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Figure 23.6 Data collection sheet for Failure Hazard Analysis (FHA).

Figure 23.6 Data collection sheet for Failure Hazard Analysis (FHA).

added to each of the existing branches, resulting in a total of four branches. Additional components are added in a like manner until the entire system has been included. The paths from the trunk to the tip of each branch are then evaluated to determine the state of the entire system for every combination of failed components. Complex systems can be in a state of total success, total failure, or some variant of partial success or failure.

Example. Figure 23.7 is a simplified one-line diagram of an electric distribution system for a facility with critical loads. The main switch gear at the facility is a double-ended substation fed from two different commercial power sources. All critical loads are isolated on a single buss which can receive power from either commercial source or an on-site emergency generator. The facility engineer conducted a Failure Mode Effect and Criticality Analysis. His findings are listed in Figure 23.8. Three single point failures were identified that result in a forced outage of the critical loads if any one of these components fails: automatic transfer switch, main breaker at the critical load switch gear, and the buss in the critical load switch gear. Action items were listed for all components to improve system reliability. Most of the breakers require on-site spares for maximum reliability, however, a number of these can be shared to minimize the expense. For example, the main breakers at switch gear "A" and "B" can share a spare since they are of the same size. The most important action items are those for the single point failures.

FMECA identifies only single point failures and can significantly underestimate the risk of a forced outage since improbable events like double failures do occur. A Double Failure Matrix (DFM) was constructed in Figure 23.9 to identify the combinations of two failed components that would deprive the critical loads of electricity. Rules were defined to gauge the criticality of each double failure. Loss of any one of the three sources of electricity is a level 2 failure with marginal consequences. Loss of any two of the three sources is a level 3 failure with critical consequences and loss of all three sources is a level 4 failure with catastrophic consequences since all critical

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