Safety and Standards
Safety Distances: How Far Must the Fence Be?
Two different calculations decide fence position: reaching over or through a barrier, and approach speed to a detection device. Both are in standards, and both are frequently guessed.
Two calculations, not one. For a physical barrier, the distance comes from reach tables in ISO 13857: how far a person can reach over, through or around it. For a detection device such as a light curtain or scanner, the distance comes from ISO 13855: approach speed multiplied by total stopping time, plus allowances. Using the wrong one is the most common finding in a cell layout review.
Physical barriers: the reach question
For a fence, the question is geometric: can a person reach the hazard. ISO 13857 provides tables based on body dimensions covering reaching upwards, reaching over a barrier of a given height, and reaching through openings of a given size.
| Route | Governed by | Practical consequence |
|---|---|---|
| Over the top of the fence | Fence height against hazard height and distance | A higher fence permits a shorter distance |
| Through a mesh opening | Opening size against required distance | Finer mesh permits the fence to be closer |
| Underneath the fence | Ground clearance | Gaps above about 180 mm allow leg access |
| Around the end | Layout completeness | The most frequently overlooked route |
| Upwards to a hazard above | Standing reach height | Overhead hazards need their own treatment |
Mesh size is the lever most often available. Reducing the aperture allows the barrier to sit closer to the hazard, which recovers floor space in a cramped cell without changing the robot or its program.
Detection devices: the speed question
For a light curtain, scanner or mat, the person is not physically prevented from approaching, so the distance must be enough for the machine to stop first. The minimum distance is the approach speed constant multiplied by the total system stopping time, plus an intrusion allowance and a device tolerance.
| Term | Value | Note |
|---|---|---|
| Light curtain response | 20 ms | From the device datasheet |
| Safety controller and network | 25 ms | Configuration dependent |
| Robot reaction and stop | 185 ms | Measured at speed and payload |
| Total stopping time | 230 ms | Sum |
| Approach term at 2,000 mm/s | 460 mm | Hand speed constant for close approach |
| Intrusion allowance for 30 mm resolution | 128 mm | From the resolution formula |
| Minimum distance | 588 mm | Before any additional tolerance |
Two constants appear in these calculations. A value of 2,000 mm/s is used for hand and arm movement toward a hazard at close range, and 1,600 mm/s for whole-body approach where the distance is larger. Which applies depends on the geometry and is specified in the standard rather than chosen.
The mistakes reviewers find
- Applying the detection formula to a fence. A physical barrier is a reach problem, not a speed problem, and the two produce different answers.
- Stopping time estimated. It has to be measured at the actual programmed speed with the actual payload, and re-measured when either changes.
- Network latency omitted. Safety communication adds cycle time and a watchdog margin, and both belong in the total.
- The approach around the end. A protective device that covers the front while the side is open is not a protective device.
- Access underneath or over. Gaps below a fence and hazards reachable over the top are separate routes with their own requirements.
- Standing behind the device. A person who steps past a light curtain into the cell is no longer detected, which is why muting, restart interlocks and presence detection inside the cell exist.
Frequently asked questions
How is the safety distance to a robot calculated?
Two different ways. For a physical barrier, from reach tables in ISO 13857. For a detection device, from ISO 13855 as approach speed times total stopping time plus intrusion and tolerance allowances.
Which approach speed constant applies?
2,000 mm/s for hand and arm movement toward a hazard at close range, and 1,600 mm/s for whole-body approach over a larger distance. Which one applies is determined by the geometry and specified in the standard.
Can a finer mesh let the fence sit closer?
Yes. Reach distances depend on opening size, so a smaller aperture permits a shorter distance to the hazard. It is often the cheapest way to recover floor space in a cramped cell.
What is most often forgotten?
Access around the end of a protective device, gaps underneath a fence, and hazards reachable over the top. Each is an independent route to the hazard and needs its own treatment.
Does stopping time change over the life of a cell?
Yes. Brake and gearbox condition, payload changes and programmed speed changes all affect it, so it should be re-measured periodically and after any modification, since it feeds the distance calculation directly.
Sources
- ISO 13857, safety distances to prevent hazard zones being reached by upper and lower limbsInternational Organization for Standardization, reach tables for fixed guarding
- ISO 13855, positioning of safeguards with respect to approach speeds of parts of the human bodyInternational Organization for Standardization, approach speed constants and the distance formula
- ISO 10218-1:2025, Robotics, Safety requirements, Part 1International Organization for Standardization, guarding requirements for robot cells