A relay schematic symbol is never just one drawing; it is a split representation consisting of an electromagnetic coil (the input) and one or more switch contacts (the output). Under the globally dominant IEC 60617 standard, the coil is drawn as a simple rectangle, while North American NEMA standards often use circles or specific letter designations like 'CR' (Control Relay). Understanding these schematic symbols relay conventions is critical because misinterpreting a normally closed (NC) contact as normally open (NO) can result in immediate short circuits or failed safety interlocks upon energization.
Complete Relay Schematic Symbols Reference Table
The table below maps the most common relay components to their graphical representations across the two major global standards. Use this as your bench reference when tracing control circuits.
| Component | IEC 60617 Symbol / Designation | NEMA / US Standard | Real-World Application |
|---|---|---|---|
| Relay Coil (General) | Rectangle (labeled 'K' or 'KA') | Circle or Rectangle (labeled 'CR' or 'M') | Energizing the electromagnet to pull in the armature. |
| Normally Open (NO) Contact | Two parallel lines, one angled away (Form A) | Two parallel lines, one angled away | Start circuits, indicator lights that turn on when the relay pulls in. |
| Normally Closed (NC) Contact | Two parallel lines with a diagonal cross (Form B) | Two parallel lines with a diagonal cross | Safety interlocks, stop circuits, alarm triggers on power loss. |
| Changeover (SPDT) Contact | Common line branching to NO and NC (Form C) | Common line branching to NO and NC | Motor reversing, transferring a load between utility and generator. |
| Time-Delay ON Coil | Rectangle with a bottom 'U' bracket or arrow pointing up | Circle with 'TD' or specific timer symbol | Soft-starts, delaying a motor run until a compressor builds pressure. |
| Latching Relay Coil | Rectangle with a mechanical latch symbol (crossed lines) | Rectangle with 'L' or latch symbol | Maintaining state during a power outage without continuous coil draw. |
Regional Variants and the "Rows People Get Wrong"
While the IEC 60617 standard has largely unified global schematic drawing, you will still encounter legacy and regional variants, especially when maintaining older industrial equipment.
- IEC (International/Europe/Modern Global): Uses a rectangle for the coil. Designations are strictly letter-based (K for relay, KA for auxiliary, KM for contactor). Contacts are drawn separately from the coil, linked only by a shared alphanumeric tag (e.g., K1 coil operates K1-13/14 contacts).
- NEMA (North America): Historically used circles for coils, though modern US CAD libraries often adopt IEC rectangles. NEMA relies heavily on functional prefixes: 'CR' for Control Relay, 'M' for Motor Starter, 'TR' for Timing Relay. Contacts are usually drawn directly beneath or beside the coil in ladder logic format.
- Old UK (BS 3939): Obsolete since the 1980s but still found in legacy British plants. Used circles with internal letters (e.g., a circle with 'R' inside) and highly stylized, curved contact arms that look more like physical knife switches than modern schematic lines.
The Rows People Get Wrong
When reading schematics, two specific symbols consistently cause troubleshooting errors:
- Form C (SPDT) vs. Two Separate Contacts: A Form C symbol shows a single common line splitting into an NO and NC path. Beginners often misread this as two independent contacts. In reality, it is a single physical pole. If the common pin loses power, both downstream paths fail.
- Time-Delay Contact Arrows: A time-delay contact features a small arrow on the moving contact line. If the arrow points toward the contact (like an umbrella), it is an ON-delay (holds off, then closes). If the arrow points away (like a funnel catching drops), it is an OFF-delay (closes immediately, holds on, then drops off). Mixing these up will ruin sequential control logic.
Never guess a relay's coil voltage if the physical sticker is faded or missing. Applying 120VAC to a 24VDC coil will instantly vaporize the winding, posing a fire and shock hazard. If markings are illegible, de-energize the panel, lock out the breaker, and use a multimeter to measure the coil resistance. A 24VDC Omron MY2N coil reads approximately 650Ω, while a 120VAC coil reads around 4.4kΩ. Cross-reference the measured resistance and physical pin layout with the manufacturer's datasheet before applying power.
Pinout Mapping: Translating Schematics to Physical Relays
Schematics show logical connections, but physical wiring requires exact pin numbers. The most common industrial relay format is the 8-pin octal base (e.g., Omron MY2 series or Finder 40.52). Understanding the bridge between the IEC schematic tag and the physical socket pins is where most installation time is lost.
| Function | IEC / DIN Pin Number | Alternate Labeling | Multimeter Test (De-energized) |
|---|---|---|---|
| Coil Input | 13 and 14 | A1 and A2 | Reads specific resistance (e.g., 650Ω for 24VDC) |
| Pole 1 Common | 9 | COM1 | Continuity to 1 (NC) and OL to 5 (NO) |
| Pole 1 NC | 1 | NC1 | 0.00Ω to 9 (Common) |
| Pole 1 NO | 5 | NO1 | OL (Open Loop) to 9 (Common) |
| Pole 2 Common | 10 | COM2 | Continuity to 2 (NC) and OL to 6 (NO) |
| Pole 2 NC | 2 | NC2 | 0.00Ω to 10 (Common) |
| Pole 2 NO | 6 | NO2 | OL (Open Loop) to 10 (Common) |
Note: On 14-pin relays (like the Omron MY4), the coil moves to pins 13 and 14, but the contact pins shift to accommodate four poles (Commons at 9, 10, 11, 12). Always verify the socket diagram printed on the side of the relay base.
Frequently Asked Questions
What do the schematic symbols relay numbers like 13 and 14 mean?
The numbers 13 and 14 are standardized DIN/IEC designations for the relay coil input terminals. This standardization ensures that regardless of whether you are using a Japanese Omron, an Italian Finder, or a German Phoenix Contact relay, the control wire from your PLC or switch always lands on 13 (positive/hot) and 14 (negative/neutral). The contacts follow a strict decade system: the first digit represents the pole number (1, 2, 3, or 4), and the second digit represents the function (1/2 for NC, 3/4 for NO, 5/6 for NO, 7/8 for NC, 9/0 for Common).
How do I identify a relay symbol when the schematic has no standard letter designation?
If you are looking at a legacy or poorly drafted schematic lacking 'K' or 'CR' tags, look for the mechanical linkage line. In both IEC and NEMA standards, a dashed or solid line connects the coil symbol to its associated contacts, indicating they move together. Alternatively, trace the circuit logic: if a symbol features a rectangle or circle on a control rung, and a corresponding switch symbol on a parallel load rung that changes state simultaneously, you are looking at a relay. For deeper theory on electromagnetic switching, refer to the All About Circuits relay chapter.
Why does my relay schematic show a diode across the coil symbol?
That is a flyback (or freewheeling) diode, and it is mandatory for DC relay coils. When a DC coil (which is a large inductor) is de-energized, the collapsing magnetic field generates a massive reverse voltage spike (inductive kickback) that can easily exceed 100V, instantly destroying the driving transistor or PLC output. The diode, drawn in parallel with the coil and reverse-biased during normal operation, provides a safe loop for this spike to dissipate. If your schematic shows this diode, you must install it physically across pins A1 and A2 (cathode to positive), typically using a standard 1N4007 rectifier. For more on relay protection circuits, see this guide on relay switching circuits.
