The Single Pole, Double Throw (SPDT) relay is the workhorse of industrial control and DIY automation. It features one common contact that switches between a Normally Open (NO) and Normally Closed (NC) terminal. However, the SPDT relay symbol you see on a schematic changes drastically depending on whether the print was drawn in Europe, North America, or on a legacy machine from the 1980s. Misreading the coil pins or contact designations can instantly fry a low-voltage driver or cause a mains short.

Below is the definitive reference for identifying, interpreting, and wiring SPDT relays across all major standards, terminating in a concrete decision path for unmarked components.

The SPDT Relay Symbol & Pinout Reference Table

Use this table to match the schematic symbol or physical pin stamping to the correct standard and terminal function. This covers the three dominant standards you will encounter on the bench or jobsite.

Standard / Region Symbol Style Coil Designation Common (COM) Normally Open (NO) Normally Closed (NC)
IEC 60617 (Global, EU, Modern UK) Rectangle for coil; angled line for contacts A1 (+), A2 (-) 11 (or 1) 14 (or 3) 12 (or 2)
IEEE 315 / ANSI (US, Canada, NEMA) Circle/loop for coil; hinged arm for contacts CR (Control Relay) or just a circle C (or COM) NO NC
Old UK / BS 3939 (Legacy panels pre-1990) Rectangle with diagonal line; switch blade L1, L2 or +, - C N/O N/C
5-Pin PCB Standard (e.g., Omron G2R, Songle) Physical pin layout (not schematic) Pins 1 & 5 (or 2 & 5 depending on mfr) Center Pin (Pin 3) Pin 4 Pin 2 (or 1)
8-Pin Octal Socket (e.g., Omron LY1, Finder) Physical plug-in base (circular) Pins 2 & 7 Pin 8 Pin 6 Pin 5
Bench Tip: When reading IEC schematics, the first digit of a two-digit contact number indicates the sequence (e.g., '1' for the first SPDT pole), while the second digit indicates the function ('1' or '2' for NC, '3' or '4' for NO).

What Each Symbol and Pin Designation Means in Practice

Understanding the schematic is only half the battle; you must map it to physical hardware. Here is how the abstract symbols translate to real-world bench behavior.

  • The Coil (A1/A2 or CR): This is the electromagnet. In IEC, A1 is typically the positive or hot side, and A2 is negative or neutral. If you see a diode symbol drawn in parallel with the coil (cathode pointing to A1), it indicates a built-in flyback diode. Do not apply AC voltage to a DC coil with a built-in diode; it will short the AC cycle and destroy the coil.
  • Common (COM / 11): The moving armature. This is your line voltage input or signal source. In high-current AC applications, the COM terminal takes the brunt of the arc wear.
  • Normally Open (NO / 14): The contact that closes only when the coil is energized. Use this for "start" circuits or enabling loads.
  • Normally Closed (NC / 12): The contact that is closed when the relay is at rest (de-energized). Use this for safety interlocks, emergency stop routing, or "fail-safe" defaults.

Rows and Markings People Get Wrong

When troubleshooting or reverse-engineering a board, these specific symbol interpretations and physical markings cause the most blown components and miswired panels.

1. The 8-Pin Octal Coil Pin Trap

Many hobbyists and junior techs assume pins 1 and 8 are always the coil on an 8-pin plug-in relay because that is true for DPDT relays (like the Omron LY2). However, on a true SPDT 8-pin relay (like the Omron LY1), the coil is almost always on pins 2 and 7. Applying 120VAC to pins 1 and 8 on an LY1 will route mains voltage directly through the contact block, potentially dead-shorting the line or energizing the load unpredictably.

2. Misinterpreting the IEC "Knife Switch" Contact

In IEC 60617, the SPDT contact is drawn as a single angled line (the wiper) resting against a fixed point (NC) with a gap to another fixed point (NO). People often mistake the gap for a capacitor symbol or assume the line represents a ground. Remember: the angled line is the COM, the point it touches is NC, and the point it points toward (but doesn't touch) is NO.

3. Faded Markings on Legacy Finder and Schneider Relays

On older DIN-rail relays (like the Finder 40 series), the stamped pinout diagram on the side of the plastic housing often rubs off due to panel vibration and heat. Never guess based on the physical proximity of the pins. If the stamping is faded, you must use the multimeter decision path below.

SAFETY WARNING: Relays are frequently used to switch mains voltage (120V/230V AC). Before probing any relay terminals or removing a relay from its socket, de-energize the circuit, lock out the breaker, and verify the circuit is dead using a CAT III or CAT IV rated multimeter. Never probe for continuity on a live circuit.

Regional Standards: Which Symbol Set Applies to You?

Your region and the age of the equipment dictate which electrical relay standard you will encounter.

  • IEC 60617 (International/EU/Modern UK): The global standard. Uses the rectangle-and-angle-line format. If you are building a new panel in Europe, or working on modern PLC-controlled machinery anywhere in the world, use IEC. The UK transitioned fully to IEC harmonized colors and symbols post-1990.
  • IEEE 315 / ANSI Y32.2 (North America): The dominant standard in the US and Canada for industrial and commercial schematics. It uses the circle/loop for the coil and a hinged, gravity-dropping arm for the contacts. If you are reading prints from a US-based manufacturer (like Allen-Bradley or Cutler-Hammer), expect ANSI.
  • Old UK / BS 3939 (Legacy): Obsolete, but you will still find it in British factories built before the 1990s. It looks similar to IEC but uses older alphanumeric labeling. Treat these panels with extreme caution, as the wiring color codes inside the panel will also follow the old UK standard (e.g., black for phase, red for neutral) rather than modern brown/blue.

Decision Path: Wiring an Unmarked or Faded SPDT Relay

If you pull a relay from a board or a legacy panel and the schematic symbol or physical pinout stamping is missing, faded, or illegible, follow this exact decision tree to map the pins safely and select a replacement.

Step Condition / Observation Action to Take Expected Measurement / Result
1. Visual Pin Count Relay has 5 physical pins (PCB mount) Identify the two pins furthest apart or isolated from the center cluster. These are the coil. Proceed to Step 2.
1. Visual Pin Count Relay has 8 physical pins (Octal plug-in) Locate the keyway (the blank spot or notch) on the circular base. The pins immediately adjacent to the keyway are usually the coil. Proceed to Step 2.
2. Find the Coil (Multimeter) Set DMM to Resistance (Ohms). Probe suspected coil pins. Look for a specific resistance range based on nominal voltage. 12VDC: 70-150Ω. 24VDC: 300-650Ω. 120VAC: 2kΩ-10kΩ. If it reads 0.0Ω (short) or OL (open), the coil is dead. Trash the relay.
3. Map the Contacts Set DMM to Continuity/Beep mode. Probe remaining pins. Find the pair that beeps (NC). Then apply rated DC voltage to the coil to hear the click, and check which new pair beeps (NO). The pin common to both the NC and NO pairs is your COM terminal.
4. Check for Diode Set DMM to Diode Test mode. Probe the coil pins. Check for a 0.5V - 0.7V drop in one direction and OL in the other. If present, it's a DC coil with a flyback diode. Polarity matters: A1 must be positive.
5. Final Replacement Pick You need a reliable, modern replacement part. Order the industry-standard equivalent based on form factor. 5-Pin PCB: Omron G2R-1-E (10A, 250VAC).
8-Pin Plug-in: Omron LY1-AC120 (15A, 120VAC coil) + PYF-08A socket.

By mapping the physical pins with a multimeter rather than guessing based on a faded plastic stamping, you eliminate the risk of wiring mains voltage into a low-voltage microcontroller GPIO. When in doubt, default to the IEEE 315 / ANSI standard for North American documentation, and always terminate your troubleshooting path with a verified, name-brand replacement like the Omron G2R or LY series to ensure contact ampacity ratings are actually met.