The Fence Gate That Was Too Far from the Robot
We installed a safety fence around a robot cell. The fence was 500 mm from the robot’s maximum reach. The interlocked gate was on one side. On the safety audit, the inspector measured the safe distance. The robot’s maximum speed was 2 m/s, and the safety stop time was 0.5 seconds. The required safe distance was: S = K × C + 8 × (mm), where K = 2,000 mm/s (approach speed), C = stop time in seconds. S = 2000 × 0.5 + 8 × (1200mm extension factor) = 1,000 + 9,600 = 10,600 mm? No, that’s the standard formula. Let me recalculate. The standard EN ISO 13855 formula: S = K × T + C. K is 2,000 mm/s (for approach by walking), T is the total stop time (safety system response + machine stop), and C is the additional distance (based on the sensor’s detection capability). For a fence gate (a solid barrier), the distance is simpler: the fence must be far enough that the operator can’t reach the hazard before the machine stops. We had 500 mm. The stop time was 0.5 seconds. The operator’s hand approach is 1,600 mm/s. Required distance = 1,600 × 0.5 = 800 mm. We were 300 mm short. We moved the fence out. The inspector signed off. The mistake was estimating the fence distance visually, not calculating it.
Machine guarding fence design isn’t putting a fence around the robot. The fence distance, gate interlocks, and material must meet the safety standard. This article covers the design.
The Safety Distance Formula
The fence (or light curtain) must be far enough from the hazard that the operator can’t reach the moving part before the machine stops. The standard formula (EN ISO 13855):
S = K × T + C
- S: Minimum safe distance (mm) from the hazard to the fence/sensor.
- K: Approach speed. 1,600 mm/s for hand approach (reaching through a barrier). 2,000 mm/s for walking approach.
- T: Total stop time. The safety system response time (relay, 20–50 ms) plus the machine’s stop time (how long it takes to coast to a halt, 0.2–2 seconds depending on inertia).
- C: Additional distance (based on how far the hand can reach past the sensor, typically 0–850 mm depending on the sensor).
For a fence with an interlocked gate (solid barrier), C is 0 (the fence blocks reaching). S = K × T. For a 0.5 second stop time: S = 1,600 × 0.5 = 800 mm. The fence must be at least 800 mm from the robot’s maximum reach.
| Stop Time T | Safe Distance (hand approach, K=1600) |
|---|---|
| 0.2 s (fast stop) | 320 mm |
| 0.5 s (typical) | 800 mm |
| 1.0 s (large inertia) | 1,600 mm |
| 2.0 s (heavy robot) | 3,200 mm |
Fence Material and Height
The fence must prevent reaching over, under, or through.
- Height: Minimum 1,800 mm (standard). For tall machines, 2,000–2,200 mm. The operator can’t step over it.
- Mesh opening: The fence mesh must be small enough that fingers can’t reach through. For a fence at the safe distance, the mesh can be larger. But if the fence is closer (less than the safe distance), the mesh must be small (less than 12 mm opening for finger protection).
- Gap under the fence: Less than 180 mm (so a foot can’t reach under). For low hazards, less than 300 mm.
Standard safety fencing uses welded wire mesh (40×40 mm or 25×25 mm openings) on aluminum or steel posts. For high-risk (robot cells), use 25 mm mesh.
Interlocked Gates
Every access gate in the fence has an interlock switch. Opening the gate triggers the safety circuit (see article 64). The machine stops when the gate opens.
- Gate switch: A safety-rated interlock (not a standard limit switch). It has direct opening (the actuator is mechanically linked to the switch contacts).
- Guard locking: For machines that take long to stop (large robots, spinning equipment), the gate has a guard lock (the gate stays locked until the machine has stopped and the safe state is reached). The operator can’t open it while the machine is moving.
- Bypass: Don’t bypass the interlock (tie it shut). That defeats the safety. If maintenance needs to run the machine with the gate open, use a hold-to-run or teach mode (with an enable switch).
The fence design rule: Calculate S = K × T. Place the fence at least S mm from the hazard. Use interlocked gates with direct-opening switches. For machines with long stop times, use guard-locked gates. The fence that was too close wasn’t unsafe by luck — it was 300 mm short of the calculated safe distance. Move it out.
Fixed Guards vs. Movable Gates
Not every opening needs a gate. Some sections are fixed (bolted panels that can’t be opened without tools).
- Fixed panels: Bolts or tamper-proof fasteners. For sections that don’t need access. Cheaper than interlocked gates.
- Interlocked gates: For sections that need regular access (loading, unloading, maintenance). Each gate has a safety switch.
Use fixed panels where possible (they’re cheaper and can’t be bypassed). Put interlocked gates only where access is needed.
Robot Reach: Measure the Maximum
The safe distance is measured from the hazard to the fence. The hazard is the robot’s maximum reach (the farthest point the tool can reach). Not the robot’s base, but the farthest point of the arm at full extension.
Walk the cell through the robot’s full program (all positions). Find the maximum reach. The fence must be S mm beyond that. If the robot has a gripper that extends further, measure from that point.
Light Curtains vs. Fences
For loading openings (where the operator puts parts in but doesn’t enter the cell), a light curtain is more practical than a fence gate. The operator reaches through the light beam to load the part. Breaking the beam stops the robot.
The light curtain’s safe distance uses the same formula (S = K × T). But light curtains have a resolution (the beam diameter). A finger-resolution curtain (14 mm beams) can be closer than a body-resolution curtain (300 mm beams).
| Guard Type | Best For | Safe Distance |
|---|---|---|
| Safety fence (mesh) | Full cell enclosure | S = K × T (800 mm typical) |
| Interlocked gate | Access doors | Same as fence (gate is part of it) |
| Light curtain (finger) | Loading openings | S = K × T + C (closer) |
| Safety mat | Walking-up hazard zones | Under the operator’s feet |
| Fixed guard (fixed cover) | Covering a hazard | Must be removed with tools |
Color and Visibility
Safety fencing is typically yellow (machinery safety yellow) or orange. It’s visible. The operator sees the hazard zone. Don’t use transparent panels (acrylic) for robot cells — the robot can break through them. Use welded mesh (which contains parts if they fly).
Add warning signs: “Robot Cell — Authorized Personnel Only” and “Do Not Enter While Operating.”
A Fence Design Checklist
- What is the machine’s stop time? (Seconds.)
- What is the hazard’s maximum reach? (mm from the hazard point.)
- S = K × T (K=1600 for hand approach).
- Is the fence at least S mm from the hazard?
- Fence height? (1800 mm minimum.)
- Mesh opening? (Small enough — 25 mm for close guards.)
- Gap under fence? (<180 mm.)
- Which gates need interlocks? (All access gates.)
- Are gate switches safety-rated (direct opening)?
- Is guard locking needed? (For long stop times.)
- Is there a light curtain for loading? (Instead of a gate.)
- Are signs posted? (Warning, authorized personnel.)
The Bottom Line
Machine guarding fence design starts with the safe distance formula S = K × T. The fence that was too close was estimated visually, not calculated. Measure the robot’s maximum reach, calculate the stop time, and place the fence S mm beyond. Use interlocked gates with safety switches, fixed panels where access isn’t needed, and light curtains for loading openings. The cell that passes the safety audit wasn’t fenced the highest — it was fenced the right distance.
