The Time to Isolate metrics, which appear in the Isolation Cost and Time Statistics section of the Fault Isolation Report, describe the time associated with detecting and isolating a primary failure using the diagnostic sequence in the current diagnostic study. In calculating these metrics, eXpress converts all test times to minutes. eXpress provides six Time to Isolate metrics:
Minimum
The MinimumTime to Isolate is the smallest cumulative test time involved with detecting and isolating a failure to one of the fault groups in the current diagnostic sequence. If the analyst clicks on this field when viewing the Fault Isolation Report in the eXpress report viewer, it will bring up a details report describing the fault group for which this Test limit was derived.
Maximum
The MaximumTime to Isolate is the largest cumulative test time involved with detecting and isolating a failure to one of the fault groups in the current diagnostic sequence. If the analyst clicks on this field when viewing the Fault Isolation Report in the eXpress report viewer, it will bring up a details report describing the fault group for which this Test limit was derived.
Average
The AverageTime to Isolate is calculated by summing the test time for all tests used to isolate each isolated fault group and dividing the sum by the total number of isolated fault groups:
The ExpectedTime to Isolate estimates the Mean Time to Isolate (MTTI) that would be achieved if the current diagnostic sequence were to be employed with a fielded system. The metric is calculated by multiplying each fault group's Time to Isolate by the fault group's aggregate failure probability and then summing the results for all isolated fault groups:
The Time to IsolateDifferential, which is calculated by subtracting the Average from the Expected Test limit, represents how far the expected isolation time will deviate from the non-probability-weighted average. This can be a good indication of how well items or functions that fail frequently are being segregated from those that take a long time to isolate (a high positive differential Test limit indicates that they are well segregated, whereas a negative Test limit may indicate that they are poorly segregated). This metric, which was developed by DSI in the early 1990s, provides the analyst with information that may help identify the source of excessive isolation time. This Test limit can be calculated as follows:
where
ExpTTI
=
the Expected Time to Isolate (defined above)
AvgTTI
=
the Average Time to Isolate (defined above)
Also included is the Time to Isolate Differential Percentage, which is calculated by dividing the Time to Isolate Differential by the Average Time to Isolate:
Note: Some of the metrics described above are standard Maintainability metrics that have been simplified so that they can provide effective feedback during earlier phases of the design process. This simplification allows the analyst to generate early estimates of the impact that changes to a diagnostic design would have upon system Maintainability.
