《风险评价技术及方法》 14._Fault_Hazard_Analysis

Chapter

14

Fault Hazard Analysis

14.1

INTRODUCTION

Fault hazard analysis (FaHA)is an analysis technique for identifying those hazards arising from component failure modes.It is accomplished by examining the poten-tial failure modes of subsystems,assemblies,or components and determining which failure modes can form undesired states that could result in a mishap.

14.2BACKGROUND

The FaHA technique falls under the detailed design hazard analysis type (DD-HAT)analysis.The basic hazard analyses types are described in Chapter 3.The purpose of FaHA is to identify hazards through the analysis of potential failure modes in the hardware that comprises a subsystem.

The FaHA is applicable to analysis of all types of systems and equipment.FaHA can be implemented on a subsystem,a system,or an integrated set of systems.The FaHA can be performed at any level from the component level through the system level.It is hardware oriented and not suited for software analysis.

The FaHA is a thorough technique for evaluating potential failure modes.How-ever,it has the same limitations as the FMEA.It looks at single failures and not com-binations of failures.FaHAs generally overlook hazards that do not result entirely from failure modes,such as poor design,timing errors,and the like.

The conduct of an FaHA requires a basic understanding of hazard analysis theory,failure modes,and a detailed understanding of the system under analysis.The meth-odology is similar to failure mode and effects analysis (FMEA).Although the FaHA

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Hazard Analysis Techniques for System Safety ,by Clifton A.Ericson,II Copyright #2005John Wiley &Sons,Inc.

262FAULT HAZARD ANALYSIS

is a valuable hazard analysis technique,the subsystem hazard analysis(SSHA)has replaced the FaHA.The SSHA methodology includes considering failure modes for safety implications,and thus it accomplishes the same objective as the FaHA.

The FaHA technique is not recommended for general usage.Other safety analysis techniques are more cost effective for the identification of hazards and root causes, such as the SSHA.The FaHA should be used only when a rigorous analysis of all component failure modes is required.The FaHA technique is uncomplicated and easily mastered using the worksheets and instructions provided in this chapter. 14.3HISTORY

The Boeing Company developed the FaHA in1965for the Minuteman program as a variation of the FMEA technique.It was developed to allow the analyst to stop the analysis at a point where it becomes clear that a failure mode did not contribute to a hazard,whereas the FMEA requires complete evaluation of all failure modes. 14.4THEORY

The FaHA is a qualitative and/or quantitative analysis method.The FaHA can be used exclusively as a qualitative analysis or,if desired,expanded to a quantitative one for individual component failure modes.The FaHA requires a detailed investi-gation of the subsystems to determine which components can fail leading to a hazard and resultant effects to the subsystem and its operation.

The FaHA answers a series of questions:

.What can fail?

.How it can fail?

.How frequently will it fail?

.What are the effects of the failure?

.What hazards result as a consequence of failure?

The FaHA considers total functional and out-of-tolerance modes of failure.For example,a5percent,5000-V(+250-V)resistor can have as functional failure modes“failing open”or“failing short,”while the out-of-tolerance modes might include“too low a resistance”or“too high a resistance.”

To conduct an FaHA,it is necessary to know and understand the following sys-tem characteristics:

.Equipment mission

.Operational constraints

.Success and failure boundaries

.Realistic failure modes and their probability of occurrence