When reading a schematic, the base representation for any relay is a simple rectangle denoting the coil. However, when you are looking for a symbol time delay relay, that rectangle is modified with specific timing indicators to denote the delay function. Under the global IEC 60617 standard, this is typically an "X" drawn inside the coil box or a crossed diagonal line. Under the North American ANSI/IEEE C37.2 standard, it is represented by a rectangle with a specific arrow or notch indicating the delay direction. Understanding these variations is critical, as misinterpreting an off-delay symbol for an on-delay can result in catastrophic sequencing failures in motor controls and HVAC systems.
The Master Time Delay Relay Symbol Reference Table
The table below maps the four primary time delay functions across the three dominant global standards. Use this as your bench reference when cross-referencing legacy schematics with modern replacement units.
| Timing Function | IEC 60617 Symbol Indicator | ANSI/IEEE C37.2 Indicator | NEMA ICS 18 Code | Typical Industrial Use Case |
|---|---|---|---|---|
| On-Delay (TON) | Rectangle with an "X" inside; contacts show a right-pointing arrow. | Rectangle with a right-pointing arrow on the contact arm. | TD-1 (or TR-1) | Soft-starting large blower motors; delaying compressor engagement after power restoration. |
| Off-Delay (TOF) | Rectangle with an "X" and a diagonal slash; contacts show a left-pointing arrow. | Rectangle with a left-pointing arrow on the contact arm. | TD-2 (or TR-2) | Keeping cooling fans running after a main drive motor is shut down to dissipate heat. |
| Interval (One-Shot) | Rectangle with an "X" and double slashes; bidirectional arrows on contacts. | Rectangle with a bidirectional arrow or a specific notch. | TD-3 (or TR-3) | Dosing pumps in water treatment; actuating a solenoid valve for a precise 3-second burst. |
| Repeat Cycle | Two overlapping rectangles with "X" markers and opposing arrows. | Not explicitly standardized in basic C37.2; usually drawn as a custom block diagram. | TR-4 (Asymmetric) | Flashing warning beacons; automated lubrication cycles on CNC machinery. |
In practice, the IEC standard focuses heavily on the contact behavior (the arrows on the switch symbol indicate whether the contact delays making or breaking). The ANSI standard focuses more on the coil symbol itself. If you are working on equipment imported from Europe (like Siemens or ABB panels), expect IEC 60617. If you are troubleshooting older US-manufactured Allen-Bradley or Cutler-Hammer MCCs (Motor Control Centers), you will encounter ANSI and NEMA designations.
Regional Standard Variants and Octal Pinouts
While the schematic symbols tell you what the relay does, the physical pinout tells you how to wire it. The vast majority of plug-in time delay relays use either an 8-pin or 11-pin octal base. The pin numbering is standardized globally, but the internal routing varies by manufacturer.
8-Pin vs 11-Pin Octal Base Mapping
Below is the standard pinout for the most common plug-in TDRs, such as the ubiquitous Omron H3Y-2 (8-pin) and the Schneider Electric RXM/H3Y equivalents (11-pin). Note that the coil pins are the most critical to identify correctly, as applying 120VAC to a 24VDC coil will instantly destroy the internal timing circuitry.
| Pin Function | 8-Pin Octal (DPDT) | 11-Pin Octal (3PDT) | IEC Terminal Designation |
|---|---|---|---|
| Coil Power | Pins 2 and 7 | Pins 2 and 10 | A1 and A2 |
| Common (C) | Pins 3 and 6 | Pins 3, 6, and 11 | 11, 21, 31 (or x5/x6) |
| Normally Closed (NC) | Pins 1 and 4 | Pins 1, 4, and 7 | 12, 22, 32 (or x7/x8) |
| Normally Open (NO) | Pins 8 and 5 | Pins 9, 12, and 5 | 14, 24, 34 (or x5/x6) |
Source reference: For detailed functional block diagrams of these timing modes, consult the Macromatic Time Delay Relay Functions guide, which remains the industry benchmark for North American TDR logic.
Rows People Get Wrong and Faded Marking Protocols
Even experienced technicians misinterpret specific rows in relay logic tables. The most common point of failure is the Off-Delay (TOF) function.
Safe Interpretation When Markings are Faded or Missing
On older panels, the printed schematic on the side of a Finder or Omron relay often rubs off due to heat and vibration. If you cannot read the symbol time delay relay schematic on the casing, do not guess the pinout. Follow this bench procedure:
- De-energize and Verify: Shut off the control circuit breaker. Use a CAT III/IV multimeter to verify zero voltage across the coil terminals and the socket base.
- Locate the Coil: Set your multimeter to Resistance (Ω). Probe the pins. On a 24VDC relay, the coil will typically read between 100Ω and 800Ω. On a 120VAC relay, expect 2kΩ to 10kΩ. The two pins that show this specific resistance are your coil (A1/A2).
- Map the Contacts: Switch your meter to Continuity/Diode test. Probe the remaining pins. The pins that beep (near 0Ω) are your Common and NC terminals. The pins that read "OL" (Open Loop) are your Common and NO terminals.
- Identify the Function: If the relay has a small dial for "Seconds/Minutes" but only two coil pins and no separate trigger pins, it is almost certainly an On-Delay or Interval timer. If it has three control pins (e.g., A1, A2, and a trigger like Y1), it is likely a multi-function or true Off-Delay unit.
Real-World Selection and Timing Edge Cases
When replacing a legacy pneumatic or motor-driven timer with a modern digital equivalent, the physical symbol on the old schematic might not perfectly match the DIP-switch settings on the new unit. As of 2026, solid-state digital TDRs like the Schneider Zelio RE17 or the Siemens SIRIUS 3RP20 have largely replaced older mechanical timers, dropping the cost of a multi-function DIN-rail unit to roughly $45–$75.
However, swapping these out introduces specific edge cases you must account for:
- Voltage Sag Resets: Older electromechanical timers would ride through a 20% voltage sag. Modern microprocessor-based TDRs have strict brownout thresholds. If your 120VAC control line dips to 95VAC during a large motor start, a digital TDR may interpret this as a power loss and reset its On-Delay timer to zero. Fix: Use a TDR with a wide input voltage range (e.g., 24-240V AC/DC universal input) and a built-in supercapacitor or EEPROM memory for ride-through.
- Trigger Wire Polarity: When wiring the trigger input (often labeled Y1 or B1) on a DC multi-function timer, polarity matters. While the main AC coil is non-polarized, the DC trigger inputs on units like the Omron H3DE are strictly polarized. Reversing the trigger wires will result in the timer simply ignoring the start signal.
- Minimum Load Current: Solid-state time delay relays often use internal triacs or MOSFETs instead of mechanical silver-alloy contacts. These require a minimum "wetting" current (often 10mA to 50mA) to remain stable. If you are switching a high-impedance PLC input (drawing only 2mA), the TDR contact may chatter or fail to register. Always verify the minimum switching load on the datasheet, not just the maximum ampacity.
For comprehensive symbol libraries and standard definitions, the Electrical Engineering Portal's relay symbol database provides excellent cross-references between legacy NEMA diagrams and modern IEC functional schematics. Always verify your local AHJ (Authority Having Jurisdiction) requirements when modifying control circuits in permitted industrial facilities.
