What do the timing range and accuracy figures on a timing relay datasheet actually mean? The timing range is the span of delay a device can produce, from fractions of a second to hundreds of hours, selected with a rotary range switch and fine-tuned by a potentiometer or DIP-set value per IEC 61812. Read the wrong number on the sheet — setting accuracy instead of repeat accuracy, or full-scale range instead of usable resolution at your set point — and a star-delta transition or pump-alternation sequence drifts out of tolerance after months in service. This article covers how timing ranges and multi-range selection are structured, the difference between setting accuracy and repeat accuracy, minimum trigger pulse and recovery time, how tolerances stack across two timers in one circuit, and how to read a manufacturer's range/accuracy table without guessing.
Timing Range: Spans, Steps and Multi-Range Selection
A single-function timer such as a Schneider Zelio Time RE17 on-delay unit typically covers one fixed range, for example 0.1 s to 10 s, set by a dial calibrated 0 to 10. A multifunction, multi-range device — Zelio RE22 or ABB CT-MFD — covers several discrete ranges, commonly spanning 0.05 s up to 100 h, selected with a rotary range switch (0.1 s / 1 s / 10 s / 1 min / 10 min / 1 h / 10 h, and so on) and then fine-set with a 0-10 or 0-100% dial within that range.
The range switch and the set-value dial are two separate controls, and that separation is where selection errors happen. Setting "5" on a dial calibrated 0-10 means 5 s in the 10 s range, but 5 min in the 10 min range — the dial number alone tells you nothing without the range switch position.
Setting Accuracy vs the Dial Position
Setting accuracy describes how closely the actual delay matches the value the dial or DIP switches indicate, at the moment of setting. It is usually expressed as a percentage of full-scale range, not of the set value — so a timer with 5% setting accuracy on a 10 s range can be off by up to 0.5 s at any point on that dial, whether you set 1 s or 9 s. Set a short delay near the bottom of a wide range and the same absolute error becomes a much larger fraction of your target.
That is why range selection matters: pick the range where your target delay sits in the upper half of the scale, not near the bottom, whenever the device offers a choice.
Repeat Accuracy: The Number That Actually Matters
Repeat accuracy — sometimes called repeatability — is the variation between successive timing cycles at the same set point, same supply voltage, and same temperature. It is expressed as a percentage of the set value itself, typically ±0.2% to ±1%, and it is the figure that determines whether a star-delta transition or a sequencing step behaves the same way on cycle 50,000 as it did on cycle one. Setting accuracy tells you where the dial puts you on day one; repeat accuracy tells you how far the timer wanders around that point over its life.
Formula: Repeat Accuracy Tolerance Band — Source: IEC 61812, timing performance clause
Δt = ± (a% × tset)
| Symbol | Description | Unit |
|---|---|---|
| Δt | Maximum cycle-to-cycle deviation from the set delay | s (or min/h, matching tset) |
| a | Repeat accuracy class stated on the datasheet | % of set value |
| tset | Set delay value (dial or DIP setting) | s, min or h |
A 10 s star-delta run-up delay at ±0.5% repeat accuracy wanders ±0.05 s cycle to cycle — negligible on its own. Stack that with a second timer for the transition dead-time and the combined drift stops being negligible, which the next section covers.
Minimum Trigger Pulse and Recovery/Reset Time
The minimum trigger pulse is the shortest input signal the timer will reliably recognize as a valid start command. Feed a shorter pulse — a bouncing pushbutton contact, a noisy PLC output — and the timer may miss the trigger, or worse, start and reset erratically. Flasher and cyclic timers, which retrigger continuously, are the most exposed to this: a marginal pulse width shows up as an irregular blink rate rather than a hard fault.
Ignore recovery time in a fast pump-alternation or conveyor-indexing sequence and the second cycle can start early, off-spec, or not at all. Check it against your shortest expected off-period, not just the timing delay itself.
How Tolerances Stack Up in Star-Delta and Sequencing Circuits
A star-delta timing relay combines two intervals in series: the run-up delay (star contactor closed, typically a few seconds to around 30 s) and the transition dead-time (tens of milliseconds, safety-critical to prevent star and delta contactors overlapping). If both intervals are generated by two separate single-function timers rather than one two-output device, their repeat-accuracy tolerances add — worst case, arithmetically.
What we see in the field: panels built with two mismatched single-function timers for star-delta duty drift apart faster than panels using one purpose-built two-output star-delta timer or multifunction unit, simply because two independent tolerance bands are in play instead of one calibrated pair. This does not mean single timers are wrong for the job — it means the transition setting deserves the tighter-accuracy device of the pair, since an undersized transition dead-time is the failure mode that actually damages equipment.
The same stacking logic applies to pump alternation, staircase-lighting handover, and conveyor indexing — anywhere one timer's output feeds the next timer's trigger. Read the timing relay selection guide for how to weigh accuracy class against the rest of the spec sheet for a given application.
Reading a Manufacturer's Range and Accuracy Table
A typical datasheet table lists, per model: timing range(s), number of range positions, setting accuracy (% of full scale), repeat accuracy (% of set value), recovery time, and minimum trigger pulse — sometimes on one row, sometimes split by function for multifunction and multi-range timing relays that support several modes at once. Two traps recur. First, setting accuracy and repeat accuracy are not the same number and are not interchangeable — a datasheet that lists only one usually means setting accuracy, the looser figure. Second, "range" is the maximum span the range switch offers, not a guarantee that every point on it holds the same accuracy class; accuracy near the bottom of a wide range is proportionally worse than near the top.
This depends on the exact series — some ranges are genuinely linear across their full span, others taper — so when a delay is safety- or process-critical, size the range so the working set point sits in the upper two-thirds of the scale rather than trusting the headline range figure alone.
Frequently Asked Questions
What's the difference between setting accuracy and repeat accuracy?
Setting accuracy is how close the dial reading matches the actual delay at the moment you set it, expressed as a percentage of the full-scale range. Repeat accuracy is how much that same setting drifts cycle to cycle over the timer's life, expressed as a percentage of the set value. Repeat accuracy is the more important figure for cyclic or safety-related delays.
Why choose a narrower timing range instead of a wide multi-range device?
A device covering one narrow range, such as 0.1-10 s, keeps every practical set point in the upper part of the dial, where setting accuracy is proportionally tighter. A wide multi-range device is more flexible for stock reduction but needs the range switch set correctly to keep the working point off the low end of its scale.
What happens if the trigger pulse is shorter than the minimum specified?
The timer may fail to start, start and then reset before completing the interval, or in cyclic/flasher functions produce an irregular output rate instead of a clean fault. This is a common cause of "intermittent" timing complaints traced back to a bouncing pushbutton or a marginal PLC output pulse.
How does temperature affect timing relay accuracy?
Electronic timing relays specify accuracy over a stated ambient range, and drift outside that band is not covered by the datasheet's repeat accuracy figure. Panels in unconditioned enclosures near heat sources or outdoor cabinets should use a timer rated for the actual ambient swing, not the catalog default range.
Do older pneumatic or motor-driven timers have the same accuracy class as electronic ones?
No. Pneumatic (air-bleed) and motor-driven timers generally hold looser repeat accuracy and are more sensitive to ambient temperature and mechanical wear than electronic oscillator/counter-based timers. Some panel builders still use pneumatic timers on legacy retrofits, but new designs default to electronic units for tighter, more stable tolerances.
Does recovery time apply to on-delay timers used only once per machine cycle?
It still applies whenever the machine can restart before the timer has fully reset — for example, a fast stop/restart sequence. For a slow, single-shot start-of-shift delay it rarely matters; for anything cycling faster than a few seconds it should be checked against the datasheet figure.
Conclusion
Timing range tells you what a device can cover; setting accuracy tells you how close the dial gets you on day one; repeat accuracy tells you how stable that setting stays over the timer's working life. For star-delta transitions, sequencing chains, and any delay feeding a safety-related interlock, repeat accuracy and recovery time matter more than the headline range number on the front of the datasheet. Start from the timing relay engineering guide for the full picture of functions and formats, browse timing and control relays for current stock, and see what a timing relay is and how it works or the overview of types of timing relays for the functions these specs apply to.