When safeguarding your machinery, it is not enough to simply install a light curtain(s) and assume that the machine is now safe. Calculating safety distance, as defined by the American National Standards Institute (ANSI), is a crucial step for any light curtain installation. This post explains how to calculate safety distance for light curtains.
According to ANSI B11.19-2010, safety distance is, “The distance a safeguard is installed from a hazard such that individuals are not exposed to the hazard.” It is best practice to calculate the theoretical value for the safety distance before installing a light curtain.
Then, once installed, use a stop time measurement (STM) device to calculate how quickly the machine stops, and use this value to determine the minimum safety distance required. Taking the time to calculate safety distances upfront not only ensures compliance with global safety regulations, but also helps prevent long term costs of injury and machine downtime.
For example, a mechanical power press cannot immediately come to a complete stop. Rather, it takes time for the machine to respond to the safeguarding device and all interfaced components before achieving a complete stop. If the light curtain is placed too close to the crushing point, the machine may not have sufficient time to respond to a person’s finger or hand approaching the hazard.
Additionally, some machines may have a coast down time due to a rapid process that cannot be stopped quickly without damaging the machine or products. (In these cases, guard locking interlocks may be a better solution. Interlocks keep the movable guards locked, preventing access by personnel until the hazardous motion has ceased. Consult a safety professional to determine the appropriate safety solution for your machine.)
How to Calculate Safety Distance
The ANSI calculation takes into consideration a number of factors, including a constant value representing the approach speed of the human hand, the stopping time of your machine, the response time of the light curtain, and the response time of the safety related parts of the control system. It is also important to work in extra time as a buffer for any variations in the machine stop time.
To determine the stopping time of the machine (Ts), you would typically take 10 stop time measurements, calculate the mean, then add 3 standard deviations. This calculated value, or the highest of the 10 measured values, should then be used as the value of Ts in the formula below.
Another important element of the calculation is the depth penetration factor (Dpf). This refers to how far into the protective field an object can travel before it is detected by the light curtain. The calculation is dependent on the mounting orientation and resolution (in millimeters) of the light curtain. In vertical applications, the calculation is as follows (where S is the minimum object sensitivity / resolution of the light curtain):
Dpf = 3.4 x (S – 7)
Therefore, a light curtain with 14 mm resolution (finger detection) has a Dpf of 23.8 mm (about an inch). Meanwhile, a light curtain with 30 mm resolution (hand detection) has a Dpf of 78.2 mm (about 3 inches).
ANSI Minimum Safety Distance (DS) Calculation
A safety professional can help you determine the stopping time (Ts) of your machine. Then use the following formula to calculate the minimum safety distance for your application.
Ds = K x (Ts + Tc + Tr + Tspm) + Dpf
K = Hand speed constant (63 inches/second)
Ts = Stop time of your machine (measured with a stop time measurement device)
Tc = Safety Related Parts of the Control System response time
Tr = Response time of the safety light curtain (provided by the supplier)
Tspm = Additional time to compensate for normal stop time variations
Dpf = Depth penetration factor
For more information, download SICK’s Safety Guide for the Americas: Six Steps to a Safe Machine. See Section 3c for more on safety distance.
This blog post is meant as a guideline only and is accurate as of the time of publication. When implementing any safety measures, we recommend consulting with a safety professional.