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8.1.7 Risk Control During Design

8.1.7.1 - Failure Modes and Effects Analysis (FMEA)

•Estimates overall reliability of a detailed or existing product design in terms of probability of failure

•basically, each component is examined for failure modes, and the effects of each failure is con-

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sidered. In turn, the effects of these failures on other parts of the system is considered.

• the following is a reasonable FMEA chart.

•the basic steps to filling one out is,

1.consider all critical components in a system. These are listed in the critical items column.

2.If a component has more than one operation mode, each of these should be considered individually.

3.estimate failure probability based on sources such as those listed below. Error bounds may also be included in the FMEA figures when numbers are unsure. These figures are entered in the “Failure Probability” column.

-historical data for similar components in similar conditions

-published values

-experienced estimates

-testing

-etc.

4.The failures in a particular operation mode can take a number of forms. Therefore, each mode of failure for a system is considered and its % of total failures is broken down.

5.In this case the table shows failures divided into critical/non-critical (others are possible). The effects are considered, and in the event of critical failures the probabilities are listed and combined to get the overall system reliability.

•Suitable applications include,

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-analyze single units or failures to target reliability problems.

-identify,

-redundant and fail-safe design requirements

-single item failure modes

-inspection and maintenance requirements

-components for redesign

•This technique is very complete, but also time consuming.

•not suited to complex systems where cascaded errors may occur.

8.1.7.2 - Critical Items List (CIL)

•This list can be generated from an FMEA study

•This might look like the table below,

8.1.7.3 - Failure Modes, Effects, and Criticality Analysis (FMECA)

•This is basically FMEA with greater analysis of criticality

•this involves additional steps including,

-determining the means of control

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- the results of the FMEA are reconsidered with the control factors

8.1.7.4 - Hazard Causal Analysis (HCA)

•A process where hazards are considered for their causes and their effects. The results of this analysis is used for control of hazards.

•The causes and effects can be extensive, and must be determined by a person/team with a good knowledge of a system.

•the analysis may focus on whole systems, or subsystems.

•it can be helpful to trace causes and effects both forwards and backwards in a system.

•Sensitivity analysis can be used to determine the more significant causes/effects.

•Some categories of this analysis are,

System Hazard Analysis - the entire system is considered at once, including interactions of components, interfaces to operators, modes of operations, etc. This is meant for global system failures creating hazards.

SubSystem Hazard Analysis - individual subsystems are examined individually. The effect of a failure of one subsystem on the entire system is considered. This evaluates individual system failures creating hazards.

Operational Hazard Analysis - an analysis of the detailed procedures of operation, and how a deviation from these procedures could lead to a hazard. Variations in the procedure could be unexpected events, operator errors, etc.

8.1.7.5 - Interface Analysis

•relationships between modules can be categorized as,

-physical

-functional

-or flow

•typical problems that arise are,

-a unit or connection fails, resulting in a loss of data across the interface

-a partial failure of a unit or connection results in a reduced flow across the interface

-there is an intermittent or unstable flow across the interface

-there is an excessive flow across the interface

-unexpected flow could result in unexpected operation, or functional failure

-undesired effect - the interface is operating as specified, but additional undesired effects