The Safety Critical Systems Handbook, 4th Edition

Appendix 4

Assessing Safe Failure Fraction and Diagnostic Coverage

In Chapter 3 Safe Failure Fraction (SFF) and random hardware failures were addressed and reference was made to FMEA.

1. Failure Mode and Effect Analysis

Figure A4.1 shows an extract from a failure mode effect analysis (FMEA) covering a single failure mode (e.g., OUTPUT FAILS LOW).

Columns (A) and (B) identify each component.

Column (C) is the total part failure rate of the component.

Column (D) gives the failure mode of the component leading to the failure mode (e.g., FAIL LOW condition).

Column (E) expresses Column (D) as a percentage (e.g., 20% for U8).

Column (F) multiplies Column (C) by (D) to produce the hazardous failure rate.

Column (G) shows the assessed probability of that failure ...