An electronic relay symbol represents an electromechanical or solid-state switch controlled by a low-power input, isolating the control circuit from a high-power load. Because global manufacturing blends components from different regions, reading these symbols requires fluency in both the dominant international standard and legacy North American formats.
The Complete Electronic Relay Symbol Reference Table
The table below maps the most common relay representations you will encounter on schematics, panel layouts, and PCB silkscreens. Use this as your primary bench reference.
| Component / Function | IEC 60617 Symbol Style | ANSI/IEEE 315 Symbol Style | Practical Meaning & Bench Notes |
|---|---|---|---|
| Relay Coil | Rectangle (often with 'K' or 'CR' designation) | Circle or semi-circle with two connection lines | The electromagnetic actuator. In IEC, the rectangle is universal for coils, inductors, and solenoids; context and the 'K' prefix confirm it is a relay. Typical DC coil resistances range from 15Ω (5V) to 650Ω (24V). |
| SPST-NO (Normally Open) | Two parallel lines with a diagonal line bridging them upon actuation | Switch blade drawn open, often with a dotted line linking to the coil | Single Pole Single Throw, Normally Open. Circuit is open until the coil energizes. Commonly used for motor start circuits or load switching. |
| SPST-NC (Normally Closed) | Two parallel lines with a diagonal line crossing them, breaking upon actuation | Switch blade drawn closed, breaking when coil energizes | Single Pole Single Throw, Normally Closed. Circuit passes current until the coil pulls in. Used for safety interlocks and stop circuits. |
| SPDT (Changeover) | Three terminals; moving contact bridges one of two stationary contacts | Single blade switching between two fixed points | Single Pole Double Throw. The 'Common' (COM) pin switches between NO and NC. Standard on 8-pin relays (e.g., Finder 40.52). |
| DPDT (Double Pole) | Two SPDT symbols stacked, linked by a dashed mechanical tie line | Two switch blades mechanically ganged together | Two independent SPDT switches actuated by one coil. Standard 8-pin layout: Pins 1-8 (Pole 1), Pins 4-5 (Pole 2), Pins 2-7 (Coil). |
| Solid State Relay (SSR) | Rectangle with an internal optocoupler symbol (LED and phototransistor) and a triac/thyristor output | Similar opto-isolator block, sometimes with a diamond or specific SSR block outline | No moving parts. Input is typically 3-32VDC (LED), output switches AC or DC via semiconductor. Look for zero-crossing indicators on AC output SSRs. |
| Time-Delay (On-Delay) | Standard coil rectangle with a small 'X' or crosshatch block indicating the delay mechanism | Circle with a clock face or specific delay arc symbols on the contacts | Contacts change state only after a preset time once the coil is energized. The 'X' in IEC denotes the pneumatic or electronic dashpot mechanism. |
Regional Standards: IEC 60617 vs. ANSI/IEEE 315 vs. Old UK
When troubleshooting a machine imported from Germany or maintaining a 1980s American control panel, the electronic relay symbol you see will shift dramatically based on the drafting standard used. Knowing which standard applies to your region prevents dangerous miswiring.
IEC 60617 (International / Modern Global)
Used across Europe, Asia, and modern global OEM designs. IEC favors simple geometric shapes. The coil is always a rectangle. Contacts are drawn as simple lines. The mechanical link between the coil and its contacts is usually implied by the component designator (e.g., K1 for the coil, K1.1, K1.2 for the contacts) rather than drawn with dashed lines. This keeps schematics clean but requires you to cross-reference designators.
ANSI/IEEE 315 & NEMA (North America / Legacy Industrial)
Dominant in the US and Canada, especially in older industrial plants and NEMA-rated motor control centers. ANSI uses circles or semi-circles for coils. Crucially, ANSI often draws a dashed mechanical line directly connecting the coil to its associated contacts on the page. Contacts look more like physical switch blades. If you are working on a US-built CNC machine or HVAC system from before 2010, expect this format.
Old UK (BS 3939)
Largely superseded by IEC 60617 in the UK, but you will still find BS 3939 drawings in older British infrastructure, marine panels, and legacy railway systems. It shares similarities with early ANSI but uses unique contact annotations (like 'make' and 'break' text labels instead of purely graphical representations). If you encounter a schematic with heavy text annotations inside the switch symbols, suspect BS 3939.
Rows People Get Wrong & Faded Marking Protocols
Schematics are only half the battle. On the bench, you are dealing with physical components where silkscreen fades, plastic melts, and pinouts get confusing.
The Most Common Schematic Misreads
- Confusing the Coil with an Inductor: In IEC, both are rectangles. If the rectangle is labeled 'L' (e.g., L1), it is an inductor or choke. If it is labeled 'K' or 'CR' (Control Relay), it is a relay coil. Always check the designator prefix.
- SPDT Common Terminal Misidentification: On a standard 14-pin octal relay (like the Omron LY4), beginners often wire the load to the wrong pole. The common pins are 9, 10, 11, and 12. The NO pins are 1, 2, 3, 4. The NC pins are 5, 6, 7, 8. The coil is always on pins 13 (+) and 14 (-). Wiring a load to pin 13 will feed your load voltage directly into your control logic, likely destroying your PLC output card.
- Assuming Dashed Lines Mean 'Delayed': In ANSI schematics, a dashed line linking a coil to a contact just means 'mechanically operated by'. A dashed line drawn parallel to the contact blade, however, indicates a time-delay function.
Safe Interpretation When Markings are Faded or Missing
Industrial environments destroy relay labels. When you pull a dust-caked, 15-year-old relay from a DIN rail and the pinout diagram is gone, follow this protocol:
- Identify the Coil: Set your multimeter to resistance (Ω). Probe the pins. On a standard 24VDC relay (like an Omron G2R-1), you will read approximately 650Ω across the coil pins. AC coils will read much lower (often 10Ω to 50Ω depending on VA rating). Pins showing infinite resistance (OL) or a dead short (0.0Ω) are not the coil.
- Map the Poles: Switch your meter to continuity/diode mode. Find the Common (COM) pin by locating the pin that shows continuity to one other pin (the NC contact) in the unenergized state.
- Verify Actuation: If it is a sealed relay with a manual test lever, use a non-conductive plastic spudger to press the lever. The continuity should shift from the NC pin to the NO pin. If it's a solid-state relay (SSR), you cannot test it this way; you must apply the rated DC input voltage (e.g., 5V) and measure the AC output side under load.
For authoritative guidance on physical relay testing and internal construction, refer to the All About Circuits relay tutorial or manufacturer datasheets from Electronics Tutorials.
Frequently Asked Questions
What is the schematic symbol for a solid state relay (SSR)?
The electronic relay symbol for an SSR typically features a rectangle divided into two halves. The input side shows an LED symbol (indicating the optocoupler input), and the output side shows a triac, thyristor, or MOSFET symbol depending on whether it switches AC or DC. A crucial detail often missed is the 'zero-crossing' indicator—a small sine wave with a line through it—found on AC output SSRs, which tells the designer the relay will only switch on when the AC waveform crosses 0V to minimize inrush current and EMI.
How do you represent a time-delay relay on an electrical drawing?
In IEC 60617, a time-delay relay coil looks like a standard rectangle but includes a small black crosshatch or 'X' block inside or adjacent to it, representing the delay mechanism (pneumatic dashpot or electronic timer). The contacts will have a specific arrow indicating the direction of the delay. An arrow pointing toward the contact line means 'on-delay' (delays closing after energization), while an arrow pointing away means 'off-delay' (delays opening after de-energization).
Why does my relay symbol show a diode across the coil?
That is a flyback diode (or freewheeling diode), and it is critical for DC relay circuits. When the control circuit cuts power to the relay coil, the collapsing magnetic field generates a massive reverse voltage spike (inductive kickback) that can easily exceed 100V, instantly destroying the driving transistor or PLC output. The symbol shows a diode (like a 1N4007) wired in reverse bias across the coil to safely clamp and dissipate this spike. If you see this symbol on a schematic, ensure the physical diode is installed with the cathode (stripe) facing the positive supply.
What does a latching relay symbol look like compared to a standard relay?
A latching (or bistable) relay maintains its contact position even after power is removed from the coil. On schematics, the coil symbol is often drawn with a small mechanical latch symbol (looking like a tiny hook or a cross inside a box) adjacent to it. Alternatively, it may show two separate coils: one labeled 'Set' (S) and one labeled 'Reset' (R). In IEC formats, you will frequently see a permanent magnet symbol integrated into the coil rectangle to indicate magnetic latching.
