The Reliability Properties of Equipment and Parts in Series System Arrangement

Equipment and components in a series system arrangement have three reliability properties that arise because it is a series system.

These properties lead to several important business and operational implications when your plant and equipment is made of items arranged in series.


Slide 31 – The Three Reliability Properties of Equipment and Parts in a Series System Arrangement


A set of equipment in series system arrangement would have a Reliability Block Diagram like the one in the slide above. One item feeds the next, and that one feeds another, and so on until the entire process is completed. A series system has three interesting reliability properties.

Series System Arrangement Reliability Property 1

Property 1 is that the system reliability can never be higher than the reliability of the least reliable component.

Say, you had two items of equipment in series each at 0.9 reliability. The reliability of the system is the multiplication of each equipment’s reliability. The calculation is, 0.9 x 0.9 = 0.81, which results in a system reliability below the least reliable equipment item.

Series System Reliability Property 1 is further explained in one of the coming slides.

Series System Arrangement Reliability Property 2

The second property of equipment in series system arrangement is the system reliability falls if you include more items into the series.

If the two-item series mentioned above includes one more item of 0.9 reliability, the three-item system reliability becomes, 0.9 x 0.9 x 0.9 = 0.729. The system was 0.81 reliable but with one more item added it fell to 0.729.

As more equipment is included into a series configuration there are more things that can go wrong to stop the system.

To return the system back to the reliability of the two-item arrangement, the reliability of each equipment item must rise above the required system reliability (which is Series System Property 1). We can show the effect numerically: 0.93 x 0.93 x 0.93 = 0.81. Each of the individual elements in the system must be outstandingly reliable if you want to have a high system reliability.

An explanation of Series System Reliability Property 2 is given in one of the coming slides.

Series System Arrangement Reliability Property 3

The third property of equipment in series system arrangement is a system-wide improvement brings a greater proportionate rise in system reliability than the rise in individual equipment reliability.

Say, we had a three-equipment system with reliability 0.93 x 0.93 x 0.93 = 0.81 that was increased to 0.95 x 0.95 x 0.95 = 0.857. The individual equipment reliability rose from 0.93 to 0.95, which is a 2.2% rise. The system reliability went from 0.81 to 0.86, which is about a 6% rise. That is a three times greater system-wide effect than the effect on individual items in the system. A small universal, system-wide effort to improve a series system delivers a vastly greater value from the whole system.

Series System Reliability Property 3 is explained with an example in an upcoming slide.

Implications of the Three Reliability Properties of a Series System

These three properties of equipment in series system arrangement lead to valuable implications for businesses.

  1. Making system-wide reliability improvements brings vastly greater benefit than improving individual equipment reliability.
  2. Improve the reliability of the least reliable equipment to lift the reliability of the system.
  3. Keep spare parts for equipment in a series system immediately available so they are at hand to quickly repair breakdowns.
  4. Standardise the equipment everywhere possible.
  5. Eliminate component failure modes everywhere possible
  6. Provide connections into series systems so temporary replacements can be utilised if breakdowns occur.
  7. Get rid of all unnecessary equipment and items: simplify, simplify, simplify the system.


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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