How do you select a timing relay? Match five parameters in a fixed order — function (on-delay, off-delay, true off-delay, star-delta, or multifunction), timing range against the process interval, supply voltage and trigger type against the control circuit, contact rating against the load, and mounting format (DIN-rail modular or 8/11-pin plug-in socket) — per IEC 61812 and IEC 60947-5-1. Skip a step and the failure shows up later: a unit sized right on time range but wrong on contact rating arcs and welds within months, and a supply-triggered off-delay wired where a capacitor-backed true off-delay was needed loses its timing the instant power drops. This guide walks the five checks in order, gives a margin formula for the timing range step, and covers when a single multifunction timer replaces three single-function models.
Step 1: Confirm the Function Before Anything Else
Every other decision is downstream of function. Get this wrong and no amount of correct voltage or contact rating saves the application. Four functions cover most panel work.
ON-DELAY (delay-on-operate, function A/E) applies power, holds the output for the set interval, then switches and stays switched — used for sequencing a second load after a first one has stabilized, or blocking a signal until a startup transient clears. OFF-DELAY (delay-on-release, function B/R) switches the output immediately on trigger and holds it through a set delay after the trigger drops, common on exhaust fans and cooling pumps that need to keep running after the main machine stops. STAR-DELTA timers run two outputs — star, a short dead-time, then delta — for reduced-voltage motor starting. MULTIFUNCTION units let a rotary switch select on-delay, off-delay, or several other modes from one part number, which is where multifunction and multi-range timing relays earn their keep on panels with mixed sequencing needs.
What we see in the field: a surprising number of "faulty timer" complaints are function mismatches, not defects — someone installed an on-delay where the sequence actually needed off-delay. Read the full timing relay types breakdown before assuming the unit is bad.
Step 2: Match the Timing Range and Accuracy to the Process
Once function is fixed, size the timing range. A dial or DIP range that only covers 0.1-10 s is useless for a 5-minute pump alternation cycle, and an oversized range (say 1-100 h) sacrifices setting resolution on a 3 s delay — the dial mechanically can't be read that fine. Multifunction/multi-range models such as Schneider Zelio Time RE22 or ABB CT-MFD cover several ranges (typically spanning roughly 0.1 s to 100 h across range positions) from one device, at the cost of a rotary range-select step the installer must set correctly.
Accuracy matters more on star-delta transition and process-critical delays than on non-critical ones. Repeat accuracy (how much a single unit's output timing varies from cycle to cycle at a fixed setting) is normally quoted as a percentage of the set value, roughly ±0.5% to ±1% on decent electronic timers; setting accuracy (how close the actual delay lands to the dial's printed value) is usually looser. On a transition dead-time set to 50 ms, ±1% is trivial; on a 30-minute interval, ±1% is close to 18 seconds of drift, which may or may not matter depending on what's downstream.
Formula: Timing Range Selection Margin — Source: general engineering practice per IEC 61812 accuracy classes
Tset = Tprocess × (1 + M)
| Symbol | Description | Unit |
|---|---|---|
| Tset | Minimum dial/range value the selected timer must be able to reach | s, min, or h |
| Tprocess | Measured or specified process interval to be timed | s, min, or h |
| M | Safety margin fraction — typically 0.1-0.2 for non-critical delays, 0.2-0.3 where the load has run-in wear (motor bearings, mechanical brakes) | dimensionless |
Applied to star-delta: if run-up measured on the bench is 4 s, don't select a range that tops out at 5 s — a worn bearing or a slightly heavier load pushes run-up past the ceiling and the motor starts on star indefinitely, tripping overload. See the dedicated star-delta timing relay guide for transition-time detail.
Step 3: Check Supply Voltage and How the Timer Is Triggered
Two separate questions here, and installers conflate them. First: does the timer's power supply band cover the panel's control voltage? Multi-voltage models (commonly 24-240 V AC/DC on one part) simplify stock but cost more per unit than a single fixed-voltage variant; a panel builder running 24 VDC exclusively can standardize on a cheaper fixed-voltage timer instead.
Second, separately: is this a control-voltage off-delay or a true off-delay? A control-voltage (or "signal") off-delay keeps its supply energized throughout and times from a separate trigger input dropping — if the supply itself is cut, timing stops immediately, no delay happens. A TRUE off-delay stores energy internally (a capacitor bank) so it can complete its timed release even after the main supply is removed. These are not interchangeable. Specifying a control-voltage off-delay for an application that needs the fan to keep running after the whole panel loses power is a design error that only shows up during a real outage.
This depends on how the load is actually wired more than on the catalogue description — check whether "off-delay" in the datasheet assumes supply-present or supply-removed before ordering.
Step 4: Size the Contact Rating to the Actual Load
Timing relay outputs are almost always change-over (SPDT or DPDT), rated for a few amps at AC-15/DC-13 duty — enough to drive a contactor coil or a small pilot load directly, not enough to switch a motor or a large lighting bank without an interposing contactor. Read the AC-15 rating specifically for coil loads, not the higher resistive AC-1 figure printed alongside it; a coil's inrush current at pickup is several times its holding current, and sizing off the wrong column shortens contact life fast.
Where the timer is only ever going to pilot a contactor coil, a single-pole output is fine. Where the sequence needs one contact for indication and one for interlock at the same time, specify a DPDT unit up front — retrofitting a second timer later to add a contact is a wasted panel slot and an extra point of failure. Both timers and the downstream contactors they pilot should be checked against the actual coil inrush, not the nameplate holding current alone.
Step 5: Pick the Mounting Format — DIN-Rail or Plug-In Socket
DIN-rail modular timers (17.5 mm or 22.5 mm wide, the Schneider RE17/RE22 and ABB CT-D families both follow this format) mount directly to the rail and wire with screw or spring terminals — the standard choice for new panel builds where the timer is a permanent fixture. Plug-in (8-pin or 11-pin socket) formats such as Schneider REXL sit in a separate base wired once; the timer itself unplugs for replacement without disturbing field wiring, which matters on equipment where downtime for a swap has to be measured in seconds, not minutes — municipal pumping stations and some machine OEM panels favor this for that reason.
Some panel builders still default to whatever format they last stocked, but the swap-without-rewiring advantage of plug-in bases is real on equipment serviced by non-specialist staff, and worth the marginally higher installed cost.
When One Multifunction Timer Replaces Three Single-Function Units
A multifunction/multi-range timer (Schneider RE22, ABB CT-MFD) selects function and range via rotary switches instead of being fixed at manufacture. The trade-off is panel stock versus setup risk: one SKU covers what would otherwise be three or four single-function parts, cutting spare-parts inventory, but every function/range switch is a field-configurable setting that can be set wrong during commissioning — unlike a single-function timer, which is correct by definition once the right part is installed.
| Criteria | Single-function on/off-delay | Star-delta timer | Multifunction/multi-range |
|---|---|---|---|
| Function set | Fixed at manufacture, one function | Fixed: star, dead-time, delta | Field-selectable, multiple functions |
| Timing range | One fixed range, best resolution | Run-up + short dead-time range | Several ranges via rotary select |
| Best for | Single, well-known repeated task | Reduced-voltage motor starting | Mixed sequencing, spares consolidation |
| Commissioning risk | Low — correct by part selection | Moderate — transition setting critical | Higher — function/range must be set correctly |
| Typical width | 17.5 mm modular | 22.5-45 mm (two-output) | 17.5-22.5 mm modular |
For a panel with one repeated, well-understood sequence, the single-function part is still the safer choice — it can't be misconfigured on the bench. For a service van stocking spares against unknown fault calls, the multifunction unit wins on flexibility. Full wiring detail for either type is in how to wire a timing relay, and the broader function map is in the timing relay engineering guide.
Frequently Asked Questions
What's the difference between on-delay and off-delay timing relays?
On-delay switches its output the set time AFTER power or trigger is applied and stays switched. Off-delay switches immediately on trigger and reverts to its rest state the set time after the trigger is removed. Confusing the two is the most common timing relay selection error.
Can I use a standard off-delay timer if the supply might drop out?
Not reliably. A control-voltage off-delay needs continuous supply and times from a separate trigger input, so it stops timing if the supply is cut. A true off-delay stores energy internally and can complete its delay even with the supply removed — specify that variant if the application depends on it.
Do I need a multifunction timer or is a single-function model enough?
If the panel has one repeated, well-defined task, a single-function timer is simpler to commission and can't be misconfigured. A multifunction/multi-range model makes more sense when stocking spares for several different sequences or when the exact function needed may change during commissioning.
What contact rating do I need for a timing relay driving a contactor coil?
Check the AC-15 (inductive/coil) rating, not the AC-1 (resistive) figure. Contactor coils draw several times their holding current at pickup, and an output sized off the resistive rating alone will wear out faster than expected.
Should I choose DIN-rail or plug-in timing relays?
DIN-rail modular is the default for new panel builds — lower cost, permanent mounting. Plug-in (socket-mounted) formats cost more installed but let the timer be swapped without disturbing field wiring, which is worth it on equipment where fast, non-specialist replacement matters.
How much margin should I add when selecting a timing range?
A working rule is 10-20% above the measured or specified process interval for non-critical delays, and 20-30% where the load has run-in wear, such as a motor's run-up time drifting as bearings age. Never select a range where the target interval sits right at the maximum dial value.
Conclusion
Selection order matters: function first, then timing range and accuracy, then supply voltage and trigger type, then contact rating, then mounting format. Each step depends on the one before it, and skipping ahead — picking a part on voltage and range alone, say — is how "faulty timer" service calls happen months after commissioning. For function-specific detail, see what a timing relay is and how it works; for the broader relay category including monitoring and interface relays, browse timing and control relays.