The Fence That Didn’t Stop the Operator
A robot cell had a fence around it. The fence was 2 m high, steel mesh, bolted to the floor. But the gate in the fence didn’t have an interlock — it had a simple latch. The operator could open the gate, reach into the cell to clear a jam, and the robot kept running. We found this during a safety review. The fence looked right. It just wasn’t functional as guarding — the gate didn’t stop the hazard. We added an interlocked gate (safety switch that cuts power when opened). The fence wasn’t the problem; the gate was.
Machine guarding design standards are about more than putting a fence around the machine. The fence must prevent access to the hazard, the gate must be interlocked, and the guarding must be designed to the standards (ISO 14120, ANSI B11). This article is how I design guarding that meets the standards and actually works.
The Hierarchy of Guards: Fix the Hazard First
Guarding is the last resort. Before you put a fence around a machine, ask: can the hazard be eliminated or reduced?
- Eliminate: Remove the hazard (replace a sharp blade with a non-cutting method).
- Substitute: Use a less hazardous process (hand tools instead of power tools).
- Engineering controls: Guard the hazard (fences, covers, interlocks).
- Administrative: Training, procedures, warning signs.
- PPE: Gloves, glasses, ear protection (last resort).
The fence is step 3. If the robot arm can be redesigned to not present a hazard in the loading zone, that’s better than fencing the operator out. But for most custom machines, the hazard is inherent (moving parts, pressing force), and guarding is required.
Fixed Guards vs. Interlocked Guards
Fixed Guards
A permanent barrier (welded, bolted, or screwed) that can’t be removed without tools. It stays in place. It’s the simplest and most reliable guard.
Best for: Hazards that never need access (robot cells, conveyor enclosures, gearboxes). Once the machine is running, nobody needs to enter.
Interlocked Guards
A movable guard (gate, door, cover) with a safety interlock. When the guard opens, the interlock stops the hazardous motion. The machine can’t run while the guard is open.
Best for: Access needed for loading, unloading, setup, or maintenance. The gate must be opened to use the machine.
The interlock is the critical part. It must be a safety-rated switch (not a simple proximity sensor). When the gate opens, the safety circuit breaks, and the machine stops. The switch must be welded or bolted on (not just screwed) so it can’t be defeated by a simple adjustment.
| Guard Type | Access | Interlock | Best For |
|---|---|---|---|
| Fixed fence | No access (tools to remove) | None | Permanent enclosures, robot cells |
| Interlocked gate | Controlled access | Safety switch on gate | Loading stations, setup access |
| Light curtain | Optical (no physical barrier) | Light curtain + safety controller | Frequent access, operator loading |
| Presence-sensing mat | Pressure-sensitive floor | Mat switches safety circuit | Walking-up hazards, operator zones |
Fence Design: Height, Mesh, and Gap
The fence isn’t just a panel. Its dimensions determine whether it prevents access.
Fence Height
The standard minimum fence height is 2,000 mm (about 6.5 feet). Above 2,000 mm, an average adult can’t reach over. For hazards that require climbing (robot arms at height), the fence should extend higher or the hazard should be lowered.
If the fence is below 2,000 mm, the operator can reach over. The standard specifies that the top of the fence must be at a height where a person can’t reach the hazard over it. This depends on the hazard distance (how far the hazard is from the fence).
Mesh Size (Finger Access)
The mesh opening must be small enough that fingers can’t reach through. The standard (ISO 13857) specifies the gap size based on what’s behind it.
- Behind the mesh: no hazard (just the machine frame): 12 mm mesh (fingers can’t reach through).
- Behind the mesh: moving parts or pinch points: 8 mm mesh (or solid panel).
- Reach-over (above the fence): The fence height matters more than mesh size.
A 25 mm mesh looks fine but lets fingers reach through. For robot cells with moving parts behind the fence, use 12 mm mesh or smaller. The cost difference is small; the safety difference isn’t.
Fence-to-Floor Gap
The gap between the bottom of the fence and the floor must be small enough that fingers or feet can’t reach under. Standard: less than 180 mm (6 inches) from the floor. If the gap is larger, add a toe kick or extend the fence to the floor.
Interlock Design: The Gate Is the Weak Point
A fence with an interlocked gate is only as safe as the gate. The gate must fail safe — when it opens, the machine stops.
Safety Switch Types
- Solenoid interlock: The gate is locked closed (electrically) while the machine runs. The operator can’t open it until the machine stops. The solenoid releases after the hazardous motion stops. This is the highest level — the gate can’t be opened during operation.
- Non-contact safety switch: A magnetic or RFID switch that detects the gate position. No physical contact (no wear). Used for gates that are opened frequently. If the gate opens, the switch breaks the circuit.
- Limit switch (positive opening): A mechanical switch that is physically actuated by the gate. The switch has a positive-opening mechanism (it’s forced open, not just losing contact). This is the simplest and most reliable for low-frequency access.
Duplicated (Redundant) Circuits
For high-risk machines, the interlock circuit is duplicated. Two independent switches in series. If one fails closed (welded contacts, broken wire), the other still stops the machine. This is required for higher safety categories (PLd / SIL2 and above).
Bypass Prevention
The interlock must be hard to bypass. A switch that can be defeated by taping it closed is not guarding. Use:
- Guarded switch mounting: The switch is behind a cover, not accessible from outside.
- Non-contact switches: Can’t be bypassed by taping (they need a coded target).
- Solenoid locking: The gate is physically locked, not just monitored.
The test: Can the operator defeat the guard with a screwdriver, a piece of tape, or by propping the gate open? If yes, it’s not guarding. It’s a visual barrier. A real interlock can’t be bypassed without tools and knowledge.
Safety Distance: How Far the Fence Must Be
The fence must be far enough from the hazard that, when the interlock trips, the machine stops before the operator reaches the danger. This is the safety distance (ISO 13855).
The formula: d = K × C + 1,200 mm, where K is the approach speed (1.6 m/s for walking, 2.0 m/s for reaching), and C is the machine stopping time in seconds.
Example: a robot that stops in 0.5 seconds. The operator walks at 1.6 m/s. Distance = 1.6 × 0.5 + 1.2 = 2.0 m. The fence must be at least 2 meters from the robot. If the fence is closer, the operator could reach the robot before it stops.
Calculate this, don’t guess. The fence that “looks safe” but is 1 meter from a 0.5-second robot is 1 meter too close.
Access for Maintenance: Gates and Locks
Maintenance needs to enter the cell. The guarding must allow controlled access without defeating the safety.
- Maintenance gate: A separate gate with its own interlock. When maintenance opens it, the machine goes to a safe state (robot stops, air pressure released, drives disabled).
- Lockout / Tagout (LOTO): Maintenance applies a padlock to the energy isolation (air supply, electrical disconnect). The machine can’t be restarted until the lock is removed. This is OSHA / ISO 14118.
- Hold-to-run for setup: When setup, the machine runs only while a button is held. Release, and it stops. No automatic cycle.
A Machine Guarding Checklist
- Has the hazard been eliminated or reduced before guarding?
- What type of guard? (Fixed fence, interlocked gate, light curtain?)
- Is the fence height ≥ 2,000 mm?
- Is the mesh small enough (≤12 mm for moving parts)?
- Is the fence-to-floor gap ≤ 180 mm?
- Are the gates interlocked with safety-rated switches?
- Is the interlock redundant (two channels) for high risk?
- Is the safety distance calculated (ISO 13855)?
- Can the interlock be bypassed? (It shouldn’t be easy.)
- Is LOTO provided for maintenance?
- Is the guarding documented in the safety file (risk assessment)?
- Has the design been reviewed by a safety engineer?
The Bottom Line
Machine guarding design isn’t putting a fence around the machine. It’s eliminating hazards first, then guarding the rest with fences, interlocks, and safety distance. The fence must prevent finger access (mesh size), reach-over (height), and foot-under (floor gap). The gate must be interlocked with a safety-rated switch that stops the machine when opened. The safety distance must be calculated, not guessed. The cell that looks safe but has a non-interlocked gate isn’t guarded — it’s decorated. Guard to the standards, verify the interlocks, and make maintenance access controlled. The operator’s hand is faster than the robot’s stop time. The fence has to account for that.
