the same relay tag, it represents a latching (bistable) relay. One coil is the 'Set' coil (often pulsed positive) and the other is the 'Reset' coil (pulsed negative or via a separate circuit). Unlike standard monostable relays that spring-return when power is removed, latching relays maintain their last position indefinitely without continuous power, making them ideal for battery-backed memory circuits or power-fail safety interlocks.
What is the difference between a standard relay symbol and a solid-state relay (SSR) symbol?
An electromagnetic relay symbol uses mechanical switch blades. A Solid-State Relay (SSR) symbol, governed by the same IEC/ANSI standards, replaces the mechanical blade with a semiconductor symbol (usually a thyristor, TRIAC, or MOSFET pair) and includes an optocoupler representation (two opposing arrows or a light-source/detector box) to show the galvanic isolation between the low-voltage control input and the high-voltage output. SSRs have no moving parts, meaning they lack the 'click' and mechanical bounce of electromagnetic relays, but they require heat sinking due to semiconductor voltage drop.
