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?

07/02/2014


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?

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

Control Engineering, Safety control systems: Essential considerations, costs

Contact: www.jbtitus.com for “Solutions for Machine Safety.”



No comments
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Intelligent, efficient PLC programming: Cost-saving programming languages are available now; Automation system upgrades; Help from the cloud; Improving flow control; System integration tips
Smarter machines require smarter systems; Fixing PID, part 3; Process safety; Hardware and software integration; Legalities: Integrated lean project delivery
Choosing controllers: PLCs, PACs, IPCs, DCS? What's best for your application?; Wireless trends; Design, integration; Manufacturing Day; Product Exclusive
PLCs, robots, and the quest for a single controller; how OEE is key to automation solutions.
This article collection contains several articles on improving the use of PID.
Learn how Industry 4.0 adds supply chain efficiency, optimizes pricing, improves quality, and more.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Special report: U.S. natural gas; LNG transport technologies evolve to meet market demand; Understanding new methane regulations; Predictive maintenance for gas pipeline compressors
Cyber security cost-efficient for industrial control systems; Extracting full value from operational data; Managing cyber security risks
Drilling for Big Data: Managing the flow of information; Big data drilldown series: Challenge and opportunity; OT to IT: Creating a circle of improvement; Industry loses best workers, again