Standards & Calculators: ISO 13855, IEC 61496 Type 2/4, Test Rods
Practical, standards-aligned guidance for specifying and validating safety light-curtain applications: how to compute ISO 13855 safety distance, when to pick IEC 61496 Type 2 vs Type 4, and how to conduct acceptance with 14/25/30 mm test rods. Written for controls/safety engineers and auditors.
Use this overview to choose a task, then consult the machine safety standards index for the relevant device, control-system and application requirements. It is not a substitute for the applicable standard or a machine-specific validation record.
1) ISO 13855 — Safety distance calculation
Formula For approach-sensing devices such as light curtains (AOPD), the expression below introduces the distance calculation. The numerical example uses the legacy ISO 13855:2010 vertical upper-limb reach-through method; ISO 13855:2024 is the current edition. Reach-over, reach-under, whole-body access, field orientation and machine-specific requirements need separate assessment:
S = K × T + CParameters
| Symbol | Meaning | Typical values / notes |
|---|---|---|
| K | Approach speed | For hand/arm approach on AOPD, 2,000 mm/s is commonly applied per ISO 13855. |
| T | Total stopping time | T = tsensor + tinterface + tmachine. Measure tmachine with a stop-time meter at worst case speed/load. |
| C | Additional distance | Accounts for reach-through in this limited example, not reach-over. For light curtains with resolution d = 14–40 mm, a commonly used relation is C = 8 × (d − 14) mm. (Example: d=30 mm → C=128 mm.) |
Worked example
Given: Type 4 light curtain, resolution d=30 mm; sensor response 15 ms; interface relay 10 ms; measured machine stop 120 ms.
T = 0.015 + 0.010 + 0.120 = 0.145 s
C = 8 × (30 − 14) = 128 mm
S = 2,000 mm/s × 0.145 s + 128 mm = 290 + 128 = 418 mmS = 418 mm
This illustrative 418 mm result is not an approved mounting distance. The legacy method starts at K = 2,000 mm/s with a 100 mm minimum; only if the result exceeds 500 mm may its specified 1,600 mm/s branch be considered, retaining a 500 mm minimum. Evaluate all other access routes and the current edition. Re-measure stop time after any maintenance that can affect braking.
What to record (for audits)
- Device model/serial, safety Type, resolution d, protective height.
- Response times tsensor, tinterface, measured tmachine (raw logs).
- Calculated S with method and date; actual guard distance installed.
- Operator visibility and reset position (no automatic restart).
Calculator: open the scope-limited arithmetic tool → ISO 13855 Safety Distance Calculator
2) IEC 61496 — Type 2 vs Type 4 (how to select)
Scope IEC 61496 defines requirements for electro-sensitive protective equipment (ESPE) such as AOPD light curtains. The “Type” describes the capability to detect faults and resist common-cause failures.
| Aspect | Type 2 | Type 4 |
|---|---|---|
| Fault tolerance | Periodic testing within the specified test architecture | High; continuous self-checking |
| Typical risk level served* | Up to PL c (context-dependent) | Can support up to PL e / SIL 3 when correctly integrated |
| Use cases | Applications whose required performance is within the exact device capability | Applications requiring the certified Type 4 device capability, subject to the full system assessment |
| Environmental robustness | Verify specified optical, EMC and environmental limits | Verify specified optical, EMC and environmental limits; Type is not an IP or temperature rating |
| Price/availability | Lower | Higher |
*Performance Level (PL) per ISO 13849-1 and SIL per IEC 62061 depend on the entire safety function: sensors + logic + actuators + diagnostics. A Type 4 light curtain makes PLe/SIL3 feasible but does not guarantee it alone.
Deep dive with diagrams and migration notes → IEC 61496: Type 2 vs Type 4
3) Acceptance using 14/25/30 mm test rods
Test rods simulate fingers/hands during commissioning and periodic verification. Use rods matching the installed resolution and application limits.
| Rod Ø | Typical protection | Notes |
|---|---|---|
| 14 mm | Finger protection | High resolution; usually short ranges and higher costs. |
| 25 mm | Detection capability must match the intended body-part access assessment | Check standard applicability and blanking rules. |
| 30/40 mm | Hand protection | Most industrial retrofits; pair with correct C value in ISO 13855. |
Step-by-step acceptance
- Verify model, Type and detection capability against the risk assessment; test restart interlocking and final-element feedback wherever required by the validated control design and supported by the selected equipment.
- Measure worst-case stopping performance under the approved controlled test plan, using a calibrated stop-time meter and the relevant operating conditions without exposing a person to hazardous motion.
- Apply the relevant positioning method and all access-path checks. Mark the validated guarding position and verify the installed distance; the legacy K×T+C subtotal alone is insufficient.
- With hazardous motion prevented, use the specified test rod across the complete protective field and boundaries. Confirm the protective outputs remain in the safe state throughout the manufacturer-specified path near the emitter, receiver and relevant intermediate positions. Do not substitute a fixed 50–100 mm sampling interval for continuous coverage.
- Record evidence: photos, meter logs, operator visibility check, reset position, maintenance schedule.
Procedural details and printable templates → Test rods 14/25/30 mm and application limits
4) Common pitfalls & checks
- Using catalog stop times: always measure tmachine onsite; brakes age.
- Ignoring interface delay: include relay/SSC reaction times and all relevant downstream stopping delays without double-counting. Feedback monitoring is a separate diagnostic check, not automatically an extra serial stopping delay.
- Reach-over/under: ensure protective height and mounting prevent circumvention.
- Wrong resolution vs hazard: fingers need finer resolution than hands; adjust C.
- EMC/optical interference: separate from VFD cables, add shrouds for welding arcs.
- Missing records: store S-calcs, stop-time logs, test-rod checklists, photos, and dates.
5) FAQ
Do I always use K = 2,000 mm/s?
No. This example starts at 2,000 mm/s for the limited ISO 13855:2010 vertical upper-limb reach-through method. Its lower-speed branch is conditional, not a free preset. Other geometries and ISO 13855:2024 require their applicable method; a calculator result alone does not approve an installation.
How often must I re-measure machine stop time?
At commissioning, after any change affecting stopping performance, and at defined maintenance intervals. Keep the worst-case value on file.
Is Type 2 ever acceptable?
Type 2 may be suitable where the required safety performance, applicable machine standard and exact device evidence allow a capability up to PL c / SIL 1. Type 4 can support up to PL e / SIL 3 but does not establish the performance of the entire safety function. Machine names or access frequency alone are not a selection rule.
6) Tools & further reading
Edition references: ISO 13855:2024 scope and IEC 61496-1:2020. Consult the licensed documents and the device instructions for project requirements.