The single number that decides whether a safety light curtain installation is compliant or a written-up incident waiting to happen is the safety distance S calculated under ISO 13855 — get that wrong and nothing else on the bill of materials matters [S3].
This guide walks through resolution selection, the K × T + C formula with worked examples, mounting geometry, electrical integration via OSSD outputs, and the on-site acceptance tests that prove the installation actually stops the machine in time [S1][S2][S3][S4][S5].
Pick the Resolution Before You Mount Anything
Detection capability (d) drives both the formula and the C term, so it must be locked before purchase: Pepperl+Fuchs lists a 14 mm SLC14 finger-protection variant with a 5 m sensing range and 750 mm protective field height, a 30 mm SLC30 hand-protection variant with a 15 m range and 1650 mm height, and a 60 mm SLC60 body-detection variant with a 15 m range and 1800 mm height, all self-monitoring to Type 4 of IEC/EN 61496-1 [S1][S2][S4]. For light-duty Type 2 cells, the Pepperl+Fuchs SLCT30 series delivers 30 mm resolution up to 2400 mm height at SIL 1 per IEC 61508 across a –30 to +60 °C window [S5]. Type 4 devices with OSSD semiconductor outputs and IP67 housings remain the default on hydraulic presses, robotic cells, and palletisers where a single internal fault must not silence the curtain [S1][S2][S4].
ISO 13855 Safety Distance: The Formula and Two Worked Examples
ISO 13855 reduces the geometry to S = K × T + C, where K is the approach-speed constant, T is the total stop time (machine + curtain response), and C is the penetration-distance offset [S3]. For finger and hand detection with d ≤ 40 mm, K = 2000 mm/s and C = 8 × (d – 14) mm; for body detection with 40 < d ≤ 70 mm, K drops to 1600 mm/s and C is fixed at 850 mm to approximate an arm's reach [S3]. A GL-R60H with d = 25 mm, response time 0.0157 s, and a 0.1 s machine stop yields S = 2000 × 0.1157 + 88 = 319.4 mm; a GL-R08L with d = 45 mm and 0.0069 s response yields S = 1600 × 0.1069 + 850 ≈ 1021 mm [S3]. Whenever S ≥ 500 mm the standard forces a recalculation with K = 1600 mm/s, and if that second pass returns ≤ 500 mm you clamp S to 500 mm — a guard band the standard imposes so a near-threshold layout cannot creep under the limit [S3].
Compare Resolution Options Against the Decision Criteria That Matter
Three variables decide which curtain family to specify: detection capability d, the resulting safety distance S, and the cell's worst-case stop time. Finger-protection at 14 mm keeps C at zero but multiplies K up to 2000 mm/s, so a 0.4 s total stop time already pushes S past 800 mm [S1][S3]. Hand-protection at 30 mm is the workhorse balance: C = 128 mm, K still 2000 mm/s, and a 0.2 s stop time lands near 528 mm — comfortably above the 500 mm clamp on most cells [S2][S3]. Body-detection at 60 mm takes K down to 1600 mm/s but C jumps to 850 mm, which is why perimeter and reach-through applications accept the larger S in exchange for taller protective fields up to 1800 mm and longer 15 m sensing ranges [S3][S4]. The trade-off is concrete: each 1 ms of curtain response time adds 2 mm to S at K = 2000 mm/s, and every extra second of machine stop time adds 2000 mm at the same constant [S3].
Mounting Geometry and Mechanical Alignment
Curtain columns must be mounted on rigid, vibration-isolated structures so beam alignment is not lost when the machine cycles; Pepperl+Fuchs SLC-series columns are rated IP67 and accept three-sided mounting, which lets the integrator frame the hazard from above, below, or both [S1][S2][S4][S5]. For reach-over applications the bottom beam must sit no higher than 300 mm above the floor per ISO 13855 reach allowances — the standard's C = 850 mm term exists precisely because operators can reach around a poorly framed light field [S3]. Master/Slave topologies let you cascade two columns into a single OSSD pair, which solves long openings without doubling the wiring back to the safety relay [S1][S2]. For hazardous-area deployments, Pepperl+Fuchs offers ATEX variants for zone 2 and zone 22 with the same Type 4 self-monitoring, so the same mounting discipline applies inside paint booths and grain handling [S1][S2][S4].
Electrical Integration: OSSD, EDM, and Restart Logic
OSSD (Output Signal Switching Device) pairs are the modern interface: two potential-separated semiconductor outputs that must pulse-test low within milliseconds of each other, so a cross-fault or welded short fails the system instead of latching it green [S1][S2][S4]. External Device Monitoring (EDM) checks the downstream contactors' normally-closed feedback, so a contactor that welds closed is detected on the next cycle and the safety relay refuses to restart [S1][S2]. Start/Restart disable (manual reset) must be enforced on any cell where a person can stand inside the protected field undetected; a keyence SL-C series column ships with this wired in and a dedicated restart interlock [S6]. Operating-reserve indication and the integrated function display let maintenance staff see margin loss from contamination or misalignment before a nuisance trip becomes a real one [S1][S2][S4].
Acceptance Tests and When to Escalate, Not Repair
Acceptance begins with a stop-time measurement using a calibrated tachometer or encoder on the dangerous moving part, recorded as t2 in the ISO 13855 formula; the response time t1 comes from the curtain's datasheet and the two are summed to T [S3]. Next, walk the standardised test rod (14, 30, or 60 mm depending on resolution) through every beam at the calculated S distance and at three points along the protective field height — top, middle, bottom — verifying the OSSDs drop within the published response time on every pass [S3][S1]. If any beam fails to detect the rod at the documented S, do not adjust the mounting to compensate: either the machine stop time has drifted (t2 grew), the resolution is wrong for the application, or the curtain is misaligned, and each is a re-engineering decision, not a repair [S3]. Annual re-verification is the norm; safety light curtains do not drift benignly, so a passing test today is meaningless without a recorded trend [S3]. For a wider look at how Type 2, Type 4, IP rating, and resolution line up, see Safety Light Curtain Types and Classifications: Type vs Resolution vs IP.
Spec-level background on the components involved: linear guide, and crossed roller guide.