Relay Symbol Guide: IEC, ANSI & NEMA Schematic Standards Explained

When reading or drafting schematics, the symbol of a relay changes depending on whether you are following IEC (global/Europe), ANSI/IEEE (North America), or legacy BS (UK) standards. While the coil and contact representations differ visually, the underlying logic—specifically the de-energized state representation—remains universal. Below is the complete reference matrix to decode any schematic you encounter.

Standard Relay Schematic Symbols Reference

This table maps the most common relay components across the three major drafting standards. Use this to cross-reference legacy prints with modern CAD outputs.

The Rows People Get Wrong (Common Schematic Mistakes)

Misinterpreting a relay symbol usually results in a dead short or a machine that fails to start. Watch out for these three specific errors:

• The 'Energized' vs 'De-energized' State: Schematics always show relay contacts in their de-energized (shelf) state. A common mistake among junior drafters is drawing the contacts in the state they will be in when the machine is running. If a contact is drawn as NO on the schematic, it is physically open when the relay has zero power applied to the coil.
• Coil vs Inductor Confusion: In ANSI/IEEE standards, a relay coil (semicircles) looks nearly identical to an inductor. The distinguishing factor is the IEC rectangular box, or in ANSI prints, the presence of a dashed mechanical link line connecting the coil to a separate switch symbol. If there is no link line, it is a passive inductor, not a relay.
• DPDT vs Two Separate SPDTs: If you see two SPDT switches on a schematic, they are independent. To represent a single DPDT relay, the two poles must be connected by a mechanical link line (dashed in IEC, solid in ANSI). Missing this line tells the panel builder to wire two separate relays, doubling your cost and panel space.

Safe Interpretation When Markings are Faded or Missing

Physical relays in industrial control panels—especially older octal 'ice cube' styles—often have faded silkscreens after years of heat cycling. Guessing the pinout will destroy your control board.

Step 1: Identify the Coil Pins
Set your digital multimeter (e.g., Fluke 87V) to the Ohms (Ω) range. Probe pairs of pins. You are looking for specific resistance ranges based on the coil voltage:

• 5VDC Coil: ~50Ω to 70Ω
• 12VDC Coil: ~150Ω to 275Ω
• 24VDC Coil: ~600Ω to 1,100Ω
• 120VAC Coil: ~4,000Ω to 10,000Ω

Note: If you read 0.1Ω to 0.5Ω, you are measuring across a closed contact, not the coil.

Step 2: Map the Contacts (Using the 8-Pin DPDT Standard)
Most unmarked 8-pin DPDT relays follow the industry-standard octal pinout. Once you find the coil (typically Pins 7 and 8), set your meter to Continuity mode (the diode/beep setting).

• Pole 1: Probe Pin 2 (Common) against Pin 1 and Pin 3. The pin that beeps without power applied is your Normally Closed (NC) contact (usually Pin 1). The pin that does not beep is Normally Open (NO) (usually Pin 3).
• Pole 2: Probe Pin 5 (Common) against Pin 4 (NC) and Pin 6 (NO).

Regional Standard Variants: Which Applies to You?

Your physical location and the origin of the equipment dictate which symbol set you must use. Mixing them on a single schematic is a violation of drafting standards and causes dangerous misinterpretations on the shop floor.

• IEC 60617 (International Electrotechnical Commission): The default for Europe, Asia, and modern global CAD software (like KiCad, Altium, and EPLAN). It uses strict rectangular geometry for coils and orthogonal lines for contacts. If you are exporting schematics for global manufacturing, use IEC.
• ANSI/IEEE 315 & NEMA: The standard for North America. It relies heavily on circles, semicircles, and specific 45-degree switch blade angles. Required for UL-listed control panels built in the US and Canada. You can review the baseline standards via the IEEE 315 directory.
• BS 3939 (British Standard): Largely obsolete. The UK officially adopted IEC standards decades ago, but you will still encounter BS 3939 symbols when troubleshooting legacy 1970s and 1980s UK plant documentation. Recognize it by the distinct hinge dots on contact wipers.

Decision Tree: Picking the Exact Relay Part Number

Don't leave your schematic as a generic 'DPDT Relay' block. Terminate your design with a specific, purchasable part number based on your application constraints.