A relay schematic symbol consists of two distinct parts: the coil (input control) and the contacts (output switching). The exact drawing you see on a blueprint depends entirely on whether your region follows the IEC 60617 standard (Europe, Asia, and most global industrial projects) or the ANSI/IEEE C37.2 standard (North American power and heavy industry). Below is the direct reference data you need to read, draw, and troubleshoot these symbols on the bench or jobsite.
Master Relay Schematic Symbol Reference Table
The table below maps the most common relay configurations to their IEC and ANSI representations, including standard pin numbering and real-world component examples. Use this to cross-reference your schematic with the physical relay in your panel.
| Relay Type / Function | IEC 60617 Symbol Description | ANSI/IEEE C37.2 Symbol | Standard Pinout (IEC) | Real-World Component Example |
|---|---|---|---|---|
| Standard Coil | Rectangle (often with diagonal lines for AC or polarity marks for DC) | Circle or Rectangle with Device Number (e.g., '21' for distance, '86' for lockout) | A1 (+/L1), A2 (-/L2) | Omron G2R-1 (10A PCB/Plug-in) |
| SPST-NO (Form A) | Line with a gap and a diagonal slash crossing the gap (Normally Open) | Same as IEC, or two parallel lines with a gap and slash | 11 (Common), 14 (NO) | Finder 38.51 (SPST-NO Master) |
| SPST-NC (Form B) | Line with a gap and an overlapping diagonal line resting on the contact (Normally Closed) | Same as IEC, overlapping line indicates closed state | 11 (Common), 12 (NC) | Schneider Electric RXM (NC contact) |
| SPDT (Form C) | Common line branching to two diagonal lines (one NO, one NC) | Changeover symbol, often drawn as a single pole switching between two throws | 11 (C), 12 (NC), 14 (NO) | Omron LY2N (DPDT, uses 11-14 & 21-24) |
| Time-Delay (On-Delay) | Standard contact symbol with an arrow pointing away from the contact line | Timer symbol (circle with '62' or '63') linked to standard contacts | A1, A2 (Coil); 15/18 (Timed NO) | Macromatic TD-116 (On-Delay Timer) |
| Latching / Pulse Relay | Standard coil with a mechanical latch symbol (small rectangle/block) or dual opposing coils | Latching relay device number '86' with mechanical hold indicator | A1 (Set), A2 (Reset), 11/14 | Finder 20.21 (Step/Pulse Relay) |
Regional Standards: IEC 60617 vs. ANSI/IEEE C37.2
Knowing which standard applies to your schematic prevents catastrophic wiring errors, especially when retrofitting imported machinery or working on legacy North American panels.
- IEC 60617 (Global Standard): Uses geometric shapes. Coils are strictly rectangles. Contacts are drawn using angled lines to indicate their resting state (NO vs NC). Pin numbering is highly standardized (e.g., A1/A2 for coils, 11/12/14 for the first SPDT pole, 21/22/24 for the second). If you are working on modern PLC panels, European imports, or residential/commercial HVAC, you will almost exclusively see IEC symbols.
- ANSI/IEEE C37.2 (North American Power/Heavy Industry): Originating from power generation and transmission, this standard relies heavily on 'Device Numbers'. Instead of drawing a complex latching relay symbol, an ANSI schematic might just draw a circle with the number 86 inside it. A circle with 49 means a thermal overload relay. The physical wiring diagram is often separated from the logical schematic. You will see this in US substations, large motor control centers (MCCs), and older NEMA-style ladder logic.
- NEMA (Legacy US Industrial): Older NEMA ladder diagrams use a mix of literal pictorial representations and simplified line art. Coils are often drawn as circles or parentheses
( ), and contacts as simple parallel lines| |for NO or|/|for NC. While largely superseded by IEC in new builds, NEMA symbols are still prevalent in US manufacturing plants built before the year 2000.
When troubleshooting a panel that has been modified over decades, you may find IEC physical relays wired into a NEMA-style logical schematic. Never assume the pinout based on the schematic symbol alone. A physical IEC Finder 55.34 relay (14-pin) will have completely different pin assignments than a legacy NEMA octal relay, even if the schematic draws them both as generic DPDT contacts. Always verify physical pin mappings against the manufacturer's datasheet before applying control voltage.
The 'Rows People Get Wrong' Guide
Even experienced technicians misread specific relay symbols when skimming a dense schematic. Here are the most common interpretation failures and how to avoid them.
1. Time-Delay Contact Arrows (On-Delay vs. Off-Delay)
The direction of the arrow on a time-delay contact is the most frequently misread element in IEC schematics. The Rule: The arrow always points in the direction the contact moves to reach its timed state. If the arrow points away from the contact line (upward), it is an On-Delay (the contact waits to close after the coil energizes). If the arrow points toward the contact line (downward), it is an Off-Delay (the contact closes immediately, but waits to open after the coil de-energizes). Swapping these in a motor starter circuit can cause the motor to start before the safety brakes release.
2. Latching (Pulse) Relay Coils
A standard relay drops out when power is removed. A latching relay (like the Finder 20 series step relays) stays in its last state. On schematics, a latching coil is often drawn with a small solid rectangle attached to the side of the coil box, or as two separate coils (one for 'Set', one for 'Reset'). Technicians often mistake the mechanical latch symbol for a thermal overload block, leading them to search for a non-existent bimetallic strip when the relay fails to reset.
3. Form A, B, and C vs. NO/NC
North American datasheets frequently use 'Form' designations instead of drawing the symbol. Form A = SPST-NO (Normally Open). Form B = SPST-NC (Normally Closed). Form C = SPDT (Changeover, one Common, one NO, one NC). If a schematic note says 'K1: DPDT (Form C)', it means the relay has two completely isolated SPDT poles. Do not confuse 'DPDT' with 'two separate SPST relays'—they share a single physical coil.
Safe Interpretation When Markings Are Faded or Missing
On the jobsite, you will eventually encounter a dusty Allen-Bradley 8501 or a generic unbranded relay where the printed schematic on the side has faded away, and the original panel drawings are lost. You must safely map the relay before replacing it or energizing the circuit. Here is the exact bench procedure to reverse-engineer an unknown relay using a digital multimeter (DMM).
Step 1: Identify and Test the Coil (A1 / A2)
Set your DMM to resistance (Ohms). Look for pins labeled A1 and A2, or '+' and '-'. If markings are totally gone, look for the two pins that are physically isolated from the switching contacts (often on the bottom or top edge of the base). Expected Resistance Values:
- 12V DC Coil: Typically reads between 100Ω and 300Ω.
- 24V DC Coil: Typically reads between 400Ω and 1,200Ω.
- 120V AC Coil: Typically reads between 3,000Ω (3kΩ) and 10,000Ω (10kΩ).
- 240V AC Coil: Typically reads between 10kΩ and 25kΩ.
Diagnostic: If your meter reads 'OL' (Open Loop), the internal coil wire is broken; the relay is dead. If it reads near 0Ω, the coil is shorted and will blow the control circuit fuse the moment you apply power. Replace the relay.
Step 2: Map the Contacts (Common, NO, NC)
Switch your DMM to Continuity mode (the diode/beep setting). Probe the remaining pins to find the 'Common' (C) terminal. The Common pin will show continuity (beep) to exactly one other pin in the resting state—that is your Normally Closed (NC) pin. The pin that shows 'OL' (no continuity) when measured against the Common is your Normally Open (NO) pin. In IEC numbering, if you find a Common pin labeled '11', the NC will be '12' and the NO will be '14'. If you find a second set (e.g., 21, 22, 24), you have a DPDT relay.
Step 3: The Safe Energize Test
Never apply mains voltage (120V/240V) to an unverified coil to 'see if it clicks'. If you guessed the voltage wrong, the coil will catch fire or explode. Instead, use a variable bench power supply. Start at 5V DC and slowly increase the voltage while listening for the audible 'click' of the armature. Once it clicks, verify with your DMM that the continuity has swapped from the NC pin to the NO pin. Note the voltage required to hold the relay closed, and purchase a replacement matching that exact coil voltage and contact ampacity rating.
For a comprehensive breakdown of ANSI device numbers used in heavy power schematics, refer to the IEEE C37.2-2022 Standard for Electrical Power System Devices Function Numbers. Always defer to the manufacturer's specific datasheet when physical pinouts conflict with generic schematic symbols.
