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.
1) ISO 13855 — Safety distance calculation
Formula For approach-sensing devices such as light curtains (AOPD), the safeguarding distance is calculated as:
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 resolution/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
Round up and add project tolerances. 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 online tool with presets → 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 | Basic; periodic tests required | 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 | Lower risk, limited hazard exposure | High-risk machines (presses, press brakes, robot cells) |
| Environmental robustness | Moderate | Higher immunity; stricter EMC/light immunity |
| 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 | Between finger/hand | 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/resolution vs the risk assessment; confirm manual reset and EDM are active.
- Measure stop time at worst case (warm machine, max speed, max load) using a calibrated stop-time meter.
- Compute S (K×T + C), mark the physical guarding line, and check real installation ≥ S.
- Insert the correct test rod along the entire protective height; verify OSSD drop and machine stop every 50–100 mm.
- 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 the EDM loop.
- 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?
For hand/arm approach with light curtains, 2,000 mm/s is commonly applied. Other device types (e.g., scanners) or approaches (whole-body walking speed) can use different K. Follow ISO 13855 tables for the specific device and scenario.
How often must I re-measure machine stop time?
At commissioning, after any change affecting stopping performance (brakes, drives, tooling), and at defined maintenance intervals. Keep the worst-case value on file.
Is Type 2 ever acceptable?
Yes—on lower-risk applications where the risk assessment supports PLc (or similar) and where environmental/diagnostic requirements are met. For high-risk tasks or frequent access, choose Type 4.