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Regardless of the process, downtime always means a loss of productivity for a process application. Unplanned downtimes, whether due to component failure or human error, are the most costly ones.

Consequently, fault tolerant systems were developed to reduce the occurrence of one of the root causes of unplanned downtime – component failure. When it comes to discrete and process control, controller and power supply redundancy are the first keystones that come to mind to achieve a highly available architecture, along with other components such as networking and server redundancy. Originally costly and complex to engineer, controller and power supply redundancy are increasingly becoming a commodity, with more and more PLC vendors now offering these features natively embedded in their products. Selecting the most appropriate vendor for any given application can be quite tricky as offers look remarkably similar on the surface.

One common and easy-to-use selection criteria is the overall system’s availability rate. Typically expressing this criteria as a percentage (i.e. “99.9967%”) makes it easier to compare the different solutions as engineers, especially, tend to rationalize everything down to equations and numbers in order to make the “right” choice. However, the catch is not to forget what lies behind this number, namely other factors which are key to truly maintaining the availability of a process system.

One such factor that should be taken into account is the “MTBF” (Mean Time Between Failures) of each component. A shortcut often utilized by system integrators, or in certain project specifications, to avoid complex availability calculations is to compare the MTBF of CPUs from different vendors in order to see which would be the best option. This is a mistake. First of all, application availability rates depend on every single product used, so a CPU MTBF of close to one million hours – as some vendors advertise – is useless if its power supply only has an MTBF of 200,000 hours.

Secondly, MTBF and availability are linked, though not directly. Consider this scenario: A candle has an infinite MTBF (when lit, and without any external factors, it is impossible for it to fail by itself). However, the availability of light from the candle may o...