The Guard That Was Too Close
We installed a safety fence (article 74) around a robot cell. The fence was 300 mm from the robot’s reach. The fence’s interlock switch (article 74) stopped the robot when the gate opened. On the floor, an operator reached through the fence’s mesh (30 mm holes). The operator’s hand reached the robot (the robot was too close to the fence). The fence was at 300 mm (inside the safety distance). The problem: the fence was too close to the hazard. The operator could reach the robot through the mesh. We moved the fence to 500 mm (the safety distance, per the standard). The operator couldn’t reach the robot. The mistake was not calculating the safety distance (the fence must be far enough that the operator can’t reach the hazard).
Guard safety distance is how far the fence (or light curtain, article 160) must be from the hazard. This article covers the calculation.
The Safety Distance Formula
The safety distance (S, mm) is:
S = K × T + C
Where K is the hand speed (2000 mm/s, per ISO 13855), T is the total stop time (s, the time from the stop signal to the hazard stopping), and C is the additional distance (based on the resolution, e.g., 8 × (resolution – 14) mm for a 14 mm curtain).
For a robot (T = 0.5 s, the robot stops in 0.5 s): S = 2000 × 0.5 + C = 1000 + C. For a 30 mm mesh fence (C = 0, the mesh blocks the reach): S = 1000 mm. The fence must be at least 1000 mm from the robot. The fence at 300 mm was too close.
The safety distance rule: Calculate S = K × T + C. The fence at 300 mm was too close (the operator could reach). For a robot that stops in 0.5 s, S = 1000 mm. Move the fence to 1000 mm (or more). The fence that was too close didn’t account for the stop time (the robot doesn’t stop instantly).
Step 1: The Stop Time (T)
The stop time (T) is the time from the stop signal (the gate opening) to the hazard actually stopping. For a robot, the stop time is 0.3–0.5 s (the robot decelerates). For a press (heavy flywheel), it’s longer (1–2 s, the flywheel coasts). The longer the stop time, the farther the fence must be (S increases with T).
Step 2: The Mesh Size (C)
The fence’s mesh (hole size) affects C. A small mesh (30 mm holes) blocks the reach (C = 0). A large mesh (100 mm holes) lets the arm reach through (C is negative — the fence must be farther). For a fence with small mesh (30 mm), C = 0 (the mesh blocks). For a guard with a larger opening, add C (the additional distance for the reach through the opening).
Step 3: The Gate’s Interlock (Article 74)
The fence’s gate (interlock switch, article 74) stops the robot when the gate opens. But the safety distance is still needed (even with the interlock). The interlock stops the robot, but the robot doesn’t stop instantly (T). The fence must be far enough that the operator can’t reach the robot before it stops.
Step 4: Floor Marking
Mark the floor (the danger zone) with a yellow line. The operator knows where the hazard is. The fence is inside the marked zone. Don’t let the operator step inside (the fence blocks).
| Hazard | Stop Time (T) | Safety Distance (S) |
|---|---|---|
| Robot (fast stop) | 0.3 s | 600 mm |
| Robot (normal stop) | 0.5 s | 1000 mm |
| Press (flywheel coast) | 1.5 s | 3000 mm |
| Conveyor (fast stop) | 0.2 s | 400 mm |
A Safety Distance Checklist
- What is the hazard? (Robot? Press?)
- What is the stop time (T)? (Robot spec?)
- Calculate S = K × T + C? (K = 2000 mm/s?)
- Is the fence at S? (Or more?)
- Is the mesh small? (Blocks the reach?)
- Is the gate interlocked? (Article 74?)
- Is the floor marked? (Danger zone?)
- Can the operator reach the hazard? (Test?)
- Is the safety relay monitoring? (Article 127?)
- Is the reset monitored? (Article 150?)
The Bottom Line
Guard safety distance calculates S = K × T + C. The fence at 300 mm was too close (the operator could reach). For a robot (T = 0.5 s), S = 1000 mm. Move the fence to S. The fence that protected the operator wasn’t the closest one — it was at the safety distance.
