Electrical relay symbols are standardized graphical shorthand used in schematics to represent relay coils, contacts, and timing mechanisms. The exact symbol you see depends entirely on whether your schematic follows IEC 60617 (global/European) or NEMA/ANSI (North American) standards. Below is the master reference table to decode them.
The Complete Electrical Relay Symbols Reference Table
Use this table to cross-reference the schematic symbol with the physical component on your bench. For deeper foundational reading on relay mechanics, refer to the comprehensive guides at Electronics Tutorials.
| Component | IEC 60617 Symbol Description | NEMA / ANSI Symbol Description | Practical Meaning in the Field |
|---|---|---|---|
| Relay Coil | Rectangle with diagonal lines or letter 'K' | Circle or semicircle with 'CR' (Control Relay) | The electromagnetic actuator. Energizing this pulls the mechanical armature. |
| Normally Open (NO) Contact | Two parallel lines with a gap and a diagonal wiper | Two parallel lines with a gap and an intersecting 'X' or angled line | Circuit is open (off) when the coil is de-energized. Closes when energized. |
| Normally Closed (NC) Contact | Two parallel lines bridged by a diagonal wiper | Two parallel lines bridged by an intersecting line | Circuit is closed (on) when the coil is de-energized. Opens when energized. |
| Changeover (SPDT) Contact | Wiper resting between an NO and NC contact line | Angled line resting between an NO and NC contact line | Single Pole Double Throw. Common pin switches between two paths (break-before-make). |
| Time-Delay ON (TON) | Contact symbol with an arrow pointing TOWARD the gap | Contact symbol with a small clock or 'TON' label | Contact changes state only after a set time delay once the coil is energized. |
| Time-Delay OFF (TOF) | Contact symbol with an arrow pointing AWAY from the gap | Contact symbol with a small clock or 'TOF' label | Contact changes state only after a set time delay once the coil is de-energized. |
| Latching Relay Coil | Rectangle with a small mechanical latch indicator (cross-hatch) | Circle with a latch indicator or 'L' designation | Requires only a brief pulse to change state. Removes continuous coil heating. |
Regional Standards: IEC vs. NEMA vs. Old UK
Knowing which standard applies to your region prevents catastrophic wiring errors when reading imported machinery schematics or legacy plant drawings.
IEC 60617 (Global / European / Modern UK / Australia)
The International Electrotechnical Commission standard dominates global automation. It uses minimalist geometry: rectangular boxes for coils and simple line gaps for contacts. Terminal numbering strictly follows the IEC convention (e.g., A1/A2 for coils, 11/12/14 for the first SPDT contact block). If you are working on modern Siemens, ABB, or Schneider Electric equipment, you will use IEC symbols.
NEMA and ANSI/IEEE 315 (North America)
In the US and Canada, NEMA (National Electrical Manufacturers Association) and ANSI/IEEE 315 standards prevail. These symbols are more pictorial. Coils are often drawn as circles or specific geometric shapes, and contacts use explicit intersecting lines. You will see these heavily in Allen-Bradley, Eaton, and legacy North American motor control centers (MCCs). For a detailed breakdown of North American graphic symbols, consult the IEEE 315 standard documentation.
Old UK (Pre-BS EN 60617 / Legacy BS 3939)
Before the UK harmonized with European IEC standards, British Standard 3939 was the norm. You will still encounter these in older UK water treatment plants and legacy rail systems. BS 3939 used distinct semicircle notations for coils and highly stylized, almost cursive, contact wipers. If you see a symbol that looks like a stylized 'S' or a series of overlapping semicircles, you are likely looking at a pre-1990s British schematic.
Rows People Get Wrong (And How to Fix Them)
Mistake 1: Confusing SPDT (Changeover) with DPST
The IEC changeover symbol has a distinct wiper shape resting between two contacts. Technicians often misread this as two separate contacts and wire the common pin incorrectly on physical relays like the Omron LY2N or Finder 40.52. Remember: SPDT has 3 pins per pole (Common, NO, NC). DPST has 4 pins per pole (two isolated NO or NC pairs). Count the physical pins on the relay base to confirm.
Mistake 2: Reversing Time-Delay Arrows
In IEC schematics, an arrow pointing toward the contact gap means delay-on-energize (TON). An arrow pointing away means delay-on-de-energize (TOF). Reversing these in a safety interlock circuit—such as a ventilation exhaust delay for a confined space—can cause a machine to start before toxic fumes are cleared, or stop before the cycle is safely complete.
Mistake 3: Treating Latching Coils as Standard Coils
A latching relay symbol includes a small mechanical latch indicator (often a small rectangle or cross-hatch on the coil box). If you treat it as a standard coil and apply continuous voltage via a standard PLC output, you will burn out the relay's internal reset circuit or the PLC output transistor. Latching coils require a brief pulse (typically 50ms to 200ms) to set, and a separate pulse to reset.
Safe Interpretation When Markings Are Faded or Missing
In industrial environments, UV exposure, oil mist, and heat routinely fade the pinout diagrams printed on the sides of physical relays (like the ubiquitous Schneider Electric RXM or Omron G2R series). Never guess the pinout based on a faded diagram. Follow this bench procedure:
- De-energize and Lock Out (LOTO): Ensure the control circuit is completely dead. Verify with a known-working multimeter.
- Identify the Coil: Set your multimeter to resistance (Ohms). Probe the suspected coil pins (usually A1/A2 in IEC, or pins 2 and 7 on an 8-pin octal base). A 24VDC coil will typically read between 400 and 800 ohms. A 120VAC coil will read much higher, often between 3k and 10k ohms. If you read 0 ohms, the coil is shorted; if you read infinite (OL), the coil is open and the relay is dead.
- Map the Contacts: Switch your meter to continuity mode (with audible beep). Find the Common (C) pin by locating the pin that shows continuity to one other pin (the NC) while the relay is at rest.
- Actuate Manually: Most industrial relays have a small plastic test button on the armature. Press it with an insulated tool. The continuity should shift from the NC pin to the NO pin. Label the base immediately with a fine-tip permanent marker.
Frequently Asked Questions
What do the numbers on electrical relay symbols mean?
In IEC-standard schematics, the numbers next to the relay symbols designate specific terminal functions, not just sequential pin counts. For the coil, you will see A1 (positive/line) and A2 (negative/neutral). For contacts, the first digit indicates the contact block number (1, 2, 3, etc.), and the second digit indicates the function: 1 and 2 denote a Normally Closed (NC) pair, while 3 and 4 denote a Normally Open (NO) pair. Therefore, '13' and '14' means the NO contact on the first pole. This numbering system allows electricians to trace wires across massive schematics without needing to look at the physical relay.
How do you read a time-delay electrical relay symbol?
Look for the arrow or clock notation adjacent to the contact gap. If using the IEC arrow method, visualize the armature moving. If the arrow points in the direction the contact moves to close (toward the gap), it is a Time-Delay ON (TON). The contact waits to close after the coil receives power. If the arrow points in the direction the contact moves to open (away from the gap), it is a Time-Delay OFF (TOF). The contact closes immediately on power, but waits to open after power is removed. In NEMA schematics, look for a small clock face inside the contact symbol, often accompanied by 'ON' or 'OFF' text.
Why does my relay symbol show a diagonal line through the contact?
A solid diagonal line crossing the contact gap in an IEC schematic typically indicates a time-delayed contact or a thermal overload trip. If the diagonal line is accompanied by a small semicircle or a bimetallic strip symbol (a zigzag line), it represents a thermal overload relay (like a TeSys LRD series) rather than a standard electromagnetic control relay. Thermal overloads are designed to trip slowly under sustained overcurrent conditions to protect motors, whereas standard control relays switch instantaneously. Always check the device designation letter (e.g., 'F' for protection/overload vs. 'K' for control relay) next to the symbol to confirm.
