The standard schematic symbol for a relay consists of a rectangle (or circle) representing the electromagnetic coil, paired with switch symbols representing the contacts. However, the exact drawing changes drastically depending on whether your blueprint follows IEC 60617 (international) or ANSI/IEEE 315 (North American) standards. Misreading these symbols is the number one cause of wiring errors in control panels. Below is the definitive reference to decode any relay symbol you encounter, map it to physical pins, and select the exact replacement part.
Master Relay Schematic Symbol Reference Chart
Use this table to translate the geometric shapes on your schematic into physical relay behaviors. The IEC standard separates the coil and contacts visually, linking them only by an alphanumeric tag (e.g., K1), while ANSI often draws them mechanically linked.
| Component / Function | IEC 60617 Symbol Description | ANSI/IEEE 315 Symbol Description | Practical Bench Meaning |
|---|---|---|---|
| Relay Coil | Rectangle, labeled 'K' (e.g., K1) | Circle or rectangle, often with a diagonal line or 'CR' label | The electromagnet. Apply rated voltage (e.g., 12VDC) here to energize. |
| SPST-NO (Normally Open) | Two terminals with a bridging line angled away | Two terminals with a bridge angled away, often linked to coil | Circuit is OPEN until coil is powered. Used for start buttons and primary loads. |
| SPST-NC (Normally Closed) | Two terminals with a bridging line angled across, plus a slash | Bridge angled across with a slash or 'X' through it | Circuit is CLOSED until coil is powered. Used for stop buttons and safety interlocks. |
| SPDT (Single Pole Double Throw) | Three terminals: one COM, one NO, one NC | Same three-terminal layout, mechanical linkage drawn to coil | Switches a single input between two outputs. The most common general-purpose relay type. |
| Time-Delay (On-Delay) | Standard NO/NC symbol with an 'X' or double-slash on the contact arm | Standard symbol with an inward-pointing arrow or 'TD' label | Contacts change state only after a set time (e.g., 5s) AFTER the coil is energized. |
| Latching / Bistable | Rectangle with a small mechanical latch symbol or two opposing coils | Coil symbol with a small mechanical detent or 'L' designation | Requires only a pulse to change state. Maintains position when power is removed. |
Regional Standard Variants: IEC vs. ANSI/IEEE vs. Old BS
Before you start tracing wires, you must identify which drafting standard the engineer used. Mixing these up leads to catastrophic miswiring, especially with time-delay and latching circuits.
- IEC 60617 (Global/Europe/Modern): The coil is always a simple rectangle. The contacts are drawn completely detached from the coil, scattered across the schematic where they are logically needed. They are tied together solely by a reference designator (e.g., Coil K1 operates contacts K1-1, K1-2). This is the standard for modern PLC wiring diagrams.
- ANSI/IEEE 315 (North America): Older schematics draw the coil as a circle with a diagonal line; newer ones use a rectangle. Contacts are often drawn physically adjacent to the coil, connected by a dashed mechanical linkage line. This makes it easier to see the whole relay in one glance but clutters complex diagrams.
- Old UK BS 3939 (Legacy British): Obsolete since the 1990s but still found in legacy UK industrial plants and marine panels. The coil is drawn as a circle with a diagonal line, but the contacts use unique, highly stylized arcs that do not match modern IEC NO/NC conventions. If you are working on a pre-1990 UK machine, consult the IEC conversion charts or trace the circuit physically.
Rows and Symbols People Get Wrong (And How to Fix Them)
Even experienced technicians misinterpret specific relay symbol nuances. Here are the most common trapdoors and how to avoid them.
1. The Dashed Line is NOT a Wire
In ANSI schematics, a dashed line connecting the coil to the contacts represents mechanical linkage, not an electrical connection. If you wire 120VAC into that dashed line thinking it is a neutral return, you will short the control circuit. Treat dashed lines as invisible physical gears.
2. NO vs. NC Slash Confusion
People frequently misread the slash on a Normally Closed (NC) symbol as a 'closed' gate. In both IEC and ANSI, a line drawn diagonally across the contact bridge (often looking like a strike-through) means NC. If the bridge is simply angled away without the strike-through, it is NO. When in doubt, verify with a multimeter: an unpowered NC contact reads < 1 ohm; an unpowered NO contact reads OL (Open Loop).
3. Time-Delay 'X' Direction
An 'X' or double-slash on the movable contact arm indicates a time delay. But which way? According to Macromatic's relay symbol guide, if the 'X' is drawn on the side of the contact that closes when energized, it is an On-Delay. If it is on the side that opens, it is an Off-Delay. Always check the timing diagram in the corner of the schematic rather than guessing from the symbol alone.
Decision Tree: Selecting the Right Physical Relay for Your Schematic
When your schematic calls for a relay but doesn't specify a manufacturer part number, use this decision path to select the exact physical component you need to buy.
| IF your schematic requires... | AND your load is... | THEN buy this exact part number: |
|---|---|---|
| Standard SPDT (Form C) | < 10A at 120/240VAC or 30VDC | Omron G2R-1-E (Specify coil voltage, e.g., 12VDC) |
| DPDT (Form C, dual pole) | Motor reversing or dual isolated circuits | Finder 40.52 or Omron G2R-2-S |
| Latching / Bistable | Battery-powered or state-memory needed | Panasonic TQ2-L2-12V (Dual coil latching) |
| High Inrush (Motor/Transformer) | Inductive loads with 10x inrush current | Omron G7L-2A-TUB (High inrush SPST-NO) |
Safe Interpretation When Markings Are Faded or Missing
In older control panels or salvaged equipment, the physical relay label might be sun-faded, painted over, or entirely missing, and the original schematic is long gone. You must safely map the pins before applying power.
Follow this bench procedure to map an unknown 8-pin (DPDT) or 5-pin (SPDT) relay using a standard digital multimeter (DMM):
- Find the Coil: Set your DMM to resistance (Ohms). Probe pairs of pins. The coil pins will typically read between 50 and 400 ohms (for 5V to 24V DC coils) or 2k to 10k ohms (for 120VAC coils). Mark these two pins as A1 and A2 (or Coil + and -).
- Find the COM and NC: Switch the DMM to continuity mode (beep). Probe the remaining pins. You will find one pin (COM) that has continuity (< 1 ohm) to a second pin (NC). Mark them.
- Identify the NO: The remaining pin on that pole is the Normally Open (NO) contact. It should read OL (Open Loop) against the COM pin.
- Verify with Power: Connect a variable bench power supply to the coil pins. Set it to the suspected coil voltage (start at 5V, step up to 12V, then 24V). When you hit the correct voltage, you will hear an audible 'click'. While holding the voltage, re-test the contacts with your DMM. The COM-to-NC should now read OL, and COM-to-NO should read < 1 ohm.
By combining a firm understanding of IEC and ANSI schematic symbols with physical multimeter verification, you eliminate the guesswork from relay wiring. Always default to socketed, industry-standard parts like the Omron G2R series for new builds, ensuring that when a relay eventually fails, replacing it takes 10 seconds rather than requiring a full panel rewire.
