Glossary · Guards and machine motion
Minimum distance
In machine safety, the minimum distance is the calculated separation needed between a protective device or guard opening and a hazard, so that a person cannot reach the hazard before hazardous motion has ceased. ISO 13855 calculates it from approach speed, overall stopping time and an intrusion allowance.
- Machine safety
- Risk assessment
In one sentence
The minimum distance of a light curtain or interlocked guard follows from approach speed and overall stopping time; it must be recalculated after changes.
Example
A light curtain on a press brake is mounted at a minimum distance calculated from the measured stopping time of the ram, the response time of the curtain and its detection capability.
How it applies
- Machine safety: The general formula is S = (K × T) + C: S the minimum distance, K the approach speed of the body or body part, T the overall system stopping performance (response time of the device plus stopping time of the machine) and C an additional distance that depends on how far a body part can intrude before detection.
- Evidence: T must be measured on the real machine, not taken from a brochure. Keep the stopping-time measurement, the device data and the calculation in the verification records.
- Change control: A slower brake, a heavier tool or a changed control program increases T. Treat the minimum distance as a parameter to recheck under change control.
- Technical documentation: State the calculated distance and the stopping time it assumes, and tell users to have stopping time checked at the specified interval.
Minimum distance vs. safe distance
Minimum distance is calculated for devices that react to an approaching person. Safe distance under ISO 13857 relates to reaching over, around or through a physical barrier.