How to Calculate Safety Distances for Light Curtain Installation

in Machine Safety Blog by

Estimated reading time: 9 minutes

Last Updated: 2024-04-12

Safety light curtains are an essential component of industrial safety systems. They detect the presence of people or objects in a hazardous area and prevent equipment from starting or operating if there is a risk of injury. However, to ensure that light curtains provide effective protection, we must calculate the safety distances correctly before installing them.

First Step: Calculate the Safety Distance!

Actually, the first step should be to identify all the hazards and assess the risk. However, for this article, we assume you have already performed a risk assessment and are looking specifically for help with safety distance calculations.

When safeguarding machinery, it’s important to take all necessary precautions to prevent accidents and injuries.  One crucial step in installing light curtains, commonly used as machinery safeguards, is calculating the safety distance.  According to the American National Standards Institute (ANSI), safety distance is:

“…the minimum distance an engineered control must be installed from a hazard to ensure individuals are not exposed to it.”

A mechanical power press is one example of a machine that requires careful consideration of safety distance.  These machines cannot immediately come to a complete stop and need time to respond to the safeguarding device and components.  If you place the light curtain too close to the crushing point, the machine may not have sufficient time to respond to a person’s finger or hand.

Calculating the safety distance involves considering factors such as an individual’s approach speed, the machine itself, the reaction time of the light curtain, and the reaction time of the safety-related parts of the control system. It is also important to add extra time as a buffer for any variations in the machine’s reaction time.

Use Tables 1.3 and 1.4 from ANSI B11.19-2019 to determine the reaching distance associated with devices. These tables tell you how far over, under, or through someone can move through the protective field before being detected by the light curtain. It’s essential to consult with a safety professional to determine the appropriate safety solution for your machine. Ferndale Safety can help you decide which type of light curtain you need and how to install it.

Drawing of a light curtain on a saw depicting safety distance calculation.

How To Calculate the Safety Distances

OSHA Recommended Safety Distance

The OSHA safety distance formula, as specified in OSHA 29 CFR 1910.217, is as follows:

Ds = K x Ts

  • Ds – is the Safety Distance.
  • K – The hand speed constant.  63 is the OSHA recommended hand rate constant in inches per second.  (See “The K hand speed constant”)
  • Ts is the total stop time of all devices in the safety circuit, measured in seconds.  This value must include all components involved in stopping the hazardous motion of the machinery.  For a mechanical power press, it is the stopping time measured at approximately the 90º position of the crankshaft rotation.

Note: The Ts number must include the response times of all devices, including the safety light curtain, the safety light curtain controller, the machine’s control circuit, and any other devices that react to stop the hazardous motion of the machinery.  

Not including the response time of a device or devices in the stop time calculation will result in insufficient safety distance for the application and may result in operator injury.

EN ISO 13855 Definition

The safety distance is generally defined in EN ISO 13855 as s = K x (t1 + t2) + C, similar to the OSHA formula but also adds C, which is the intrusion distance. This is equivalent to the reaching distance; see below.

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  • Learning how safety distances work
  • Choose the right light curtain or sensor
  • Do an on-site visit and assessment of your machinery.

    A drawing of light curtain safety distance

    ANSI Safety Distance Formula

    The 2019 edition of the ANSI safety distance formula, which Ferndale Safety recommends, is as follows:

    D = ( K x T ) + dds + Z

    • D – The minimum safety distance between the safeguarding device and the nearest point of operation hazard, in inches.  It is defined as “the minimum distance an engineered control (guard or device) is installed from a hazard such that individuals are not exposed to it.”
    • K – Hand speed constant in inches per second.  (See “The K hand speed constant” below)
    • T – Total time to achieve a safe condition.  We can break down T into multiple sub-variables (see “The Total Time Variables” below)
    • dds Reaching distance associated with devices – To decide on a suitable safety distance for engineering control devices, the reaching distance related to these devices is crucial.  This distance is the maximum reach an individual can make before the device detects them.  In older versions of ANSI B11.19, this was called the “depth penetration factor” (Dpf).  At the same time, international standards refer to it as the “intrusion factor” (C). (See “The Reaching Distance” below)
    • Z – Additional time allowed to compensate for variations in normal stopping time, such as mirror-based errors and tolerance ranges.

    The Total Time Variables (T)

    The total time to achieve a safe condition (T) is the sum of all components’ response times and the machine’s stop time. These variables include:

    • Ts – Stop time of the machine tool and type of actuators such as motors, brakes, etc. – Measured at the final control element and measured by a stop time device
    • Tc – Response time of the machine actuator control system
    • Ti – Response time of the safety-related control system and logic interface
    • Td – Response time of the presence sensing device (safety light curtain).  The device manufacturer generally states this value, or the user can measure it.
    • Tscm – Used when a safe condition monitoring system is used; this value would be the calculated factor that must be added to the total time.

    The (K) Hand Speed Constant

    AKA The typical approach speeds of parts of the human body.

    The principle of the hand-speed constant ensures that any hazard on a machine is stopped before a person can get hurt. We want to know how fast a person can move to determine safe distances.

    The ANSI standard value is 63 inches per second when the operator begins reaching the point of operation hazard from rest.  

    Note: ANSI B11.19 1990 E4. states, “The value of the hand speed constant, K, has been determined by various studies, and although these studies indicate speeds of 63 inches/second to over 100 inches/sec., they are not conclusive determinations.

    The employer should consider all factors, including the operator’s physical ability, when determining the value of K. Also, these standards do not mention anything like a person running into consideration. For example, if a person runs at full speed to catch something, the typical approach speed will be much higher.

    What does EN ISO 13855:2010 Say?

    EN ISO 13855: 2010 standard states the following values for approach speeds:

    • Walking speed: 1600mm/s (63 inches per second), or where the safeguard is over 500mm(20″) in the distance from the hazard.
    • Upper limb movement speed: 2000mm/s (79 inches per second), or where the safeguard is placed less than 500mm from the hazard.

    If you have calculated a safety distance of less than 500mm, redo the calculation using a value of 2000mm/s to account for faster upper limb movement speed.

    The Reaching Distance (dds)

    The “reaching distance” in the context of safety systems, especially light curtains or similar protective devices, refers to the maximum distance an individual can extend or move a part of their body (like a hand or arm) over, under, or through a protective field without being detected. It measures how close a person can come to a hazard without triggering a safety response from the protective system.

    If a person can reach too far into a dangerous zone without the machine stopping or the system giving a warning, there’s a high risk of injury. Hence, determining and ensuring a safe reaching distance helps design systems and workplaces that prevent individuals from getting too close to potential hazards before the protective mechanisms are activated.

    1. Determine ddo, ddu, and ddt:
      ddo : The distance over the protective field.
      ddu : The distance under the protective field.
      ddt : The distance through the protective field.
    2. Use Reference Tables: Tables I.3 and I.4 from ANSI B11.19-2019 can help determine ddo and ddu. These tables provide values based on the hazard zone’s height and the sensing field’s positioning.
    3. Calculate ddt: For vertical applications, using the formula ddt=3.4×(de−7) mm, where de is the light curtain’s effective detection capability or resolution.
    4. Determine the Overall Reaching Distance: The reaching distance, dds, is the greatest value among ddo, ddu, and ddt: dds≥max(ddo,ddu,ddt)

    By following these steps and employing the appropriate reference materials, you can determine the reaching distance for a given safety system. Always follow the manufacturer’s guidelines and consider any additional factors unique to the specific application or environment.

    Safety Distance is Only Part of the Equation

    Now that you know how far away from the hazard you should install the light curtains, you may need to consider pass-through risks.  A dangerous situation can arise when a person is between the light curtain and the hazardous area.  In fact, the safety distance may be large enough to fit multiple people in that area.  Add additional guarding, horizontal light curtains, or area scanners to prevent pass-through risks.

    Proper installation and sizing of light curtains are crucial to minimizing the risk of injury to employees operating machinery. Establish the safety distance first, followed by sizing the light curtains appropriately. The height of the light curtains may vary based on the height of employees, and the mode of operation can affect workers’ safety.  

    Around, Under, Through, and Over (AUTO)

    Pay special attention to persons who can reach hazards around the light curtains, underneath, or overtop.

    Use guarding in areas not covered by light curtains to prevent workers from reaching over or under the light curtain or barriers.  Bolster height, and the use of platforms should also be taken into consideration when installing light curtains.

    The bottom beam should not be too high or the top beam too low for light curtains. We recommend that the bottom beam be no more than 12” or 300mm from the floor. A lower beam that is too high can tempt a person to crawl underneath. Likewise, if the top beam of a light curtain is too low, a person can reach over it.

    Safety Standards

    Here are the safety standards that you can reference:

    1. ISO 13855:2010 (Replaced EN 999: 1998 + A21:2008)
    2. ANSI B11.19
    3. OSHA 29 CFR 1910.217

    When in Doubt, Contact a Professional

    Calculating safety distance is a crucial step in safeguarding machinery with light curtains.  It not only ensures compliance with global safety regulations but also helps prevent long-term costs of injury and machine downtime.  Consult the safety professional at Ferndale Safety to determine the appropriate safety solution for your machine and ensure you take all necessary precautions to prevent accidents and injuries.

    Schedule a call with us today.

      Images (credits)
      Light curtain installed on a gantry saw – Source: REER