In this video, we’re going to talk about a very important part of
any automated industrial process – the Safety PLC.
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 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.
The Safety PLC is referred to
as the “Logic Solver” in the Safety Instrumented System.
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).
OK…, back to the Safety PLC.
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 hardwired relay logic in both Process and Safety PLCs.
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.
The logic in the Safety PLC is “locked”.
It has “safety signatures” ensuring that coding has not been changed.
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 multichannel 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.
For a more detailed look at SIS and SIL,
check out our videos called
“What is a Safety Instrumented System?”
and “What is an Emergency Shutdown System”.
You can find the links to these videos in the description.
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