What is Hot standby in PLC?

What is PLC?

A PLC is a digitally operated device that would utilize programmable memory to store the instructions internally, by executing certain functions such as timing, counting, arithmetic, and logic sequencing. These functions are executed to control various kinds of machines or processes. So it can be used to control the equipment in an industrial process.

Industrial PLC

What is the hot standby in PLC?

The major components of the PLC are the input interface, memory section, CPU, programming tool, and also a programming language. So the PLC would check the input with the help of Input modules and the CPU would process the logic and then the output will be given to the output module. CPU is the major part of the PLC it consists of a microprocessor, memory, and also the power supply.

In earlier days there was only a single CPU in a PLC, so if we are using this PLC to control a process, then the process would be entirely dependent on this single CPU. In case there is any failure in the CPU then it could affect the operation of the whole process. In case there is a power failure then it can also affect the CPU, CPU also has communication ports to other peripherals. So in order to achieve the continuity of the process hot standby feature is introduced to the PLC. In PLC hot standby means a backup CPU is used, so there will be two CPUs sharing the same I/O.

How can a Hot standby PLC improve the plant operation?

As we discussed above, the CPU is the major part of the PLC so we can’t afford a failure in the CPU. In case there is a CPU failure then it could affect the process and also the plant. In case the PLC is not functioning properly due to the failure of the CPU, then it could affect the field devices and also the process. Due to this maintenance costs would be increased, so by using a hot standby PLC we won’t face any of this problem. In case the primary CPU fails then the backup CPU would take over the place of the primary CPU and there won’t be any interruption in the process of the plant operation. The backup CPU would takeover so quickly that we won’t even notice the switchover. When the backup CPU takes over the primary CPU then it would carry out the logic functions and also the I/O control which was assigned to the primary CPU.

What is the difference between a redundant PLC and hot standby PLC?

A redundant PLC means there would be two PLCs to control a process, while the hot standby PLCs would have two CPU’s primary and the backup CPU. The redundant PLC would have different signal paths from input to output due to the individual PLCs, in the case of the hot standby PLC the input has two paths one is to the primary CPU and the other one is for the backup CPU.

Redundant PLC system

What is a duplex PLC system?

The duplex PLC system is created to increase the reliability of the PLC system.  In this PLC system, there would be double CPU units. So if one CPU shuts down then the other one would function and thus there won’t be any process interruption. So the two CPUs would simultaneously perform the calculations. In this system, the two CPU units would be synchronized so that a smooth transition can occur between them.

What are the major applications of hot standby PLC?

  • Chemical and petrochemical industry
  • Water and wastewater
  • Power generation
  • Oil and gas
  • Infrastructures like tunnels, buildings, etc.

What are the advantages of Hot standby PLC?

  • Process reliability can be improved
  • Less maintenance
  • Production cost can be decreased
  • We can avoid the plant idle time
  • We can reduce the equipment failure


post-graduate in Electronics & communication.

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