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Updated on 11/7/2019
Relyence User Guide
Performing RBD Calculations
Direct link to topic in this publication:

Performing RBD Calculations

Once your RBD is complete, you can calculate the reliability and availability metrics by clicking Calculate from the Sidebar. The Calculate RBD dialog is shown to allow you to select the options you prefer.


Depending on the complexity and layout of your RBD, and the selection of metrics to compute, the time to perform the RBD calculations will vary. The Relyence RBD calculation engine determines the optimal approach for analyzing your diagram.

The following describes the available RBD calculation options. Note that Relyence remembers your selections from run to run.

1. End time and Number of display steps: RBD calculations all begin at time zero, and will be computed out to the time your select as the End time. The interval between time point results is determined based on the Number of display steps selected. In the example above, the time point results will show 10 results plus time point 0: time 0,  100, 200, 300, 400, 500, 600, 700, 800, 900, 1000. Times are in hours.

2. Reliability: The probability that the system has not failed when no repairs are made.

3. Failure rate: The rate that the system fails, given that it is currently operating, when no repairs are made.

4. Availability: The probability that the system is operating when repairs are taken into account.

5. Mean availability: The average availability the system has experienced.

6. Hazard rate: The rate that the system fails, given that it is currently operating, when repairs are taken into account.

7. Total downtime: The cumulative total of the time the system has been in a failed state.

8. Unreliability: The probability that the system has failed when no repairs are made.

9. Equivalent failure rate: The failure rate a constant failure rate system would have to achieve to meet the same Reliability value of the system under analysis.

10. Unavailability: The probability that the system is not operating when repairs are taken into account.

11. Mean unavailability: The average unavailability the system has experienced.

12. Failure frequency: The expected frequency that failures will occur in a repairable system, whether it is operating or not.

13. Steady-state results:

  • MTTF: Mean Time To Failure, assuming no repairs are made.
  • MTTR: Mean Time To Repair for a repairable system.
  • MTBF: Mean Time Between Failures: The average time between successive system failures, including the time to fail and the time to repair.
  • Availability: The availability the system will stabilize at if it operates sufficiently long enough.

14. Expected Number of Failures: The expected number of failures within the time interval from 0 to End time. The number of failures is calculated based on the failure frequency.

15. Path sets: Determines the path sets, or successful paths from the Start Node through the End Node.

16. Number of iterations: The number of iterations for the simulator to run. Increasing the number of simulations increases the accuracy of the results, up to a point, but also increases computational time.

17. Number of failures to reach steady-state: Sets the number of failures components can experience before considered to have reached steady-state. By default, this is set to 1. 

18. Set random number seed: Select this checkbox and enter the random number seed if you wish to specify the random number seed for the simulator engine. Otherwise, Relyence will obtain a random number seed.

19. Always use simulation: If possible, Relyence will use an analytical (non-simulated) calculation method to obtain exact results. If you prefer to always use the simulator engine for your RBD calculations, select this checkbox.

20. Do not update subdiagram results: When clear, the results of calculated subdiagrams will be saved and can be viewed on the subdiagram. When selected, the newly calculated results of subdiagrams will not be saved, and any old results on the subdiagrams will still be available to view.  Using this option allows you to calculate the subdiagrams independently and keep those independently calculated results unchanged when you later calculate the parent diagram. If a subdiagram is used in multiple places, only one set of results are saved. If you are not using subdiagrams in more than one place, it is recommended to clear this option so that your subdiagram results are saved.

21. Calculate Reliability Prediction before calculating RBD: When enabled, your Reliability Prediction Analysis will be calculated before initiating the RBD calculations. Enabling this setting ensures that the failure information in your RBD that is linked to your Reliability Prediction will be up to date. Disabling this setting will perform the RBD calculations using the last saved reliability prediction calculation data. For example, if your Reliability Prediction analysis is complete and will not be modified, you can disable this option because the reliability prediction values will not change.

Once you have set the calculation options to your preferences, click Calculate to start the RBD calculations.

A progress bar will appear to indicate calculation progress. Once the calculation is complete, the Results dialog is displayed. 


The Results dialog shows the overall results for your RBD. You can also view the individual block results for each block in your RBD. To do this, drag the Results dialog over until you can see the "Motors" block. Click on the "Motors" block to see the Results dialog update and display the calculated results for the Motors. To view the overall RBD results, select the Start or End Block.

Click Close when you have completed viewing the calculation results. Also, Reliability of each of the blocks (at the specified end time point) will be shown on all the blocks of your RBD. The result value displayed on the RBD blocks can be set using RBD Properties and is set to Reliability by default.

Anytime you want to view the Results again, click View Results from the Sidebar to re-display the Results dialog.