Explanation of Series System Reliability Property 2 for Plant and Equipment Reliability Improvement

Series System Reliability Property 2: As new items are added into a series the total system reliability falls—to maintain the system reliability other item’s reliability must rise.

Simplify, Simplify, Simplify your series systems to keep them simple and short.


Slide 33 – The Implications of Series System Reliability Property 2 When You Want Maximum Plant and Equipment Reliability


An unfortunate aspect of Series System Reliability Property 2 is shown in the slide above—when you add an item into a series system the system reliability falls. In the slide, a third item of equal reliability is added into a series of two items. The numbers indicate what happens to the series reliability—it drops from 0.81 down to 0.73.

Having long series systems automatically leads to having more problems with equipment, and with the added work tasks that need to be performed because of the extra equipment. As a series system grows there are more components that can fail. The only way to keep system reliability the same when adding items to a series system is for each new item to be perfectly reliable, i.e. R = 1. Currently in the 21st Century, it is impossible for equipment to forever be perfect and reliable. Also, with added equipment in a longer series system operators and maintainers will work on them. When people do work on the extra items more opportunities are added where human error causes things to go wrong.

The Business Effects of Series System Reliability Property 2

The message from the discussion above about Series System Reliability Property 2 is clear: if you add more items to a series arrangement the series system reliability will drop.

To return the system reliability back to what it was before the addition of new items the reliability of other items in the series must also be improved.

There are options as to how to restore the three items series in the slide back to the original system reliability of 0.81. The reliability of each item, new and old, can all be raised to 0.932. Alternately, the reliability of selected items in the series can be raised so that in combination the effect is to deliver 0.81 system reliability.


This slide is a companion to the new Industrial and Manufacturing Wellness book. The book has extensive information, all the necessary templates, and useful examples of how to design and build your own Plant Wellness Way enterprise asset life cycle management system-of-reliability. Get the book from its publisher, Industrial Press, and Amazon Books.

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