Approaches to FMEAs
Product and process FMEAs can be further categorised by the level on which the failure modes are to be considered.
• Functional FMEAs–Focus on the functions that a product, process or service is to perform rather than on the characteristics of the specific implementation. When developing a functional FMEA, a functional block diagram is used to identify the top-level failure modes for each functional block on the diagram. For example, two potential failure modes for a heater would be: “Heater fails to heat” and “Heater always heats.” Because FMEAs are best begun during the conceptual design phase, long before specific hardware information is available, the functional approach is generally the most practical and feasible approach by which to begin a FMEA, especially for large, complex products or processes that are more easily understood by function rather than by the details of their operation. When systems are very complex, the analysis for functional FMEAs generally begins at the highest system level and uses a top-down approach.
• Interface FMEAs–Focus on the interconnections between system elements so that the failures between them can be determined and recorded and compliance to requirements can be verified. When developing interface FMEAs, failure modes are usually developed for each interface type (electrical cabling, wires, fibre optic lines, mechanical linkages, hydraulic lines, pneumatics lines, signals, software, etc.). Beginning an interface FMEA as soon as the system interconnections are defined ensures that proper protocols are used and that all interconnections are compliant with design requirements.
• Detailed FMEAs–Focus on the characteristics of specific implementations to ensure that designs comply with requirements for failures that can cause loss of end-item function, single-point failures, and fault detection and isolation. Once individual items of a system (piece-parts, software routines or process steps) are uniquely identified in the later design and development stages, FMEAs can assess the failure causes and effects of failure modes on the lowest level system items. Detailed FMEAs for hardware, commonly referred to as piece-part FMEAs, are the most common FMEA applications. They generally begin at the lowest piece-part level and use a bottom-up approach to check design verification, compliance and validation.
Variations in design complexity and data availability (along with time and money) will dictate the analysis approach to be used. Some cases may require that part of the analysis be performed at the functional level and other portions at the interface and detailed levels. In other cases, initial requirements may be for a functional FMEA that is to later progress to an interface FMEA, and then finally progress to a detailed FMEA. Thus, FMEAs completed for more complex systems often include worksheets that employ all three approaches to FMEA development.