Time Delay Relay (Off-Delay) Symbol
Definition: The Time Delay Relay (Off-Delay) symbol represents an electromechanical or electronic timing relay—drawn as a standard relay coil symbol with its associated contacts annotated with a timing symbol indicating delayed de-energisation—that keeps its output contacts in the operated state for a preset time interval after the coil is de-energised, as standardised in IEC 60617 and ANSI/IEEE 315.
Also known as: TOF relay, off-delay timer, delay-on-de-energise relay, time delay on release, time delay relay off delay, TOF.
What the Time Delay Relay (Off-Delay) symbol means
The Time Delay Relay (Off-Delay) symbol denotes a relay whose contacts change state immediately when the coil is energised, but delay returning to their normal state for a set time after the coil is de-energised. The off-delay relay (also called TOF — Timer OFF) is used wherever a controlled process must continue running for a defined period after the initiating signal disappears.
In control schematics, the off-delay relay coil is drawn with a standard rectangle or circle (IEC/ANSI) and its contacts are marked with a special timer contact symbol: in IEC 60617, a normally-open off-delay contact shows the contact with an arrowhead pointing downward toward the contact line, indicating delay on opening. The coil terminals are labelled A1 and A2 (control supply connections) per IEC terminal numbering.
How to identify the Time Delay Relay (Off-Delay) symbol
The off-delay timer relay contact symbol in IEC 60617 is drawn as a standard contact (NO or NC) with a small semicircle or arc on the contact moving arm, with the arc opening facing the direction of delayed movement: for a normally-open off-delay contact (NODO), the arc faces down (delay on opening, meaning the contact delays before it opens after coil dropout). In ANSI/IEEE 315, a dashed line or a designated timing symbol (arrow or hourglass) is placed adjacent to the contact. The coil is a standard relay coil rectangle labelled TOF or with the time-delay qualifier. A1 = coil supply (+) and A2 = coil supply (−).
Function in a circuit
When the coil of an off-delay relay is energised (A1-A2 powered), the contacts switch immediately to their operated position. When the coil is de-energised, the relay starts an internal timer. The contacts remain in the operated position for the duration of the preset delay time, then return to their normal (de-energised) state. This allows a downstream process — such as a motor, fan, or valve — to keep running after the control signal is removed. Common applications include motor cooling fans that must run after the motor stops, conveyor belts that must clear product before shutting down, and pneumatic valve controls requiring timed exhaust.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-07 defines the graphical symbols for time-delay relay contacts. An off-delay (delay on de-energising) normally-open contact is shown with a semicircle arc on the moving contact element, open side facing the contact gap (delay on opening). IEC 60947-5-1 covers the construction and performance of control circuit devices including timer relays. IEC 61131-3 uses TOF as the standard function block identifier for off-delay timers in PLC programming. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 315-1975 defines time-delay contact symbols. A delay-on-dropout contact is annotated with a specific timing symbol. NEMA ICS 5 covers control circuit and pilot devices including timer relays in North American practice. The functional designation TOF is used in PLC programming under IEC 61131-3 in both IEC and ANSI environments. |
| Key difference | IEC 60617 uses arc/semicircle annotations on the contact arm to indicate which direction of contact travel is delayed (arc opening direction). ANSI/IEEE 315 uses a different geometric annotation, sometimes a dashed horizontal bar, adjacent to the contact. The functional meaning is identical: the off-delay contact delays returning to normal state after coil dropout. |
Terminals / pins
| Pin | Name |
|---|---|
| a1 | A1 |
| a2 | A2 |
Typical values
Coil voltage: typically 24 V DC, 24 V AC, 110 V AC, or 230 V AC. Delay range: 0.1 s to 10 minutes (solid-state types), 0.1 s to 100 hours (electronic digital timer relays). Contact rating: typically 8 A to 16 A at 250 V AC (NO/NC contacts). Reset: automatic (timer restarts when coil is re-energised).
Where the Time Delay Relay (Off-Delay) symbol is used
- Motor cooling fan control — the fan runs for a set time after the motor is switched off to dissipate residual heat
- Conveyor system sequencing — the conveyor belt continues running for a timed period after a stop command to clear in-process product
- Pneumatic and hydraulic valve control — solenoid valves held open for a defined exhaust time after a machine cycle ends
- Lighting control — outdoor or stairwell lights remain on for a timed period after the occupancy trigger is removed
- HVAC systems — supply fans continue to run after a heating or cooling call ends to distribute residual conditioned air
- Industrial process control — chemical dosing pumps or mixing agitators run for a fixed time after a batch process stop signal
Example
In a motor-control panel, a TOF timer relay coil is wired in parallel with the main motor contactor coil (A1 and A2 terminals sharing the same 24 V AC supply). When the motor STOP button de-energises both the contactor and the TOF coil simultaneously, the TOF normally-open contact (wired in series with the cooling fan contactor coil) remains closed for the preset delay time (e.g. 120 seconds), keeping the cooling fan running after the main motor has stopped.
Key facts
- The Time Delay Relay (Off-Delay) symbol uses a standard relay coil (A1/A2 terminals) with contacts annotated by a timing symbol indicating delayed return to the de-energised state; in IEC 60617-07, the arc on the moving contact arm opens toward the delayed direction of travel.
- An off-delay relay (TOF) switches its contacts instantly when energised and delays the return to normal state after the coil is de-energised by a preset time interval.
- The functional designation TOF (Timer Off-Delay) is the standard identifier in IEC 61131-3 PLC function block programming for the off-delay timer function.
- Coil terminals are labelled A1 (supply +) and A2 (supply −) per IEC 60947 terminal numbering convention; contact terminals follow the standard 1–2 (NO), 1–4 (NC), 3–4 (NO), etc. numbering.
- Off-delay relays are distinguished from on-delay relays (TON) by which direction of coil state change triggers the timing: TOF starts timing on coil dropout; TON starts timing on coil pick-up.
- Modern solid-state and digital timer relays combine both TON and TOF functions in a single unit, selectable via a mode switch, with a wide delay-time adjustment range (typically 0.1 s to 100 h).
- The off-delay timer relay is listed as a standard protective and control device in IEC 60947-5-1 (control circuit devices and switching elements).
- A common application mistake is confusing the off-delay (TOF) with the on-delay (TON): in TOF, the contact change is immediate on pick-up and delayed on dropout; in TON, the change is delayed on pick-up and immediate on dropout.
Frequently asked questions
What does the time delay relay off-delay symbol look like?
The off-delay (TOF) relay symbol consists of a standard relay coil rectangle with A1 and A2 terminal labels, plus associated contact symbols annotated with a timing indicator. In IEC 60617, the off-delay contact has a small semicircle arc on the moving contact arm, with the arc opening facing the direction of delayed movement (toward the contact gap for a normally-open contact, indicating delay on opening). In ANSI/IEEE 315, a distinct timing annotation is placed adjacent to the contact.
What is the difference between an on-delay (TON) and off-delay (TOF) relay?
An on-delay relay (TON) delays the closure of its output contacts after the coil is energised: the contacts switch after a preset time. An off-delay relay (TOF) switches its contacts immediately when the coil is energised, but delays the return to the normal de-energised state after the coil drops out. TON is used to delay the start of a process; TOF is used to extend or run-on a process after a stop command.
What does TOF stand for in relay and PLC terminology?
TOF stands for Timer Off-Delay. It is the standard function block identifier used in IEC 61131-3 PLC programming to describe an off-delay timer whose output remains TRUE for a preset time after the input signal goes FALSE. The same designation appears in relay ladder diagrams and control panel documentation.
What are typical applications for an off-delay timer relay?
Common off-delay relay applications include motor cooling fan run-on (fan continues for a set time after motor stop), conveyor belt clearing (belt runs on after a stop command to clear product), HVAC fan purging (fan continues after heating/cooling call ends), and pneumatic valve timed exhaust. Any application requiring a controlled run-on time after a stop signal uses an off-delay relay.
What standard governs the off-delay relay symbol?
The graphical symbols for time-delay relay contacts (including off-delay) are defined in IEC 60617-07. The physical construction and performance requirements of timer relays are covered by IEC 60947-5-1 (control circuit devices). In North America, ANSI/IEEE 315-1975 defines the equivalent schematic symbols, and NEMA ICS 5 covers the devices.
What are the terminal designations on a time delay relay?
The coil terminals of an IEC-style time delay relay are labelled A1 (positive/line supply) and A2 (negative/neutral), following IEC 60947 terminal numbering. Contact terminals follow standard relay numbering: 1-2 (normally-open), 1-4 (normally-closed), or 3-4 (normally-open) depending on the relay type. Some timer relays also have an instant contact (no delay) alongside the timed contacts.
How is the off-delay time set on a timer relay?
Most electromechanical and solid-state off-delay relays have a potentiometer or rotary switch on the front face of the relay that sets the delay time over a defined range (e.g. 0.1–10 s, 1–60 s, or 1–60 min). Digital timer relays allow precise digital entry of the delay time. The relay's nameplate and wiring diagram show the available time ranges and the unit of measurement.
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