The time delay relay symbol on a schematic dictates whether a circuit triggers immediately, waits to engage, or holds after power drops. Because North American (NEMA/IEEE) and International (IEC) drafting standards use entirely different graphical conventions, misreading this symbol is a primary cause of miswired control panels and tripped breakers. Below is the definitive cross-reference to decode these symbols, verify unlabeled hardware, and select the correct physical module for your bench or jobsite.

The Core Time Delay Relay Symbol Reference Table

Before wiring any 8-pin octal or 11-pin DIN-rail relay, confirm its function against this master table. The timing ranges assume modern digital solid-state or microprocessor-based relays (typically $40–$90), not legacy pneumatic dash-pot timers.

Function IEC 60617 Symbol Description NEMA / IEEE 315 Symbol Description Practical Meaning Common Part Example
On-Delay (TON) Standard coil rectangle with an 'X' inside; contact shows an arrow pointing UP (delay on make). Coil with a dashed line; contact arm intersects a semi-circle opening UP. Power applied → timer starts → contact changes state after delay. Macromatic TR-65121
Off-Delay (TOF) Standard coil rectangle with an 'X' inside; contact shows an arrow pointing DOWN (delay on break). Coil with a dashed line; contact arm intersects a semi-circle opening DOWN. Power removed → timer starts → contact reverts after delay. Omron H3Y-2
One-Shot (Pulse) Coil with an 'X'; contact shows a single downward arrow with a horizontal line (fixed pulse). Coil with a dashed line; contact shows a specific 'pulse' notch symbol. Trigger applied → contact closes for a fixed time, then opens (ignores sustained trigger). Schneider RE22R2AMU
Interval (On-Delay + Auto-Off) Coil with an 'X'; contact shows an UP arrow and a DOWN arrow (make and break delay). Contact arm intersects two opposing semi-circles. Trigger applied → delay → contact closes → second delay → contact opens automatically. Macromatic TR-60322
Repeat Cycle (Flicker) Coil with an 'X' and a sine-wave or dual-arrow notation; contact shows alternating make/break arrows. Contact arm shows a zig-zag or repeating pulse notation. Power applied → cycles ON and OFF continuously at set intervals. Timerblox TBL-100

Regional Standards: IEC vs. NEMA vs. Legacy UK

Knowing which standard applies to your region prevents catastrophic misinterpretation of control schematics.

IEC 60617 (International / EU / Modern UK / AU)

The International Electrotechnical Commission standard uses a minimalist approach. The coil is always a basic rectangle. The magic happens at the contact symbol. An arrow pointing toward the contact line indicates a delay on make (On-Delay), while an arrow pointing away indicates a delay on break (Off-Delay). Pinouts strictly follow the A1/A2 (coil) and 15/16/18 (contacts) numbering convention for 11-pin relays.

NEMA ICS 19 & IEEE 315 (North America)

US and Canadian schematics rely on the semi-circle and dashed-line method. The coil is often drawn with a specific internal notation (like a clock face or dashed box), and the contacts use semi-circles to represent the 'dashpot' physical mechanism of older pneumatic timers. Pinouts typically follow the 8-pin octal standard (pins 2 and 7 for coil, 1/3/4 and 5/6/8 for contacts).

Legacy UK (BS 3939)

If you are retrofitting a plant in the UK built before 1992, you will encounter BS 3939. This obsolete standard used distinct cross-hatching inside the relay coil to denote time delays. Warning: Never mix BS 3939 and IEC 60617 symbols on the same as-built drawing. The cross-hatching looks dangerously similar to IEC thermal overload symbols.

Rows People Get Wrong (And How to Avoid Miswiring)

Even experienced panel builders mix up specific time delay functions when reading complex schematics. Here are the most common traps:

⚠️ WARNING: The Off-Delay vs. Interval Trap
The most frequent miswire occurs when a designer specifies an Off-Delay (TOF) for an HVAC blower run-on, but the builder installs an Interval relay.
The Failure Mode: An Off-Delay relay requires continuous power to the coil to keep the contacts closed. When the thermostat drops the 24VAC signal, the timer starts, and the blower runs for 3 minutes. An Interval relay, however, only needs a momentary pulse. If you wire 24VAC continuously to an Interval relay's trigger pin, it will time out and shut the blower off while the thermostat is still calling for cooling, resulting in a frozen evaporator coil and a flooded compressor.

The 'One-Shot' Re-trigger Edge Case: Many cheap, unbranded one-shot relays from online marketplaces are 're-triggerable' (the timer resets if the trigger pulses again before timing out). Industrial-grade one-shots (like the Schneider Zelio series) are typically 'non-retriggerable'. Always check the datasheet's timing diagram for a secondary trigger pulse response before using a one-shot for safety-critical lockout circuits.

Faded Markings & Missing Labels: Safe Interpretation Protocol

When you are troubleshooting a legacy panel and the schematic is missing, or the heat from a nearby VFD has baked the label off the physical relay, do not guess. Use this empirical testing protocol to identify the relay's function safely.

Required Tools: Bench power supply (or isolated 24VDC transformer), Fluke 87V (or equivalent CAT III multimeter), stopwatch.

  1. Isolate and Identify: Remove the relay from the live panel. Identify the coil pins (A1/A2 for DIN, 2/7 for Octal). Identify the common (C), normally open (NO), and normally closed (NC) pins using the multimeter's continuity mode while the relay is unpowered.
  2. Setup the Test Circuit: Connect your isolated 24VDC supply to the coil pins through a momentary pushbutton. Connect the multimeter (in continuity/beep mode) across the Common and NO pins.
  3. Apply Power (The 'Make' Test): Press and hold the pushbutton.
    • If the multimeter beeps immediately, it is NOT an On-Delay relay.
    • If the multimeter beeps after a delay (e.g., 5 seconds), it is an On-Delay (TON).
  4. Remove Power (The 'Break' Test): Release the pushbutton while monitoring the multimeter.
    • If the beep stops immediately, it is a standard On-Delay or One-Shot.
    • If the beep continues for several seconds after power is removed, then stops, it is an Off-Delay (TOF).
💡 Pro Tip: Inductive Kickback Protection
When bench-testing older electromagnetic time delay relays (not solid-state), always wire a 1N4007 flyback diode in reverse-parallel across the DC coil pins (cathode to positive). Without it, the inductive voltage spike when you release the pushbutton can exceed 100V, potentially damaging your multimeter's continuity circuit or arcing across the pushbutton contacts.

Decision Tree: Picking the Right Relay Function for Your Circuit

Stop guessing which module to order. Use this decision matrix to lock in the exact function, form factor, and part number for your application. All prices reflect typical 2026 industrial distributor pricing for single units.

Application Scenario Required Logic / Function Concrete Part Pick Estimated Cost
HVAC Blower Run-On: Fan must keep running for 3 mins after thermostat stops calling for heat/cool. Off-Delay (TOF)
Needs continuous power to hold, times out on power loss.		 Schneider Electric RE22R2AMU
(Multifunction, set to TOF, 24-240VAC/DC)		 $65 - $75
Conveyor Start Warning: Horn must sound for 5 seconds before the conveyor motor contactor pulls in. On-Delay (TON)
Power applied, waits, then triggers the motor contactor.		 Macromatic TR-65121
(Dedicated On-Delay, 24VDC, 0.1s-100h range)		 $45 - $55
Sump Pump Anti-Short-Cycle: Prevent pump from restarting immediately after a rapid float-switch bounce. On-Delay with Anti-Flutter
Ignores micro-second contact bounces on the trigger.		 Omron H3Y-2 AC120
(Fixed or adjustable On-Delay, dual pole)		 $50 - $60
Automated Lubrication Pump: Must run for exactly 30 seconds every 4 hours, regardless of machine state. Repeat Cycle (Asymmetric)
Independent ON and OFF timing loops.		 Timerblox TBL-100-AD
(Dual-knob asymmetric flicker, DIN mount)		 $80 - $95

For further reading on standardizing your control panel schematics, refer to the EC&M guide on time-delay relay applications and the Fluke relay testing methodology. Always verify your local AHJ requirements for control circuit overcurrent protection, as NEC Article 430.72 dictates specific fuse sizing for motor control circuit transformers feeding these relays.