Preliminary hazard analysis design phase

The major hazard analysis and control effort is concentrated in the design phase (or phases). The first and in many ways the most important hazard analysis is the preliminary hazard analysis (PHA). 

Preliminary hazard analysis (PHA) As described in NHB 1700.1(V1-A) and this document. The PHA is to identify safety-critical areas, to identify and evaluate hazards, and to identify the safety design and operation requirements needed in the program concept phase (NSTS 22254). 

In the analysis of system safety, the initial process begins with the development of the preliminary hazard list during the project or system concept phase. Although it is not always compiled in aU cases, an available PHL can become the working foundation for the development of the preliminary hazard analysis during the design phase of the project life cycle. 

The SSHA evaluates hazardous conditions, on the subsystem level, which may affect the safe operation of the entire system. In the performance of the SSHA, it is prudent to examine previous analyses that may have been performed such as the preliminary hazard analysis (PHA) and the failure mode and effect analysis (FMEA). Ideally, the SSHA is conducted during the design phase and/or the production phase, as shown in Chapter 3, Figure 3.4. However, as discussed in the example above, an SSHA can also be done during the operation phase, as required, to assist in the identification of hazardous conditions and the analysis of specific subsystems and/or components. In the event of an actual accident or incident investigation, the completed SSHA can be used to assist in the development of a fault tree analysis by providing data on possible contributing fault factors located at the subsystem or component 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 purposefiil 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, a 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). 

This example will develop a hardware FMEA for a proposed system that is well into the design phase of the product life cycle. For informational purposes, it is assumed that a preliminary hazard analysis was previously performed during the early stages of the design phase of this system. The information from the PHA will be used to assist in the development of the hardware FMEA. It should also be noted that the nature of a FMEA requires evaluation of subsystems, subassemblies, and/or components. For this reason, more detailed and specific descriptive information is provided here than that supplied for previous examples discussed in this text. 

Preliminary Hazard Analysis (PHA) is a widely used method during the concept design phase. This is an unstructured approach that is used when there is a lack of definitive information such as functional flow diagrams and drawings. The mefhod has proved to be an effechve tool to take early measures to identify and eliminate possible hazards when all the necessary data are unavailable. Its findings are also considered useful to serve as a guide in potential detailed analysis. 

Very little real analysis is completed during the concept phase because analysis detail and data are generally not available. A preliminary risk assessment code (RAC) is determined, however, as part of the preliminary hazard list. This initial RAC is used to aid in determining the initial scope of the system safety effort and in the early evaluation of alternative designs and approaches. 

Since we have begun this analysis during the concept phase of the project life cycle, it would be prudent to develop a preliminary hazard list of basic safety concerns associated with the project concept. Then, as the project moves into the design phase, a PHA can be performed on the basis of the information contained on the PHL. 

The preliminary hazard list and the customer requirements form the basis for the Functional Hazard Assessment (FHA). While considering the required functionality and the respective system enviromnent the FHA shall answer the question How safe does the system need to be The analysis performed during the FHA provides the safety objectives and a first set of system safety requirements which are necessary in order to fulfil the safety goals and to prevent the identified hazards from occurring. The system requirements are amended by the system safety requirements. Doing this in the early project phase assures that the safety requirements are appropriately considered in the system design. 

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