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Although question is a little bit unclear however, possible answers are:
1) PLC AS PLANT FAIL SAFE SYSTEM: At many process plants in case of abnormality in any parameter PLCs are used to shift that part or complete plant to FAIL SAFE condition. Fail safe logic is also normally referred to as Interlock and its diagrams are called T-diagrams. This system is also known as Emergency Shutdown System (ESD).
2) PLC REDUNDANT OR FAIL SAFE CPU: Control system availability in hour of need is primary requirement for 24/7 running plants. Therefore, many customers prefer PLCs in redundant configuration. In this scenario we have two CPUs in same chassis or redundant chassis also. with usually redundant (02) power supplies. One CPU acts as primary and other acts as secondary or backup CPU. Secondary performs calculations on input data and is continuously synchronizing itself with primary. However, secondary cannot send outputs to output channels. In case of fault in primary CPU control shifts to secondary promptly. This is bumpless transfer.
In TMR , 03 CPUs are used and all three CPUs share their output tables with each other and any error in calculations is removed this way.
A PLC fail-safe in engineering is a design feature that in the event of a specific type of failure, inherently responds in a way that will cause no or minimal harm to other equipment, the environment or to people.
A system's being "fail-safe" means not that failure is impossible or improbable, but rather that the system's design prevents or mitigates unsafe consequences of the system's failure. That is, if and when a "fail-safe" system "fails", it is "safe" or at least no less safe than when it was operating correctly.
Example:
Isolation valves and control valves - if those valves are used in systems containing hazardous substances, then it can be designed to close upon loss of PLC power, for example by spring force. This is known as fail-closed upon loss of power.
Fail system means if there is any problem in your safety loop it will inform you prior.
For example if your emergency switches are not working properly, then may be at the time of emergency it will not work.
So fail safe PLC continuously monitor the safety devices loops if there is any problem in this it will shutdown your process safely or it'll give you alarm for safety failure.
As suggested by the name fail-safe CPU, understanding the fact that if in any abnormality occur while carrying out a process using a PLC, the system should be safe enough for minimal damage or none if possible.
Honestly, this is the first time I heard term of "PLC fail-safe CPU". How come there is fail safe CPU ?.
Comparing to "fail-safe" term at field instrument, if there is loss of manipulated energy for example electric power then the device (valve for example) will move (or stay) at its fail safe position.. that is its initial position without manipulated energy.
PLC of course needs power for "thinking" at least it needs battery backup.
Here, we use redundant power supply (one from normal power and another from UPS) to ensure it runs all the time. Is this the fail-safe CPU you talk about ?
