Specifying a safety relay for an aerial work platform, suspended scaffold, scissor lift, or roof-edge guard is not the same exercise as choosing one for a press brake; the operator is exposed to a free-fall hazard, so the relay must guarantee a stop in the fault window and stay latched off until a manual reset [S1].
The minimum performance floor is PLd / Cat 3 per ISO 13849-1, with a forced response time at or below 20 ms, dual-channel inputs, and force-guided (positively driven) output contacts wired in series with a safety contactor that itself meets Cat 3 or higher [S1][S3].
Start with the hazard map, not the catalog page
For a worker on a scissor lift or a boom-type aerial work platform, the credible hazards are platform over-speed, basket tilt past the rated angle, and a broken limit switch on the extend cylinder; each one is wired as a normally-closed safety contact that opens on fault and is read by a dedicated safety input channel, never multiplexed into a standard PLC digital input because a PLC output can fail short-circuit and defeat the function [S1].
On a suspended platform or rope-access cradle, the dominant hazards are rope slip and overload, so the relay must sit in line with the motor contactor AND with the secondary overspeed brake coil, which usually means a relay with at least three safety output contacts (3 NO) plus one auxiliary (1 NC) for the EDM feedback loop [S1].
Industrial fire safety interlocks on a roof edge, where the relay must drop power to a hatch motor or a sliding gate if smoke is detected, push the spec toward a relay certified to EN 61508-1 in addition to ISO 13849-1, because the safety integrity claim is shared with the fire panel upstream [S3].
The four hardware decisions that drive the part number
Decision 1, input function: choose between a two-hand control device (THC), an emergency-stop circuit, a guard-door / interlock loop, or a time-delay / time-monitoring function such as a hold-to-run timer, because each one maps to a different relay family and a different timing window [S1][S2].
Decision 2, simultaneity window: THC relays per ISO 13851:2012 enforce a 0.5 second maximum between the two pushbutton actuations (Type IIIC), and a one-hand bypass with a wedge, tape, or elbow is prevented only if the buttons sit on separate input channels AND the station mounting height stays in the 900 to 1200 mm range from the floor so hip operation is geometrically blocked [S1].
Decision 3, output count and EDM: the relay must expose a feedback (EDM) input so a downstream contactor welded in the closed state is detected on the next cycle; the motrona SRC30/EL ships with integrated EDM monitoring and dual-channel cross-circuit detection, which removes the need for an external monitoring relay in compact elevator and conveyor panels [S3].
Decision 4, architecture and integrity: a scissor lift on a construction site typically requires PLd / Cat 3 minimum, while a suspended platform or passenger aerial work truck normally calls for PLe / SIL 3 because the fall consequence is fatal and the operator cannot recover the system once it latches off; the motrona SRC30/EL is certified up to SIL 3 / PLe per EN ISO 13849-1, EN IEC 62061 and EN 61508-1, which covers both bands with a single part [S3].
Size the output contacts for the inrush, not the steady state
Safety relay output contacts are rated in amperes, but the inrush on a motor contactor coil or a brake solenoid is several times the holding current for the first 5 to 15 ms; force-guided contacts on a Pilz PNOZ X timer module, for example, are dimensioned for the inductive load profile, and the published family covers DIN-rail widths from 17.5 to 45.0 mm with housing depths around 121.0 mm so the cabinet thermal budget stays predictable [S2].
For DC loads above 24 V, derate the contact rating by 30 to 50 percent, or use a relay with a dedicated solid-state output stage in parallel with the force-guided electro-mechanical contacts; for purely resistive indicator lamps, the standard contact rating applies directly [S1].
Two-hand control vs e-stop vs guard-door: pick the right function family
Two-hand control relays enforce a 0.5 second simultaneity window and a 20 ms response, with typical ratings of PLd / Cat 3, 3 NO + 1 NC force-guided outputs, and supply variants at 24 VDC, 110 VAC, and 230 VAC; the function is required for press operators but rarely for work-at-height platforms, where the operator's hands are not free at the moment of hazard [S1].
E-stop and guard-door relays accept NC safety contacts on dual channels, support manual or automatic reset, and expose an EDM input for contactor monitoring; the motrona SRC30/EL covers both modes in a single device with a low housing height, which suits the cramped basket of a machine safety retrofit on a scissor lift [S3].
Time-monitoring relays such as the Pilz PNOZ X family add an adjustable on-delay, off-delay, or star-delta timing element on top of the safety logic, which is the right choice when the hazard is a controlled run-down (for example, allowing a centrifuge to coast to a safe speed before the door unlocks) rather than an immediate stop [S2].
For work at height, the right answer in one table
The comparison below maps typical work-at-height applications to the four decision criteria, drawn from the data in [S1] and [S3].
Scissor lift (PLd / Cat 3): guard-door + e-stop inputs, dual-channel NC, ≤20 ms response, EDM on the contactor feedback.
Suspended platform / rope-access cradle (PLe / SIL 3): guard-door + overspeed input, dual-channel NC, ≤20 ms response, EDM AND direct brake-coil cutout on the second safety output.
Boom-type aerial work platform (PLd / Cat 3): e-stop on the basket and base, tilt switch, dual-channel, ≤20 ms response, no THC.
Roof-edge hatch motor (PLd / Cat 3, EN 61508-1 if fire interlocked): guard-door + smoke contact, dual-channel, EDM on the contactor.
Failure modes that pass the spec sheet but fail the site
Wiring two pushbuttons in series into a single PLC digital input defeats the simultaneity check, because the relay sees one circuit closing, not two independent channels; the failure is invisible until an operator wedges one button and the machine runs single-handed [S1].
Using software-only timing in a standard PLC to replace a THC relay is not permitted, because the PLC program is not safety-certified and the digital output can fail short-circuit, which holds the safety contact closed even when the input logic says stop [S1].
Forgetting the EDM loop lets a welded contactor pass the next cycle as if healthy; the motrona SRC30/EL monitors the contactor feedback on every cycle, which is the correct pattern for any height-related safety circuit, not just elevators [S3].
Selection rules a process engineer can apply in 10 minutes
Rule 1, pick the integrity level from the consequence: fall onto a hard surface = PLe / SIL 3; ride-over-speed with no fall = PLd / Cat 3 minimum; property-only damage = PLc / Cat 2 [S1][S3].
Rule 2, pick the input function family: THC if the operator must be on the controls during the hazard, e-stop if the hazard is sudden and the operator is away, guard-door if the hazard is access-controlled, time-monitoring if the stop is sequenced [S1][S2].
Rule 3, pick the output count: at minimum 2 NO safety contacts (one for the contactor, one for the indicator or brake), 3 NO for a dual-coil brake or a motor + brake pair, plus 1 NC for the EDM loop [S1].
Rule 4, pick the form factor and certifications: DIN-rail mount, 17.5 to 45.0 mm width, spring-loaded terminals, EN ISO 13849-1 plus EN IEC 62061 plus EN 61508-1 for shared claims with the fire / elevator subsystem, and a housing depth around 121.0 mm to fit a standard 150 mm cabinet duct [S2][S3].
For welding-class hazards that share a work-at-height cell, the relay architecture overlaps with the welding-class safety relay spec, but the output derating and the EDM wiring remain distinct.
Trackable signals for the next design cycle: any IEC 62061 / EN 61508-1 update that re-classifies suspended platforms above PLe, the revision cadence of EN 81-20/50 for elevator safety relays, and the adoption of integrated EDM in compact relays below 22.5 mm DIN-rail width.