What types of timing relays exist? Timing relays split by trigger point and output behavior into on-delay (delay-on-operate), off-delay (delay-on-release, with a true off-delay sub-type that runs from stored energy), star-delta (two sequential outputs), impulse/interval/flasher, and multifunction/multi-range units that combine several of these in one IEC 61812 DIN-rail or plug-in housing. Picking the wrong type means the output changes at the wrong moment in the sequence — a star contactor that never drops out, or a fan that cuts off the instant a thermostat opens instead of after a purge cycle. This article covers each function, how star-delta and multifunction types differ from the rest, contact and format options, and how to match a type to an application.
On-Delay (Delay-on-Operate) Timing Relays
An on-delay timer starts counting the moment supply or the trigger signal is applied. The output contact stays in its rest state for the set interval, then changes over and stays there as long as the trigger is present. Function code A (or E on some Zelio Time parts) marks this behavior on Schneider timers; ABB's CT-ERD range is a dedicated single-function on-delay module. Typical use: delaying a pump start after a level switch closes, so a brief splash on the sensor does not start the motor.
Setting range is usually selectable between a handful of bands — 0.1 s to 10 s, 1 s to 100 s, and so on — via a rotary switch, then fine-tuned with a potentiometer or DIP combination. Repeat accuracy on electronic on-delay units runs in the +-0.5% to +-1% of set-value class; the older pneumatic (air-bleed) on-delay timers drift more with temperature and are largely out of new designs now. For the full mechanism read our on-delay timing relay guide.
Off-Delay (Delay-on-Release) Timing Relays
Off-delay reverses the sequence: the output changes over as soon as the trigger is applied, then reverts the set time after the trigger is removed. Function code B (or R) on Zelio Time timers, and ABB's CT-AHD range, cover this. Ventilation is the classic case — a fan relay closes instantly when a machine starts, then keeps running a set 5-15 minutes after stop to clear fumes or heat.
True Off-Delay vs Control-Signal Off-Delay
There are two off-delay variants, and panel builders confuse them often. A control-signal off-delay keeps its own auxiliary supply live and times from a separate trigger input; remove the trigger and the timer counts down, but only while its own power stays on. A true off-delay stores energy internally, usually in a capacitor, so it can complete the delay even after both the trigger and the supply are removed together.
If a design needs a fan to keep running after someone kills the main disconnect, only the true off-delay type does that; a control-signal version stops immediately with the supply. See the dedicated off-delay timing relay guide for wiring detail.
Multifunction and Multi-Range Timing Relays
A multifunction timer packs on-delay, off-delay, interval, and often flasher modes into one housing, selected by a front rotary switch alongside the timing range. Schneider's RE22 and ABB's CT-MFD are the reference parts in each brand's line. Multi-range means the same physical unit covers several timing bands — 0.1 s up to 100 hours on the widest CT-MFD variants — instead of one fixed span.
What we see in the field: panel shops standardize on one or two multifunction part numbers to cover a whole cabinet's delay needs, instead of stocking a dedicated on-delay, off-delay, and interval timer separately. The trade-off is a busier front-panel setup. Function select, range select, and the potentiometer all have to be set correctly, and a misread rotary switch is a common commissioning fault. Full selection logic is in our multifunction and multi-range timing relay article.
Star-Delta Timing Relays: A Two-Output Type
Star-delta timers are a distinct category because they drive two outputs in sequence instead of one. On start, the star output closes and the motor accelerates on reduced (star) voltage for a set run-up time, typically a few seconds up to around 30 s depending on load inertia. The star output then opens, the timer holds a short transition (dead-time) of tens of milliseconds, and only then does the delta output close. Schneider's RE22R and ABB's CT-VWD/CT-YDD/CT-SDD are dedicated two-output star-delta parts; some multifunction timers also offer a star-delta mode.
Formula: Star-Delta Total Starting Time — Source: Star-delta motor starting practice (IEC 60947-4-1)
ttotal = trun-up + ttransition
| Symbol | Description | Unit |
|---|---|---|
| trun-up | time the motor runs on the star (reduced-voltage) contactor, sized to acceleration | s |
| ttransition | dead-time between star contactor opening and delta contactor closing | ms |
| ttotal | total time from start command to full delta run | s |
The transition setting is the one figure on the whole timer that is safety-critical. Set it too short and the delta contactor can close before the star contactor's arc fully clears, a line-to-line short across the two contactors. Set it too long and the motor coasts un-powered longer than needed, with extra current draw when delta finally picks up. This function pairs directly with the contactors it switches; see how to wire a contactor for star-delta starting and the dedicated star-delta timing relay for motor starting guide. On new designs, some panel builders now reach for a soft starter instead of star-delta for the same reduced-inrush goal; it depends on the motor size and whether stepped torque during transition is acceptable.
Impulse, Interval and Flasher (Cyclic) Timing Relays
Three more functions round out most multifunction rotary selectors. An interval timer closes the output for exactly the set time once triggered, then drops out on its own, a fixed-duration pulse regardless of how long the trigger stays present. A single-shot (impulse) timer gives one short output pulse per trigger edge, useful for driving a counter input or a momentary indicator. A flasher (cyclic) timer alternates the output on and off continuously while triggered, either symmetrical (equal on/off time) or asymmetrical (separately set on and off periods). Staircase lighting and warning beacons use this function.
None of these three time from a fixed reference the way on-delay or off-delay do; they generate a self-contained pulse pattern instead. That distinction matters when specifying a replacement part: a flasher swapped in for an interval timer keeps cycling instead of dropping out once, and the fault will not show up until the next machine cycle confuses an operator.
Comparing the Types: Which One Fits Which Job
Function is the first filter, not brand. The table below lines up the four most-specified types against the questions that actually decide a part number.
| Criteria | On-Delay | Off-Delay | Multifunction | Star-Delta |
|---|---|---|---|---|
| Output changes | Set time AFTER trigger applied | Immediately, reverts set time AFTER trigger removed | Selectable — any of the above | Star closes, delta closes after run-up + transition |
| Outputs | 1 change-over | 1 change-over | 1-2 change-over | 2 dedicated (star, delta) |
| Typical use | Delayed pump/motor start | Fan overrun, purge cycles | Mixed panel needs, one stocked part | Reduced-voltage motor starting |
| Setup complexity | Low, one range dial | Low, one range dial | Higher, function + range select | Two settings, transition is safety-critical |
Format and supply matter next. DIN-rail 17.5 mm modules (Schneider RE17/RE22, ABB CT-D) fit fixed panel builds; plug-in 8-pin or 11-pin socket timers (Schneider REXL) let a field tech swap a failed unit without removing wiring, at the cost of one more contact interface that can loosen. Multi-voltage input timers accept a wide AC/DC band, which cuts SKU count on international panel builds where 24 V DC and 110-240 V AC control circuits both show up. Full selection logic is in how to select a timing relay for your application.
Timing relays are also easy to confuse with two adjacent device classes. A monitoring relay measures a quantity, such as voltage or phase sequence, and trips on a threshold; it has no adjustable time delay as its primary function, though some models add one. A PLC software timer is flexible and free once the I/O exists, but it depends on the PLC's own supply and program logic staying healthy. A hardwired timing relay keeps working as a fail-safe even if the PLC is down for maintenance.
For the underlying mechanism common to all these types, start with what a timing relay is and how it works, or browse the full timing and control relays range. This article is part of the timing relay engineering guide.
Frequently Asked Questions
What is the difference between an on-delay and an off-delay timing relay?
An on-delay timer waits the set time after the trigger before its output changes over, then holds. An off-delay timer changes its output immediately when triggered and reverts the set time after the trigger is removed. The delay sits on opposite sides of the trigger event.
What makes a true off-delay timer different from a normal off-delay timer?
A true off-delay stores energy internally, usually in a capacitor, so it can finish its delay even if the control supply is removed along with the trigger. A control-signal off-delay needs its own supply to stay live throughout the delay period.
What is a multifunction timing relay?
A multifunction timer combines several functions, such as on-delay, off-delay, interval, and flasher, plus several timing ranges in one device, selected with front-panel rotary switches. It reduces the number of distinct timer part numbers a panel builder needs to stock.
How is a star-delta timer different from other timing relay types?
A star-delta timer drives two sequential outputs, star then delta after a run-up time and a short transition dead-time, instead of the single output most other timing relay types use. The transition setting prevents the two motor contactors from overlapping.
What is a flasher or cyclic timing relay used for?
A flasher timer alternates its output on and off repeatedly while triggered, symmetrically or with separately set on/off periods. Staircase lighting timers and warning beacons are typical applications.
Conclusion
Function decides the part, not the brand. On-delay and off-delay cover the two single-output cases split by which side of the trigger the delay sits on; true off-delay adds capacitor-backed hold-through for supply loss. Multifunction and multi-range units trade a busier front panel for fewer stocked part numbers. Star-delta stands apart with two sequential outputs and a safety-critical transition setting, and impulse/interval/flasher functions generate self-contained pulse patterns rather than timing from one reference point. Match the control sequence first, then pick the DIN-rail or plug-in format and the voltage band that fits the panel.