Glossary · Guards and machine motion
Safe distance
Also known as: safety distance
In machine safety, a safe distance is a distance selected to prevent a person from reaching a hazard before hazardous motion has ceased. For physical barriers ISO 13857 gives safety distances based on reach; for protective devices ISO 13855 gives the calculated minimum distance.
- Machine safety
- Risk assessment
In one sentence
A safe distance keeps the hazard out of reach until motion has stopped; ISO 13857 covers reach past barriers and ISO 13855 approach toward protective devices.
Example
The safe distance between the fence of a robot cell and the robot's working envelope is taken from the ISO 13857 table for reaching over protective structures.
How it applies
- Machine safety: For guards, the safe distance depends on body reach and does not change with machine speed. For protective devices, it depends on how quickly the machine stops; see minimum distance.
- Terminology: ISO 13857 speaks of “safety distance”. Documents often mix “safe distance”, “safety distance” and “minimum distance”. Define which one is meant and cite the standard behind the value.
- Evidence: Keep the measured distances and the tables or calculation used in the verification records, and recheck after layout changes.
- Technical documentation: When users may reposition fences, stands or devices, the instructions must state the required distance and that it must not be reduced.
Safe distance vs. stopping distance
Safe distance is the separation between a person and the hazard. Stopping distance is how far the machine part travels after a stop command, which feeds into the safe distance.