A relay electrical symbol on a schematic is never drawn as a single physical block. Instead, it is split into two distinct logical components: the coil (the electromagnetic input) and the contacts (the switched output). In North American NEMA schematics, the coil is typically a circle, while in European IEC drawings, it is a rectangle. Understanding this split representation is the first step to accurately tracing control circuits and replacing failed components on the bench.
Complete Relay Electrical Symbol Reference Chart
The table below maps the most common relay symbols to their physical meanings. Keep this reference open when tracing control panels or designing PCB layouts.
| Component Function | NEMA (US/Canada) Symbol | IEC 60617 (EU/Global) Symbol | Physical Meaning on the Bench |
|---|---|---|---|
| Relay Coil (Standard) | Circle or plain rectangle | Rectangle with a diagonal line | The electromagnetic winding. Apply rated voltage here to pull in the armature. |
| SPST-NO Contact | Two parallel lines with a movable bridge, open | Similar open bridge, often with a specific angle | Single Pole Single Throw, Normally Open. Circuit is open until coil is energized. |
| SPST-NC Contact | Two parallel lines with a movable bridge, closed | Similar closed bridge, overlapping | Normally Closed. Circuit conducts power until the coil pulls the armature and breaks it. |
| SPDT Contact | Common line switching between NO and NC | Common line switching between NO and NC | Single Pole Double Throw. One common pin routes to either NO or NC depending on coil state. |
| Time-Delay ON (TON) | Standard NO contact with an upward-pointing arrow | Standard NO contact with a specific delay bracket | Contacts close only after the coil has been energized for a set duration. |
| Time-Delay OFF (TOF) | Standard NO contact with a downward-pointing arrow | Standard NO contact with an off-delay bracket | Contacts close instantly on energization, but remain closed for a set time after power is removed. |
| Solid State Relay (SSR) | Rectangle with an optocoupler symbol inside | Rectangle with a thyristor/triac symbol inside | No moving parts. Uses an LED and phototransistor/triac to switch loads silently. |
Regional Standards: NEMA, IEC, and Legacy UK Symbols
Before you start tracing wires, you must identify which drafting standard the schematic uses. Mixing standards on a single drawing is a leading cause of miswiring in imported machinery.
- NEMA (National Electrical Manufacturers Association): Dominant in the US and Canada. NEMA symbols are highly pictorial and standardized independently of physical device construction. Reference designators usually use 'CR' for Control Relays and 'M' for Motor Starters.
- IEC 60617 (International Electrotechnical Commission): The global standard used in Europe, Asia, and increasingly in modern US PLC panels. IEC symbols are more abstract and rely heavily on alphanumeric reference designators (e.g., 'K1' for a relay, 'KM' for a contactor). For a deep dive into IEC logic, consult the Electronics Tutorials relay guide.
- Legacy BS 3939 (Old UK Standard): You will still see this on older British machinery. It uses unique graphical representations for contacts that differ slightly from modern IEC, particularly in how time-delay dashes are drawn. If you are retrofitting a pre-1990s UK panel, cross-reference the original OEM manual before swapping components.
The 'Rows People Get Wrong' Troubleshooting Notes
When reading relay schematics, three specific symbol interpretations consistently cause bench and jobsite errors. Memorize these edge cases to avoid short circuits or safety interlock failures.
1. The Definition of 'Normal' (NO vs NC)
The most common mistake is assuming 'Normally Open' means the state the relay is in during normal machine operation. Normal refers strictly to the de-energized, shelf state. If a safety interlock circuit uses an NC contact, it conducts power when the machine is off and the relay is unpowered. When the relay energizes (e.g., a guard door is closed), the contact opens. Always read the symbol as drawn on the page, assuming the coil has zero voltage applied.
2. Coil and Contact Separation
Beginners often look for a single symbol that combines the coil and the switch. In both NEMA and IEC, they are drawn separately to keep schematics readable. They are linked by a reference designator (like K1, K2, or CR1). If you see K1-1 and K1-2 on different pages, those are the contacts belonging to the coil labeled K1. For more on schematic reading conventions, see the All About Circuits relay chapter.
3. Time-Delay Arrow Direction
On NEMA schematics, the arrow on a time-delay contact indicates the direction of the delay. An upward-pointing arrow means On-Delay (delays pulling in). A downward-pointing arrow means Off-Delay (delays dropping out). Reversing these in a motor starter circuit can cause severe mechanical shock or contact welding.
Decision Path: Identifying and Replacing Unknown Relays
Use this decision tree to move from a schematic symbol to a physical part number in your cart.
| Schematic Clue / Condition | Action Required | Result / Concrete Pick |
|---|---|---|
| Coil is drawn as a circle; contacts labeled 'CR1' | Use NEMA pinout conventions; look for octal or 11-pin plugs. | Source a NEMA-style plug-in relay (e.g., Schneider Electric 8501 series). |
| Coil is a rectangle with 'K1'; contacts show DIN pin numbers (A1/A2, 13/14) | Use IEC conventions; look for 14-pin rectangular or 8-pin DIN bases. | Source an IEC DIN relay (e.g., Finder 55.34 series). |
| Symbol shows a rectangle with an optocoupler; load is >2A AC | Do not buy a mechanical relay. You need a Solid State Relay (SSR) with a zero-crossing detector. | Concrete Pick: Crydom D2425 (25A, 24-280VAC output). |
| Schematic shows generic SPDT PCB relay symbol; no specific OEM part listed; 12V DC coil | Specify a high-reliability, general-purpose PCB power relay with a Class F insulation rating. | Concrete Pick: Omron G2R-1-E DC12 (16A, SPDT, PCB mount). |
Safe Interpretation When Markings Are Faded or Missing
On the bench, you will frequently encounter relays where the printed pinout diagram on the plastic casing has been baked off by heat, or you are reverse-engineering a board with no schematic. Do not guess the pinout based on the physical shape of the pins. Use your multimeter to map the relay electrically.
Step-by-step mapping for an unmarked 5-pin SPDT relay:
- Find the Coil: Set your multimeter to resistance (Ohms). Probe pairs of pins until you find two that read between 100Ω and 400Ω (typical for a 12V DC coil) or 2kΩ to 10kΩ (for a 24V DC coil). Mark these as your coil pins.
- Find the Common (C): Set the meter to continuity mode. With the coil unpowered, probe the remaining three pins. One pin will show continuity to a second pin (this is your NC and Common pair). The third pin will read 'OL' to everything (this is your NO). The pin shared in the continuity reading is your Common.
- Verify the Throw: Apply the rated DC voltage to the coil pins using a bench power supply. You should hear a distinct click. Re-check continuity: the Common pin should now show continuity with the previously 'OL' pin (NO), and read 'OL' with the previous NC pin.
By treating the relay electrical symbol as a logical map rather than a physical picture, and verifying with a meter when the physical markings fail, you eliminate the guesswork that leads to blown fuses and fried control boards.
