An off-delay timer (often called a drop-out delay or TOFF) maintains its actuated contact state for a preset duration after the control coil is de-energized. Reading the correct timer symbols delay off on a schematic is critical for troubleshooting motor controls, HVAC fan run-ons, and safety interlock circuits. Below is the definitive reference for identifying these symbols across global standards.
The Complete Timer Symbols Delay Off Reference Table
The table below maps the exact contact behaviors for off-delay timers. Use this as your bench reference when tracing ladder logic or panel schematics. Note that the physical state of the contact is defined when the timer coil has zero power.
| Standard Nomenclature | IEC 60617 Visual Marker | NEMA / IEEE 315 Visual Marker | Action When Coil De-energized | Common Application |
|---|---|---|---|---|
| NOTO (Normally Open, Timed Open) |
NO contact with arrow pointing away from the gap (outward) | NO contact with downward-pointing wedge or arrow | Stays closed for the set duration, then opens. | Post-purge cooling fans, lube oil pumps. |
| NCTC (Normally Closed, Timed Closed) |
NC contact with arrow pointing away from the gap (outward) | NC contact with downward-pointing wedge or arrow | Stays open for the set duration, then closes. | Anti-reclosing lockouts, alarm horn silencing. |
Regional Standards and Faded Markings
Schematic symbols are not universal. The standard applied to your drawing depends entirely on the region and the era of the machine's manufacture. Assuming a single standard applies globally is a primary cause of miswiring during panel retrofits.
Which Standard Applies to Your Region?
- IEC 60617 (Global / Europe / Modern US): Uses rectangular blocks for coils and specific directional arrows on contact crossbars. This is the dominant standard for modern PLCs and OEM machinery exported worldwide.
- NEMA ICS 2 / IEEE 315 (North America): Uses circles or squares for coils and relies on "wedges" or directional arrows attached to the contact arm. Still heavily used in US-based industrial motor control centers (MCCs).
- BS 3939 (Legacy UK): Obsolete since the 1980s but frequently encountered in older British manufacturing plants. It used graphical representations of the physical dashpot mechanism (a piston in a cylinder) rather than abstract arrows.
Safe Interpretation of Faded or Missing Markings
On 30-year-old NEMA prints, the tiny wedges indicating "delay off" vs "delay on" are often faded by UV exposure or oil stains. Never guess the timing behavior of a safety interlock based on a faded print. According to Automation Direct's technical guidelines, you must verify the physical behavior:
- Trace the coil wires to the physical time-delay relay on the panel (e.g., a Macromatic TR-60522 or Omron H3Y).
- Read the physical part number and pull the manufacturer datasheet.
- If the print is entirely unreadable and the relay is unmarked, use a multimeter in continuity mode. Energize the coil, then de-energize it. Observe which contacts drop out immediately and which hold for the dial's duration. Document this on the schematic before proceeding.
Rows People Get Wrong (And How to Fix Them)
Even experienced technicians misread timer schematics. Here are the most common traps when interpreting timer symbols delay off, and how to avoid them.
Confusing the IEC Arrow Direction
The most frequent error is mixing up on-delay and off-delay arrows in IEC 60617. The rule is based on the physical movement of the contact arm relative to the gap:
On-Delay (Make): The arrow points towards the contact gap (inward), symbolizing that the contact is "delayed" in making the connection.
Off-Delay (Break): The arrow points away from the contact gap (outward), symbolizing that the contact is "delayed" in breaking the connection. If you see an outward-pointing arrow, it is an off-delay.
Misunderstanding "Normally" in Timed Contacts
In standard relay logic, "Normally" refers to the shelf state (coil de-energized). However, in NEMA timer nomenclature, the "Timed" descriptor defines the transition that is delayed. For an off-delay NOTO (Normally Open, Timed Open), the contact is physically open on the shelf. When energized, it closes instantly. When de-energized, it delays opening. Technicians often wrongly assume the "Timed Open" means it delays opening when power is applied. Always anchor your logic to the de-energized falling edge.
Assuming the Coil Symbol Defines the Contact Type
A single off-delay timer coil (e.g., a Siemens SIRIUS 3RP25) can output both NOTO and NCTC contacts simultaneously. The coil symbol only tells you the timer's function class; you must read the individual contact symbols scattered throughout the ladder logic to know how each specific pole behaves. For deeper logic mapping, refer to the All About Circuits digital logic chapters on timing diagrams.
Frequently Asked Questions
What is the difference between delay off and delay on timer symbols?
The core difference lies in the trigger edge. A delay on (TON) timer initiates its timing sequence on the rising edge (when voltage is applied to the coil). Its contacts return to their shelf state instantly when power is removed. A delay off (TOFF) timer actuates its contacts instantly on the rising edge, but initiates its timing sequence on the falling edge (when voltage is removed). Visually, NEMA uses upward-pointing wedges for delay on, and downward-pointing wedges for delay off. IEC uses inward arrows for delay on, and outward arrows for delay off.
How do I wire an off-delay timer relay in a 24V DC control circuit?
Using a standard 8-pin plug-in off-delay relay (like the Omron H3Y-2) as an example: Wire your 24V DC control power to the coil terminals (A1 for +24V, A2 for 0V). For a NOTO (delay off break) application, wire your load through the common terminal (15) and the NO timed contact (16). For an NCTC (delay off make) application, use the common (15) and the NC timed contact (18). Always place a flyback diode (e.g., 1N4007) across A1 and A2 (cathode to A1) to protect your PLC output transistor from inductive kickback when the coil de-energizes. For comprehensive relay specs, check the Macromatic Technical Resources library.
Why does my PLC ladder logic use a different symbol for timer delay off?
Physical electrical schematics use IEC or NEMA contact symbols because they represent hardwired, physical metal contacts moving in space. PLC ladder logic uses abstract instruction blocks (like the TOF block in Allen-Bradley Studio 5000 or Siemens TIA Portal). In ladder logic, the TOF block operates on bit-level logic rather than physical coils. When the rung feeding the TOF instruction goes false, the internal bit stays true for the preset accumulator time before flipping false. While the underlying logic is identical to a hardware off-delay timer, the visual representation is a software function block with an enable-in (IN), preset (PT), and output (Q), completely abandoning the physical contact crossbar symbols used on paper prints.
