Light curtains have become a popular choice for safeguarding hazardous machinery. They offer a clear view of the work area while providing a reliable shutdown mechanism when an intrusion is detected. However, improper installation, particularly regarding the minimum safe distance, can render them ineffective and leave workers vulnerable.
We provide practical guidance to ensure your light curtains are positioned for optimal worker protection, and explore the two main formulas for determining minimum safe distance.
Why Minimum Safe Distance Matters
Light curtains are classified as non-separating safeguards. This means they lack a physical barrier to prevent accidental contact with hazards. Their effectiveness hinges on proper placement. Installing a light curtain too close to the hazard does not allow sufficient time for the machine to stop completely before reaching the intrusion point. This can lead to severe injuries, even if the light curtain activates.
Understanding Safety Standards
Several international safety standards govern the proper installation and use of light curtains. The most common ones in the United States include:
- OSHA 29 CFR 1910.217(c)(3)(iii)(e): This standard outlines the minimum requirement for calculating the safety distance.
- ANSI B11.19 1990 E4.2.3.3.5: This standard builds on OSHA’s requirements by incorporating additional factors for a more comprehensive calculation of the safe distance.
- ISO 13857: This international standard provides a framework for machine safety, including light curtain placement.
Calculating the Minimum Safe Distance
This OSHA formula offers a basic approach to calculating the minimum safe distance (Ds):
Ds = 63 inches/second x Ts
To break down this safe distance formula:
- Ds = minimum safety distance (inches)
- 63 inches/second = assumed hand speed constant
- Ts = stopping time of the machine measured at approximately 90° position of crankshaft rotation (seconds)
This formula considers the operator’s hand reaching speed and the machine’s stopping time. However, it doesn’t account for crucial factors like:
- Total system response time: This includes the combined response times of the control system, light curtain, interface, and brake monitor.
- Minimum object sensitivity: This refers to the smallest object the light curtain can detect.
- Hand or object speed: Depending on the application, the operator’s hand movement or object might exceed the assumed 63 inches/second.
ANSI B11.19 addresses these limitations with a more comprehensive formula:
Ds = K x (Ts + Tc + Tr + Tbm) + Dpf
To break down this safe distance formula:
- K = Hand speed constant (typically 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 (light curtain) 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) of the light curtain
Example Using the ANSI B11.19 Formula:
- 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″.
Additional Considerations
- Stop Time Measurement: A calibrated stop-time measuring (STM) unit is crucial for accurately determining the machine’s stopping time (Ts).
- Minimum Mounting Distance: Regardless of the calculated distance, never mount a light curtain closer than 6 inches from the hazard zone.
Key Takeaways from ANSI and OSHA
- Light curtains are effective safeguards when installed correctly.
- Minimum safe distance is paramount for ensuring worker safety.
- OSHA and ANSI standards provide formulas for calculating the minimum safe distance.
- The ANSI formula offers a more comprehensive approach by considering additional factors.
- Stop time measurement and minimum mounting distance are crucial aspects.
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.
Comprehensive and Compliant Safeguarding Solution
Our team at Arrow have the in-depth knowledge and extensive experience to design, build, and install a light curtain system that adheres to all applicable safety regulations and seamlessly integrate with your specific operational needs. We will meticulously handle the safety distance calculations, ensure optimal placement for maximum effectiveness, and provide a complete safeguarding solution that prioritizes worker safety while optimizing operational efficiency.
Contact us today to schedule an initial consultation. Our team is dedicated to collaborating with you to understand your unique requirements and to develop a customized plan to safeguard your machinery and empower your workforce to operate with confidence.