What is an on-delay timing relay? An on-delay (function A, delay-on-operate) timing relay holds its output contact in the rest position after power is applied, then switches it once a set interval elapses — anywhere from 0.1 s to several hours depending on the range selected — per IEC 61812. The output stays switched for as long as supply remains, so the delay is one-directional: nothing happens early, and nothing reverses until power is removed. This article covers how the A/E function counts its interval, timing range and repeat accuracy, contact behavior during and after the delay, 2-wire versus 3-wire control wiring, how on-delay differs from off-delay and interval modes, and where panel builders actually specify it.
How the On-Delay Function Works
Energize the timer and an internal oscillator or digital counter starts immediately. The output change-over contact does nothing until the count reaches the dial setting — then it operates and stays operated for as long as the supply stays on. Remove power before the interval finishes and the counter resets to zero; the output never switches. That reset behavior is what separates a timer from a plain time-delay switch built from a capacitor and a comparator: a timing relay re-arms cleanly every cycle, with no partial-charge drift carried into the next start.
Terminology varies by brand. Some catalogs label this function "A", others "E" or "delay-on-operate" in plain text — the underlying behavior is identical across manufacturers building to IEC 61812. What we see in the field: technicians sometimes confuse on-delay with off-delay because both use a change-over contact and both look identical sitting on the DIN rail. The only way to tell them apart without a label is to power the unit and watch which contact state is default.
Timing Range, Setting and Repeat Accuracy
Single-function on-delay units typically offer one fixed range (say 1-30 s) set by a potentiometer or dial. Multi-range and multifunction devices add a rotary range switch (0.1 s, 1 s, 1 min, 1 h, and so on) so one part covers several jobs. The number that matters for repeatable process timing is repeat accuracy — how much a single unit's actual switching time drifts cycle to cycle at a fixed dial setting, not how close the dial reading is to the true delay (that is setting accuracy, a separate spec).
Formula: Repeat accuracy error — Source: IEC 61812, accuracy clause
Δt = ± (a% × tset)
| Symbol | Description | Unit |
|---|---|---|
| Δt | Maximum deviation between repeat timing cycles at the same dial setting | s |
| a | Repeat accuracy stated by the manufacturer (typically 0.5% to 1% for electronic timers) | % |
| tset | Timer setting value read off the dial or rotary switch | s |
At a 1% repeat accuracy, a 10-minute on-delay can wander by up to 6 seconds cycle to cycle — negligible for a pump start delay, not negligible for a process step timed against a downstream sensor window. Electronic (digital, quartz-referenced) units hold tighter repeat accuracy than the older pneumatic or motor-driven timers they replaced; that is one reason pneumatic timers have mostly disappeared from new panel designs, though some panel builders still keep a few in service on legacy machinery where nobody wants to touch a working sequence.
Contact Behavior and Output State During the Delay
The output is a change-over (SPDT or DPDT) contact rated for a few amps at control voltage, not motor load. Before the delay: the normally-open path is open, normally-closed is closed — the rest state. During the count: nothing changes, regardless of dial position. After timeout: the contact transfers and stays transferred until supply is removed, at which point it drops back to rest instantly, not after another delay. That instant drop-out on power loss is what distinguishes on-delay from true off-delay, where the whole point is to hold the output through a power interruption.
Wiring an On-Delay Timer: 2-Wire and 3-Wire Control
Two-wire control ties the trigger to the supply: apply power and the count starts on the same two terminals that feed the timer. Simple, fewer terminals, but the timer cannot be re-armed without cycling its own supply — awkward on a DC control rail shared with other devices that must stay powered continuously.
Three-wire control keeps the auxiliary supply on at all times and adds a separate start input (often a normally-open contact from a pushbutton or a PLC output) that begins the count. This lets the timer sit ready, ready to fire on command, without dropping and re-establishing its own power feed each cycle — the more common arrangement in a modern panel where 24 VDC is distributed to everything and nothing gets switched at the supply rail. Full terminal-by-terminal diagrams and a walkthrough of both schemes are covered in how to wire a timing relay.
On-Delay vs Off-Delay and Interval Timing
On-delay switches late on the way in and drops instantly on the way out. Off-delay is the mirror image — the output switches the instant the trigger is applied, then reverts the set time after the trigger is removed; a full breakdown of that behavior, including the energy-storage requirement for true off-delay, is in off-delay timing relay. Interval (single-shot) mode looks like on-delay's opposite twin: output switches on immediately, holds for the set time, then drops on its own regardless of the trigger — useful for a fixed-duration pulse rather than a startup delay.
Star-delta timers combine two on-delay-style intervals in one device: a run-up delay before opening the star contactor, then a short transition dead-time before closing delta. The run-up portion behaves exactly like a standalone on-delay function; the transition portion is safety-critical and covered separately in star-delta timing relay. Multifunction units fold on-delay, off-delay, interval and cyclic modes into one rotary-selectable part; see multifunction and multi-range timing relays for how the selection works and when a single-function unit is the better call anyway. Background on the full function family is in the timing relay engineering guide.
Where On-Delay Timers Are Used
Staggered starting is the most common application: bringing up motors, pumps or fan banks a few seconds apart so inrush current does not stack and trip an upstream breaker. Sequencing conveyor sections so the downstream belt runs before the upstream one dumps material onto it is another. On lighting circuits, an on-delay gives a warm-up period before a contactor closes onto ballasts that draw a high inrush cold. In control logic, on-delay timers filter noisy digital inputs — a sensor has to stay made for the set interval before the timer output confirms it, which rejects short glitches that a straight-through relay would pass along.
These same functions sit alongside the timers on a control-circuit BOM: the on-delay output usually drives a contactor coil (see contactors) or feeds a PLC input, and the timer itself is part of the broader timing and control relays range stocked for panel builds.
Selecting an On-Delay Timing Relay
Match the timing range to the job with margin — a 30 s delay set at the top of a 0-30 s dial has worse repeat accuracy in absolute seconds than the same 30 s set in the middle of a 0-60 s range, because accuracy is a percentage of full scale on some analog dials (check whether the spec is percent-of-setting or percent-of-range before assuming). Check contact rating against the actual coil or PLC input current it drives, not just voltage. Confirm the supply is 2-wire or 3-wire compatible with the panel's control philosophy, and whether a single-function or multifunction part makes more sense for the inventory the panel shop already carries — Schneider's Zelio Time RE17 line covers single-function on-delay in a 17.5 mm footprint, ABB's CT-ERD does the same job in its CT-D range, and both sit in the same relay collection alongside their multifunction siblings. A structured walk through range, accuracy, contact and supply selection criteria is in how to select a timing relay.
Frequently Asked Questions
What does "on-delay" mean on a timing relay?
It means the output contact stays in its rest state after power is applied and only switches once the set time interval finishes. The output then holds until supply is removed, at which point it drops back to rest instantly.
What is the difference between function A and function E?
Both labels describe the same on-delay behavior — brands differ in how they letter or number their function list on the dial or in the catalog. Confirm against the specific datasheet rather than assuming a universal code.
Does an on-delay timer reset if power is interrupted mid-count?
Yes. Losing supply before the set interval elapses resets the internal counter to zero. The next power-up starts a fresh count rather than resuming where it left off.
Can an on-delay relay hold its output through a power loss?
No. The output drops the instant supply is removed, regardless of how long it had been operated. Holding an output through a power interruption is what true off-delay (energy-storage) timers are built for instead.
Is 2-wire or 3-wire control better for an on-delay timer?
Neither is universally better — 2-wire uses fewer terminals but forces a full power cycle to re-arm the timer, while 3-wire keeps the timer's own supply live and starts the count from a separate trigger input, which suits panels running continuous 24 VDC control rails.
What repeat accuracy should I expect from an electronic on-delay timer?
Most modern electronic units state 0.5% to 1% of the set value, cycle to cycle. Older pneumatic or motor-driven timers typically run wider than that, which is a factor in why they have mostly been replaced in new panel designs.
Conclusion
An on-delay timer is defined by one behavior: hold the output at rest, count from power-up, switch once, hold that state, drop instantly on power loss. Everything else — range selection, repeat accuracy, 2-wire versus 3-wire wiring, single-function versus multifunction — is a variation on that core function. Getting the range and accuracy right for the actual process window, not just the nearest dial marking, is what keeps a staggered start or a sequencing step landing where the panel was designed to put it.