When performing a symbolic analysis of relay and switching circuits, the first rule is identifying the governing standard. Globally, IEC 60617 dictates rectangular coils and numeric pin designations (A1/A2, 11/12/14), while North American IEEE 315/NEMA schematics often use circular coils and alphanumeric letter designations (CR, M). Below is the direct reference data you need to read, draw, and troubleshoot these circuits without guessing.
The Master Relay & Switch Symbol Reference Table
This table maps the physical component to its schematic representations across the two dominant global standards. Use this as your bench-side cheat sheet when tracing control wiring.
| Component / Function | IEC 60617 Symbol & Pins | NEMA / IEEE 315 Symbol & Pins | Practical Meaning & Bench Notes |
|---|---|---|---|
| Relay Coil (Standard) | Rectangle. Pins: A1 (+), A2 (-) | Circle or Semi-circle. Label: CR (Control Relay) | The electromagnet. A1/A2 are universal IEC designations. Polarity matters only if a built-in flyback diode or LED indicator is present. |
| SPST-NO (Form A) | Line with slash, open. Pins: 13, 14 | Two parallel lines, open. Label: 1CR-1 | Normally Open. Closes when coil energizes. Used for start circuits and holding seals. |
| SPST-NC (Form B) | Line with slash, closed. Pins: 11, 12 | Two parallel lines, closed with diagonal. Label: 1CR-2 | Normally Closed. Opens when coil energizes. Used for interlocks, stop buttons, and safety chains. |
| SPDT (Changeover / Form C) | Common line splitting to NO/NC. Pins: 11 (Com), 12 (NC), 14 (NO) | Common line splitting. Label: 1CR-3 | The most common industrial contact block. Pin 11 is the wiper/common. |
| DPDT (Double Pole) | Two SPDT blocks mechanically linked (dashed line). Pins: 21/22/24 | Two NEMA SPDT blocks linked. Label: 2CR | Switches two isolated circuits simultaneously. Second pole uses the '20' series numbers. |
| Time-Delay (On-Delay) | Rectangle with inward arrow on contact. Pins: 15/16/18 | Circle with 'TD' and inward arrow | Contacts change state only after the preset time elapses post-energization. Arrow points toward the contact line. |
| Time-Delay (Off-Delay) | Rectangle with outward arrow on contact. Pins: 25/26/28 | Circle with 'TD' and outward arrow | Contacts change state immediately on energization, but delay returning to normal when power is cut. Arrow points away. |
| Latching (Bistable) | Rectangle with a small latch/block symbol inside | Circle with 'L' or mechanical latch symbol | Remains in last state after power is removed. Requires a secondary pulse (often to A2/A3) to reset. |
Regional Standard Variants: IEC vs. NEMA vs. Old UK
A schematic drawn in Germany looks fundamentally different from one drawn in Ohio, even if the physical wiring is identical. Knowing which standard applies to your region prevents catastrophic miswiring.
- IEC 60617 (Global/EU/Modern UK/AUS): Uses strict numeric pin designations. The coil is always A1 and A2. Contacts use a two-digit system: the first digit indicates the pole number (1, 2, 3, 4), and the second digit indicates the function (1/2 for NC, 3/4 for NO, 5/6 for NC time-delay, 7/8 for NO time-delay). Example: Pin 14 is Pole 1, NO.
- NEMA / IEEE 315 (North America): Relies heavily on alphanumeric device function numbers and letters. A control relay is 'CR', a motor starter is 'M', a timer is 'TR'. Contacts are identified by the device letter and a sequence number (e.g., 1CR-1, 1CR-2). Physical pin numbers are rarely shown on the schematic; the electrician relies on the manufacturer's wiring diagram on the panel door.
- Old UK (BS 3939 - Legacy): Largely superseded by IEC 60617, but still found in panels built before the late 1990s. It used similar rectangular coils but often represented contacts with older, more stylized mechanical linkage lines rather than the clean IEC slashes. If you see a mix of BS 3939 and IEC, treat the numeric pinouts as IEC but verify physical continuity.
The 'Rows People Get Wrong' Trap
Even experienced technicians misread specific relay symbols and pinouts. Here are the most common traps and how to avoid them.
1. Confusing the 'Common' Pin on Changeover (SPDT) Relays
In IEC numbering, the common pin is always the lowest odd number of the pole group. For Pole 1, common is 11. For Pole 2, common is 21. People frequently wire the load to 12 (NC) or 14 (NO) thinking it is the common feed, resulting in a circuit that is either permanently dead or bypasses the relay logic entirely. Always trace the 'wiper' line on the schematic back to the numbered common terminal.
2. Time-Delay Arrow Direction
The arrow on a time-delay contact symbol is not decorative; it dictates the physical timing mechanism.
Rule: The arrow points in the direction of the delayed movement.
If the arrow points toward the contact line (closing the gap), it is an On-Delay (the closing is delayed). If the arrow points away from the contact line (opening the gap), it is an Off-Delay (the opening is delayed). Reversing these in a safety interlock circuit can cause machinery to start before guards are fully locked.
3. Assuming Coil Polarity Doesn't Matter
While a raw electromagnetic coil will pull in the armature regardless of DC polarity, modern industrial relays (like the Omron G2R series or Finder 55) often include built-in suppression diodes or LED status indicators. If you wire A1 to negative and A2 to positive on a relay with a built-in flyback diode, you will create a dead short across your DC power supply the moment the transistor switches on, instantly blowing the control fuse.
Safe Interpretation When Markings Are Faded or Missing
Industrial environments are harsh. UV exposure, oil mist, and heat frequently obliterate the printed pinout diagrams on relay sockets. Never guess. Use this multimeter procedure to safely map an unmarked 8-pin or 11-pin DIN relay base.
- Locate the Coil (A1/A2): Set your multimeter to resistance (Ohms). Probe pairs of pins. A standard 24VDC industrial relay coil will read between 400Ω and 800Ω (e.g., an Omron G2R-1 24VDC coil is ~650Ω). A 12VDC coil will read roughly 100Ω to 200Ω. If you read 'OL' (Open), the coil is burned. If you read near 0Ω, it is shorted. The two pins showing this specific resistance range are your coil.
- Map the Commons: Switch to continuity mode (beep). Find the pins that show continuity to multiple other pins when the armature is manually actuated. On an 11-pin relay (3PDT), the commons are typically pins 1, 4, and 7 (or 11, 21, 31 in IEC socket numbering).
- Identify NO vs NC: With the relay unpowered (armature at rest), probe the common against the remaining pins in that pole group. The pin that beeps is Normally Closed (NC). The pin that is silent is Normally Open (NO). Actuate the armature manually with a small flathead screwdriver; the states will reverse, confirming your mapping.
Decision Path: Choosing the Right Relay Symbol & Part for Your Circuit
Stop defaulting to whatever is in the parts bin. Use this decision matrix to select the exact relay architecture and a concrete, reliable part number for your next build or panel upgrade.
| Circuit Condition / Load Profile | Required Symbol Architecture | Concrete Part Recommendation (Default Pick) |
|---|---|---|
| Low Voltage / PCB Logic (< 2A, 5V-12V DC, switching microcontrollers or small solenoids) |
SPST-NO or SPDT (IEC: 13/14 or 11/12/14) |
Omron G6B-1114P-US (12VDC) Compact, 8A rating handles unexpected inrush, gold-clad contacts prevent low-voltage oxidation. |
| Standard Industrial Control (5A - 10A, 24VDC/120VAC coil, DIN rail, switching contactor coils or indicator banks) |
DPDT or 4PDT (IEC: 2-pole or 4-pole changeover) |
Finder 55.34.9.024.0000 The gold standard. 4PDT, 24VDC coil, 7A per pole. Plug-in base makes troubleshooting trivial. Use with Finder 94.74 socket. |
| High Inrush / Motor Loads (> 10A, highly inductive, switching 3-phase motors, heavy ballasts, or capacitive banks) |
3-Pole NO (Contactor) (IEC: L1/L2/L3 to T1/T2/T3) |
Schneider TeSys D (LC1D09) Standard relays will weld their contacts shut under high inrush. You must step up to a dedicated contactor with arc chutes. |
| Safety / E-Stop Interlocks (Must guarantee contact separation even if welded) |
Force-Guided (Mechanically Linked) (IEC: NO/NC with inverted 'U' link symbol) |
Pilz PNOZ p or Omron G7SA Standard relays can fail with NO and NC contacts closed simultaneously. Force-guided relays physically prevent this, satisfying ISO 13849 safety requirements. |
The Default Industrial Pick: If you are building a standard 24VDC control panel and need a reliable, universally understood switching component, standardize on the Finder 55.34 series (4PDT, 24VDC). Its 4-pole architecture gives you maximum flexibility (you can use two poles for NO logic and two for NC interlocks on the same device), and the plug-in socket with integrated LED/clip eliminates the need for complex symbolic analysis when troubleshooting in the field—you simply pull the relay, swap it, and verify the light.
