The symbol of relay switch components on a schematic dictates how control logic flows, but misreading a North American NEMA diagram as an international IEC diagram will result in miswired control cabinets and shorted coils. Below is the exact breakdown of relay symbols, regional standards, and practical troubleshooting steps for when physical markings fail.
The Complete Relay Switch Symbol Reference Table
Use this spec-sheet-table to decode the most common relay symbols found on industrial and residential control schematics. The 'Normal' state of any contact refers to the shelf state (when the coil is de-energized and no external force is applied).
| Component | NEMA / JIC Symbol Description | IEC 60617 Symbol Description | Common Part Example | Practical Meaning in Circuit |
|---|---|---|---|---|
| Relay Coil | Circle with designation (e.g., CR1, M1) | Rectangle with designation (e.g., K1, KM1) | Omron LY2N (AC/DC coil) | The electromagnetic actuator. When voltage is applied across A1/A2, it generates the magnetic field to pull the contacts. |
| Normally Open (NO) Contact | Two parallel lines, top line curved upward | Two parallel lines, top line angled away (gap open) | Schneider RXM4AB2 (Pin 13-14) | Blocks current when the coil is off. Closes to pass current only when the coil is energized. |
| Normally Closed (NC) Contact | Two parallel lines crossed by a diagonal slash | Two parallel lines, angled, crossed by a diagonal slash | Schneider RXM4AB2 (Pin 11-12) | Passes current when the coil is off. Opens to break the circuit when the coil is energized. |
| DPDT Contacts (Form C) | Stacked NO and NC symbols linked by a dashed line | Stacked NO and NC symbols linked by a solid mechanical line | Omron MY2N (8-pin base) | A single pole that switches between two throws. Common pin used as the pivot point (e.g., Pin 9 to 1 or 5). |
| Time-Delay (On-Delay) | Standard NO/NC with an arrow pointing away from the contact | Rectangle with an 'X' and an arrow pointing away | Macromatic TE-7011 | Contacts change state only after a set time after the coil receives power. Returns instantly when power is removed. |
NEMA vs. IEC: Regional Standards and How to Read Them
Knowing which standard applies to your region—and more importantly, which standard the original equipment manufacturer (OEM) used when designing the panel—is critical. A machine imported from Germany to the US will use IEC symbols on its schematics, even if it is plugged into a 480V NEMA-configured outlet.
| Criteria | NEMA / JIC (North America) | IEC 60617 (Europe / Global) |
|---|---|---|
| Governing Body | National Electrical Manufacturers Association / Joint Industrial Council | International Electrotechnical Commission |
| Coil Designation | CR (Control Relay), M (Motor Starter) | K (Relay/Contactor), KM (Motor Contactor) |
| Contact Numbering | Often sequential or arbitrary (1, 2, 3, 4) | Strict 2-digit system: Unit digit (1/2=NC, 3/4=NO), Tens digit (contact sequence) |
| Physical Footprint | Larger, heavier, designed for harsh environments and high fault currents | Compact, modular, DIN-rail mounted, optimized for space efficiency |
| Schematic Style | Ladder logic (vertical power rails, horizontal rungs) | Often linear or cross-reference grid layouts with interrupted line tracking |
For deep dives into standard definitions, the All About Circuits reference chapter on relays provides excellent visual breakdowns of these historical divergences. When specifying replacement parts for IEC panels, always verify the IEC 60617 compliance on the datasheet to ensure the physical terminal numbering matches your schematic.
Symbol Elements & Table Rows People Get Wrong
Even experienced technicians misinterpret specific relay symbols, leading to logic faults that are incredibly difficult to troubleshoot under pressure. Here are the most common pitfalls associated with the reference table above:
1. The 'Normal' State Confusion
The most frequent error is assuming 'Normally Open' means the state the contact is in during normal machine operation. 'Normal' strictly means the de-energized, shelf state. If a safety circuit relies on a relay that is energized 99% of the time the machine is running, its NO contact is actually closed during 'normal' operation. Always trace the schematic based on the coil's unpowered state.
2. Time-Delay Arrow Direction
Look closely at the On-Delay row in the table above. The arrow pointing away from the contact indicates the delay happens on energization (the contact is pushed away). If the arrow points toward the contact, it is an Off-Delay relay: the contact changes state immediately when powered, but delays returning to its normal state when power is cut. Swapping these two in a motor braking circuit can cause severe mechanical shock.
3. DPDT Mechanical Link Lines
In IEC schematics, the solid line linking multiple contact poles indicates a mechanical interlock—meaning if one contact moves, they all move simultaneously. In older NEMA drawings, this was sometimes represented by a dashed line. Do not confuse a dashed mechanical link with a dashed line indicating a delayed-action or limit-switch trigger.
Safe Interpretation When Markings Are Faded or Missing
In legacy panels, UV exposure, heat, and oil mist destroy relay labels and schematic stickers. When you cannot rely on the symbol of relay switch diagrams printed on the cabinet door, you must empirically map the physical relay using a multimeter.
Follow this exact sequence to map an unmarked 8-pin or 11-pin ice-cube relay (like the common Omron MY or LY series):
- Identify the Coil: Set your multimeter to resistance (Ω). On an 8-pin relay, pins 2 and 7 (or A1 and A2 on IEC bases) are typically the coil. You should read a specific resistance (e.g., 10kΩ to 15kΩ for a 120VAC coil, or 150Ω to 300Ω for a 24VDC coil). A reading of 'OL' (Open Loop) means a blown internal coil; a reading near 0Ω means a shorted coil.
- Map the Common Pins: Switch the meter to Continuity (the diode/beep setting). On an 8-pin DPDT relay, pins 1 and 8 (or 9 and 12 on 11-pin) are the 'Common' (C) pivot points.
- Find the NC Contacts: With the relay completely unpowered and removed from the base, place one probe on the Common pin. Probe the adjacent pins. The pin that yields a continuous beep (typically < 0.5Ω) is your Normally Closed (NC) contact.
- Find the NO Contacts: While keeping the probe on the Common pin, manually press the relay's plastic test armature down with your finger (simulating coil energization). The pin that now beeps is your Normally Open (NO) contact. The NC contact should simultaneously go 'OL'.
By combining a firm understanding of schematic symbols with empirical multimeter verification, you eliminate the guesswork from control panel troubleshooting. Always cross-reference your physical findings with the OEM wiring diagrams, and if the physical relay behavior contradicts the schematic, trust the physical test and update the documentation accordingly.
