The control relay symbol is the standardized schematic representation of an electrically operated switch, used to isolate low-voltage control circuits from high-power loads. In modern schematics, the coil is depicted as a rectangle (IEC standard) or a circle (NEMA standard), while contacts are always drawn in their de-energized, or 'shelf', state. Understanding these symbols is the baseline requirement for reading ladder logic, wiring industrial control panels, and debugging automated machinery.
Complete Control Relay Symbol Reference Table
The table below maps the most common control relay symbols to their physical terminal designations and practical circuit functions. All contact states are shown in the de-energized (coil off) condition, which is the universal rule for reading electrical schematics.
| Component / Function | IEC 60617 Symbol & Designation | NEMA / ANSI Symbol | Practical Meaning & Pinout |
|---|---|---|---|
| Relay Coil (Standard) | Rectangle labeled 'K' or 'CR' (e.g., K1) | Circle labeled 'CR' or 'M' | The electromagnetic actuator. IEC pins: A1 (+/L) and A2 (-/N). NEMA pins: X1 and X2. |
| Normally Open (NO) Contact | Two parallel lines, top line angled away. Designation: 13-14 | Two parallel lines with a gap, bridged by a diagonal line. | Form A (SPST-NO). Circuit is open until coil energizes. IEC pins: 13 (Common), 14 (NO). |
| Normally Closed (NC) Contact | Two parallel lines, top line angled inward, crossing the bottom. Designation: 11-12 | Two parallel lines bridged by a diagonal line with a slash through it. | Form B (SPST-NC). Circuit is closed until coil energizes. IEC pins: 11 (Common), 12 (NC). |
| Changeover (Form C / SPDT) | Combination of NO and NC sharing a common line. Designation: 11-12-14 | Single pole with a double throw switch symbol. | Selects between two circuits. IEC pins: 11 (Common), 12 (NC), 14 (NO). |
| On-Delay Timer Coil | Rectangle with an 'X' inside, or 'ON' arrow pointing up. | Circle with 'OND' or a clock face symbol. | Contacts change state only after a set time post-energization. Drops out instantly. |
| Off-Delay Timer Coil | Rectangle with an 'X' and 'OFF' arrow pointing down. | Circle with 'OFFD' or clock face with slash. | Contacts change state instantly on energization, but delay returning to shelf state. |
| Latching (Impulse) Relay | Rectangle with a mechanical latch symbol (small box with a line). | Circle with 'L' or 'LTCH'. | Requires only a momentary pulse to change state; maintains state without continuous power. |
Regional Variants: IEC 60617 vs. NEMA vs. Legacy UK
When troubleshooting a machine imported from overseas or reading legacy prints, you will encounter different drafting standards. Assuming one standard applies universally is a primary cause of wiring errors.
IEC 60617 (International / European / Modern Global)
The IEC standard uses alphanumeric designations. Coils are rectangles, and the physical pin numbers are printed directly on the schematic symbol (e.g., A1, A2 for the coil; 11, 12, 14 for a Form C contact). This makes cross-referencing the physical relay (like a Schneider RXM or Omron MY2N) to the drawing straightforward.
NEMA / ANSI/IEEE 315 (North America)
Common in US and Canadian industrial panels governed by NFPA 79. NEMA uses circles for coils and relies heavily on ladder logic rungs. Instead of pin numbers, NEMA uses device prefixes (e.g., 1CR-1 for the first NO contact of Control Relay 1). You must reference the manufacturer's pinout sheet to map '1CR-1' to physical pins.
Legacy UK (BS 3939)
Largely superseded by IEC 60617 (adopted as BS EN 60617), but still found in British facilities built before the late 1990s. BS 3939 used distinct, often more complex graphical representations for contacts, including specific cross-hatching for time-delay contacts. If you see a schematic with unfamiliar, highly detailed contact symbols, check the title block for BS 3939 before applying IEC logic.
The 'Rows People Get Wrong' Notes
Even experienced technicians misinterpret specific relay symbols on the bench. Here are the most common pitfalls:
- Form C (SPDT) Common Pin Confusion: On IEC relays, the middle pin (11) is the Common (COM). Beginners often wire the load to 12 (NC) and 14 (NO), leaving 11 floating, which results in a dead circuit. Always wire your incoming feed or load return to pin 11.
- DC Coil Polarity and Flyback Diodes: While AC coils (A1/A2) are non-polarized, DC coils are not. If the schematic shows a diode symbol across the coil rectangle, it indicates a built-in flyback diode. If you wire A1 to negative and A2 to positive, you will short the power supply through the internal diode and likely destroy the relay or blow the control fuse. Always verify A1 is positive on DC circuits unless the datasheet specifies otherwise.
- Time-Delay Arrow Direction: In IEC schematics, an arrow pointing toward the contact line indicates an on-delay (the contact delays moving). An arrow pointing away from the contact line indicates an off-delay (the contact delays returning). Mixing these up will completely break sequential motor starters.
Safe Interpretation When Markings Are Faded or Missing
Industrial environments are harsh. Heat, ozone, and oil mist frequently erase the printed pinout diagrams on the sides of clear-cased relays like the Finder 38.51 or Omron LY series. When the visual symbols are gone, use a digital multimeter (DMM) to map the pins safely:
- Identify the Coil: Set your DMM to resistance (Ohms). Probe pairs of pins. The coil will typically read between 100Ω and 10,000Ω (e.g., a 24VDC coil often reads around 650Ω; a 120VAC coil might read 4,000Ω). Pins that read 'OL' (Open Loop) are not the coil.
- Identify the Common (COM):strong> Switch the DMM to continuity/diode mode. Find the pin that shows continuity (less than 0.5Ω) to one pin, and 'OL' to another adjacent pin. The shared pin is your Common (e.g., pin 11).
- Map NO and NC: The pin showing continuity to the Common in the de-energized state is Normally Closed (NC, pin 12). The pin showing 'OL' to the Common is Normally Open (NO, pin 14).
- Verify: Apply the rated control voltage (e.g., 24VDC) to the coil pins using a bench power supply. You should hear an audible click. Re-check continuity; the NO pin should now show continuity to Common, and the NC pin should read 'OL'.
FAQ: Control Relay Symbol Questions
What does the control relay symbol 'A1 and A2' mean?
In the IEC 60617 standard, A1 and A2 designate the terminals for the relay's electromagnetic coil. A1 is typically the line or positive terminal, and A2 is the neutral or negative terminal. On a physical 14-pin octagonal relay, these might map to specific base pins (like 7 and 8), but on standard DIN-rail relays, A1 and A2 are clearly printed on the top or front of the housing.
How do I read a Form C (SPDT) control relay symbol on a schematic?
A Form C symbol shows a single common line splitting into two paths: one Normally Open (NO) and one Normally Closed (NC). In practice, this means the relay acts as a transfer switch. When the coil is off, current flows through the NC path. When the coil energizes, the internal armature physically breaks the NC connection and bridges the NO path. It is physically impossible for both paths to be closed simultaneously.
Why does my control relay symbol have a diode across the coil?
This indicates a flyback (or freewheeling) diode, used exclusively on DC control circuits. When a DC coil is de-energized, the collapsing magnetic field generates a massive reverse voltage spike (inductive kickback) that can destroy sensitive PLC outputs or solid-state switches. The diode provides a safe loop for this spike to dissipate. If you see this symbol, you must strictly observe coil polarity; reversing it will short the power supply.
What is the difference between a control relay symbol and a contactor symbol?
While both use coils and contacts, a contactor symbol (often designated 'KM' or 'K' with higher amp ratings) includes an arc-chute symbol or indicates three-phase main power contacts (L1/T1, L2/T2, L3/T3) alongside auxiliary control contacts. Control relays (designated 'KA' or 'CR') are strictly for low-current logic and signaling (usually rated 5A to 10A max) and do not feature arc suppression hardware for switching heavy inductive motor loads.
