The standard electrical symbol for a relay consists of two distinct parts: a rectangle or circle representing the electromagnetic coil, and a set of switch contacts (Normally Open or Normally Closed) linked by a dashed mechanical line. In North America, NEMA/ANSI standards (IEEE 315) dominate industrial schematics, while the rest of the world relies on IEC 60617. Knowing which standard your schematic uses prevents costly wiring errors and blown fuses when replacing common components like an Omron G2R, Schneider Zelio RXM, or Finder 38 series relay.
The Complete Relay Symbol Reference Table
The table below maps the most common relay symbols to their physical counterparts. Use this as a bench reference when tracing faults or designing control panels.
| Component | NEMA / ANSI (US) Symbol Description | IEC 60617 (International) Symbol Description | Practical Application & Bench Notes |
|---|---|---|---|
| Relay Coil | Circle with 'CR' (Control Relay) or a rectangle with a diagonal line. | Rectangle with 'K' or 'KA' inside. Terminals marked A1 and A2. | The load that actuates the relay. Always check the voltage rating (e.g., 24VDC vs 120VAC) before applying power. |
| Normally Open (NO) Contact | Two parallel lines with a bridging line angled away (like an open knife switch). | Two parallel lines with a bridging line angled away. Terminals 13 and 14 (or NO/COM). | Closes when the coil is energized. Used for start circuits and holding seals. |
| Normally Closed (NC) Contact | Two parallel lines with a bridging line crossing them (closed knife switch). | Two parallel lines with a bridging line crossing them. Terminals 11 and 12 (or NC/COM). | Opens when the coil is energized. Used for interlocks, stop buttons, and safety limits. |
| Changeover (Form C) | Combination of NO and NC sharing a common pivot point. | Combination of NO and NC sharing a common pivot. Terminals 11, 12, 14. | Single pole, double throw (SPDT). Common in reversing motor circuits and signal routing. |
| Time-Delay ON (NOTC) | NO contact with an arrow pointing INWARD toward the hinge. | NO contact with an inward arrow or a small 'on-delay' box symbol. | Delay-on-energize. Contact closes X seconds AFTER the coil receives power. |
| Time-Delay OFF (NCTO) | NC contact with an arrow pointing OUTWARD away from the hinge. | NC contact with an outward arrow or 'off-delay' box symbol. | Delay-on-de-energize. Contact stays closed for X seconds AFTER coil power is removed. |
| Latching Relay Coil | Standard coil symbol with a small mechanical latch or 'L' designation. | Rectangle with a mechanical latch symbol inside or dual coils (Set/Reset). | Maintains state without continuous power. Requires a pulse to set and a pulse to reset. |
Regional Standards & Variants: NEMA vs. IEC
One of the most frequent causes of miswiring on the bench is assuming a schematic follows the local standard when it was actually drafted overseas. The visual language of relays differs significantly between the two dominant frameworks.
NEMA / ANSI (North America): Heavily relies on literal, pictographic representations. A relay coil looks like a circle or a box with a diagonal stripe, and contacts look like physical knife switches. Device designators are usually functional: CR for Control Relay, M for Motor Starter, TR for Timing Relay. You will see this in older US manufacturing plants, Allen-Bradley legacy panels, and HVAC control boards.
IEC 60617 (International / Modern Global): Uses abstract, geometric shapes. The coil is always a simple rectangle, and the internal letter designates the function (e.g., K for a standard relay, KA for an auxiliary relay, KM for a contactor). Contacts are drawn as simple parallel lines with a diagonal wiper. IEC schematics separate the coil and contacts entirely on the page, linking them only by an alphanumeric tag (e.g., Coil K1 is linked to Contacts K1-13/14 and K1-11/12). This is the standard for modern PLC I/O diagrams, Siemens panels, and European machinery.
Rows People Get Wrong & Faded Marking Protocols
The Most Misinterpreted Symbols
The biggest trap for junior technicians and hobbyists is the Time-Delay arrow direction. A common mistake is wiring an off-delay circuit when an on-delay was specified, causing a motor to start immediately and coast to a stop, rather than ramping up. Remember the hinge rule: if the arrow points toward the hinge (inward), it restricts the contact from moving immediately upon energization (On-Delay). If it points away from the hinge (outward), it restricts the contact from returning immediately upon de-energization (Off-Delay).
Another frequent error is confusing a Form C (Changeover) symbol with two independent contacts. A Form C contact shares a single common terminal (the pivot point). If you wire the NO and NC loads to separate power sources without isolating the common, you will create a dead short when the relay switches states.
Safe Interpretation of Faded or Missing Markings
On older machinery, relay labels fade, or the physical schematic is lost. If you have an unmarked 8-pin octal or 14-pin blade relay pulled from a live panel, do not guess the pinout. Follow this bench protocol:
- De-energize and LOTO: Lock out the panel. Never test continuity on a live circuit; you will blow your multimeter's internal fuse or destroy the meter's ADC.
- Identify the Coil: Set your multimeter to Ohms (Ω). Probe the suspected coil pins. For a standard 24VDC relay (like an Omron MY2N), you should read between 100Ω and 800Ω. For a 120VAC coil, expect 2,000Ω to 10,000Ω. If you read 0.0Ω, the coil is shorted. If you read OL (Open Loop), the internal winding is burned out.
- Map the Contacts: Set the meter to Continuity (the diode/beep setting). Probe the remaining pins. You should find a Common (COM) pin that shows continuity (< 0.5Ω) to one pin (the NC contact) and OL to another (the NO contact).
- Verify the Throw: If the relay has a manual test button (common on Schneider and Finder models), press it while monitoring the meter. The continuity should shift from the NC pin to the NO pin, confirming the mechanical throw.
Frequently Asked Questions
What is the electrical symbol for a relay coil vs contact?
The coil symbol represents the input (the electromagnet that consumes power to create a magnetic field), while the contact symbol represents the output (the physical switch that routes the load current). On a schematic, they are linked by a dashed line in NEMA diagrams, or by a shared alphanumeric tag (like 'K1') in IEC diagrams. The coil is always placed in the control circuit (left side of the ladder logic), while the contacts are placed in the logic or power rungs (right side).
How do you read a time-delay relay symbol on a schematic?
Look at the base contact type (NO or NC) and the direction of the arrowhead. An NO contact with an inward-pointing arrow is an On-Delay (NOTC): it stays open when powered, then closes after the set time. An NC contact with an outward-pointing arrow is an Off-Delay (NCTO): it opens immediately when powered, but delays returning to the closed state when power is removed. Always check the margin notes for the specific time setting (e.g., 'T=5.0s').
What does a solid state relay (SSR) symbol look like compared to electromechanical?
A Solid State Relay (SSR) symbol replaces the mechanical coil and switch with semiconductor symbols. The input side is drawn as an LED or diode symbol (representing the internal optocoupler), while the output side is drawn as a TRIAC (for AC loads) or a MOSFET (for DC loads) with a light arrow striking the gate. Unlike electromechanical symbols, SSR symbols emphasize the electrical isolation barrier between the control voltage (e.g., 3-32VDC) and the load voltage (e.g., 24-280VAC). For deeper reference on component symbols, consult the Electrical Engineering Portal symbol database or the All About Circuits reference textbook.
