Page 75 - Electric Drives and Electromechanical Systems

68 Electric Drives and Electromechanical Systems

the operator. Several techniques can be used to identify the unknown hazards,
including hazard and operability studies (HAZOP), hazards analysis (HAZAN), fault
tree analysis (FTA), and failure modes and effects analysis (FMEA).
4. Determine the characteristics of hazards. This stage attempts to determine the
frequency, relative severity, and characteristics of each hazard. By doing so, the
designer can focus initially upon those hazards that can result in the most damage
and/or those that have the greatest risk associated with them.
5. Elimination and reduction of the hazard. Following identification of the hazard,
they can be ranked in order of severity and occurrence; the designer can concen-
trate on their elimination.

HAZAN seeks to identify the most effective way in which to reduce the threat of
hazards within a design by estimating the frequency and severity of each threat and
developing an appropriate response to these threats. Although there are some similar-
ities between HAZAN and HAZOP (e.g., both focus upon hazards, and both try to
anticipate the consequences of such hazards), nevertheless there are clear distinctions
between the two methods. In particular, HAZOP is qualitative in nature, in contrast to
HAZAN, which is quantitative. The stages of HAZAN in the form of three brief questions:

How frequently will the hazard occur?
How large is the possible consequences of the hazard?
What action is to be taken to eliminate or reduce the hazard?
HAZAN is based upon probabilistic analysis in estimating the frequency with which
some threat to safety may occur, together with the severity of its consequences. Through
such analysis, engineers can focus their initial efforts toward reducing those hazards
with the highest probabilities of occurrence and/or the most severe consequences.
Failure modes and effects analysis When using failure modes and effects analysis
(FMEA) to troubleshoot a design, one begins by focusing upon each basic component one
at a time and tries to determine every way in which that component might fail. All com-
ponents of a design should be included in the analysis, including such elements as warning
labels, operation manuals, and packaging. One then tracks the possible consequences of
such failures and develops appropriate corrective actions. As part of a FMEA exercise an
analysis of all the system’s components are produced. A format can be used through which
all components or parts can be listed, together with the following information:

Failure models, identifying all ways in which the part can fail to perform its inten-
ded function should be identified.
Failure causes, identifies the underlying reasons leading to a particular failure.
Identifying how that a particular failure mode has occurred.
Details of the protective measures that have been incorporated to prevent any
failure.
A weighted value of the severity, occurrence and detection of the event.