Choosing a limit switch by actuator head (roller lever, plunger, adjustable rod), body format and contact action — snap versus slow-break, and why positive-opening NC contacts are the safety requirement — across the stocked Telemecanique XCKJ/XCMD and Siemens SIRIUS 3SE5 modular ranges, plus when to go non-contact instead.
Industrial limit switches are two-part machines. The actuator head — roller lever, dome or roller plunger, adjustable rod, spring whisker — converts machine motion into a defined stroke, and on the quality ranges it rotates in 90° steps and replaces separately. The body carries the contact block and cable entry. Both stocked systems are modular this way: Telemecanique's classic metal XCKJ110513 roller-lever unit and compact XCMD series (plunger versions like the XCMD21F0L5 included), and Siemens' SIRIUS 3SE5 where body and actuator head are literally separate order numbers — which is also why a worn lever is a cheap fix, not a new switch.
| Action | Behaviour | Choose it for |
|---|---|---|
| Snap action | Contacts flip at a fixed point regardless of actuator speed | Slow or creeping cams — the snap guarantees a clean switch point |
| Slow-break | Contacts travel with the actuator; break point follows the stroke | Faster motions; gives overlapping NC/NO sequencing options |
| Positive opening (⊖) | Actuator mechanically forces the NC contact apart | Mandatory for safety functions — welded contacts get torn open |
Safety interlocks on guards and hatches use positive-opening NC channels as a rule — the stocked 3SE5 metal safety switch with 1 NO + 2 NC is the dual-channel shape, same logic as the e-stop contacts in the pushbutton guide. Environmental details finish the selection: metal bodies and IP66/67 for washdown and deck duty, cable entry size (M20-entry XCKJ variants are stocked), and approach speed/angle inside the head's rated window.
High cycle rates, wet abrasive environments, or targets that would batter a roller are the signal to go non-contact — an inductive sensor has no moving parts to wear (the proximity-sensor guide covers the swap), at the cost of needing supply voltage and metal targets. Close to 900 limit-switch line items are stocked across Telemecanique, Siemens and others, legacy references included — quote the number off the old body or head and the current equivalent, or the head alone, is confirmed within 24 hours.