Safety Fence Design: Safe Distance Formula S=KxT and Gate Interlocks

The Fence Gate That Was Too Far from the Robot: Safety Distance Calculation

A robot cell with a safety gate. The gate was 500 mm from the robot. The safety engineer said it was too close. The customer thought 500 mm was fine. The safety engineer was right. This is about safety distance calculation per ISO 13855, not about the gate being defective.

The safety distance formula

ISO 13855 defines the minimum safety distance between a safeguard (gate, light curtain) and a hazard (robot):

S = K × T + C

Where K is the hand speed (2000 mm/s for approach from above, 1600 mm/s for approach from the side), T is the total stop time (reaction time of the safeguard + robot stop time), and C is the intrusion depth (850 mm for a horizontal approach, 200 mm for vertical).

For our robot: T = 0.5 s (light curtain reaction 0.05 s + robot stop 0.45 s). K = 1600 mm/s (horizontal approach). C = 200 mm (intrusion depth for a gate). S = 1600 × 0.5 + 200 = 1000 mm. The gate needs to be at least 1000 mm from the robot. We had it at 500 mm. It was half the required distance.

What we did

1. Moved the gate. We moved the fence gate from 500 mm to 1200 mm from the robot. The safety distance is now 1200 mm (more than the 1000 mm required). The gate opens into a maintenance aisle, not directly at the robot.

2. Added a light curtain. At the point where the robot reaches the fence, we added a light curtain. The light curtain triggers when the operator reaches in. The robot stops in 0.3 s. The safety distance for the light curtain is 1600 × 0.3 + 200 = 680 mm. We set the light curtain 800 mm from the robot.

3. Used a Category 3 safety relay. The gate switch and light curtain feed into a dual-channel safety relay. The relay stops the robot’s drive when triggered. The stop time is 0.45 s (the robot’s deceleration time).

The intrusion depth

The intrusion depth (C) accounts for how far a person can reach past the safeguard. For a gate that opens horizontally, the operator can reach 200 mm past the gate opening. For a light curtain, the operator can reach 850 mm past the beam plane. The C value is critical — it adds to the required distance.

The safety distance rule: Calculate S = K × T + C. Don’t guess. A gate at 500 mm from a robot that stops in 0.5 s is too close. The required distance is 1000 mm. Move the gate. Add a light curtain. The safety engineer who flagged it was right — the distance wasn’t generous, it was unsafe.