The standard relay switch symbol consists of a rectangle or circle (representing the electromagnetic coil) and switch contacts (NO/NC) linked by a dashed mechanical line. However, the exact geometry changes drastically depending on whether you are reading an IEC (International), ANSI/NEMA (North American), or legacy schematic. Misreading these symbols in a control panel can lead to catastrophic short circuits or logic failures. Below is the direct reference data you need to interpret these schematics accurately.
Relay Switch Symbol Reference Chart
This table maps the most common relay components to their respective international and North American graphical standards. Use this as your bench reference when tracing control logic.
| Component / Function | IEC Symbol (60617) | ANSI/NEMA Symbol (IEEE 315) | Function in Practice & Standard Pinouts |
|---|---|---|---|
| Relay Coil | Rectangle (often with A1/A2 labels) | Circle or Rectangle with a diagonal line | Energizes the electromagnet. IEC pins: A1 (+/L), A2 (-/N). |
| SPST-NO (Form A) | Line angled away from contact point | Line with a hinge/paddle not touching contact | Normally Open. Closes when coil energizes. IEC pins: 13 & 14. |
| SPST-NC (Form B) | Line angled touching the contact point | Line with hinge overlapping the contact point | Normally Closed. Opens when coil energizes. IEC pins: 11 & 12. |
| SPDT (Form C) | Two lines sharing a common pivot point | Hinged paddle between two stationary contacts | Changeover contact. Switches from NC to NO. IEC pins: 11, 12, 14. |
| DPDT Relay | Two Form C symbols linked by dashed line | Two Form C symbols linked by dashed line | Two isolated changeover circuits. Pins: 9-12-14 and 10-11-13. |
| Latching Relay Coil | Rectangle with a diagonal arrow inside | Circle with a mechanical latch indicator | Maintains state without continuous power. Requires pulse to reset. |
| Time-Delay (On-Delay) | Rectangle with arrow pointing toward contact | Circle with a clock face or specific arrow | Delays contact closure after coil receives power. |
Regional Standard Variants and Faded Marking Protocols
Electrical schematics are not universal. The standard that applies to your region dictates how the IEC graphical symbols or NEMA electrical symbols are drawn on the panel door.
IEC vs. ANSI/NEMA vs. Legacy UK
- IEC 60617 (Global/Europe/Modern US): Highly minimalist. Relies on rectangles for coils and simple angled lines for contacts. It assumes the reader understands the alphanumeric pin designations (A1/A2, 13/14) printed next to the symbol.
- ANSI/IEEE 315 & NEMA ICS 19 (North America): More pictorial. Coils are frequently drawn as circles. Switch contacts feature distinct 'hinges' or 'paddles' to visually represent the physical armature movement. This is standard in older US industrial panels and HVAC control boards.
- Legacy UK (BS 3939): Largely superseded by IEC standards in 1996, but still found in older British industrial installations. These symbols often used semicircles and specific cross-hatching that can easily be confused with modern capacitor or resistor symbols if you aren't aware of the panel's vintage.
Safe Interpretation of Faded or Missing Markings
If you are troubleshooting a legacy control panel where the sun or oil mist has faded the schematic, do not guess the relay logic based on half-visible symbols. Follow this physical verification protocol:
- De-energize and Lockout/Tagout (LOTO): Kill the mains feed and verify zero voltage with a CAT III/IV multimeter (e.g., Fluke 117).
- Locate the Physical Relay: Match the physical component to the faded schematic box.
- Map the Coil: Look for A1 and A2 (or 2 and 7 on older octal bases). Apply the rated control voltage (e.g., 24VDC) temporarily using a bench power supply to hear the mechanical 'click'.
- Continuity Test: With power removed, set your multimeter to continuity. Probe the common (COM) pin against the adjacent pins. The pin that beeps before energizing is NC; the pin that beeps after energizing is NO. Map these physical pins back to your schematic manually.
Rows People Get Wrong: Common Misinterpretations
When reading complex ladder logic or PLC schematics, even experienced technicians misinterpret specific relay switch symbols. Here are the most common pitfalls documented in standard electrical symbol references.
1. Confusing Form C (SPDT) with Two Separate Relays
A Form C (SPDT) symbol shows a common line branching into two contacts. In dense ANSI schematics, this can look like two separate SPST relays drawn close together. The tell: Look for the dashed mechanical linkage line connecting the contact arm to the coil rectangle. If the dashed line connects back to a single coil, it is one SPDT relay. If there are two distinct dashed lines going to two different coils, they are independent relays.
2. Latching Relay Diagonal Line vs. Standard Coil Diagonal
In ANSI schematics, a standard relay coil is sometimes drawn as a circle with a diagonal line through it. However, a latching relay coil in IEC schematics is a rectangle with a diagonal arrow. If you mistake a latching relay for a standard continuous-duty relay, you will wire the control logic incorrectly. A latching relay requires a momentary pulse to set, and a separate pulse (or reverse polarity) to reset. Wiring it as a continuous hold will fry the coil.
3. Time-Delay Arrow Direction
Time-delay relays feature a small arrow inside or next to the coil box. The direction of this arrow is critical:
- Arrow pointing TOWARD the contact line: On-Delay. The timer starts when the coil is energized, and the contact changes state after the preset time.
- Arrow pointing AWAY from the contact line: Off-Delay. The contact changes state immediately upon energizing, but delays returning to its normal state after power is removed.
Frequently Asked Questions
What is the difference between a relay switch symbol and a contactor symbol?
While both use electromagnetic coils to move contacts, a contactor symbol includes additional graphical elements indicating arc suppression. In IEC schematics, contactor power contacts often feature a small 'bridge' or 'loop' drawn over the contact line, representing an arc chute. Additionally, contactors always include auxiliary contact symbols (usually labeled with 13/14 or 21/22) drawn separately from the main power poles to indicate feedback loops for the PLC.
How do I identify the relay switch symbol for a solid-state relay (SSR)?
Solid-state relays do not have moving mechanical parts, so they lack the dashed mechanical linkage line. Instead, the SSR symbol features an optocoupler design: an LED symbol on the input side (control) pointing toward a phototransistor or TRIAC symbol on the output side (load). If you see a relay symbol with a light beam or diode arrow crossing an isolation barrier, you are looking at an SSR.
What does a dashed line mean in a relay switch symbol?
The dashed line represents a mechanical linkage. It indicates that the movement of the switch contact is physically driven by the adjacent coil (or by the same mechanical armature in a multi-pole relay). If a switch symbol lacks a dashed line connecting it to a coil, it is a manually operated switch (like a pushbutton or limit switch) or a sensor, not a relay contact.
Which relay switch symbol standard applies to my US-based home automation panel?
If you are wiring a standard 120V/240V residential panel in the US, the electrician will likely use ANSI/NEMA symbols. However, if your home automation setup utilizes imported DIN-rail PLCs (such as Siemens LOGO! or Phoenix Contact relays), the manufacturer's documentation and the schematic printed on the side of the relay will strictly use IEC 60617 symbols. Always check the manufacturer's datasheet for the specific relay model to confirm the standard used.
