System Safety The Concept

System safety professionals have achieved notable successes, and generalists in the practice of safety can learn valuable lessons from them. In MORT Safety Assurance Systems, William G. Johnson made the following comments, with which I agree, about accompKshments that could not have been achieved without applying system safety concepts. 

The system safety programs used in aerospace, nuclear, and military projects provided a well-ordered guide to some requirements for a superlative effort. Indeed, they are a route to accomplishing things which would otherwise be beyond human reach [p. 127]. 

Those accomplishments are a matter of fact, and they are immense. Anyone who has an understanding of the complexity of the hardware, the demands on the personnel, and the attendant risks must marvel at the success of a space shot. Space travel could not have been achieved without application of the principles of system safety. 

I believe that generalist safety practitioners will improve the quality of their performance by acquiring knowledge of what system safety is all about. I do not say that those generalists must become specialists in system safety, although trends indicate that they will be expected to apply at least the fundamentals of system safety hazard analysis and risk assessment. 

INFLUENCES ENCOURAGING ADOPTION OF SYSTEM SAFETY CONCEPTS 

In Chapter 5, Transitions Affecting the Practice of Safety, comments were made on the revisions taking place with respect to the priorities and significance for certain elements within an Operational Risk Management Plan. In a sense, this chapter on the system safety concept continues that discussion. Transitions in progress have led to increased prominence for  

On the Practice of Safety, Fourth Edition. Fred A. Manuele. 

To influence safety generalists to acquire and apply system safety concepts, this chapter  

RELATING THE GENERALIST PRACTICE OF SAFETY TO SYSTEM SAFETY 

In the American National Standard ANSI/ASSE Z590.2—2003. Criteria for Establishing the Scope and Functions of the Professional Safety Position, a part of section 3 reads as follows  

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System Safety The Concept This chapter outlines the system safety idea in terms that relate well to the definition of the practice of safety given in Chapter 2. Lessons can be learned from the successes attained by system safety practitioners. System safety is hazards and design based. So is the entirety of the practice of safety. As opportunities arise for generalist safety professionals to participate in the design processes, the need for system safety skills will be apparent. 

Other possible preliminary safety analysis methods are concept safety review (CSR), critical examination of system safety (CE), concept hazard analysis (CHA), preliminary consequence analysis (PCA) and preliminary hazard analysis (PHA) (Wells et al., 1993). These methods are meant to be carried out from the time of the concept safety review until such time as reasonably firm process flow diagrams or early P I diagrams are available. 

Throughout the design of a chemical plant, issues relating to safety, economics and environmental impact must be considered. By doing so, the risks associated with the plant can be minimised before actual construction. The same principle applies whatever the scale of the process. The field of process control (Chapter 8) considers all these issues and is, indeed, informed by the type of hazard analyses described in Chapter 10. The objectives of an effective control system are the safe and economic operation of a process plant within the constraints of environmental regulations, stakeholder requirements and what is physically possible. Processes require control in the first place because they are dynamic systems, so the concepts covered in the earlier chapters of this book are central to process control (i.e. control models are based on mass, energy and momentum balances derived with respect to time). Chapter 8 focuses on the key aspects of control systems. 

Mil STD-882B (11) outlines the general concepts of Systems Safety. Notice 1 (12) appended to that document specifically covers requirements for software systems safety. The Air Force also has a Software Systems Safety Handbook (13). A number of recent articles in Hazafd Prevention (14), the Journal of the Systems Safety Society, expanded on the technical content of this handbook. The concepts were covered in a more explicit and useful manner by Erwin Schoitsch of the Austrian Research Center Seibersdorf in a recent forty-two page, two-part, article (15). 

As to new safety system elements, we don t have any. But we do try different things. You know that we use a lot of chemicals and we are reexamining our permissible exposure levels because we are determined to do better than the world standards. You ll be interested in this one new approach we re taking—we are addressing the gap between theory and reality, trying to do a better job of incorporating elements in our environmental, health, and safety systems within the concept of operational excellence. [Editorial note Wow ]... 

As every loss event results from the interactions of elements in a system, it follows that all safety is systems safety . The safety community instinctively welcomed the systems concept when it appeared during the stagnating performance of the mid-1960s, as evidenced by the ensuing freshet of symposia and literature. For a time, it was thought that this seeutingly novel approach could reestablish the continuing improvement that the public had become accustomed to however, this anticipation has not been fulfilled. 

A notion that is analogous to state-observability and, at the same time, is more directly related to the safety performance of a system is the concept of diag-nosability. In particular, the concept of diagnosability describes the intrinsic property of a system to reveal to an outside observer the concealed past behaviors of the system that may affect the future safety performance of the system. A number of techniques that implement failure diagnosis have appeared in the literature within the last three decades. For a discussion on failure diagnosis / detection techniques for dynamical systems the reader is referred to Willsky (1976) and Frank (1990) and the references therein. 

We define intrinsic safety as safety designed and built into a system. Yes, this is an overlap with system safety. The two concepts are converging. Intrinsic safety is certainly a noble goal and one that should be continually pursued. 

A sixth problem confronting the system safety effort is the lack of qualified system safety engineers and managers, even for system safety efforts in place at the beginning of the 1990s. If the system safety effort is to expand to meet the challenges of the next century, many more personnel will be required. Additionally, they will all need to know system safety objectives, concepts, and methods in order to participate in SSWGis and to interface with the overall effort. 

This chapter presents the fundamental principles of probabiUty theory and briefly examines the use of statistical analysis in the practice of system safety. The information discussed here should provide the reader with a very basic understanding of these concepts, which, by some accounts, is essential to the overall understanding of the system safety discipline. It should be noted that it is not within the scope of this Basic Guide to System Safety to provide aU there is to know regarding probability theory and statistical analysis. However, a certain level of understanding is essential and will therefore be discussed here. 

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. 

Life Cycle A phased concept to explain the various stages of product or system progression consisting of the concept phase, design phase, production phase, operational phase, and disposal phase. In system safety, the product or system life cycle is often used to indicate the timing of certain types of analytical evaluations. 

In September 1963, the USAF released MIL-S-38130. This specification broadened the scope of the system safety effort to include aeronautical, missile, space, and electronic systems. This increase of applicable systems and the concept s growth to a formal Mil-Spec were important elements in the growth of system safety during this phase of evolution. Additionally, MIL-S-38130 refined the definitions of hazard analysis. These refinements included system safety analyses system integration safety analyses, system failure mode analyses, and operational safety analyses. These analyses resulted in the same classification of hazards, but the procuring activity was given specific direction to address catastrophic and critical hazards. 

The basic approach is to direct the system to the safest operating level relative to people or the environment when any emergency condition is detected, including power loss. An important concept of process control safety is to have adequate redundancy to reduce unwanted shutdowns and maintain an adequate level of certainty that a safe state will result if a real emergency does occur. As far as possible, instruments should be of the fail-safe type. 

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