MTBF (Mean Time Between Failures)
MTBF is a basic measure of reliability for repairable items. The average time used in the MTBF is the time for which all of the parts of a defined item perform within their specified limits, during a particular measurement period, under stated conditions.
The equation for calculating MTBF is:
For system tree items that have zero failures, you can either ignore these items (default) or use of the Chi-Square distribution for zero failure calculations.
• If you ignore system tree items that have zero failures, the MTBF for these items is stored as a null value (not as zero).
• If you use of the Chi-Square distribution, you must define a confidence level between 0 and 100. The default confidence level is 75. The lower bound chi-square value is computed using two degrees of freedom. The equation for calculating MTBF for zero failures is:
For general theory associated with zero failure calculations, pages 63 through 72 in the following book are helpful: Applied Reliability, Second Edition, Paul A. Tobias and David C. Trindade.
The following table demonstrates the calculation and roll up of MTBF values when items that have zero failures are ignored.
|
System Tree Structure
|
Incidents
|
Time
|
Item MTBF
|
Rolled-UP MTBF
|
MTBF/60
|
||
|---|---|---|---|---|---|---|---|
|
System
|
0
|
1000
|
125
|
2.083333333
|
|||
|
A1
|
0
|
1000
|
125
|
2.083333333
|
|||
|
A1-S1
|
5
|
1000
|
200
|
200
|
3.333333333
|
||
|
A1-S2
|
3
|
1000
|
333.3333333
|
333.3333333
|
5.555555556
|
||
|
A2
|
0
|
1000
|
|||||
|
A2-S1
|
0
|
1000
|
|||||
This next table demonstrates the calculation and roll up of MTBF values when items having zero failures use the Chi-Square distribution. For more information, see Chi-Square Distribution.
|
System Tree Structure
|
Incidents
|
Time
|
Item MTBF
|
Rolled-UP MTBF
|
MTBF/60
|
||
|---|---|---|---|---|---|---|---|
|
System
|
0
|
1000
|
721.35
|
73.82711753
|
1.230451959
|
||
|
A1
|
0
|
1000
|
721.35
|
106.5383707
|
1.775639511
|
||
|
A1-S1
|
5
|
1000
|
200
|
200
|
3.333333333
|
||
|
A1-S2
|
3
|
1000
|
333.3333333
|
333.3333333
|
5.555555556
|
||
|
A2
|
0
|
1000
|
721.35
|
360.675
|
6.01125
|
||
|
A2-S1
|
0
|
1000
|
721.35
|
721.35
|
12.0225
|
||
Various MTBF values can also be calculated for a set of items by applying a filter to the System Tree Items table.