What Is a Timing Relay and How Does It Work
What is a timing relay? A timing relay switches its output contact a set interval after a trigger condition instead of instantly, with the delay set by a dial or DIP/rotary selector and built to IEC 61812, the standard covering time-delay relays for industrial control circuits. Pick the wrong function or timing range and a star-delta starter can close the delta contactor before the star contactor has fully opened, or a pump alternation circuit can let two pumps start together and trip a breaker. This guide covers how the internal timing mechanism works, the core function types (on-delay, off-delay, star-delta, multifunction), contact ratings and configuration, the accuracy specs that separate one timer from another, and where a timing relay stops making sense against a PLC software timer.
How Does a Timing Relay Generate the Delay?
Inside a modern timing relay, an electronic oscillator or RC network drives a counter circuit. The dial position (or DIP switch range plus potentiometer) sets a target count; once the trigger condition is met, the counter runs and the output relay energizes when it reaches the target. Older designs used a small synchronous motor and cam, or a pneumatic bellows that bled air through an adjustable orifice — both mechanical, both still found in legacy panels and both less precise than an electronic timer.
Power for the timing circuit comes from an auxiliary supply terminal (A1/A2) or, on some models, directly from the control voltage that also drives the load side. That distinction matters for off-delay behavior — a timer with no separate supply loses its logic the instant the control voltage drops, which rules out a true off-delay function.
What Are the Core Timing Functions?
Every timing and control relay line builds from a handful of base functions, and knowing the letter code on the datasheet saves a wrong order.
On-Delay (Delay-on-Operate)
Apply power, wait the set time, then the output contact changes state and stays changed until power is removed. Function code A or E on most European catalogs. See the full breakdown in our on-delay timing relay guide.
Off-Delay (Delay-on-Release)
The output changes immediately on trigger, then reverts the set time after the trigger is removed. A control-signal off-delay needs the supply present throughout the timing period; a true off-delay stores energy so it keeps timing even if the supply drops mid-cycle. Details in the off-delay timing relay guide.
Star-Delta
Two outputs instead of one: the star contactor closes, the motor runs up for a set time (typically a few seconds to around 30 s), the star contact opens, a short transition dead-time holds (tens of milliseconds) so star and delta never overlap, then the delta contactor closes. The transition setting is safety-critical — get it too short and both contactors can bridge phase-to-phase. Full mechanics in star-delta timing relays for motor starting.
Single-Shot, Interval and Cyclic (Flasher)
Single-shot fires one pulse of set duration per trigger. Interval mode is similar but re-triggerable. Symmetrical or asymmetrical flasher functions cycle the output on and off continuously — used for lamp flashing or intermittent duty cycling, not motor control.
Multifunction and Multi-Range: Why Stock Fewer Part Numbers?
A multifunction timer selects several functions (on-delay, off-delay, star-delta, flasher) and several time ranges (commonly spanning 0.1 s to 100 h across the range switches) from one physical device, using a pair of rotary selectors. Multi-voltage supply options (typically covering both common AC and DC control voltages in one part) reduce this further. The trade-off: a multifunction unit costs more per piece than a dedicated on-delay timer, but a panel builder stocking six functions across three voltage classes can replace that with two or three multifunction part numbers. Our multifunction and multi-range timing relay guide covers the selector logic in detail.
What we see in the field: panels built before multifunction timers became common still carry a drawer of single-function RE17-style relays as spares, because swapping in a multifunction unit means re-verifying every DIP setting against the original drawing — not a five-minute job during a shutdown window.
Contact Types and Ratings — What Actually Switches?
The output is a change-over contact, SPDT on compact single-function timers or DPDT on larger multifunction and star-delta units, rated for a few amps at control voltage — enough to drive a contactor coil or an interposing relay, not a motor load directly. DIN-rail formats (17.5 mm or 22.5 mm modular width) wire directly to terminals; plug-in formats use an 11-pin or 8-pin socket so the relay itself can be swapped without disturbing field wiring, at the cost of one more connection point that can develop resistance over years of vibration.
Timing Range and Accuracy: Reading the Spec Sheet
Two numbers decide whether a timer fits an application: the timing range (the span the dial or rotary selector covers) and the repeat accuracy (how much a given setting drifts cycle to cycle). A timer set near the bottom of its range on a wide multi-range switch position gives worse practical resolution than one set mid-range on a narrower position — the dial graduations get compressed at one end.
Formula: Repeat Accuracy — Source: IEC 61812, accuracy of operation clause
terr = ±(a% × tset)
| Symbol | Description | Unit |
|---|---|---|
| t_err | maximum deviation between repeated timing cycles at a fixed setting | s |
| a | repeat accuracy rating of the relay (commonly ±0.5% to ±1% of set value) | % |
| t_set | time value set on the dial or rotary range selector | s |
A relay rated ±1% repeat accuracy set to 10 s will drift roughly ±0.1 s cycle to cycle — negligible for a lighting delay, but worth checking against the transition-time tolerance on a star-delta application where too much drift risks contactor overlap. Setting accuracy (how close the dial reading matches the actual delivered time) is a separate figure and usually looser than repeat accuracy.
Timing Relay vs Monitoring Relay vs PLC Timer
These three get confused because they all sit in a control circuit and all look like a relay in a socket.
A PLC software timer runs inside the controller logic — flexible, easy to change from the programming software, but dependent on the PLC being powered, scanned, and not faulted. A hardwired timing relay keeps working if the PLC is down, which is why safety-related sequencing (like star-delta transition) is often kept on a dedicated relay even in a fully PLC-controlled panel. An impulse or latching relay is a different device entirely — bistable, switched by pulses, with no timing function at all.
This depends on the application's failure-mode tolerance: a conveyor sequencing delay that lives in PLC logic is fine to lose along with the PLC, but a star-delta transition timer failing open with the PLC is a contactor-overlap risk most panel builders won't accept.
Where a Timing Relay Fits in the Panel
Timing relays sit alongside contactors in most motor-starting and sequencing circuits, and increasingly alongside soft starters where a project moves from star-delta to electronic starting but still needs a delay somewhere in the control chain (bypass contactor pickup, for example). For a full walkthrough of function types, construction, contact ratings, and brand-specific series from Schneider Zelio Time and ABB CT-D, see the timing relay engineering guide. For the different function families side by side, our types of timing relays article breaks each one down individually.
Frequently Asked Questions
What is the difference between a timing relay and a regular relay?
A regular relay switches its contact the instant its coil is energized or de-energized. A timing relay inserts a deliberate, adjustable delay between the trigger and the contact change, set by a dial, DIP switch, or rotary range selector.
Can a timing relay replace a PLC timer?
For a single fixed delay in a hardwired circuit, yes — and it keeps working independent of PLC power or scan status. For a delay that needs to change often or depends on other program logic, a PLC timer is more practical.
What does "true off-delay" mean?
A true off-delay relay stores energy internally, usually in a capacitor, so it can complete its timing cycle even if the control voltage is removed at the same moment as the trigger. A control-signal off-delay needs the supply to stay present through the full delay period.
How accurate is a typical timing relay?
Repeat accuracy on most industrial electronic timers falls in the ±0.5% to ±1% of set-value range. Setting accuracy — how closely the dial reading matches delivered time — is usually a looser figure and should be checked separately on the datasheet.
Why does my star-delta timer need a transition setting?
The transition dead-time keeps the star and delta contactors from being closed at the same instant, which would bridge two phases together. Even tens of milliseconds of dead-time prevents the short circuit; setting it to zero defeats the purpose of a dedicated star-delta timer.
DIN-rail or plug-in timing relay — which should I choose?
DIN-rail units wire directly and take up less panel depth. Plug-in units let you swap the relay without disturbing field wiring, useful where the timer is expected to need replacement or reprogramming during commissioning, at the cost of an extra socket connection.
Conclusion
A timing relay's job is narrow — delay a contact change by a set, repeatable interval — but the function code, timing range, accuracy rating, and contact configuration all have to match the circuit for that delay to do its job safely. Star-delta and off-delay applications carry the most risk if the wrong variant goes in. When the choice isn't obvious, our how to select a timing relay guide and how to wire a timing relay guide walk through the decision and the terminal connections in order.
