When troubleshooting a control panel or designing a PCB, misreading a relay circuit symbol can lead to dead shorts, unswitched loads, or destroyed coils. The exact mapping of schematic symbols to physical relay contacts depends heavily on the standard used to draft the print. Below is the direct reference data you need to decode the schematic and select the correct physical component.

The Master Relay Circuit Symbol Reference Table

This table maps the core relay functions to their visual representations in the two dominant global standards, alongside the physical contact designations you will see on datasheets.

Function IEC 60617 Symbol Shape ANSI/IEEE 315 Symbol Shape Contact Designation 14-Pin Base Mapping
Relay Coil Rectangle (often with diagonal lines for AC) Circle (with letter 'K' or 'CR' inside) N/A Pins 2 & 7 (or 13 & 14 on 8-pin)
Normally Open (NO) Two parallel lines, one with a gap and angled line Two parallel lines, top line angled up Form A / SPST-NO Pins 9 & 5 (Pole 1), 12 & 8 (Pole 2)
Normally Closed (NC) Two parallel lines connected by a diagonal cross-line Two parallel lines, top line angled down with cross Form B / SPST-NC Pins 9 & 1 (Pole 1), 12 & 4 (Pole 2)
Changeover (SPDT) Combination of NO and NC sharing a common pivot point Combination of NO and NC sharing a common line Form C / SPDT 9-1-5 (Pole 1), 12-4-8 (Pole 2)
Time-Delay (On-Delay) Rectangle with an inward-pointing arrow on the armature Circle with an inward-pointing arrow Delayed Form A/B Varies by specific timer module
Time-Delay (Off-Delay) Rectangle with an outward-pointing arrow on the armature Circle with an outward-pointing arrow Delayed Form A/B Varies by specific timer module
Latching (Magnetic) Rectangle with a small mechanical latch symbol (crossed boxes) Circle with a mechanical latch symbol Latching Form A/B/C Requires dual-coil or pulse base

Regional Standard Variants: IEC vs. ANSI/IEEE vs. Old UK

A relay circuit symbol is not universal. The visual language changes depending on where the schematic was drafted and when. Knowing which standard you are looking at prevents catastrophic miswiring.

Bench Tip: If the coil is drawn as a rectangle, you are looking at an IEC 60617 drawing (standard in Europe, Asia, and modern global projects). If the coil is drawn as a circle, you are looking at an ANSI/IEEE 315 or NEMA drawing (standard in North America).
  • IEC 60617 (Global/EU): Uses rectangular blocks for coils and distinct geometric lines for contacts. It heavily relies on alphanumeric pin designators (e.g., A1/A2 for coils, 11/12/14 for contacts) printed directly next to the symbol.
  • ANSI/IEEE 315 (North America): Uses circles for coils, often labeling them with device function numbers like 'CR' (Control Relay) or 'K'. Contacts are drawn with more literal 'switch' angles and are often linked to the coil via dashed lines rather than alphanumeric codes.
  • BS 3939 (Old UK - Obsolete): You will only see this on legacy British industrial panels pre-dating the 1990s. It uses highly stylized, almost pictographic representations of the physical relay armature. If you encounter this, treat it as an archaeological artifact and trace the physical wires; do not trust the symbol geometry to match modern IEC logic.

The 'Rows People Get Wrong' Notes

Even experienced technicians misinterpret specific relay symbols when reading complex ladder logic or schematic diagrams. Watch out for these three common traps.

1. Form C (SPDT) vs. Two Separate Relays

A single Form C (changeover) contact is drawn with a common pivot point feeding both a NO and NC terminal. Beginners often mistake this for two separate Form A (NO) relays drawn close together. The fix: Look for the mechanical linkage line (a dashed or solid line connecting the armatures). If the NO and NC contacts share a single physical pivot point and a single common pin, it is one SPDT relay. If they have separate coils, they are two distinct relays.

2. Time-Delay Arrow Directions

In IEC symbols, the arrow on the time-delay relay indicates the direction of the delay, not the physical movement of the contact. An inward-pointing arrow (pointing toward the coil) means On-Delay (the contact delays closing after the coil is energized). An outward-pointing arrow (pointing away from the coil) means Off-Delay (the contact delays opening after the coil is de-energized). Reversing these in a safety interlock circuit will defeat the purpose of the timer.

3. Latching vs. Standard Coils

A standard relay requires continuous voltage to hold the contacts. A latching relay (often drawn with a small cross-hatched box next to the coil) only requires a momentary pulse to change state, and it stays mechanically locked until a reset pulse is applied. Wiring a continuous 24VDC signal to a dual-coil latching relay will burn out the coil in seconds, as they are rated only for momentary duty (typically < 1 second).

Decision Path: Identifying a Relay with Faded or Missing Markings

When you are troubleshooting a panel and the physical relay's label is faded, burned, or missing, and you don't have the original schematic, use this multimeter decision tree to map the pins and determine the replacement.

Safety Warning: Always de-energize the panel, lock out the breaker, and verify dead with a tested CAT III/IV multimeter before probing relay pins. Never probe for continuity on a live circuit.
Step Multimeter Action Expected Reading Conclusion / Next Step
1. Find the Coil Set DMM to Resistance (Ohms). Probe all pin combinations looking for a specific resistance range. 70-300 Ω (12VDC), 600-1200 Ω (24VDC), or >2000 Ω (120VAC). Pins showing this resistance are your Coil (usually 2 & 7 or 13 & 14). Pins showing 0.0 Ω or OL are contacts.
2. Find the Common (Pole) Set DMM to Continuity (Beep). With relay unpowered, probe the remaining pins to find which pin connects to two others. Pin X beeps with Pin Y, AND Pin X beeps with Pin Z. Pin X is your Common. Pin Y is NC. Pin Z is NO.
3. Verify NO/NC Apply rated voltage to the coil pins (use a bench supply, not mains). Re-test continuity between Common and the other two pins. Common now beeps with Pin Z, and is open (OL) with Pin Y. Confirmed: Pin Y is Normally Closed, Pin Z is Normally Open. You now have the complete pinout.

Concrete Part Mapping: From Symbol to Real-World Component

Once you have decoded the relay circuit symbol and mapped the physical pins, you need to source the exact replacement. Do not buy generic '14-pin relays' without verifying the coil voltage and contact rating. Here are the exact part numbers for the most common industrial configurations based on standard schematic symbols.

  • Schematic shows: DPDT (2x Form C), 10A contacts, 24VDC Coil.
    Buy: Omron LY2N-DC24 or Finder 55.34.9.024.0040. Both are industry-standard 14-pin plug-in relays with bifurcated contacts suitable for PLC output switching.
  • Schematic shows: DPDT (2x Form C), 10A contacts, 120VAC Coil.
    Buy: Omron LY2N-AC120. Ensure you use the AC-specific part number; the coil impedance and shading ring design are entirely different from the DC variant.
  • Schematic shows: 4PDT (4x Form C), 5A contacts, 24VDC Coil.
    Buy: Omron MY4N-DC24. This uses the same 14-pin base but breaks the contacts into 4 poles instead of 2. Note the lower 5A per-pole current rating compared to the LY series.

For deeper study on standard electrical schematic conventions, refer to the All About Circuits relay chapter, and always cross-reference physical replacements with the manufacturer's official datasheets, such as the Omron Relays family documentation, to verify exact terminal numbering and derating curves for inductive loads.