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?


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?

Machine Safety: When should 2 mechanical safety switches be used on an access door?

Control Engineering, Safety control systems: Essential considerations, costs

Contact: for “Solutions for Machine Safety.”

Tisha , MI, United States, 12/20/16 01:32 PM:

I know this is years after the post but I think it is an important discussion to have. I just had one similarly with the owner of a company that does risk assessment and safety design. I asked him the question "why not just always design to the highest safety design techniques (ex dual channel, feedback monitoring signals, etc.)". He mentioned that it may not be a problem with electrical systems but can become very expensive with hydraulic and pneumatic systems. I do machine design and I haven't seen a situation where cost because outrageous. Usually the only pitfall to good practice in safety design, that I have witnessed, is the lack of competence in doing so.

I think there is a great benefit to talking about these things within the community. It is almost never discussed among the controls engineers I work with and when it is, there is clearly a lack of expertise.
Tisha , MI, United States, 12/20/16 01:50 PM:

What is the big deal about using two switches? By two switches, I am assuming we are talking about one gate locking mechanism that has two contacts inside of it. Is this correct? If so, even if it is not so, are we just concerned about the added cost of an extra switch in exchange for a better degree of protection?
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