System safety concept performance measures

When objective measurement of performance capacities has been incorporated into many clinical trials, concepts and tools from human performance engineering can facilitate the selection of variables and shed some Hght on issues noted above. In either safety- or efficacy-oriented studies, study variable selection can be characterized as a two-step process (1) identification of the factors in question (Table 82.1) and (2) selection of the relevant performance capacities to be measured and associated measurement instruments. This Hnk between these two steps often represents a challenge to researchers for a number of reasons. First, duality in terminology must be overcome. Concerns about an intervention are typically initially identified with negative terms such as dizziness and not in terms of performance capacities such as postural stability. Human performance models based on systems engineering concepts [Kondraske, 1995] can be used to facilitate the translation of both formal and lay terms used to identify adverse effects to relevant performance capacities to be measured, as shown in Table 82.1. 

The concept of safety integrity levels (SELs) was introduced during the development of BS EN 61508 (BSI 2002) as a measure of the quality or depen bility of a system which has a safety iimction - a measure of the confidence with which the system can be expected to perform that fimctioa It is also used in BS lEC 61511(BSI 2003), the process sector specific application of BSEN 61508. 

The research into safety culture has led to the use of safety climate measurement tools as a Safety Performance Indicator (Guldenmund, 2000) but the results are largely intangible and may not help organizations identify where they need to focus attention in order to improve safety (Sorenson, 2002). The lack of a generally accepted definition for safety culture or safety chmate also limits their usefulness as concepts since they do not support a systemic methodology for their measurement (Zhang et al., 2002). 

Emerging concepts of system analysis, accident causation, human factors, error reduction, and measurement of safety performance strongly suggest the practicality of developing a higher order of control over hazards (than currently exists). 

The contribution of each structure, system or component to the facihty hazard can be measured through the probabilistic concept of the performance goal. The performance goal for a structure, system or component in relation to a specific external event is defined as the probability of failure (Pp) of the structure, system or component to perform its required safety function in the case of that external event. The performance goal for an external event may be lower than the performance goal for internal accidents. 

The high reliability of the barrier system of a VVER is ensured by applying the concept of defence in depth. Adequate engineering of the barriers and the measures used to protect them, coupled with feedback to maintain operation within the optimal range, lead to long-term smooth and stable NPP performance in producing electricity on demand. Indeed, the most important indicator of a successful defence in depth strategy is continuous operation with little or no need to call on built-in safety systems. 

In Chapter 5, we introduced the concept of SHE culture. It deals with shared values and beliefs of the members of an industrial organisation that determine their commitment to the organisation s SHE management systems and achievements. We will here use the term safety climate to denote such aspects of an organisation that are possible to measure by use of a questionnaire-based survey and where the results meet statistical criteria for aggregation to the organisational level (Cox and Elin, 1998). Results of such attitude surveys are used as performance indicators at the organisational level. 

This point of view is consistent with modern safety approaches, e.g, the deviation concept (Kjellen 1984) or the systems approach (Summala 1985). Human performance variables such as the reliability of monitoring, fault detection, and action are expected to vary to a normal extent. The range of variation from excellent to poor is determined by physiological, psychological, and social causes. The boundaries are established by task requirements. A performance variable is classified as a deviation when its value falls outside the boundary or norm, and together with other technical deviations may lead to an accident. This may not only be valid for individual and technical factors, but also for organizational measures (Kjellen Larsson 1981). 

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