What is an off-delay timing relay? An off-delay (delay-on-release) timing relay switches its output the moment a trigger is applied, then holds that state for a set interval after the trigger is removed, per the timing functions defined in IEC 61812. The consequence is a controlled shutdown: a fan, pump, or lamp keeps running past the point where the operator or process signal says "stop," instead of cutting out instantly. This article covers the difference from on-delay operation, the split between true off-delay and control-signal off-delay circuits, timing range and accuracy, contact and wiring practice, and where the function gets used in real panels.
How an Off-Delay Relay Differs From an On-Delay Relay
On-delay (function A/E) waits after power is applied, then changes the output and stays changed. Off-delay (function B/R) does the opposite: the output changes as soon as the trigger arrives, and the delay sits on the back end, timing out only after the trigger disappears. Both use the same electronic oscillator/counter core and the same 17.5 mm DIN-rail or 8/11-pin plug-in housings. What changes is which edge of the signal starts the clock.
This matters for anything with thermal or mechanical inertia. A motor that just ran hot needs its cooling fan to keep turning after the motor stops, not stop with it. An on-delay timing relay solves the opposite problem — holding a load off until something else is ready. Mixing the two up in a wiring diagram is a common documentation error; the relay body often looks identical, so the function code on the label is the only reliable check.
True Off-Delay vs Control-Signal Off-Delay: Two Different Circuits
Not every "off-delay" relay behaves the same way when power drops. A true off-delay relay stores energy internally — a capacitor sized to the maximum timing range — so it can complete its timing cycle even if the supply is removed at the same instant as the trigger. Cut the power to a true off-delay unit mid-cycle and the output still holds for the programmed interval, running off the stored charge.
A control-signal off-delay relay works differently. It keeps its auxiliary supply connected at all times and times from a separate trigger input, not from the supply itself. Removing the supply on this type kills the output immediately, delay or not — there is nothing storing energy for it. Confusing the two on a fail-safe shutdown circuit (a cooling fan that must keep running if the control cabinet loses power mid-cycle, for example) picks the wrong relay for the job.
What we see in the field: control-signal off-delay units are more common because they are cheaper and simpler to wire into an existing 24V control rail. True off-delay is specified specifically when the trigger and the supply can be lost together and the delay still has to run.
Timing Range, Repeat Accuracy and What the Numbers Mean
Off-delay relays are specified with a timing range (commonly spanning from a fraction of a second to tens of hours, selected by rotary or DIP range switches) and a repeat accuracy, expressed as a percentage of the set value rather than an absolute figure. A relay repeating at ±1% of a 10-minute setting drifts roughly six seconds run to run; the same ±1% on a 30-second setting drifts a fraction of a second. Longer settings need looser absolute tolerance to hit the same percentage, not tighter.
Setting accuracy (how close the dial position matches the actual delivered time) is a separate figure from repeat accuracy (how consistent that same dial position is across cycles). A panel builder tuning a process to the second cares about setting accuracy on first commissioning and repeat accuracy for every cycle after.
Formula: Repeat Accuracy Error — Source: IEC 61812, timing performance clause
Δt = tset × (a / 100)
| Symbol | Description | Unit |
|---|---|---|
| Δt | Maximum cycle-to-cycle timing deviation | s (or same unit as tset) |
| tset | Programmed off-delay time | s, min, or h |
| a | Rated repeat accuracy of the relay | % of set value |
Check timing range and accuracy against the sibling article on timing relay ranges and accuracy specs when the process tolerance is tight — sequencing steps under a few seconds apart is where accuracy class actually decides which relay fits.
Contact Configuration and Wiring for Off-Delay Timers
Output is a change-over contact, SPDT on compact single-function units, DPDT on many multifunction and multi-range parts, rated for a few amps resistive at 250V AC — enough for a contactor coil or an indicator, not for switching a motor directly. Two wiring patterns show up in the field. A 2-wire connection ties the trigger to the same terminals as the supply, so trigger and supply arrive and leave together; this is the simpler layout and it is what a control-signal off-delay relay expects. A 3-wire connection keeps the supply permanently applied and brings the trigger in on a separate terminal, which is how a true off-delay relay is normally wired so it can keep timing after the trigger terminal goes low.
Get the two backwards and the relay either times out instantly (trigger removed at the same moment as supply, no stored energy to draw on) or never releases (supply held on a control-signal unit that was expected to drop with the trigger). Check the wiring diagram on the relay's own label, not a generic diagram from a different series — terminal numbering is not standardized across brands even when the function code matches.
Where Off-Delay Timing Relays Are Used
Motor cool-down fans on VFD cabinets and compressor rooms: the fan needs to keep clearing heat for minutes after the drive stops switching. Cooling tower and chiller fan overrun, timed the same way. Lighting delay-off in stairwells and warehouse aisles, where a push-button trigger should hold the light for a set walk-through interval rather than switching it off instantly. Post-run lubrication and purge cycles on machine tools, where a pump or blower has to finish a cycle after the main sequence stops. Door or gate hold-open timers, where the off-delay keeps a lock released briefly after the release button is let go.
Most of these share a pattern: something with thermal or mechanical inertia needs its supporting system to outlast the main command by a fixed, repeatable margin. A star-delta timing relay solves a related but different problem — sequencing two contactors during motor starting rather than extending a shutdown — and the two functions occasionally sit in the same starter panel.
Selecting and Setting an Off-Delay Relay Correctly
Start with the trigger source. If it is a maintained control signal that a PLC or pushbutton holds and releases cleanly, a control-signal off-delay relay is enough. If the supply itself can disappear before or with the trigger — a common condition on E-stop and safety-adjacent shutdown circuits — move to true off-delay and confirm the maximum hold time the stored-energy design supports, since capacitor-based storage has a practical ceiling that a longer control-signal timing range does not share.
Check the minimum trigger pulse the relay accepts. A trigger shorter than the rated minimum pulse may not register at all, particularly on units built for control-signal operation where the trigger input is read on an edge, not held. Confirm the contact rating covers the actual coil or load current with margin, and confirm the supply voltage band covers the control rail — most current timing and control relays ship multi-voltage, but older stock and some plug-in variants do not. For the full walkthrough on terminal-by-terminal connection, see how to wire a timing relay; for picking between functions and ranges more broadly, see how to select a timing relay.
Schneider's Zelio Time RE17 range covers single-function off-delay in the compact 17.5 mm body; ABB's CT-AHD covers the same function on the CT-D platform, with true off-delay behavior on the parts built for it. Both sit alongside multifunction parts (Zelio RE22, ABB CT-MFD) that include off-delay as one selectable mode among several — worth checking before stocking a dedicated single-function part if the panel already carries a multifunction range elsewhere.
Frequently Asked Questions
What is the difference between an on-delay and off-delay timing relay?
On-delay changes its output a set time after power or trigger is applied and stays changed. Off-delay changes its output immediately on trigger and reverts a set time after the trigger is removed. The delay sits on opposite edges of the signal.
What is a true off-delay timing relay?
A true off-delay relay stores energy internally, typically in a capacitor, so it can complete its timing cycle even after both the trigger and the supply voltage are removed at the same time. Control-signal off-delay relays cannot do this — they need the supply to stay connected.
Can an off-delay relay hold its contact after a power failure?
Only a true off-delay type, and only for the duration its stored energy supports, which is bounded by the capacitor sizing behind the timing range. A control-signal off-delay relay drops its output as soon as the supply is lost, regardless of the delay setting.
What is the minimum trigger pulse an off-delay relay needs?
It depends on the model and is listed on the datasheet as a minimum pulse duration, typically tens of milliseconds. A trigger shorter than that minimum may not be registered, which matters most on control-signal units reading the trigger as an edge rather than a held level.
Where are off-delay timing relays commonly used?
Motor and drive cabinet cooling fan overrun, cooling tower and chiller fan delay-stop, stairwell and warehouse lighting delay-off, post-run lubrication or purge cycles, and door or gate hold-open timing. The common thread is a load that needs to keep running briefly after the main command stops.
Conclusion
Off-delay timing is the mirror image of on-delay: the output changes on trigger and releases later, not the other way around. The choice that actually matters in the field is true off-delay versus control-signal off-delay — get that wrong on a fail-safe cooling or shutdown circuit and the delay either never runs or runs when it should not. Confirm the trigger source, the minimum pulse, the timing range and accuracy class, and the wiring pattern against the relay's own label before it goes in the panel. For the broader picture across all timing functions, see the timing relay engineering guide, and pair off-delay fan and pump control with properly rated contactors sized for the switched load.