An electric linear actuator is almost never a continuous-duty product, so its nameplate must declare either an S2 short-time rating with a fixed run minutes figure (for example S2 15 min or S2 30 min) or an S3 intermittent periodic duty expressed as a percentage, typically 10 to 25 % on standard catalog units [S7].
IEC 60034-1 splits the two ratings cleanly: S2 runs at constant load only until it is deliberately de-energised before thermal equilibrium, then rests until the winding returns to ambient [S4][S1]. S3 instead runs a repeating cycle of identical load and rest pulses in which thermal equilibrium is never reached, and the rating is the ratio of run time to cycle time [S4][S3].
S2 Short-Time Duty: Definition and Where It Fits on an Actuator
S2 is described in IEC 60034-1 as constant-load operation short enough that the motor does not reach thermal equilibrium, with a de-energised period long enough for the machine to cool back to ambient before the next start [S1][S4]. The duty designation is written S2 followed by the run minutes, so the common catalog values are S2 10 min, S2 15 min, S2 30 min, and S2 60 min [S1][S5].
For linear actuators, the practical translation is a one-shot stroke: the linear actuator drives from one end of travel to the other, holds, then powers down and is allowed to cool fully. Typical S2 applications are hoist drives, gate operators, and the emergency or test stroke of a damper, where the stroke is short relative to the cool-down interval [S3]. Because the winding never equilibrates, an S2-rated machine can be mechanically smaller for the same peak torque than an equivalent S1 continuous motor, and its overload headroom in the first few minutes of operation is the design asset the specifier is buying [S2][S5].
S3 Intermittent Periodic Duty: Definition and Where It Fits on an Actuator
S3 is defined as a sequence of identical cycles, each containing a running period at constant load and a rest period, with the cycle short enough that thermal equilibrium is not reached and starting current treated as negligible [S4][S6]. The duty is written S3 followed by a percentage, for example S3 25 %, meaning 25 % of each cycle is on-load and 75 % is de-energised [S1][S3].
For catalog linear actuators, S3 10 to 25 % is the standard published band, and on a typical stroke time of 30 s that corresponds to roughly 1 to 2.5 minutes of continuous run per cycle window [S7]. S3 is the natural rating for any application that demands repeated, predictable strokes, including valve actuation, solar tracker drives, and indexing gates, and is the rating used as the baseline when actuator suppliers publish a 'duty cycle' figure on their datasheets [S7][S3].
Decision Matrix: S2 vs S3 for Linear Actuator Selection
The two ratings are not interchangeable, and the wrong one will either burn out a motor sized for one-shot use or waste money on an oversized frame for a repeating application. A direct comparison on the four criteria that drive the buy: [S3]
Run profile. S2 = single fixed run duration, then a long rest back to ambient. S3 = repeating identical cycle, never reaching equilibrium, expressed as % on-time [S4][S1]. Thermal target. S2 designs optimise for short-term overload and accept a high initial temperature rise. S3 designs target a steady average winding temperature under cyclic load [S2][S5]. Catalog use on linear actuators. S2 is published on hoists, gate drives, and one-shot positioning units; S3 is the standard published duty for general-purpose rod actuators and valve or damper actuators [S3][S7]. Sizing consequence. S2 lets the specifier pick a smaller frame for a given peak torque because the cool-down to ambient absorbs the heat. S3 requires the frame and thermal class to be sized for the cyclic average, not the peak [S2][S5].
Specifiers comparing valve or damper actuation profiles should treat the actuator duty selection in the same way they treat the motor selection in the related spec work covered in NEMA vs IEC AC motor frame sizes: shaft-height codes, mounting flanges, and the 3-4 mm difference, because the IEC rating system is the language used in both cases.
What the Label Numbers Actually Mean on a Nameplate
Reading the plate correctly is part of the spec: an S2 plate will show the cycle minutes (S2 30 min means rated to deliver rated torque for 30 minutes from a cold start, then must rest to ambient), while an S3 plate will show the percentage (S3 25 % means rated to deliver rated torque for 25 % of any cycle, provided the cycle is short enough that thermal equilibrium is not reached) [S1][S4].
For an actuator, the cycle time on an S3 plate is normally implied by the stroke time and the published 'on' percentage, so a 30 s stroke on an S3 20 % plate implies a 150 s cycle window between successive energisations [S7][S3]. Misreading the percentage as 'percent of full load torque' rather than 'percent of cycle time on' is one of the more common mistakes that turns a working S3 spec into an early winding failure [S3].
Linear Actuator Reality: Where S3 25 % and S2 10 / 30 min Actually Sit
Most catalog electric linear actuators are published in the 10 to 25 % S3 band, which on a 30 s stroke gives roughly 1 to 2.5 minutes of permissible continuous run per cycle [S7]. This is the S3 figure to use for general valve, damper, and solar-tracker stroking where strokes repeat on a known cadence.
Where the actuator performs a one-shot positioning task, for example a hatch, a louver, a gate, or a hoist-style lift, S2 with a fixed minutes rating (S2 10 min, S2 15 min, S2 30 min) is the correct label, and the specifier can usually downsize the frame relative to an S3-equivalent unit because the cool-down to ambient carries the heat away [S3][S1]. If the same physical actuator is being repurposed into a higher-stroke-frequency service, the S3 percentage on the same nameplate is the ceiling, not the S2 figure, and the unit must be re-evaluated against the new cycle [S7][S5].
Limitations, Failure Modes, and What the Rating Does Not Cover
IEC 60034-1 ratings describe thermal stress under the declared cycle, but they do not cover mechanical life, and an actuator with the right thermal duty can still fail mechanically if the stroke count exceeds the published cycle rating on the bearing and leadscrew [S3]. For rod-style actuators, total stroke count over service life is the second axis the spec must close, alongside the S2 or S3 rating [S7].
On S2, the hidden failure mode is operating the unit past the declared run minutes, or energising it again before ambient is restored, both of which push the winding past the design temperature rise assumed in the rating [S2][S5]. On S3, the hidden failure mode is increasing the load, the stroke time, or the cycle frequency until the on-time percentage silently exceeds the nameplate value, with no external indication until the insulation trips or chars [S3][S7]. Where a VFD is in the drive line, time relay interlocks or drive-side current limits are normally used to enforce the declared cycle rather than relying on the operator [S3].
How to Specify a Linear Actuator Duty Rating in a Purchase Document
The unambiguous spec line, drawn from IEC 60034-1, reads 'Duty: S2 15 min' for a one-shot positioning service or 'Duty: S3 25 %, cycle time not to exceed 150 s' for a repeating stroking service, with the thermal class (commonly Class F or Class H) and the ambient temperature stated alongside [S1][S4]. If the application can drift toward a higher cycle rate, write the spec at the worst-case expected percentage rather than at the typical value, because IEC 60034-1 ratings are thermal ceilings and not nominal operating points [S3][S5].
For a deeper look at how actuator duty interacts with stroke count and life expectancy, the selection guide at Linear Actuator 25% vs 100% Duty Cycle: Spec-Driven Selection Guide covers the same S3 percentage problem from the cycle-rate side. The S2 vs S3 question is essentially a thermal-load question, but the actuator's mechanical life, the drive's linear bearing arrangement, and the controller's time relay interlocks must all be specified to the same cycle, otherwise one of the three will fail before the motor does.