A schematic relay symbol consists of two primary elements: the electromagnetic coil and the mechanical switch contacts. In modern global schematics, the coil is typically drawn as a rectangle (IEC) or a circle (legacy NEMA), while contacts are drawn as standard switch symbols linked to the coil by a dashed mechanical line or a magnetic flux line. The direct answer to identifying any relay on a drawing is to locate the coil designation (e.g., K1, CR1) and trace the matching alphanumeric label to the remote switch contacts.
Standard Schematic Relay Symbols (IEC, NEMA, and Legacy)
The following table maps the most common relay components across the three major drafting standards. IEC 60617 is the modern global standard (used in the EU, Asia, and increasingly in the US). NEMA/ANSI/IEEE 315 is the traditional US industrial standard. BS 3939 is the legacy UK standard, still found on older British machinery and retrofitted panels. Note that while the NEC (NFPA 70) governs physical wiring and installation practices in the US, it does not dictate schematic drafting symbols; for that, we look to NEMA and IEEE.
| Component / Function | IEC 60617 (Global/EU) | NEMA / ANSI (US) | Legacy UK (BS 3939) | Practical Meaning & Common Part Example |
|---|---|---|---|---|
| Relay Coil (General) | Rectangle with diagonal lines or letter 'K' (e.g., K1) | Circle or rectangle with 'CR' (Control Relay) | Circle with 'R' or rectangle | The electromagnet. E.g., Omron MY2N 24VDC coil (Pins 13 & 14). |
| NO Contact (Form A) | Two parallel lines, one with a hinged diagonal bar | Two parallel lines, one with a hinged diagonal bar | Similar, often with a gap indicator | SPST-NO. Closes when coil energizes. Used for motor starters. |
| NC Contact (Form B) | Hinged diagonal bar resting against a fixed contact point | Hinged diagonal bar resting against a fixed contact point | Similar, resting on contact | SPST-NC. Opens when coil energizes. Used for safety interlocks. |
| Changeover (Form C) | Hinged bar between two fixed points (SPDT) | Hinged bar between two fixed points (SPDT) | Hinged bar between two points | SPDT. Common, NO, and NC. E.g., Finder 40.52 series. |
| Latching Relay Coil | Rectangle with a mechanical latch symbol (hook) inside | Rectangle/Circle with 'L' or latch hook | Rectangle with cross-hatching | Maintains state without power. Requires a reset pulse. |
| On-Delay Timer (TON) | Rectangle with an arrow pointing INWARD toward the contact | Circle/Rectangle with inward arrow or 'TD' | Rectangle with inward arrow | Contacts change state X seconds AFTER coil receives power. |
| Off-Delay Timer (TOF) | Rectangle with an arrow pointing OUTWARD away from contact | Circle/Rectangle with outward arrow | Rectangle with outward arrow | Contacts hold state for X seconds AFTER coil loses power. |
The "Rows People Get Wrong" Notes & Edge Cases
Even experienced technicians misinterpret specific schematic relay symbols when moving between regional standards or dealing with complex timing relays. Here are the most common pitfalls based on bench and jobsite experience:
- The Time-Delay Arrow Direction: This is the #1 error in control panel troubleshooting. In IEC 60617, an arrow pointing inward (toward the switch blade) means the contact is delayed in operating (On-Delay). An arrow pointing outward (away from the switch blade) means the contact is delayed in releasing (Off-Delay). Mixing these up will cause a motor to start immediately instead of waiting for a soft-start sequence, or fail to run a cooling fan after the main drive stops.
- Form C vs. Dual Form A/B: A Form C (SPDT) contact shares a single common terminal. Schematics sometimes draw a Form C as two separate switches (one NO, one NC) with a dashed line linking them. Beginners often wire these as two isolated circuits. If you are wiring an Omron G2R-1-E PCB relay, pins 1 (Common), 2 (NC), and 3 (NO) are physically linked; applying 240VAC to pin 1 means pin 2 and pin 3 will both see 240VAC potential depending on coil state.
- Wetting Current Failures: A schematic might show a standard 10A electromechanical relay switching a 5VDC, 10mA microcontroller GPIO signal. While the symbol is correct, the physical implementation will fail. Standard silver-alloy contacts require a minimum "wetting current" (usually 10mA to 50mA at 5V) to burn off surface oxidation. For low-voltage logic, the schematic should specify a gold-plated contact relay (like the Omron G6K series) or a solid-state relay (SSR).
- AC vs. DC Coil Symbols: Schematics often omit the AC/DC designation next to the coil rectangle, assuming the builder will check the BOM. A 24VAC coil (denoted by a '~' symbol inside the rectangle in strict IEC) has a much lower DC resistance than a 24VDC coil. Applying 24VDC to a 24VAC coil will draw excessive current, overheat the winding, and melt the bobbin within minutes.
Safe Interpretation When Markings Are Faded or Missing
On legacy machinery, the physical relay labels (e.g., "K3", "CR12") often bake off due to decades of panel heat, or the original schematic is lost entirely. When you must reverse-engineer a relay's function from the physical hardware, follow this systematic approach.
Step 1: Identify the Base and Pinout
Most industrial control panels use 8-pin (octal) or 14-pin (blade) plug-in relays. For a standard 14-pin relay (like the ubiquitous Schneider Electric RXM4AB1), the coil is always on pins 13 and 14. The four common terminals are 9, 10, 11, and 12. The NO contacts are 1, 2, 3, 4, and the NC contacts are 5, 6, 7, 8.
Step 2: Bench-Test the Coil
Remove the relay from its socket. Set your multimeter to the Ohms (Ω) setting. Measure across the coil pins. A healthy 24VDC coil typically reads between 600Ω and 1200Ω. A 120VAC coil will read much higher (often 2kΩ to 5kΩ). If you read "OL" (open loop), the internal winding is burnt and the relay is dead. If you read near 0Ω, the coil is shorted.
Step 3: Verify Contact Mechanics
Switch your meter to Continuity or Diode Test mode. Place probes on the Common and NC pins; you should hear a beep. Place probes on the Common and NO pins; it should remain open. Use a small flathead screwdriver to manually depress the relay's mechanical test lever. The continuity should swap. If the NO contact shows high resistance (>1Ω) even when manually closed, the contacts are pitted or carbon-fouled and the relay must be replaced.
Schematic Relay Symbol FAQ
What does the dashed line mean in a schematic relay symbol?
The dashed line (or sometimes a solid thin line with a mechanical linkage symbol) represents the physical or magnetic connection between the relay coil and its switch contacts. It tells the reader that when the specific coil (e.g., K1) is energized, all contacts linked by that dashed line and labeled "K1" will change state simultaneously. In modern CAD software like EPLAN or AutoCAD Electrical, this line is often omitted in favor of cross-reference grid coordinates to reduce schematic clutter.
How do I tell a relay coil from a contactor coil on a schematic?
In IEC 60617, both are drawn as rectangles, but the letter designation differs. A standard control relay coil is designated with 'K' (e.g., K1, K2). A power contactor coil (used for switching high-current motors, typically >15A) is designated with 'KM' (e.g., KM1, KM2). In NEMA standards, a control relay is 'CR' (Control Relay), while a contactor or motor starter is designated with an 'M' (Motor Starter) or '1M', '2M'. Additionally, contactor symbols often include integrated thermal overload relay symbols (drawn as a heater element in series with the contacts) which standard control relays lack.
Why does my schematic use "CR" instead of "K" for the relay coil?
This indicates the schematic was drafted using the North American NEMA/ANSI/IEEE 315 standard rather than the international IEC 60617 standard. "CR" stands for "Control Relay." You will frequently see this on older US-manufactured machinery, Allen-Bradley legacy prints, and in North American trade school curriculums. If you are replacing a "CR1" relay on a machine, you are simply looking for the first standard control relay in the BOM, regardless of whether a modern IEC print would have called it "K1".
What is the symbol for a solid-state relay (SSR) compared to an electromechanical relay?
An SSR schematic symbol replaces the mechanical switch contacts with a semiconductor symbol—typically a diode, a TRIAC, or a MOSFET symbol—enclosed in a box, with an optocoupler (light-emitting diode and phototransistor) separating the control input from the load output. Unlike the dashed mechanical linkage of an electromechanical relay, the SSR symbol uses a light beam arrow to show the galvanic isolation between the low-voltage DC control side (e.g., 3.3V from an ESP32 GPIO) and the high-voltage AC load side. Always check the SSR datasheet for the required minimum heatsink sizing, as they dissipate significantly more heat than electromechanical contacts at high loads.
