Time Delay Relay (On-Delay) Symbol
Definition: The Time Delay Relay (On-Delay) symbol represents an electromechanical or electronic timing relay—drawn as a standard relay coil (A1/A2 terminals) with associated contacts annotated with a timing symbol indicating delayed energisation—whose output contacts change state only after a preset time interval elapses following coil energisation, as standardised in IEC 60617-07 and ANSI/IEEE 315.
Also known as: TON relay, on-delay timer, delay-on-energise relay, time delay on pickup, timer on delay, TON.
What the Time Delay Relay (On-Delay) symbol means
The Time Delay Relay (On-Delay) symbol denotes a relay in which the coil, when energised, does not immediately switch the output contacts; instead, an internal timer begins counting and the contacts change state only after the programmed delay has elapsed. If the coil is de-energised before the timer expires, the contacts do not change state and the timer resets immediately. This is the most common type of timer relay used in industrial control circuits.
In IEC 60617-07 schematics, the on-delay relay contact is drawn with an arc or semicircle on the moving contact element, with the arc opening facing the direction of delayed travel: for a normally-open on-delay contact (NOTO), the arc faces upward, indicating delay on closing. The coil is labelled A1 (supply +) and A2 (supply −). In ANSI/IEEE 315 schematics, a timing notation (dashed bar or arrow) is placed on the contact. PLC function block designation: TON.
How to identify the Time Delay Relay (On-Delay) symbol
In IEC 60617, an on-delay normally-open contact (NOTO) is drawn as two parallel horizontal lines (the contact) with a small semicircle arc on the moving element, with the arc opening facing away from the contact gap — indicating the delay occurs before the contact closes. An on-delay normally-closed contact (NCTO) shows the arc on the moving element facing toward the contact gap, indicating delay on opening. In ANSI schematics, time-delay contacts use a specific annotated symbol (often a slanted line or dot with a prescribed shape per IEEE Std 315). The relay coil is a standard rectangle (IEC) or circle (ANSI) with A1/A2 labels.
Function in a circuit
When the control voltage is applied across A1 and A2 of an on-delay relay, the internal timer starts. After the preset delay period, the normally-open contacts close (or NC contacts open). If the control voltage is removed before the timer expires, the timer resets to zero and the contacts do not change state. Once the contacts change state (after the delay), they remain in that state as long as the coil remains energised. When the coil is de-energised, the contacts return to their normal state immediately (no delay on dropout in a pure TON relay). This behaviour makes the on-delay relay ideal for sequencing processes, preventing simultaneous motor starts, and implementing safety interlocks.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-07 defines graphical symbols for time-delay relay contacts. The on-delay (delay on energising) normally-open contact is shown with a semicircle arc on the moving contact arm, open side facing away from the contact gap. IEC 60947-5-1 covers the construction and performance of time-delay relays. IEC 61131-3 standardises TON (Timer On-Delay) as the function block identifier for on-delay timer functions in PLC programming. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 315-1975 defines time-delay contact symbols for North American schematics. NEMA ICS 5 covers control circuit devices including on-delay timer relays. The TON designation is universal in PLC programming environments per IEC 61131-3, used in both IEC and ANSI-convention environments. |
| Key difference | IEC 60617 uses directional arc annotations on the moving contact arm to distinguish on-delay (arc away from gap = delay on closing) from off-delay (arc toward gap = delay on opening). ANSI/IEEE 315 uses alternative geometric annotations. The functional meaning — contacts delay changing state after coil energisation — is identical in both standards. |
Terminals / pins
| Pin | Name |
|---|---|
| a1 | A1 |
| a2 | A2 |
Typical values
Coil voltage: 24 V DC, 24 V AC, 110 V AC, or 230 V AC. Delay range: 0.05 s to 60 s (typical pneumatic types), 0.1 s to 300 hours (electronic digital types). Contact rating: typically 8 A to 16 A at 250 V AC. Retentive/non-retentive: most hardware TON relays are non-retentive (timer resets on coil dropout).
Where the Time Delay Relay (On-Delay) symbol is used
- Motor starter sequencing — star-delta starter uses a TON relay to delay changeover from star to delta contactor after a preset acceleration time (typically 5–15 s)
- Interlock and anti-simultaneous-start protection — prevents two motors from starting at the same time by imposing a minimum delay between start signals
- Alarm acknowledgement and annunciation delays — a warning buzzer activates only after a fault condition persists for a set time, filtering out transient signals
- Conveyor and process startup sequencing — downstream equipment starts a set time after upstream equipment is confirmed running
- Weld timer circuits — resistance welding machines hold the welding current for a preset timed interval
- Air compressor auto-start delay — prevents compressor from starting immediately on power restoration to avoid voltage dip from simultaneous motor starts
Example
In a star-delta motor starter, the TON timer relay coil is energised simultaneously with the star contactor coil. After the preset delay (e.g. 8 seconds, allowing the motor to accelerate to near full speed), the TON normally-open contact closes, energising the transition circuit that drops the star contactor and closes the delta contactor, completing the star-to-delta changeover.
Key facts
- The Time Delay Relay (On-Delay) symbol uses a standard relay coil with A1/A2 coil terminals and contacts annotated with a timing symbol; in IEC 60617-07, the on-delay contact arc opens away from the contact gap, indicating delay on closing (for a normally-open contact).
- An on-delay relay (TON) starts its internal timer when the coil is energised; the contacts change state only after the preset delay elapses. If the coil drops out before the delay expires, the timer resets and the contacts do not change state.
- TON (Timer On-Delay) is the standard IEC 61131-3 PLC function block identifier for on-delay timer logic, used universally across PLC platforms (Siemens, Allen-Bradley, Schneider, etc.).
- IEC 60617-07 governs the graphical contact symbols for on-delay relays; IEC 60947-5-1 governs the physical construction and performance of time-delay relay devices.
- The most common industrial application of an on-delay relay is the star-delta motor starter, where it controls the transition time from star connection (reduced voltage starting) to delta connection (full voltage running).
- On-delay relays are distinguished from off-delay relays (TOF): TON delays action on coil pick-up (energisation); TOF delays return to normal on coil dropout (de-energisation).
- Standard coil terminal designations for IEC timer relays are A1 (+) and A2 (−); contact terminals follow standard relay numbering (1-2 NO, 1-4 NC, 3-4 NO).
- Adjustable on-delay ranges on modern electronic timer relays span from 0.05 seconds to hundreds of hours, set via potentiometer, rotary switch, or digital keypad.
Frequently asked questions
What does the time delay relay on-delay symbol look like?
The on-delay (TON) relay symbol consists of a standard relay coil rectangle (IEC) or circle (ANSI) with A1 and A2 terminal labels, plus contact symbols with a timing annotation. In IEC 60617-07, a normally-open on-delay contact has a small semicircle arc on the moving contact arm, with the arc opening pointing away from the contact gap — indicating the delay occurs before the contact closes. ANSI/IEEE 315 uses a different but equivalent geometric annotation on the contact.
What is the difference between a TON and TOF relay?
A TON (Timer On-Delay) relay delays switching its contacts to the energised state after the coil is energised; contacts return to normal immediately on coil dropout. A TOF (Timer Off-Delay) relay switches its contacts immediately when the coil is energised, but delays the return to the normal state after the coil drops out. TON is used to delay starting a process; TOF is used to extend or run-on a process after a stop signal.
What is the most common use of a TON on-delay timer relay?
The most common application is the star-delta motor starter. A TON relay coil is energised when the motor starts in star (reduced voltage) configuration; after the preset delay (5–15 seconds, depending on motor inertia), the TON contact triggers the changeover circuit that switches the motor from star to delta for full-voltage running. The delay allows the motor to accelerate to near-synchronous speed before the potentially disruptive delta switch-over.
What does TON stand for in relay and PLC terminology?
TON stands for Timer On-Delay. It is the standardised function block identifier in IEC 61131-3 for an on-delay timer whose output (Q) goes TRUE only after the input (IN) has been TRUE continuously for the preset time interval (PT). The same TON designation appears in control panel wiring diagrams and relay ladder logic.
What standard defines the on-delay relay contact symbol?
The graphical symbol for the on-delay time-delay relay contact is defined in IEC 60617-07. Physical construction and performance requirements for timer relays are covered by IEC 60947-5-1. In North America, ANSI/IEEE 315-1975 defines equivalent schematic symbols. PLC function block behaviour is standardised in IEC 61131-3.
Does an on-delay relay reset if the coil is de-energised before the timer expires?
Yes. A standard (non-retentive) on-delay relay resets its timer to zero immediately if the coil is de-energised before the delay time expires. The contacts do not change state. In retentive (latching) timer applications, a retentive TON (TONR) function block is used, which accumulates time across multiple coil energisation cycles until the total reaches the preset value — but this requires a specific retentive timer function, not a standard TON.
What are the terminal designations on an on-delay timer relay?
Per IEC 60947 terminal numbering: the coil terminals are A1 (supply positive/line) and A2 (supply negative/neutral). Output contact terminals follow standard relay numbering: terminals 1 and 2 for a normally-open (NO) timed contact; terminals 1 and 4 for a normally-closed (NC) timed contact; some relays provide terminals 3 and 4 as an additional NO timed contact. An instantaneous (non-timed) auxiliary contact may also be included.
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