1) Two Different Axes You Must Not Mix Up
Device Type (Type 2 / Type 4) describes the intrinsic fault-handling capability of the light curtain itself (per IEC 61496-1/-2 / GB/T 19436).
Required Performance for the whole safety function—your control system—is expressed as PLr (a–e) with design Categories Cat.1/2/3/4 under ISO 13849-1/-2, or as SIL 1–3 under IEC 62061 (use either ISO 13849 or IEC 62061, not both in parallel).
Placement distance is then set by ISO 13855 using the safety-distance equation S = K × T + C.
Rule of thumb: if your target is PLr d/e or SIL 2/3, a Type 4 curtain is not optional—it’s mandatory.
2) From Risk to Device Type: a Practical Decision Guide
Target Risk (from ISO 13849 assessment) Recommended Curtain Type Typical Control Architecture Notes
PLr a–b (low) Type 2 optional Cat.1/2 (single channel + self-check or periodic test) Light curtains are rare here; guards/limits often suffice.
PLr c (moderate) Prefer Type 2; upgrade to Type 4 if environment/impact is demanding Cat.2/3 (periodic test or dual-channel) Ensure short T, adequate S, and controlled false trips.
PLr d–e (high/very high) Type 4 required Cat.3/4 (dual channels, diagnostics, EDM) Presses, benders, robotic areas, cutting/clamping.
SIL 2–3 (IEC 62061) Type 4 required Meet PFHd objective Mirrors PLr d–e expectations.
How PLr is set (ISO 13849-1):
S (Severity): irreversible injury → d/e.
F (Frequency/Exposure): frequent or long exposure → higher PLr.
P (Possibility of avoidance): hard to avoid → higher PLr.
Fast track heuristics:
Stamping, bending, cutting, pressing, robot load/unload → Type 4 targeting PLr d/e.
Secondary guarding on conveyors or reduced-risk zones → may justify PLr c / Type 2, only after proving stop performance and trip robustness.
3) Why Type 4 Beats Type 2 at High Risk
Diagnostics & Redundancy: Type 4 has higher diagnostic coverage and redundancy; single-fault safe reaction is expected. Type 2 relies more on periodic testing and limited diagnostics.
Fault Handling: Type 4 tolerates interference, ageing, and misalignment better and fails safe.
Compliance Boundary: For PLr d/e or SIL 2/3, Type 2 cannot qualify.
4) Don’t Choose on “Type” Alone—Resolution & Height Matter
Resolution (beam spacing):
≤14 mm: finger protection (sealing knives, precision clamps)
30 mm: hand/forearm (robot infeed, conveyor nip)
40 mm: hand/arm (general industrial)
70–90 mm: whole-body (often better with safety laser scanners/fences)
Protective height: match the actual opening (typ. 450–1200 mm).
Environment: IP rating (washdown → IP67), steam/reflectivity immunity; angle the curtain and add dark backplates near shiny films if needed.
5) Safety Distance (ISO 13855)—Non-Negotiable
Formula: S = K × T + C
K (approach speed for upper limbs): 1600 mm/s (typical)
T (total stop time): safety relay/controller + drive/brake/valve
C (offset): 8 mm for fingertips (+ any encroachment allowance)
Example (heat-sealing knife): measured T = 0.12 s → S = 1600 × 0.12 + 8 ≈ 200 mm.
Install at ≥220 mm to cover tolerance and alignment drift. Use a stop-time meter, record the worst case, label the frame, and archive—this is part of ISO 13849-2 validation.
6) Control Architecture for High-Risk Functions (the “standard kit”)
OSSD1/OSSD2 dual channels → PL e safety relay or safety PLC.
EDM monitors contactor welding; no reset if welded.
Manual reset (no automatic restart): after beam restoration, a person must confirm.
Muting/Blanking where product must pass: use directional + time-window logic—let product through, not people.
HMI = status/diagnostics only; do not implement safety in a standard PLC.
7) Quick Prescriptions by Scenario
Press/brake/molding closure/cutting: Type 4 + PLr d/e, 14 or 30 mm, manual reset + EDM.
VFFS/heat-seal/packaging knives: reflective + washdown → Type 4, 14 mm, compute S per ISO 13855; apply fixed/floating blanking if fixtures intrude.
Robot load/unload, guarded entries: Type 4, 30–40 mm; for area/whole-body, consider safety laser scanners or interlocked doors.
General conveyors/low-risk portals: possibly PLr c / Type 2 after proof of stop time and false-trip control.
8) Selection & Validation Checklist (follow this to the letter)
Risk assessment → set PLr (or SIL).
Pick device type: if PLr ≤ c and conditions are controlled → Type 2 may fit; PLr ≥ d → Type 4 only.
Choose resolution/height for the body part at risk.
Measure T, compute S, label and archive per ISO 13855/13849-2.
Engineer OSSD/EDM/manual reset/muting; draw the circuit.
Check environmental robustness (IP, reflection, steam, vibration).
Validate: worst-case stop time, 14/30 mm test-rod intrusion, EDM welded-contactor test, restart test.
Document: risk report, calculations, schematics, validation records, maintenance plan.
9) Executive Takeaway
If potential harm includes severe injury, amputation, or operators are frequently and unavoidably exposed, go straight to Type 4 and design the function to PLr d/e (or SIL 2/3).
Only when a thorough assessment clearly places the task at PLr c and the application conditions are tightly controlled should Type 2 be considered.
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