Page 95 - Safety Risk Management for Medical Devices

74 Safety Risk Management for Medical Devices

more details). Using the appropriate logic gates connect the higher level fault
to the lower level events.
6. Repeat Steps (4) and (5) for every fault in the FT until the terminus of the
tree is reached. The terminus is where all the events are basic events, or unde-
veloped events, or normal events.
7. Identify the components that are susceptible to CCFs, and then properly
model the CCF contribution.
8. Look for dependencies of faults.
9. Determine the minimal cut sets. Identify any minimal cut sets that depend on
a singular basic event.
10. If quantitative data is available for the basic events, compute the probability of
occurrence of the Hazardous Situations. Also perform an importance and sen-
sitivity analysis.
Information from the FT can be used in the Preliminary Hazard Analysis (PHA;

Section 12.3). For example, probability of Hazardous Situations would inform the P 1
values; and sequence of events can be created from the causal chains in the FTs.
Passive versus Active components. Passive components’ contribution to the system is
more or less static. Examples are: wires, tubes, and welds. Active components provide
a dynamic contribution to the system. Examples of active components are valves and
switches. Historically, from a reliability perspective, passive components are two to
three orders of magnitude more reliable than active components.

Tip Because passive components are far less likely to fail than active components, you may
want to exclude passive components from the fault tree analysis, as their contribution to System
risk will be small.

It is important to develop a FT to a sufficient depth to gain meaningful knowledge
of failure mechanisms and functional/failure dependencies. Developing FTs beyond
that is a waste of effort, and potentially distracting. Another drawback to overly deep
FTs is that generally quantitative failure rate data doesn’t exist for low-level
components.
A common heuristic is to model the system to the depth necessary to identify
functional dependencies, and to a level for which failure rate data exists for the
components.

12.1.5 Ground rules
The ground rules listed in this section are intended to facilitate the creation of FTs
while minimizing confusion and wasted effort.