When reading a schematic or wiring a control panel, misidentifying a relay symbol means wiring a normally-open (NO) contact as normally-closed (NC), tripping breakers, or frying PLC outputs. Here is the exact translation of electrical relay symbols across global standards, starting immediately with the master reference.
Master Reference Table for Electrical Relay Symbols
The table below maps the physical relay function to its schematic representation. Use this as your bench-side cheat sheet when tracing control circuits.
| Symbol Name / Function | IEC 60617 Representation | NEMA / ANSI Representation | Function in Practice & Pin Mapping |
|---|---|---|---|
| Relay Coil (General) | Rectangle with 'K' or 'CR' label | Circle with 'CR' or 'M' label | Energizes the armature. Pins A1 (+/L) and A2 (-/N). |
| Relay Coil (AC Specific) | Rectangle with a sine wave inside | Circle with a sine wave or 'AC' text | AC coil. Polarity on A1/A2 does not matter. Expect 50/60Hz hum. |
| Relay Coil (DC Specific) | Rectangle with a solid line and dashed line | Circle with '+' and '-' polarity marks | DC coil. Polarity matters if a flyback diode is built-in. A1 is (+), A2 is (-). |
| Latching / Magnetic Coil | Rectangle with a mechanical latch symbol (crossed lines) | Circle with a latch indicator or 'L' | Pulse to set, pulse to reset. Draws zero holding current. |
| Normally Open (NO) Contact | Two parallel lines with a diagonal slash (Form A) | Two dots with a bridge line, open at rest | Passes current only when coil is energized. Pins 11 to 14 (or 9 to 5 on octal). |
| Normally Closed (NC) Contact | Two parallel lines with a diagonal slash and cross (Form B) | Two dots with a bridge line, closed at rest | Passes current until coil energizes. Pins 11 to 12 (or 9 to 1 on octal). |
| Changeover (SPDT) Contact | Combined NO/NC symbol sharing a common line (Form C) | Combined NO/NC symbol sharing a common pivot | Switches between NO and NC. Common is pin 11, NC is 12, NO is 14. |
| Time-Delay (On-Delay) | NO/NC symbol with an arrow pointing toward the contact | Standard contact with 'ON DELAY' or timer symbol | Contact changes state only after a set time post-energization. |
| Time-Delay (Off-Delay) | NO/NC symbol with an arrow pointing away from the contact | Standard contact with 'OFF DELAY' or timer symbol | Contact changes state immediately on energize, delays on de-energize. |
Regional Variations: IEC vs. NEMA vs. Old UK
Electrical relay symbols are not universal; they depend heavily on the engineering standard used when the schematic was drafted. Knowing which standard you are looking at prevents catastrophic wiring errors.
IEC 60617 (Global, EU, and Modern US)
The IEC 60617 standard dominates modern global schematics. It uses rectangles for all coils and represents contacts using a rigid grid system. IEC schematics separate the coil circuit from the contact circuit entirely, linking them only by a shared alphanumeric tag (e.g., Coil 'K1' operates contacts 'K1.1', 'K1.2'). This makes tracing complex logic much easier, as contacts are drawn wherever they fit logically, not physically next to the coil.
NEMA ICS 19 (Legacy US Industrial)
NEMA standards (now largely harmonized with ANSI/IEEE) use circles for relay coils. Contacts are drawn as standard switch symbols with a dashed line connecting them back to the controlling coil. NEMA diagrams are often 'ladder' style, reading strictly left-to-right, top-to-bottom. If you are working on a US machine built before 2010, you will likely encounter NEMA circles.
Old UK (BS 3939)
Mostly obsolete but still found in older British plants and marine vessels. BS 3939 used highly stylized, almost artistic representations of the physical relay armature. If you see a symbol that looks like a literal drawing of a spring and a pivoting lever, you are looking at legacy BS 3939. Treat these with extreme caution and verify physical wiring, as documentation for these panels is often lost or modified.
The 'Rows People Get Wrong' Trap
Even experienced techs misread specific relay symbols when working fast. Here are the most common schematic traps and how to avoid them.
In IEC schematics, the arrow on a time-delay contact indicates the direction of the delayed action. An arrow pointing toward the contact gap means the contact is delayed in closing (On-Delay). An arrow pointing away means it is delayed in opening (Off-Delay). People frequently wire Off-Delay circuits backwards, causing safety interlocks to drop out instantly instead of riding through a brief power dip.
The symbol for a Changeover (SPDT) contact shows three connection points. The middle line is the 'Common' (C). On a standard 14-pin DIN relay (like the Finder 40.52), pin 11 is Common, 12 is NC, and 14 is NO. A common mistake is assuming the physical layout matches the schematic drawing order. Always map the schematic 'C' to pin 11, regardless of how the symbol is rotated on the page.
Trap 3: Latching vs. Standard Coils
A latching relay symbol includes a small mechanical latch indicator (often a small crossed rectangle or a 'set/reset' dual coil notation). If you replace a standard relay with a latching relay without updating the control logic, the machine will fail to cycle because the latching relay requires a reverse-polarity pulse or a dedicated reset coil to turn off. Always check for the latch indicator before ordering replacements.
Decision Path: Picking the Right Symbol and Physical Part
Use this decision tree to translate your circuit requirements into the correct schematic symbol and a concrete, purchasable part number.
| Application Scenario | Schematic Symbol to Draw | Physical Part Recommendation |
|---|---|---|
| Switching a 10A, 120VAC single-phase motor from a 24VDC PLC output. | IEC: DC Coil rectangle, SPST-NO (Form A) contact. | Omron G2R-1-E DC24 (10A SPST-NO, 24VDC coil, PCB/Socket mount). |
| Interlocking two 3-phase contactors (prevent both from closing simultaneously). | IEC: AC Coil rectangles, SPDT (Form C) contacts with cross-referenced tags. | Schneider TeSys LC1D09 with built-in mechanical interlock and auxiliary NC contacts. |
| Maintaining a safety valve state during a total PLC power failure. | IEC: Latching DC Coil (dual coil or reverse pulse), SPDT contact. | Finder 60.13.9.024.0040 (Locking relay, 24VDC, 3PDT for redundant safety). |
| General purpose 24VDC control logic with multiple NO/NC needs. | IEC: DC Coil rectangle, multiple SPDT (Form C) contacts. | Phoenix Contact PLC-BSC-24DC/21 (Slim 6mm relay, 1 CO contact, 24VDC). |
Default Pick: If you are designing a generic 24VDC control panel and need a reliable, easily replaceable workhorse, standardize on the Omron G2R-2 series (DPDT) with the matching P2RF-08 socket. It covers 90% of industrial control logic needs and is available globally.
Safe Interpretation When Markings are Faded or Missing
On older equipment, the schematic diagram printed on the relay casing often fades, peels off, or is covered in grime. Never guess the pinout based on physical proximity. Follow this exact multimeter procedure to safely map an unmarked relay.
- De-energize and Isolate: Turn off the control power. Pull the relay completely out of its socket. Never probe a relay socket while it is live.
- Identify the Coil: Set your digital multimeter (DMM) to resistance (Ohms). For a standard 24VDC relay, expect a coil resistance between 600Ω and 1200Ω. For a 120VAC relay, expect 3kΩ to 10kΩ. Probe pairs of pins until you find this specific resistance range. On a 14-pin octal, it will be pins 13 and 14. On an 8-pin DIN, it will be A1 and A2 (or 13/14).
- Map the Common (C): Set the DMM to continuity (beep mode). Find the pin that shows continuity to two different pins when the relay is at rest. That pin is your Common (C). On a 14-pin, this is pin 9. On an 8-pin, it is pin 11.
- Identify NC and NO: The pin that shows continuity to Common at rest is Normally Closed (NC). The pin that shows no continuity to Common at rest is Normally Open (NO). You can verify this by manually pressing the plastic armature with a small flathead screwdriver; the continuity should shift from NC to NO.
By cross-referencing your physical multimeter tests with the IEC or NEMA symbols on the master schematic, you can confidently rewire or replace any relay in the field, regardless of the condition of the physical labeling.
