What is a 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.

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 vs. Regular PLC

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

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

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.

Appearance

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.

Safety Logic

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

Field device checking

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

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

Summary

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.

You might want to review 2 of our other articles:

1) What is a Safety Instrumented System?

2) What is an Emergency Shutdown System?

Want to Learn More?

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The RealPars Team

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