Difference between a normal PLC and a Safety PLC?

[astromanParticipantKarma: 14Rank: Padawan]

Hi everyone, just stumbled on this site and it looks pretty cool. I’m a noob in the PLC world but am excited to start my journey learning about them. I have a question, what is the difference between a standard or normal PLC and a safety rated PLC. Also, when should you use one over another?

Thanks.

[PLCGuruKeymaster]

Hi and welcome to the forums. I’m excited for you to get started on your journey in the fascinating world of PLC’s. Now for your question…

There are many differences (and similarities) between a “normal” PLC and a Safety PLC. Using the Allen Bradley ControlLogix platform as a comparison, let’s start with the similarities.

• Normal and Safety PLCs program the very same way and support the same common languages such as Ladder, Function Block, Structured Text.
• Both a normal and safety PLC can utilize the same “standard” I/O modules, power supplies, chassis, etc.
• A safety PLC can do everything a “normal” PLC can do with additional capabilities that make it a “safety PLC”.

Differences between a “normal” PLC and a safety PLC:

• Where normal PLCs leverage one main processor (that takes up one slot in the chassis), safety PLC’s integrate multiple processors (taking up two slots).
• These microprocessors are then assigned to their own tasks, such as, I/O, memory, safety communications etc.
• Watchdog circuits are integrated in the processors to perform diagnostic analysis of each other as either one of the two processors are capable of performing the safety functions. Great care is taken to ensure that both processors are operating in sync at the operating system level.
• The nice thing about all this added diagnostic and error checking is that it all happens at the operating system level, so the programmer need not be concerned about it. It all happens behind the scenes in the background.
• The other aspect of a safety PLC is that they have specialized instructions that are designed to mimic the function of their safety relay counterparts. Also, the instructions are well abstracted (meaning the code in the instruction has been well tried and tested) and all the programmer has to do is “wire” the safety instruction blocks up.
• These safety instructions are also certified by a third party to ensure that they meet applicable safety standards.

Well I’m sure there’s more I could add here, but hopefully there is enough to start drawing a distinction.

Fred

[astromanParticipantKarma: 14Rank: Padawan]

Hi Fred, thanks so much for that explanation!

[James CookParticipantKarma: 15Rank: Padawan]

Excellent explanation!!!

[PLCGuruKeymaster]

You’re most welcomed, and thanks for the comment James!

[Tanner SmithParticipantKarma: 19Rank: Padawan]

Thanks for the explanation! I was always curious about this subject.

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