How to Calculate Safe Distances

Calculating Safe Stop Distance for Machinery

Video Blog: Safe Stop Calculations Ds = [K x (Ts + Tc + Tr)] + Dpf

Most industrial equipment requires human interaction to function. Workers must be protected from dangerous interactions with machines. Calculating the minimum safe mounting distance is vital to ensure worker safety. In addition, excessive mounting distances uses up valuable floor space and can increase the time required to service machinery, which in turn reduces throughput.

Video Transcript

Hello, this is Austin Desmond, Automation Specialist at Horizon Solutions. Today we will be discussing safe mounting distance formulas as well as the resulting difference between a safety rated PLC solution and safety rated relay solution.

When renovating older machinery and creating new machines, devices such as: light curtains and area scanners, and other safety presence sensing devices maybe necessary. The purpose of all these devices is to safely stop a hazard to bring it to a safe state prior to an operator reaching it. Although, safely stopping hazard portions of machinery to protect the safety of workers should always be a priority, an often overlooked but extremely critical component to the equation is, the time it takes from device being triggered to the machine reaching a safe state. In this video, we will be highlighting the difference in safe stopping distance between safety relay and a safety rated PLC control system.

Here is the equation for creating a safe minimum mounting distance: Ds = [K x (Ts + Tc + Tr)] + Dpf as defined by the American National Standards Institute (ANSI).

  • Ds = the minimum safe distance between safeguarding device and the hazard
  • K= speed constant; 1.6 m/sec (63 inches/sec) minimum based on the movement being the hand/arm only and the body being stationary
  • Ts = machine/equipment stopping time
  • Tc = control system stopping time
  • Tr = safeguarding device response time
  • Dpf = maximum travel towards the hazard within the presence sensing safeguarding devices (PSSD) field that may occur before a stop is signaled. Depth penetration factors will change depending on the type of device and application.

The main reason for going through these calculations today is to highlight the difference in safety mounting distance when looking at different control systems. Now let’s focus on a relay solution and safety PLC solution. I don’t want to get too far into the math, but I do want to say, for both calculations the ONLY major factor we are going to be looking at changing today is Tc. For both solutions, Ts is the time from when the output changes state and when the machine comes to a complete stop. For this example, we are using a 15ms reaction time Tr for the light curtain. The depth penetration factor was calculated using ANSI standards. This will be the same for both situations. Now for the relay, the response time of the relay itself is 40ms from the change of input to the change of output.  Now using the Rockwell Automation provided GuardLogix® reaction time tool, a range of 20-72ms, considering no faults was found as the reaction time. This is a quick snapshot of that tool. As you can see, considering a single fault, 82ms reaction time. Considering multiple faults, which would be the worst-case scenario, you have 112ms response time. What this all really means is with the relay solution you could mount the light curtain 18 inches from the hazard. With the PLC the hazard would have to be more than 20.5 inches from the light curtain. That was calculated using a single fault tolerant, and when this calculation is typically done its done at worst-case scenario which would increase safe mounting distance of the PLC solution to 23 inches, a full 5 inches farther away than the relay.

Links & Tools Referenced in Video:

In his video, Austin mentions a few links, they are provided below…

  1. Link 1 [Referenced at the 2m 46s mark] Rockwell Automation GuardLogix Reaction Time Tool
  2. Link 2 [38 MB PDF file, specifically, Appendix C starting on page 193] Safety Accelerator Toolkit 
  3. Link 3 [Page with 8 application specific download options] Safety Accelerator Toolkit
  4. Link 4 [Information] Safety Services through Horizon

Want to contact Austin directly? You can email him or call him at (585) 274-8283. Horizon Solutions specialists have various safety accreditation including TUV Functional Safety Tech. Our team of safety experts frequently get deployed to assist with safe stop distance calculations. Horizon Solutions also has on staff Authorized OSHA Outreach Trainers. Need help? Reach out to us.

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About the author

Austin Desmond
Austin Desmond

Austin is an Automation Specialist with Horizon Solutions and supports industrial control, safety, PLC/HMI, motion and drives. He is currently undertaking formal TUV Rhineland safety certification. In his spare time, Austin enjoys welding, grinding, motorcycles, lawn mowers and building (tinkering?) in the Horizon Solutions Automation Lab.