What is a Safety PLC?

Learn about a very important part of any automated industrial process the Safety PLC

In this easy-to-follow article, we’re going to talk about a very important part of any automated industrial process the Safety PLC.

Safety Instrumented System (SIS)

A Safety PLC is one of the 3 parts of a Safety Instrumented System (SIS). The SIS can also be referred to as the Safety Shutdown System.

Safety Instrumented System or SIS

Stated simply, the SIS monitors the equipment or process and if an unacceptable condition or risk, or an unsafe condition occurs it reacts by the shutdown of the equipment or process.

Safety PLC

Safety PLCs are constantly guarding against plant failures which could result in harm to people, equipment, or the environment. When specified hazardous conditions occur, the Safety PLC will respond by placing the plant in a safe condition.

Logic Solver

The Safety PLC is referred to as the Logic Solver in the Safety Instrumented System.

Safety PLC

Safety vs. Regular PLC

A Safety PLC is just like a regular PLC and is programmed the same way.

Safety PLC and Regular PLC
But, it has extra features like redundancy and added hardware functionality that adheres to a specific Safety Integrity Level (or SIL).
Safety PLC vs. Regular PLC

Safety shutdown with hard-wired relay logic

Safety shutdown systems are not new. Before the introduction of Safety PLCs, plant shutdown conditions were monitored and operated by typical field devices and instruments and the logic was performed by hardwired relays. Gone are the days of hard-wired relay logic in both Process and Safety PLCs.

Safety Shutdown Systems


You might be thinking… Well, what is the difference between a Safety PLC and a regular Process PLC?

Physically, other than they might be a different color, they look exactly the same! But…that’s where the similarity ends.

Physical Differences

Safety Logic

The logic in the Safety PLC is locked. It has safety signatures ensuring that coding has not been changed.

Safety PLC Signatures

Field device checking

The Safety PLC can perform additional field device checking such as snooping for broken wires, failed contacts, among other features.

Safety PLC Field Device Checking

Finally, The Safety PLC has the multi-channel capability for redundant I/O monitor and operation capability.


Let’s review some key points

– A Safety PLC is an integral part of a Safety Instrumented System (SIS).

– When specified conditions happen, the Safety PLC is charged with placing the plant in a safe condition or sometimes a shutdown state.

– The Safety PLC is referred to as the Logic Solver in the SIS.

– A Safety PLC is programmed the same and looks very similar to a regular process PLC, but has many extra features.

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If you would like to get additional training on a similar subject please let us know in the comment section.

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The RealPars Team
By Ted Mortenson

By Ted Mortenson

Automation Engineer

Posted on Jul 6, 2020
Ted Mortenson

By Ted Mortenson

Automation Engineer
Posted on Jul 6, 2020
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