NR-12 Safety Distance

NR-12: Machine Safety Distance

Brazilian Regulatory Standard NR-12 covers safety in work with machinery and equipment. Fixed, movable and interlocked guards have one job: keep operators out of the hazard zone. Sometimes a moving part has to be stopped on demand instead, using a light curtain, a sensitive mat or a two-hand control. In that case the protective device has to sit far enough back that the dangerous motion stops before the operator's hand can reach it. You get that minimum distance from Ds = K · T + C. Here K = 2000 mm/s is the standard hand approach speed, T is the machine's total stop time plus the response time of the protective device (in seconds), and C is an intrusion constant tied to how fine the detection resolution is.

The formula lines up with ISO 13855, which positions safeguards based on body approach speeds, and with ABNT NBR 14152. When the device detects fingers (resolution ≤ 14 mm), use C = 8 · (d − 14); for hand detection (14 mm < d ≤ 40 mm), C = 128 mm. One thing to watch: always measure or estimate the machine stop time on a calibrated test bench, because brake wear pushes T up as the equipment ages.

Applications

Sizing safety light curtains on presses and shears. Positioning pressure-sensitive mats around robotic cells. Setting up interlocked guards on injection moulding machines. Checking the risk assessments that NR-12 Annex VIII requires. And backing up CIPA inspections or the technical reports (Laudo Tecnico) that occupational engineers put together.

FAQ

Why is K fixed at 2000 mm/s? That's the conservative reference speed for an outstretched arm reaching toward a hazard. ISO 13855 set it to cover the 95th percentile of operators.

Do I need to add the response time of the safety relay? Yes. T has to include sensor, logic and actuator delays, not just the mechanical stop time.

Is NR-12 compliance enough for export markets? NR-12 covers Brazilian sites. For CE marking you still have to apply the Machinery Directive and the harmonised EN/ISO standards, even though the formulas overlap quite a bit.