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Machine Safety: Can one channel mitigate a Cat 3/PLd functional requirement?

Category 3 and 4 architectures typically call for a control reliable (that is, redundant) circuit design no matter what safe logic solver under consideration: safety relay, safety PLC, safe drive, and so forth. So why would there be any confusion?

July 02, 2014

Figure 5 in ISO 13849-1; 2006 clearly shows how these factors can impact a safety circuit’s design.Category 3 and 4 architectures typically call for a control reliable (that is, redundant) circuit design no matter what safe logic solver you’re considering, such as a safety relay, safety PLC, safe drive, and so forth. So, why is there confusion?


I have seen people be confused at the differences between an application design and the individual design of a sub-system by a supplier, such as a safety PLC. Over the past 10 years or the term “control reliable” has drifted away. Redundancy or duality seems to have taken its place along with monitoring and diagnostics coverage. Haven’t we all seen where redundancy or duality has meant two safe inputs and two safe outputs connected by a safety certified logic solver for a Cat 3 or 4 architecture? And of course the safety certified logic solver had to meet the design requirements of IEC 61508, and it had to be listed for use in machine safety applications. In the past few years since ISO 13849-1 & 2; 2006, the extent of monitoring and diagnostics coverage needs to be considered within the safety circuit as well as within a sub-system.

Confusion introduced

Other factors also can be considered by a designer but isn’t this enough to introduce confusion? WHY? Because, not all designs by suppliers of sub-systems like safety PLCs are alike. This is where the safety data parameters from the supplier can play a role. One supplier of a sub-system can perform some of the monitoring and diagnostics coverage within their safety logic solver to have a single safe (enough) output in certain circumstances. Additionally, couldn’t that single safe output in certain circumstances be fed dually to two contactors? If the answer is yes, and the overall safety circuit can be verified as meeting the performance level requirements of PLd, haven’t we mitigated a Cat 3/PLd functional requirement with a single channel output?

As the graphic shows, The overall effect of analyzing a complete safety circuit’s performance including its MTTFd (mean time to fail dangerous) and its DCavg (diagnostics coverage average) along with the safety data parameters of all components and sub-systems can impact the answer to this question. Figure 5 in ISO 13849-1; 2006 clearly shows how these factors can impact a safety circuit’s design. Having said that, don’t we often simply error to the high side and provide redundancy or duality throughout a Cat 3/PLd safety circuit? I have found that the best recommendation is to always consult with the supplier of a safety certified sub-system regarding the design integrity of their safe outputs.

What are your thoughts about safety circuit design? Do you have related questions or suggestions? Add your comments or thoughts to the discussion by submitting your ideas, experiences, and challenges in the comments section below.

Related articles:

LinkedIn – Safety Automation Forum, When is dual channel allowed to be single channel?

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Control Engineering, Safety control systems: Essential considerations, costs

Contact: for “Solutions for Machine Safety.”

For more than 30 years, J.B. Titus has advised a wide range of clients on machine functional safety solutions, including Johnson & Johnson, Siemens, General Motors, Disney, Rockwell Automation, Bridgestone Firestone, and Samsung Heavy Industries. He holds a Bachelor of Business Administration degree from Oklahoma University in industrial management and an MBA from Case Western Reserve University in marketing and finance. He is a professional member of the American Society of Safety Engineers and is OSHA-certified in machine guarding. Titus is also TUV-certified as a Functional Safety Expert and serves on several American National Standards Institute, National Fire Protection Association, and National Electrical Manufacturers Association national safety and health standards committees. Reach him at jb(at) and via