System safety concept techniques

Toward the end of the Second World War, systems techniques such as fault tree analysis were introduced in order to predict the reliability and performance of military airplanes and missiles. The use of such techniques led to the formalization of the concept of probabilistic risk assessment (PRA). The publication of the Reactor Safety Study (NRC, 1975)—often referred to as the Rasmussen Report after the name of principal author, or by its subtitle WASH 1400—demonstrated the use of such techniques in the fledgling nuclear power business. Although WASH 1400 has since been supplanted by more advanced analysis techniques, the report was groundbreaking in its approach to system safety. 

These concepts, which are described in detail by Ericson in Hazard Analysis Techniques for System Safety , help prevent the analyst from jumping ahead and missing the required detail to methodically develop the tree. 

ABSTRACT The draft document of the NATO allied ordnance publication (AOP) 52 gives guidance on software safety design and assessment of ammunition-related computing systems. The content of the draft is reviewed and compared with the lEC 61508 standard for functional safety of electrical/electronic/programmable electronic (E/E/PE) systems. We discuss the overall development model, the safety-lifecycle model and proposed techniques and measures. We also investigate whether the functional safety concept of lEC 61508 is incorporated in the document. 

As a matter of fact, a toolbox offers a suitable analogy. Ideally, the system safety manager or engineer has a well-stocked toolbox of analysis types and techniques and is able to study the particular task at hand and select the appropriate tool or combination or tools to perform the task effectively and efficiently. This example is the correct application of the tailoring concept. 

The basic concept from which event and causal factors charts were developed can probably be traced back to Ludwig Benner and others at the National Transportation Safety Board. Benner developed a very similar technique called multilinear event sequencing (MES) and more recently sequentially timed events plotting (STEP). Event and causal factors charts were part of the overall MORT approach to system safety developed by W. G. Johnson for the Atomic Energy Commission in the early 1970s and further developed and taught by the Department of Energy s System Safety Development Center (SSDC). The use of the event and causal factors chart is sometimes referred to as causal factors analysis. 

Experienced system safety professionals are encouraged to keep an open mind—some will initially view parts of the book as heresy —and be patient. A large portion of the book will be old hat to many of you, but several new concepts, techniques, and approaches are presented. Current practitioners may benefit most from Part 3. 

In addition to updated content of the first edition, the revised second edition of the Basic Guide to System Safety has a more expanded and useful glossary of terms it also contains a new chapter describing the basic concept, utility, and function of the hazard and operability study (HAZOP) and what-if analysis. Both of these analytical techniques have been used quite routinely and successfully in the petrochemical industry for decades. As with all analytical methods and techniques presented in this text, it is suggested that the HAZOP smdy and what-if analysis have definite application to general industry operations as well. 

Part II of this Basic Guide to System Safety presents and briefly discusses some of the more common system safety analytical tools used in the performance of the system safety function. Through example analyses of hypothetical mechanical and/or electrical systems, the reader should become familiar with each type of system safety analysis method or technique discussed. However, it must be understood that it is not within the limited scope of this volume to provide a detailed explanation of each of these methods and/or techniques. The intention is to merely introduce the reader to the various tools associated with the system safety process. The value of each concept in the analysis of hazard risk will vary according to the individual requirements of a given organization or company. 

A closer look at some of the wider safety analysis techniques indicates that they do have some relationship with exposure and value of assets. The concept of risk analysis matrices [MoD 2004] explicitly includes impact analysis and frequency of exposure, and uses these to determine the criticality of the risk on a hazard by hazard basis. It does not, nor does it claim to, consider the role of on-going system reaction and response. In comparison, die Accident Tetrahedron explicitly includes these factors. 

The training covers the facility safety system (management concepts and philosophies, policy, goals and objectives, operations, tools and techniques, and system measurement). 

There is a reality in Browning s observations System safety literature at the time he wrote his book was loaded with governmental jargon, and it easily repelled the uninitiated. It made more of the highly complex hazard analysis and risk assessment techniques requiring extensive knowledge of mathematics and probability theory than it did of concepts and purposes. 

System Safety Engineering and Management, by Harold E. Roland and Brian Moriarty (1990) is a good but more involved book. It provides an extensive review of the concepts of system safety and their methods of application. An overview of a system safety program is given. The descriptions of several analytical techniques are valuable. For the application of some of them, quite a bit of knowledge about mathematics is necessary. 

The remainder of this book will go deeply into the two concepts of system safety and risk assessment. Proven tools and techniques are discussed, and actual engineering examples are shown. It will address safety from the operator and manufacturer s point of view, as well as from government regulations and oversight. But most importantly, it will help you understand how to manage the safety of your systems. 

The concept of seven HA types was intentionally developed by early system safety practitioners and has been proven successful for over 45 years for both military and commercial applications. The HA types were defined and refined in MlL-STD-882. One confusing area of system safety is that there are also seven HA techniques with the same name as the types however, their purpose is to satisfy each type. The following seven HA types provide the analysis baseline for the system safety discipline ... 

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