The timer off delay symbol represents a time-delay relay (TDR) or PLC instruction that maintains its actuated contact state for a preset duration after the control voltage or input signal is removed. Often called "delay on release" or "delay on de-energization," this function is critical for post-run cooldown fans, lubrication pump run-outs, and alarm silencing circuits. Because global machinery crosses borders, you will encounter this symbol in three distinct dialects: IEC (International), NEMA/IEEE (North American), and PLC Ladder Logic.
Master Reference Table: Off-Delay Timer Symbols by Standard
Use this table to identify the timer off delay symbol on the schematic in front of you. The visual representation changes drastically depending on the drafting standard used by the original equipment manufacturer (OEM).
| Standard / Domain | Visual Symbol Description | Designation / Label | Typical Application Context |
|---|---|---|---|
| IEC 60617 (Global / EU) | Rectangular block with a downward-pointing arrow inside, or a rectangle with a diagonal line crossing a vertical line and a downward arrow. | K (Relay) with 'off-delay' function block notation. | Modern European machinery, Siemens/ABB control panels, global IEC schematics. |
| IEEE 315 / NEMA ICS 18 (North America) | Standard relay contact symbol (two parallel lines or a hinge) with a downward-pointing arrow intersecting the contact line. Coil may be shaded. | TD (Time Delay) or specific contact number (e.g., 1TD-1). | US/Canadian industrial plants, Allen-Bradley hardwired schematics, NEMA motor control centers. |
| Legacy BS 3939 (Old UK) | Circle with a specific internal cross and a delayed drop-out mark (often a small 'd' or specific hash marks). | Varies by legacy drafter; often just 'T'. | Pre-1990s British infrastructure, older UK water/wastewater treatment plants. |
| PLC Ladder Logic (Software) | Rectangular instruction block labeled TOF (Timer Off Delay) with pins for IN (Input), PT (Preset Time), Q (Output), and ET (Elapsed Time). | TOF (Allen-Bradley), SF (Siemens S7). | Rockwell Studio 5000, Siemens TIA Portal, Modicon EcoStruxure logic routines. |
Decoding the Diagram: Row-by-Row Breakdown & Common Mistakes
What Each Symbol Means in Practice
When you see the IEC rectangular block with a downward arrow, it tells you the relay coil is likely a standard instantaneous coil, but the contacts are mechanically or electronically damped. For example, in a motor cooling circuit, when the main motor contactor drops out, the off-delay timer keeps the cooling blower running for 120 seconds to dissipate residual heat.
In NEMA/IEEE 315 prints, the downward arrow on the contact itself is the key identifier. The coil symbol might look entirely standard. The downward arrow signifies that when the coil loses power, the contact will delay its return to the normal (de-energized) state. Conversely, an upward arrow on a NEMA contact means "on-delay" (delay on energization).
In Ladder Logic, the TOF instruction behaves slightly differently than a hardwired relay. The TOF instruction requires a continuous TRUE input to keep the output TRUE. When the input transitions from TRUE to FALSE, the internal timer begins counting up to the Preset Time (PT). Once ET equals PT, the output (Q) goes FALSE. If the input goes TRUE again before the timer finishes, the timer resets instantly.
Rows People Get Wrong
- Confusing the Arrows: The most common bench and jobsite mistake is swapping on-delay and off-delay. Memorize this: Upward arrow = delays turning ON (On-Delay). Downward arrow = delays turning OFF (Off-Delay). Think of the arrow as gravity: it takes time to fall (drop out).
- The IEC 'X' Mark: In IEC 60617, a simple 'X' inside a relay rectangle denotes a general time-delay relay, but it does not specify whether it is on-delay, off-delay, or interval. If you see an 'X' without the specific downward arrow or textual note, you must check the bill of materials (BOM) or the physical relay dial to determine the exact function.
- Assuming NEMA Coils Delay: In NEMA schematics, the time delay is a property of the contact, not necessarily the coil. A single time-delay relay coil can have multiple contacts drawn across different pages of a schematic—some instantaneous, some off-delay. Always look at the specific contact symbol, not just the coil label.
Regional Standards, Faded Prints, and Field Verification
Which Standard Applies to Your Region?
If you are working in North America on domestically manufactured equipment (e.g., Rockwell, Eaton, Cutler-Hammer), you will almost exclusively encounter IEEE Std 315 / NEMA ICS 18 symbols. However, if you are maintaining a packaging line imported from Germany, Italy, or Asia, the OEM will have drafted the schematics using IEC 60617.
Multinational facilities often suffer from "schematic drift," where a US-based maintenance engineer replaces a failed IEC-style relay (like a Finder 80.01 modular off-delay timer) with a locally sourced NEMA-style plug-in relay (like a Macromatic TR-60), updating the physical panel but failing to update the CAD prints. Always trust the physical panel over a 15-year-old PDF.
Safe Interpretation When Markings are Faded or Missing
Thermal cycling, oil mist, and UV exposure often destroy the silk-screened dial markings on older analog time-delay relays. If you inherit a panel with an unmarked TDR and no updated schematics, do not guess the timing or function. Use this field-verification protocol:
- Isolate and De-energize: Lock out and tag out the main panel. Verify zero voltage at the TDR coil and load terminals.
- Identify the Pins: Use a multimeter in continuity mode. On a standard 8-pin octal plug-in relay (like an 11-pin tubular base), pins 2 and 10 are typically the coil for a 24VDC unit. Pins 1, 4, and 8 are commons. Check the datasheet for the specific brand (e.g., Omron H3Y series vs. Schneider Zelio).
- Bench Test the Curve: Remove the relay from the socket. Apply the rated coil voltage (e.g., 24VDC) using a bench power supply. Connect your multimeter (continuity/beep mode) across the Common and Normally Open (NO) contacts.
- Measure the Drop-Out: The contacts should close immediately (or after a brief on-delay). Now, remove the coil voltage and simultaneously start a stopwatch. The time it takes for the multimeter to stop beeping (contacts opening) is your exact off-delay setting.
- Check for Fixed vs. Adjustable: If the relay has a flathead screw dial, you can adjust it. If it is a solid-state DIP-switch or fixed-resistor type (common in older PCB-mounted relays), the time is hardcoded. If the measured time does not match the machine's required cooldown cycle, replace it with a modern adjustable digital TDR (typically $35–$60 USD for a quality din-rail mount unit).
Expert Tip: When replacing an old pneumatic off-delay relay (which used a physical air bellows and check valve to create the delay) with a modern solid-state electronic TDR, ensure the new relay's minimum load current rating is met. Solid-state outputs often fail to switch very low-current indicator lights (under 10mA) that old mechanical pneumatic contacts handled easily. If switching a low-current LED pilot light, install a dummy bleeder resistor in parallel.
