What is PLC redundancy?

What is PLC redundancy?

In this article, we will be discussing, What is PLC redundancy?, Do we really need PLC redundancy? What are the different types of PLC redundancy?

In engineering terms, we can say that redundancy means providing an alternative control system in the fault condition. This will provide reliability to the process control system as well as reduce downtime during a failure.

The definition of redundancy is different for every industry. Generally, redundancy is required for the process control system due to the following reasons,

  • To keep the equipment as well as people working around safe
  • To reduce downtime during a failure condition
  • To keep running the critical process without intervention
  • To prevent damage to critical machine or equipment

PLC Redundancy


Types of PLC redundancy

The amount of redundancy depends on many factors and can be classified into Cold, Warm, and Hot Redundancy.

Cold Redundancy

Cold redundancy is best suited where the process is not critical, as well as downtime and human operator intervention is acceptable.

As an example, such as in a steel plant pipe finishing line, if a pipe coating machine fails then it will set off an alarm to the operator about the problem. The operator acknowledges the alarm and requests the repair of the failed unit as well as starts the other unit and resumes the operation.

In this example, the PLC failure is not a big deal, because the plant has several units in parallel. This cold redundancy consists of an identical spare PLC or parts so that they can be easily programmed or replaced in fault conditions.


For that processes which are more critical, a warm and hot redundancy will be a better option.

Warm Redundancy

Warm redundancy design mode is best suited where time and response to the failure is important but still, a momentary outage is acceptable.

As an example, if a RAL in a bag filter system fails to operate, the particular chamber can be disabled and maintenance work will be carried out. Depending on the process there must be an acceptable time in which the product starts to deteriorate or damage. The maintenance work should be finished and the process must be restored within this time.

In the same way, warm redundancy systems usually operate in shadow mode. Two identical PLCs run in the same software and share the same IOs. One of the two PLC works as a primary and the other one works as a secondary. When the heartbeat signal from the primary processor is not received by the secondary, it will take off the control of the automation system and provide uninterrupted operation.

These require an arbitration circuit while handling the sensor and actuators to avoid conflict. There may be a chance of glitch and bump because changeover may take a few program scans to complete.


Hot Redundancy

The architecture of the warm and hot redundancy systems are similar, but unlike a warm redundancy system, a hot redundancy system provides an instant correction when a failure is detected. This makes it the best solution for the critical process.

For a hot redundancy system, the plc programming software and hardware must be exact to allow to transfer of messages and to access common data in both the processor. This will allow the secondary processor to take control smoothly and without glitches during failure.

Hot redundancy system is applicable in critical processes such as high speed running equipment like turbines, power plants, the critical power systems in hospitals, blast furnaces, etc. In such applications, if a primary processor fails, secondary will take off control immediately, so that there may be no damage to the critical equipment.

PLC Redundancy Types

Do we really need PLC Redundancy?

PLC reliability is the most important factor while choosing a redundancy. Redundancy brings multiple factors such as it requires multiple processors, racks, power supplies, etc. as well as adding multiple layers of complexity while troubleshooting and routine maintenance.

The factors to keep in mind while choosing a PLC redundancy:

1) PLC hardware failure is not very common. An identical spare of the modules with a pre-programmed on-the-shelf spare is the better choice.

2) Some brands of PLCs do not support full redundancy or it requires lots of programming to make a redundancy work. The ideal solution would be a bumpless transfer of control without any programming.

3) PLC redundancy requires more than a second PLC. Additional power supply, plc rack, communication card must be required for the second PLC.

4) A redundant PLC doesn’t protect against PLC code failures. Both the processor will run with the same program, so if one goes faulty the other one would be at fault too.

5) PLC redundancy adds multiple layers of complexity. So your maintenance staff must be trained and must know about plc programming to troubleshoot while a failure.

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