Page 96 - Safety Risk Management for Medical Devices

Risk Analysis Techniques 75

12.1.5.1 Write faults as faults
Choose the appropriate syntax. State what the fault is, and if conditions of the fault
are material, state under what conditions. You may need to be verbose. Write it in a
way that in the future, another person or even yourself can make sense of the fault
description. Example: catheter balloon bursts when inflated by the surgeon.

12.1.5.2 No gate-to-gate connections
Gate inputs should be properly defined. A direct gate-to-gate connection is a shortcut
which bypasses defining the lower level gate’s output. While it may be tempting to
take such shortcuts, it makes the FT more difficult to read and understand by others.

12.1.5.3 Mark low-likelihood faults as Basic Events
If it is clear that a fault is of very low likelihood, do not further develop it to lower
levels. Mark it as a Basic Event.

12.1.5.4 Don’t model passive components
Components are either passive, e.g., a wire, or a pipe. Or, active, e.g., a switch or a
valve. Historically, it’s known that passive components failure rate is 2 3 orders of
magnitude smaller than active components. While it is technically possible to model
passive component faults, it does not add much value to the risk analysis of a product.

12.1.5.5 Be judicious in modeling secondary faults
Since the purpose of the FTA is identification of credible Hazardous Situations, be
cautious about modeling secondary faults. Secondary faults occur under out-of-design
conditions. Out-of-design conditions are usually unlikely. Consider the added value
of modeling such faults.

Tip Use a skilled facilitator to guide the FTA work sessions. Good facilitation guides the brain-
storming, and proper fault tree construction, and also prevents confusion of participants on con-
cepts such as Hazard, Causes, and Basic Events.

12.2 MIND MAP ANALYSIS
12.2.1 Introduction

The use of diagrams to graphically map information is a very old technique. The
term “Mind Map” was first popularized by the British pop psychology author Tony
Buzan. A mind map is basically a tool for graphically organizing thoughts and ideas. It
is a very useful tool in brain storming, and is an accessible alternative to FTA (see
Fig. 12.4 for a simple example).