PLC Redundancy- How Much is Enough?

Is PLC Processor Redundancy Worth the Engineering Cost and Maintenance Cost? What Device Should I Use to Measure Flow of Liquid Sodium in Hazardous Environments?

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Ed Marszal

A: Yes. In my view and experience, especially in continuous process. In batch process, where one can shut down and change cards, it may not be required.

We go for redundancy in CPU, power and communication applications.

H. S. Gambhir

A: One needs to understand the difference between redundancy and contingency. The application of double- or triple-redundancy applies to the space shuttle. Once airborne, no Apollo 13 issues can take place. No room for error. On a very tricky process that has a huge lag time and/or a large start-up or shutdown time associated with it, then redundancy is a valid approach. This, of course, depends on the cost associated with the downtime (scheduled or not), and the switch-over is immediate and automatic.

Contingency, on the other hand, allows the process or machine to recover quickly, but with manual intervention.

Processor redundancy is easy. Most vendors do it for you. Power supplies can be redundant as well. Most systems don't design for failure, since the consequences aren't that important. The system shuts down, and it gets fixed. But if you can't do that, then the result of full system redundancy is priceless.

Be aware that most failures come from external devices like valves, drives and sensors. PLC hardware has proven to be very robust. You can go too far!

Jeremy Pollard, CET

A: Brian, that depends on the use to which the PLC is being put. If you can tolerate shutdowns, planned or not, caused by PLC failure, then you don't need a redundant PLC. If you're working in a critical control area or a safety instrumented system, you probably want all the redundancy you can get. Batch processing in the food, pharma and biopharma industries are examples of critical control, where having a fail-over redundant system might save hundreds of thousands of dollars or more.

Walt Boyes

Q: I have a very peculiar case wherein I have to measure the flow of liquid sodium in an environment that has a very high level of radiation and temperatures between 500 °C to 700 °C. I know the usual instruments will never work in such a harsh environment, but if I have to satisfy my customer, what options do I have? Is there any manufacturer who can supply such a measuring device? It has to be a non-contact type, but I am worried about the temperature and radiation part.

Wihang Bendre

A: I once used Foxboro Target flowmeter with remote mounted electronics on molten salt in a melamine plant. Operating temperature was about 450 °F. I would expect they may have a high-temperature version.


A: The only one that can be used at this high temperature is Flexim's non-contact ( Many have had success with them. They are only up to 400 °C, but the vendor may be willing to help it go higher.

Gerald Liu

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