Types of FMEAs

There are basically two types of Failure Mode and Effect Analyses. They are distinguished more by the target of the analysis than the actual analysis itself. In fact, the steps required in the performance of each are very similar, only the items being analyzed differ. Perhaps the fundamental difference between the two is in their approach. The first type, often referred to as the functional FMEA, utilizes the deductive reasoning approach (i.e., it begins by assuming a failure and focuses on the modes which 

Basic Guide to System Scfety, Third Edition. Jeffrey W. VincoU. 

The functional FMEA targets any subsystems that may exist within an entire system. The functional FMEA will evaluate each subsystem and attempt to identify the effect of any failures in these subsystems. The analyst not only looks for the possible effects of subsystem failures on the system as a whole, but also examines the effect of such failures on other subsystems within the system. Although functional FMEAs are not as common as the hardware FMEA, their basic utility should not be dismissed. When a complex system (such as a nuclear reactor, an airliner, an overhead bridge crane, or a robotic milling machine) consists of numerous secondary subsystems, each with their own set of supporting subsystems, the functional FMEA should be performed to ensure proper system safety evaluation at every level. 

The second and more common hardware FMEA examines actual system assemblies, subassemblies, individual components, and other related system hardware. This analysis should also be performed at the earliest possible phase in the product or system life cycle. Just as subsystems can fail with potentially disastrous effects, so can the individual hardware and components that make up those subsystems. As with the functional FMEA, the hardware FMEA evaluates the reliability of the system design. It attempts to identify single-point failures, as well as all other potential failures, within a system that could possibly result in failure of that system. Because the FMEA can accurately identify critical failure items within a system, it can also be useful in the development of the preliminary hazard analysis and the operating and support hazard analysis (Stephenson 1991). It should be noted that FMEA use in the development of the O SHA might be somewhat limited, depending on the system, because the FMEA does not typically consider the ergonomic element. Other possible disadvantages of the FMEA include its purposeful omission of multiple failure analysis within a system, as well as its failure to evaluate any operational interface. Also, in order to properly quantify the results, an FMEA requires consideration and evaluation of any known component failure rates and/or other similar data. These data often prove difficult to locate, obtain, and verify (Stephenson 1991). 

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A Level 3 FMECA is conducted to consider how the system failure may affect the platform (Level 4) and its operation application (Level 5). This type of FMEA is often referred to as a functional FMEA and may be very similar to an FHA (see Chapter 2). 

In this connection one may note that ISO 9001 2000 para 8.5.3 requires a documented procedure for preventive action. It also demands that preventive actions must be in proportion to the effect of the failure. The FMEA/FMECA approach satisfies ISO 9001 para 8.5.3, so for ISO 9001 2000 FMEA/FMECA may be applied, but it is not a specific ISO 9001 requirement, since the basic purpose of FMEA or FMECA is the same and the procedures are similar. However, one should note that there are some differences between them, which are listed in Table IV/2.0.3-l. There are different types of FMEA/FMECA, discussed next. 

There are three types of FMEA/FMECA functional FMEA/FMECA, DFMEA/ DFMECA, and PFMEA/PFMECA. Apart from these there are two other types of FMEA service FMEA and SWFMEA. In service FMEA the focus is on service issues. SWFMEA (discussed separately in later clauses) focuses on software issues. Functional FMEA/FMECA is also known as concept FMEA/FMECA or system FMEA/ FMECA. Some literature shows two types of FMEA/FMECA in the sense that one is functional and other is hardware FMEA/FMECA (where both PFMEA/PFMECA and DFMEA/DFMECA are considered under hardware FMEA/FMECA). These different types are a result of changes in analysis pattern and assessment, but the basic concepts/approaches are the same. 

Functional (/system/concept) FMEA/FMECA This actually determines the correct failure step. It is done at the conceptual stage when design is not frozen to fix hardware. Naturally, at this stage there will be functional analysis. The following points may be noted about this type of FMEA/EMECA ... 

Types of FMEA/FMECA. (A) Relation amongst FMEA/EMECA types, (B) design FMEA/ FMECA concept, (C) process EMEA/FMECA concept, and (D) uses of EMEA/EMECA. 

Introductory Information The analyst should provide basic information in this section of the report which describes the purpose and scope of the FMEA along with any limitations imposed on the analysis as a result (i.e., items not specifically within the scope of the analysis). The scope will also identify the type of FMEA (i.t., functional or hardware). Also included in the introduction section is an explanation of the methodology used to perform the analysis such as, but not limited to drawing reviews, examination of previous analyses (if applicable), evaluation of lessons learned, use of Preliminary Hazard List and/or Preliminary Hazard Analysis, and so on. Finally, any preestablished ground rules that may have been agreed upon should be provided here. Such ground rules typically limit or further narrow the scope of the FMEA, or just a portion of it, and should therefore be explained in the introductory pages of the report. 

Failure Modes and Effects Analysis (FMEA) - A systematic, tabular method of evaluating the causes and effects of known types of component failures, expressed in an annual estimation. 

A typical design FMEA is given in Fig. 13 for the alcohol example discussed earlier. The headings across the top of the FMEA define the types of information it requires. The DFSS team must evaluate the risk associated with failure of the CTQs. We discuss the functional requirement FMEA, but the same approach... 

There are various types of analyses that are used for a process hazard analysis (PHA) of the equipment design and test procedures, including the effects of human error. Qualitative methods include checklists, What-If, and Hazard and Operability (HAZOP) studies. Quantitative methods include Event Trees, Fault Trees, and Failure Modes and Effect Analysis (FMEA). All of these methods require rigorous documentation and implementation to ensure that all potential safety problems are identified and the associated recommendations are addressed. The review should also consider what personal protective equipment (PPE) is needed to protect workers from injuries. 

A What-If analysis can be organized in one of two ways. The first is to divide the facility into nodes, rather like a HAZOP, except that the nodes are typically bigger and more loosely defined. The second approach is to organize the analysis by major items of equipment rather like an FMEA, and then to discuss the different types of failure mode for each. These two approaches are discussed below. Guidance to do with utilities, batch processes, operating procedures, and equipment layout is also provided. 

An FMEA is a bottom-up approach to hazards analysis. When linked with a top-down method (such as FTA) a powerful synergy can ensue. The top-down method will highlight those areas which pose the greatest risk the FMEA can then be used to investigate those areas in greater detail. Like other types of hazards analysis, an FMEA should be carried out by a team. In most cases, however, only two or three team members—who are specialists in the required fields— are involved. 

Creating failure effect categories for each different type of highest system level effect, (i.e. each unique failure effect has a separate grouping of single failure modes). Because the FMEA considers single failure modes only, the various failure modes are assume to be independent. 

During the earhest conceptual stages of design, the FMEA attempts to preempt this type of problem by identifying single failure conditions and ensuring that these failures (if they do occur) do not escalate. 

Complex system A system is complex when its operation, failure modes or failure effects are difficult to comprehend without the aid of analytical methods or structured assessment methods. FMEA and ETA are examples of such structured assessment methods. Increased system complexity is often caused by such items as sophisticated components and multiple interrelationships. For example, for these types of systems, a portion of the comphance may be shown by the use of DALs such as by processes in RTCA/ DO-178B or RTCA/DO-254 or equivalent. See the definitions for conventional and simple for more information. 

FMEA is a complementary type of evaluation that utilizes a system failure-based form of analysis. Unlike PHA, the first objective of FMEA is to subdivide the facility Into several different (and, to the maximum extent possible, independent) system elements. Failure modes of each system element are then postulated and a structured examination of the consequences of each failure mode... 

Two distinctly different, yet complementary, perspectives of hazards for the HCF and associated radioactive material storage locations are obtained for the overall hazard analysis of Chapter 3 by using both PHA and failure mode effects analysis (FMEA) techniques. FMEA is a complementary type of evaluation that utilizes a system failure-based form of analysis. Unlike PHA, the first objective of FMEA is to subdivide the facility Into several different (and, to the maximum extent possible, independent) system elements. Failure modes of each system element are then postulated and a structured examination of the consequences of each failure mode follows. However, similar to PHA, FMEA documents preventive and mitigative features (failure mechanisms and compensation) and anticipated accident consequences (failure effects). Appendix 3D contains the FMEA for the HCF. 

The failure analysis (FMEA) is the privileged useful for this type of search and the results have to be made known to the office for studies that address the maintenance of the relevant amendment is made. 

For what types of analyses are FMEAs well suited ... 

A type of safety identification review that methodically analyzes the interactions between individuals and machines. It reviews the operation phase to operational phase, while considering the consequences of operator-system faults at each operating step within each phase. This analysis allows for the recognition of threats from equipment faults that may coexist with operator errors. It is considered similar to a Failure Mode and Effects Analysis (FMEA), but with increased emphasis on the steps in human procedures rather than viewing hardware exclusively. See also Failure Mode and Effects Anafysis (FMEA) Job Safety Analysis (JSA). 

Software System Hazard Analysis This type of analysis is conducted similar to a hardware system hazard analysis (SHA), analyzing software functional processing steps to determine whether they may have any particular hazardous effect on the system. The analysis utilizes a hazard-risk index to illustrate the severity of each potential failure. The main advantage to this method is in its ability to positively identify safety-critical hardware and software functions as well as consider the effect of the human element in system software operations. The results of the software SHA, which identifies single-point failures or errors within a system, can often be used to assist in the development of a software fault tree analysis or, to some degree, a system FMEA. However, as with the other various SWHA techniques briefly described above, this method is also time-consuming and costly to perform. 

Aiming to identify the product/service features that are critical to different types of failure, FMEA provides a means of recognising symptoms before a... 

Type of process Generally, an initial screening is possible from the type of process involved. As stated earlier, FMEA is more suitable for computer systems and HAZOP is more suitable for process systems. Naturally based on the same PHA method, selection may be possible and needs to be judged with respect to other characteristics, for example, size and complexity. 

In an FMEA, recommended actions are the tasks prescribed by the FMEA team that can be used to reduce or eliminate the risk associated with a potential cause of failure. Recommended actions take into account the existing controls, the relative importance (prioritization) of the issue, and the cost and effectiveness of the corrective action. There can be multiple recommended actions for each cause. The main aim of recommended actions is to reduce the value of RPN. The type of recommended action will greatly depend on the specific project. 

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