At a glance
Key points
- Measure the complete safety-function response, not only the motor stop.
- Use repeated tests under credible speed and load conditions.
- Include sensor, logic, output and machine stopping time.
- Account for reach-over, reach-through, field resolution and approach direction.
- Retest after relevant mechanical, drive, control or device changes.
1. What is included in total stopping time?
Total response begins when the protective device detects entry and ends when the hazardous movement has reached the defined safe state. It can include sensor response, safety network or input delay, logic processing, output switching, contactor or drive response and mechanical run-down.
Do not substitute a drive’s nominal stop ramp for a measured safety-function result. Brakes, payload, speed, wear and control configuration can change the real stop.
2. The safety-distance relationship
A commonly used form of the calculation is S = K × T + C, where S is minimum distance, K is an approach-speed parameter, T is total stopping time and C is an additional distance related to intrusion or detection capability. The actual formula and constants depend on the protective device, approach and current standard.
3. Plan the measurement
- 01
Define the hazard
Identify the movement and safe state that the device is intended to control.
- 02
Select test conditions
Use credible production speed, load, direction, tool state and drive condition.
- 03
Trigger consistently
Actuate the safety function at a defined point using suitable measurement equipment.
- 04
Repeat and record
Take sufficient readings to reveal variation and use an appropriately conservative result.
- 05
Calculate and verify geometry
Apply the method, then check reach-around, reach-over, under-crawl and hidden approach routes.
4. Light curtains, scanners and different approaches
Vertical light curtains often protect a direct approach, while horizontal fields can detect presence or approach across a floor. Laser scanners can create complex fields but remain sensitive to mounting, reflectors, contamination, field switching and the possibility of stepping over or standing in an unmonitored zone.
Resolution matters. A device that detects a hand or finger may use a different allowance from body detection. The lower and upper field heights also affect whether a person can pass under, over or through without detection.
5. When to retest
Retest after brake or drive work, changes to speed or payload, safety-program changes, device relocation, modifications to tooling or the guarded layout, and whenever performance is in doubt. A risk-based periodic check may also be appropriate where wear could increase stopping time.
Record the machine state, measurement equipment, readings, selected result, calculation and installed distance so future reviewers can understand the basis.
Sources and review basis
Authoritative references
This guide summarises general engineering considerations. Current legislation, guidance and standard editions should be confirmed for the specific project.
How Oxford Machine Guarding can help
We can survey the equipment, define the mechanical guarding and safety-system scope, develop the design, supply and install the solution, and provide testing and handover records appropriate to the agreed project boundary.
Important: this guide provides general technical information. It is not a substitute for a machine-specific risk assessment, legal advice, or verification by the responsible dutyholder.
