Back to Basics: Understanding Reliability Block Diagrams (RBD)

John Q. Todd, Sr. Business Consultant/Product Researcher, Total Resource Management, Inc. 

Posted 4/25/2024

Bring Time-tested Reliability Tools to Bear on Maintenance Problems

Reliability Block Diagrams have been around for a very long time, helping system engineers understand how the various elements, and their relationships, could impact the overall reliability and operation of the system. Taking the effort to diagram a system logically can provide great insight into where the weak links are. In some cases, an RBD can expose that your assumed redundancies are not actually as effective as you think they are. 

A system, when viewed logically vs. physically, can look quite different.

Let’s get to the fun! Get out your pencil and make a diagram of one of your critical systems. Simply draw blocks in paths that represent each element or module of the system. The system below is an electrically powered pump with a valve controlling the output. Of course, many other elements of the system could be added: the control system, the piping and wiring between blocks, etc. Keep it simple for now.

reliability block diagram example

Now assign a reliability number to each block. What is your guesstimate probability of each block being available for use at any point in time? Be honest… 95%? 80%? 50/50?

Try these values:

  • Commercial Power or Generator = 99%
  • Transfer Switch = 99%
  • Motor = 50%
  • Pump = 90% (given an assumed production rate)
  • Valve = 98%

Now take all these numbers and multiply them together:

P = .99 x .99 x .50 x .90 x .98 = .43

Whoa! You mean to tell me that my system is only 43% reliable at any point in time? It has a whopping 57% probability of being failed? This is frightening! Obviously that pesky motor in the middle of the diagram is killing our system reliability.

It’s ok… its just an overly simplified view of a system to help prove a point. Or is it?

Advanced Topic

Yes, we introduced a parallel path in this diagram. Our system is powered by commercial power or a backup generator at any point in time. This redundancy can (not always) greatly increase the reliability of the system since one or the other is supplying power, based upon their probability. Since they are both very high, 99%, then: P = (.99 +.99)/2 = .99 for the two as a “system.” You can see how a reliability block diagram can quickly get complicated!

What is a Reliability Block Diagram?

A reliability block diagram serves two purposes: one is to logically diagram a system to give you a good visual. The second is to provide a calculation approach, much like we did above. Blocks in series are, “and,” while blocks in parallel are, “or.”

From a diagramming perspective, you can use any software tool that makes flowcharts. (Pencils are fine too!) The diagram begins and ends with nodes that are used to indicate the bookends of the system. In an actual RBD calculation or analysis software tool set, these nodes serve a mathematical purpose as well, but for our case we can just call them start/stop nodes.

The blocks in the diagram are interconnected in either parallel or serial fashions. It is an interesting exercise to try and draw a system you are very familiar with. Making the decision as to whether the elements of the system are in parallel or serial may seem simple, but not always. Remember, you are diagramming a system… not necessarily a flow or path of production.

Parallelism indicates redundancy or multiple paths to get something done. Serial structures indicate that if any block fails, the entire system fails. Most systems are just serial strings of equipment… if any one element fails, the entire thing fails. Weakest link and all that.

Think of your car. Put aside all the non-essential pieces of it… lights, satellite radio, windows, spinning rims, etc. and diagram it out. Engine, battery, transmission, wheels (yes, you need all four!) and so on. Placing all these elements in a serial structure you can see if any fail, the car, as a system, fails. There is little to no redundancy in the typical car.

Fun with numbers

Now that you have a diagram to work with, the hard work begins to find some data that you can use to come up with “real numbers,” for each block. Your maintenance organization may have vast amounts of system and equipment availability data… or not. Either way, finding sources of this kind of data can be a challenge. What you do find may be very inconsistent and full of holes. At this point its ok. We are just looking for approximations.

Let’s use that motor as an example. With our guesstimate of 50% probability that it will be operational… maybe it is working, maybe it is not… we should find data to support our assertion. You may be in a situation where, “everyone knows,” that Motor #2 is problematic, and the team just pays attention to it each Monday morning to keep it limping along.

If that is the case, then you should be able to find maintenance records that support the situation Motor #2 is in. Let’s say that relative to other motors you have, Motor #2 seems to need corrective action once a month. Your mean time between failure (MTBF) is 30 days. (Add up the days between each failure and divide by the number of failure events.)

Now you are a Reliability Engineering professional! Remember, we are only looking for relative and qualitative values at this point. No need to dig too deep into the data nor perform true statistical analysis yet. We are just looking for obvious weak links on our equipment chains. Is it acceptable for the motor to fail each month.

Now what?

It has been said that if the only tool you have is a hammer, then everything looks like a nail! Most likely you are mentally diagramming all sorts of systems in your mind right now. 

Even better, reliability block diagrams can be used for more than just systems of equipment. They can visualize steps or functions in a process… anything that can be represented as sets of series or parallel relationships. Assign a probability of each being functional and you can quickly get a handle of the overall system effectiveness. Then you can go from the qualitative or anecdotal estimates to find real data that either confirm your gut-feelings or not. Either way, you have a much better view of what is, or can happen, to your systems of equipment.

Back to our simple diagram. Now you can see that the expense and benefit of the backup generator to increase the system reliability is being wiped out by that pesky motor. Figure out why that motor is having such a bad time and your system will serve you better. Spend your time and money on those elements that are proving themselves to be less than, “good.” You will never achieve 100% system reliability, but now you know where to go look for opportunities.

Article by John Q. Todd, Sr. Business Consultant at TRM originally published on Reach out to us at [email protected] if you have any questions or would like to discuss deploying MAS 8 or Maximo AAM for condition based maintenance / monitoring.

If you are interested in more information, let us help, email [email protected]. If you are interested in reliability consulting, contact us at we can help you with your reliability needs.


John Q. Todd

John Q. Todd has nearly 30 years of business and technical experience in the Project Management, Process development/improvement, Quality/ISO/CMMI Management, Technical Training, Reliability Engineering, Maintenance, Application development, Risk Management, & Enterprise Asset Management fields. His experience includes work as a Reliability Engineer & RCM implementer for NASA/JPL Deep Space Network, as well as numerous customer projects and consulting activities as a reliability and spares analysis expert. He is a Sr. Business Consultant and Product Researcher with Total Resource Management, an an IBM Gold Business Partner – focused on the market-leading EAM solution, Maximo, specializes in improving asset and operational performance by delivering strategic consulting services with world class functional and technical expertise.

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