Calculating a Safe Distance between to position a Light Curtain from a potential hazard.

01 August 2019

Light Curtains have become a very popular safety device since they provide a machine shutdown when properly integrated into the Safety System and an Employee breaks the protected barrier/beam, while providing clear sight into the process.  However, many times the Employee is not adequately protected because of the safe distance not being established or incorrectly applied. 

Presence sensing safety devices, such as Light Curtains, are considered non-separating guarding as there is no physical barrier between the operator and hazard.  Integrating a Light Curtain that is not positioned at the minimum distance from the hazard, or that can be reached around or over, is not effective and provides a false sense that the process is safe.   Imagine a machine with a rotating hazard protected by a Light Curtain instead of hard guarding.  If the Employee breaks the Light Curtain protecting beam, yet still has the ability to encounter the hazard prior to the machine or hazard coasts to a stop, then the position of the light Curtain is incorrect and the safety risk is not mitigated.  Mounting the Light Curtain too close to the process is a common mistake, typically because of area constraints or the desire to keep the device from being damaged.

To avoid this, the minimum safe distance or height at which a Light Curtain or protective device should be installed must be determine prior to installation.   There are a number of International standards that regulated this.  The most popular in the US are the OSHA 29 CFR 1910.217(c)(3)(iii)(e)and ANSI B11.19 1990 E4.  A common International standard is ISO 13857.

The first, the OSHA formula, is the minimum requirement for the calculation of the safety distance. The second formula is the ANSI formula, which incorporates additional factors to be considered when calculating the safety distance.

OSHA Standard: 29 CFR 1910.217(c)(3)(iii)(e)

29 CFR 1910.217(c)(3)(iii)(e):  the SAFETY DISTANCE (Ds) from the sensing field to the point of operation shall be greater than the distance determined by the following formula:

OSHA Safety Distance (Ds)


D = 63 inches/second  x   T where: 


D = minimum safety distance (inches); 


63 inches/second = hand speed constant; and 


T = stopping time of the press or machine measured at approximately 90°


position of crankshaft rotation (seconds).

The minimum safety distance is defined as the minimum distance from the light curtain's plane of light to the closest hazard or danger point where the operator could reach into the hazard. 

Note: The Ts stopping time is the time that the protective device barrier field is broken to the time the machine hazard comes to a complete stop.  In general, this would incorporate the response time of the safety system devices and machine response time.   A number of measurements should be taken to ensure that the stopping ability of the machine is consistent.  If there is variation, the longest stopping time should be used.

ANSI Standard: B11.19 1990 E4.

This minimum safety distance is based on the stopping ability of the machine and a hand speed constant. When the minimum safety distance is calculated, several other factors must be taken into account, which are not included in the in the OSHA formula. These factors include the total system response time, the minimum object sensitivity of the presence sensing device, and the hand or object speed. The total system response time includes the stopping time of the machine under worse case conditions, response time of the control system, response time of the presence sensing device as stated by the manufacturer,  the response time of the interface, and, if applicable, additional time allowed for the brake monitor to compensate for variations in normal stopping time. Another factor includes the penetration distance (Dpf), which is based on the light curtain's MOS (minimum object sensitivity). The following formula is used to compute the minimum safety distance (Ds) on mechanical power presses to meet the ANSI (American National Standards Institute) B11.1 Press Safety Standard:

ANSI Minimum Safety Distance (Ds)


Ds = K x (Ts + Tc +  Tr   +  Tbm) + Dpf  where: K  = Hand speed constant (63 inches/second) Ts = Stop time of equipment measured at the final control element Tc = Response time of the control system Tr   = Response time of the presence sensing device and its interface Tbm = Additional time allowed for the brake monitor to compensate for variations in             normal stopping time Dpf  = The added distance due to the penetration depth factor (MOS). Note: If the             channel blanking feature is used on light curtains, additional safety distance             must be enforced based on the number of channels blanked.


Using the ANSI formula, a light curtain response time (Tr) of 15ms, a machine stopping time (Ts + Tc) of 180ms, a brake monitor response time (Tbm) of 40ms and a 3.2 inch depth of penetration, the calculation would be as follows (Remember that the hand speed constant, K, is set by OSHA at 63 inches per second):

So, the minimum safe distance the safety light curtain must be mounted from the hazard is 18".


In either case, when determining the safety distance, a portable or built-in stop-time measuring unit must be used to check the stopping time (Ts) of the machine.  The STM (stop-time measurement) device should be a calibrated device and measures the time it takes a machine to stop after a signal is given. It is mainly used on reciprocating (stroking or cycling) machines, such as mechanical and hydraulic presses or press brakes. With optional accessories, it can also be used on machines that rotate, such as lathes, mills, and drills.

Note: Regardless of the calculated safety distance, Safety Light Curtains should never be mounted closer than 6 inches from the point of operation or pinch point hazard.


OSHA 29 CFR 1910.217(c)(3)(iii)(e)

ANSI B11.19 1990 E4. 

ISO 13857.