A relay symbol is a standardized schematic graphic representing an electromechanical or solid-state switch. The exact shape depends entirely on your region and the governing standard: IEC 60617 (global/Europe) uses rectangles for coils, while ANSI/IEEE 315 and NEMA (North America) traditionally use circles or rectangles. Regardless of the drawing style, the symbol always separates the coil (the input control circuit) from the contacts (the output switched circuit), linking them logically rather than physically.
Standard Relay Symbols: IEC vs ANSI vs NEMA Reference
Before wiring a control panel or debugging a PCB, you need to translate the schematic into physical pinouts. The table below maps the visual symbols to the physical terminal numbers you will find on standard industrial relays (like the ubiquitous Omron MY2N or Finder 38 series).
| Component | IEC 60617 Symbol | ANSI/IEEE 315 Symbol | Standard Pin Designations | Practical Meaning |
|---|---|---|---|---|
| Relay Coil | Rectangle (often with a diagonal line or 'K' label) | Circle or Rectangle (labeled 'CR' or 'K') | A1 (+/Line), A2 (-/Neutral) | The electromagnetic input. Energizing this pulls the mechanical armature. |
| NO Contact (Form A) | Gap with an angled bridging line | Similar gap, sometimes with a hinge dot | 13 (Common), 14 (NO) | Normally Open. Circuit is open until the coil is energized. |
| NC Contact (Form B) | Overlapping continuous lines with a break indicator | Overlapping lines | 11 (Common), 12 (NC) | Normally Closed. Circuit is closed until the coil is energized. |
| Changeover (Form C / SPDT) | Combination of NO and NC sharing a common pivot | Combination of NO and NC | 11 (Common), 12 (NC), 14 (NO) | Single Pole Double Throw. Switches a common feed between two paths. |
| Time-Delay (On-Delay) | Rectangle with an upward-pointing arrow or 'X' inside | Circle/Rectangle with specific timer notations | A1, A2, plus timed 15/16/18 | Contacts change state only after a preset delay once the coil is energized. |
Regional Standards: Which Schematic Applies to Your Bench?
Misinterpreting a schematic because you are applying the wrong regional standard is a fast track to a short circuit. Here is how to identify which rulebook your diagram follows.
IEC 60617 (International / Europe / Modern Global)
The IEC standard strictly separates the coil and the contacts. You will rarely see them drawn next to each other. Instead, the coil is labeled K1, and the contacts scattered throughout the diagram are also labeled K1 (or K1.1, K1.2). The coil is always a rectangle. If you are working on modern PLC-driven control panels, European machinery, or automotive systems, IEC is your default. For a deep dive into how these logical links work, the All About Circuits relay tutorial provides excellent foundational logic.
ANSI/IEEE 315 & NEMA (North America / Legacy Industrial)
North American schematics, particularly older NEMA motor control centers, often use a circle for the relay coil, though modern ANSI drafts frequently adopt the IEC rectangle. The designator letter is typically CR (Control Relay) or M (Motor Starter). NEMA diagrams tend to draw the coil and its associated contacts in closer physical proximity on the page, sometimes linked by dashed mechanical lines, which makes tracing easier for beginners but clutters complex diagrams.
Never assume a circle is a relay coil if you are reading an IEC diagram. In IEC 60617, a circle typically represents a motor, a rotating machine, or a generic signal lamp. Always check the title block for the governing standard (e.g., 'Drawn to IEC 60617') and verify the component designator letter (K vs CR vs M).
Rows People Get Wrong: Common Symbol & Pinout Mistakes
Even experienced makers trip over specific nuances in relay schematics and physical pin mappings. Here are the most common errors and how to avoid them.
Mistake 1: Misreading IEC Contact Numbers
On a physical IEC relay, the pins are numbered with two digits (e.g., 11, 12, 14). People often assume these are sequential pin numbers like an IC chip. They are not.
The Rule: The first digit indicates the contact set number (1 = first set, 2 = second set). The second digit indicates the function:
• 1 and 2 = Normally Closed (NC) path.
• 1 and 4 = Normally Open (NO) path.
Therefore, pins 11 and 12 are your NC pair, while 11 and 14 are your NO pair. Pin 21 and 22 would be the NC pair for the second pole.
Mistake 2: Confusing Solid State Relays (SSR) with Electromechanical (EMR)
Schematics for Solid State Relays often look identical to EMR coils, but the physical behavior is vastly different. An SSR symbol might include a small box with a diode or optocoupler symbol inside. Physically, an SSR input (pins 3 and 4 on standard pucks) is an LED, not a wire coil. If you apply 120V AC to the input pins of a 3-32V DC SSR because you misread the schematic, you will instantly vaporize the internal LED. Always check the manufacturer's relay terminology and spec sheet to confirm if the device is solid-state before applying power.
Safe Interpretation When Markings Are Faded or Missing
On the jobsite or when salvaging parts, you will frequently encounter relays where the silk-screened pinout diagram on the side has burned off, faded, or is covered in grime. Never guess the pinout based on physical pin placement alone, as manufacturers vary the layout (e.g., octal 8-pin vs. 14-pin rectangular). Use this multimeter decision path to safely identify the pins.
- De-energize and Isolate: Remove the relay from the circuit. Applying a multimeter to a live relay will blow your meter's internal fuse or give false readings due to parallel circuit paths.
- Find the Coil (A1/A2): Set your multimeter to Ohms (Ω). Probe pairs of pins. You are looking for a specific resistance. A 12V DC coil typically reads between 120Ω and 150Ω. A 24V DC coil reads around 600Ω. A 120V AC coil will read much higher, often between 2kΩ and 4kΩ. The two pins that show this resistance are your coil.
- Identify the Common (11): Switch your meter to Continuity mode (the diode/beep setting). Probe the remaining pins. The 'Common' pin will show continuity (a beep / near 0.00Ω) to the NC pin, and an Open Line (OL / no beep) to the NO pin.
- Verify the Throw: If you find a common pin that beeps to one pin and reads OL to another, you have a Form C (SPDT) contact. If it only beeps to one pin and all other pins read OL, it is a Form A (SPST-NO) or Form B (SPST-NC) relay.
Relay Symbol FAQ
What does the rectangle with a diagonal line mean in a relay symbol?
In the IEC 60617 standard, a plain rectangle represents a generic relay coil. When a diagonal line is drawn through the rectangle, it specifically denotes a slow-operating or time-delay relay. If the diagonal line has a small arrowhead or an 'X' inside the box, it indicates a specific timing function, such as an on-delay or off-delay. In older schematics, a rectangle with a diagonal line might also represent a latching relay (one that stays in its last state after power is removed).
How do I identify a time-delay relay symbol on a schematic?
Look for modifiers attached to the standard coil or contact symbols. In IEC diagrams, an on-delay contact (which closes only after a set time once the coil is energized) is drawn as a standard NO contact with a small left-pointing arrow or a bracket pointing away from the contact gap. An off-delay contact features an arrow pointing toward the gap. In ANSI/NEMA diagrams, time-delay relays are often drawn with a small clock face icon or a specific 'TD' (Time Delay) prefix next to the CR designator (e.g., 1TD-CR).
Why are the relay coil and contacts drawn in different parts of the circuit?
This is a fundamental principle of logical vs. physical schematic design. Schematics are drawn to show the logical flow of current, not the physical layout of the components. The coil belongs to the control circuit (often drawn on the left side or top of a ladder diagram), while the contacts belong to the load or logic circuits (drawn on the right side or distributed across multiple rungs). Drawing them together would create a tangled, unreadable mess of crossing wires. They are linked purely by their alphanumeric designator tag (e.g., K1, CR2), which tells the electrician that energizing coil K1 on rung 1 will close the K1 contact on rung 4.
