Timing Relay (Time-Delay Relay) Engineering Guide
What is a timing relay? A timing relay (time-delay relay) switches its output contact a set interval after a trigger condition instead of instantly, driven by an internal electronic oscillator or counter to a dial, DIP, or rotary-selected setpoint, and standardized under IEC 61812 alongside IEC 60947-5-1 for the control-circuit hardware itself. Skip the delay stage and star-delta transitions overlap, HVAC compressors short-cycle, and staircase lighting cuts out mid-flight, so the timing element does real sequencing and protection work, not cosmetic delay. This guide covers on-delay and off-delay functions, star-delta and multifunction timers, the specs that actually matter (range, accuracy, contact rating, supply), timing relay vs PLC timer vs monitoring relay, selection and wiring basics, and how Schneider Zelio Time compares to ABB CT-D.
What Is a Timing Relay?
A timing relay is a control relay whose output contact changes state a preset time after — or in relation to — a trigger, rather than the instant the trigger occurs. The setpoint comes from a front-panel dial, a DIP switch plus rotary range selector, or a combination of both on multifunction units. Internally, an electronic oscillator or RC/counter circuit measures the interval; older designs used a pneumatic air-bleed bellows or a small synchronous motor and cam instead, and both types are still found in legacy panels.
Physically, timing relays come in two formats: DIN-rail modular housings (17.5 mm or 22.5 mm wide, snapping onto standard rail next to contactors and breakers) and plug-in units seated in an 8-pin or 11-pin socket for field-swappable replacement without rewiring. Power comes either from a separate auxiliary supply terminal or directly from the control voltage that also drives the load circuit — a distinction that matters for off-delay behavior, covered below. The output is almost always a change-over (SPDT) contact, sometimes two (DPDT), rated for a few amps at 250 V AC — enough to pilot a contactor coil, not to switch load current directly.
Functionally, a timing relay family spans on-delay, off-delay, true off-delay, single-shot/interval, symmetrical and asymmetrical flasher (cyclic), and star-delta (two outputs with a dead-time between them). A single multifunction device often covers six or more of these from one rotary selector. For engineers stocking a panel shop, that range is the whole point: fewer part numbers, one spare-parts bin, one training curve for the wiring crew.
How a Timing Relay Works
Inside a modern electronic timing relay, the supply feeds a small switch-mode or transformer power stage, which in turn powers an oscillator or microcontroller-based counter. The dial or rotary switch sets a target count; when the trigger input (or the supply itself, depending on function) starts the count, the relay coil energizes once the target is reached, flipping the output contact. Multi-range units add a second rotary switch that multiplies the base range — for instance 0.1 s to 100 h across several decades — so one part covers applications from a debounce delay to an overnight soak timer.
Pneumatic timers work differently: a bellows fills or empties through an adjustable needle valve, and the mechanical travel trips a snap-action switch at a set point. They drift with temperature and valve wear, which is why most new designs are electronic — but pneumatic and motor-driven timers are still in service on older machinery, and a field tech has to recognize both formats. What we see in the field: an "intermittent" timing fault on a decades-old machine is more often a gummed-up pneumatic bellows than a wiring problem, and swapping in a modern electronic multifunction relay usually resolves it in one visit.
The trigger side matters as much as the timing element. Some relays start timing the instant power is applied to the device (supply-triggered); others stay powered continuously and only start timing when a separate control input closes (control-signal triggered). Datasheets state a minimum trigger pulse width — feed a shorter pulse and the relay may not register the start, which is a common cause of an on-delay timer that "does nothing" on a fast-closing upstream contact.
Core Timing Functions: On-Delay, Off-Delay, and Beyond
On-delay (delay-on-operate, often marked function A or E) is the simplest and most common function: apply power or trigger the input, wait the set time, and the output contact changes and stays changed until the trigger is removed. It is the default choice for staggered motor starts, alarm debounce, and startup interlocks where a downstream device needs a moment to stabilize before the next stage engages.
Off-delay (delay-on-release, function B or R) does the opposite: the output changes immediately when triggered, then reverts the set time after the trigger is removed. This is where a subtlety trips up a lot of panel builders. A control-signal off-delay relay stays powered from the control supply and times from a separate trigger input dropping — simple, but it stops timing if the supply itself is cut. A true off-delay relay stores enough energy internally to keep counting down even after its own supply is interrupted, which is what a "power-off delay" light or a fan overrun circuit actually needs. Specify the wrong one and a fan that is supposed to run on after the compressor supply drops simply stops with it.
Beyond on/off delay, single-shot (one-shot) relays produce one fixed-width output pulse per trigger regardless of how long the trigger is held — useful for converting a maintained signal into a momentary pulse for a counter or PLC input. Interval timers energize the output for a set time starting at power-up, then drop and stay dropped. Flasher (cyclic) relays alternate the output on and off repeatedly, either symmetrically (equal on/off time) or asymmetrically (independently set on and off periods), which covers everything from a beacon light to an intermittent wiper circuit. On-delay timing relays and off-delay timing relays each get a dedicated write-up in this series because the wiring and specification pitfalls differ enough to warrant it.
Star-Delta Timing for Motor Starting
A star-delta (wye-delta) timer sequences three contactors (line, star, and delta) to reduce inrush current on a motor start. The timer closes the star contactor first, connecting the motor windings in a reduced-voltage star configuration and letting it accelerate. After a set run-up time — typically a few seconds up to around 30 s, sized to the load's run-up behavior — the timer opens the star contactor, holds a short transition (dead-time) of tens of milliseconds, then closes the delta contactor for full-voltage running.
That transition setting is the safety-critical parameter in the whole sequence. Set it too short and the star contactor has not fully broken before the delta contactor makes — some contact bounce or mechanical release lag overlaps the two connections and shorts the winding phase-to-phase, which trips the branch breaker at best and welds contacts at worst. Set it unnecessarily long and the motor coasts uncontrolled between star and delta, producing a torque and current transient on delta closure that can be worse than a direct-on-line start. This depends on the motor's mechanical time constant and load inertia, which is why the transition dial on a good star-delta timer is fine-grained, not a coarse switch.
Formula: Star-Delta Total Starting Time — Source: motor starting sequence, coordinated with IEC 60947-4-1 contactor ratings
t_total = t_run-up + t_transition
| Symbol | Description | Unit |
|---|---|---|
| t_total | Total time from star contactor closing to delta contactor closing | s |
| t_run-up | Star-connection run-up time, set to let the motor approach rated speed before transfer | s |
| t_transition | Dead-time between opening the star contactor and closing the delta contactor | s |
See star-delta timing relays for motor starting for the function-by-function walkthrough, and wiring a star-delta motor starter for the contactor interlocking and terminal layout that pairs with the timer.
Multifunction and Multi-Range Timers
A multifunction, multi-range timer puts several timing functions — on-delay, off-delay, single-shot, flasher, star-delta, and more — behind one rotary function selector, paired with a second rotary switch for the time range (commonly spanning 0.1 s to 100 h across multiple decades). Multi-voltage supply inputs are standard on these units, accepting a wide AC/DC band so the same part number covers 24 V, 110 V, and 230 V control circuits without a separate SKU for each.
The tradeoff is commissioning risk. A dedicated on-delay relay can only be miswired, not misconfigured; a multifunction unit has two or three selectors that all have to be set correctly, and a function selector left on the wrong position looks, from a distance, exactly like a working timer that is simply timing the wrong thing. Some panel builders standardize entirely on multifunction relays for spare-parts simplicity; others keep dedicated single-function units for the highest-volume applications and reserve multifunction stock for one-offs and field repairs — both are defensible, and the right call depends on panel volume and how disciplined the commissioning checklist is.
Explore the multifunction and multi-range timing relay article for a function-by-function breakdown, and browse timing and control relays for the current stocked range.
Key Specifications That Determine Fit
Timing range and number of ranges set the coarse fit: a relay rated 0.1 s-10 s is not going to cover an overnight interval, and a multi-range unit only helps if its decades actually straddle the target setting rather than sitting just outside it. Repeat accuracy — how much a given delay drifts from cycle to cycle at the same setting — is usually specified as a percentage of the set value, commonly in the ±0.5% to ±1% band for electronic units, tighter than the several-percent drift typical of pneumatic timers.
Formula: Repeat Accuracy Error — Source: IEC 61812 timing performance clause
Δt = ±(a% × t_set)
| Symbol | Description | Unit |
|---|---|---|
| Δt | Maximum deviation of the actual delay from the set value across repeat cycles | s |
| a | Repeat accuracy rating from the datasheet (commonly 0.5 to 1) | % |
| t_set | Time value set on the dial or rotary selector | s |
Setting accuracy is a separate number from repeat accuracy and gets confused with it constantly: repeat accuracy tells you how consistent the relay is with itself, setting accuracy tells you how close the dial marking is to the actual delay it produces. A relay can have excellent repeat accuracy and still be set 3-4% off the dial marking — check both figures, not just the headline repeat-accuracy percentage on the front of the datasheet.
Recovery (reset) time — the minimum time the relay needs after one cycle before it can accept a new trigger and time correctly — matters in fast-cycling applications like pump alternation or flasher circuits, where a recovery time close to the cycle period itself will cause missed or shortened pulses. Minimum trigger pulse width determines whether a fast upstream contact reliably starts the timer. Contact rating and configuration (SPDT vs DPDT, resistive vs inductive load rating) determine whether the output can drive the downstream contactor coil directly or needs an interposing relay. Supply voltage band and whether the unit is control-voltage-triggered or supply-triggered close out the spec sheet, and directly determine which off-delay behavior (control-signal vs true off-delay) the relay is capable of, as covered above.
Timing Relay vs PLC Timer vs Monitoring Relay
A PLC software timer (TON/TOF instruction) is flexible — change the preset in the program, no rewiring, no panel space — but it is only as available as the controller: a CPU fault, a scan halt, or a program download in progress takes every software timer down with it. A discrete timing relay is hard-wired logic; it keeps working whether or not the PLC is scanning, which is why safety interlocks, motor-overrun protection, and star-delta sequencing on critical equipment are still commonly built on discrete timing relays even in a fully PLC-controlled panel. The tradeoff runs the other way for anything that changes often: a process step time that gets retuned monthly belongs in the PLC program, not on a physical dial that needs a technician and a screwdriver.
A monitoring relay is a different device family entirely, even though it shares a DIN-rail form factor and a change-over output with a timing relay: it measures a quantity — voltage, current, phase sequence, insulation resistance — against a threshold and trips on an out-of-range condition, rather than switching on elapsed time. Some monitoring relays include a trip-delay setting, which is where the two families blur in casual conversation, but the core function differs. See the monitoring relay engineering guide for the measurement side, and the related interface and coupling relay guide for the signal-isolation family that sits next to both on the rail.
An impulse (latching) relay is a third distinct family: bistable, no timing element at all — it toggles and holds state on successive pulses, commonly used for staircase lighting control from multiple switch points. Confusing it with a timing relay is an easy mistake on a parts list, and the two are not interchangeable in a wiring diagram.
Selecting, Wiring, and Choosing Between Schneider and ABB
Selection starts with the function: on-delay, off-delay (and which kind), star-delta, flasher, or multifunction if the application is not settled yet. From there, confirm the timing range actually covers the target setting with margin — not right at the edge of a range decade — and check the repeat accuracy against how tight the process actually needs to be; over-specifying accuracy for a non-critical delay just adds cost. Match the supply voltage to the control circuit (or pick a multi-voltage unit if the panel design is not locked), and size the contact rating and configuration to what the output actually drives: one contactor coil, or two coils and an indicator.
Wiring follows the trigger type. A supply-triggered on-delay relay only needs the auxiliary supply terminals wired — the delay starts the moment power is applied. A control-signal-triggered unit needs a separate trigger input wired from the pilot device, with the auxiliary supply wired continuously. Off-delay wiring depends on whether it is control-signal or true off-delay, per the distinction above — get that wrong and a fan overrun circuit stops the instant its compressor's supply drops, instead of continuing to run for its set cooldown period. Full step-by-step guidance is in how to wire a timing relay and how to select a timing relay.
| Criteria | Schneider Zelio Time | ABB CT-D range |
|---|---|---|
| Compact single-function timers | RE17 (on-delay, off-delay variants), 17.5 mm modular | CT-ERD (on-delay), CT-AHD (off-delay/true off-delay), 17.5 mm modular |
| Multifunction / multi-range | RE22, 22.5 mm, rotary function + range select | CT-MFD, 17.5 mm modular, DIP + rotary setting |
| Star-delta / two-output | RE22R (multifunction with star-delta) | CT-VWD, CT-YDD, CT-SDD (dedicated star-delta variants) |
| Plug-in / socket format | REXL range, 8-pin/11-pin socket | Earlier CT-S range; current CT-D line is primarily DIN-rail |
| Standards | IEC 61812 / IEC 60947-5-1 | IEC 61812 / IEC 60947-5-1 |
Both ranges cover the same functional ground — on-delay, off-delay, true off-delay, multifunction/multi-range, star-delta, cyclic — as multi-voltage DIN-rail or plug-in timers with change-over outputs. Where they actually differ is module width, how many functions and ranges combine in a single part, the granularity of the star-delta transition setting, and how much of the plug-in socket format is still current versus legacy. What we see in the field: panels standardized on one brand's contactor line tend to default to the matching timer range for spares commonality, not because of a meaningful functional gap between the two. Detailed reviews of each are in Schneider Zelio Time RE17 and RE22 and ABB CT-D range electronic timing relays, with a head-to-head in Schneider Zelio Time vs ABB CT-D. For the contactors these timers pilot, see selecting the right contactor and browse contactors and soft starters as the electronic alternative to star-delta on larger motors.
Frequently Asked Questions
What is the difference between an on-delay and an off-delay timing relay?
An on-delay relay waits the set time after the trigger before its output changes, then stays changed while the trigger is present. An off-delay relay changes output immediately on trigger, then reverts the set time after the trigger is removed — the delay is on the release side, not the operate side.
What is a true off-delay timing relay?
A true off-delay relay stores energy, typically in a capacitor, so it keeps timing down even after its own control supply is removed following the trigger. A standard control-signal off-delay relay needs its supply to stay present throughout the delay and stops timing if that supply drops.
Why does a star-delta timer need a transition (dead) time?
The transition holds both the star and delta contactors open for tens of milliseconds between switching. Without it, contact bounce or mechanical release lag can let both connections make at once, shorting the motor winding phase-to-phase and tripping the branch breaker or welding contacts.
What does ±1% repeat accuracy mean on a timing relay datasheet?
It means the actual delay produced varies by no more than about 1% of the set value across repeat operating cycles at the same setting. It is a separate figure from setting accuracy, which measures how close the dial marking is to the delay actually produced.
Should I use a timing relay or a PLC timer?
Use a discrete timing relay for interlocks and sequences that must keep working independent of the PLC's health, such as safety interlocks or star-delta sequencing. Use a PLC timer for process steps that get retuned frequently, since the preset changes in software with no rewiring.
Are Schneider Zelio Time and ABB CT-D timing relays interchangeable?
They cover equivalent functions — on-delay, off-delay, multifunction/multi-range, star-delta, cyclic — as multi-voltage DIN-rail timers, but they are not drop-in physical replacements. Module width, terminal layout, and rotary selector labeling differ, so a panel swap needs a wiring and label check, not just a functional match.
Conclusion
A timing relay's job is narrow — change a contact state a set interval from a trigger — but getting the function, trigger type, range, and accuracy right is what separates a sequence that works from one that intermittently does not. Match the function to the actual behavior needed (on-delay vs the two flavors of off-delay matter more than they look on paper), size the range and accuracy to the application rather than defaulting to the highest spec available, and treat star-delta transition settings as safety parameters. Stoklink stocks Schneider Zelio Time and ABB CT-D timing relays alongside the timing and control relays, contactors, and soft starters they pair with in a motor-starting or sequencing circuit.
