Sequence errors, human factors

Human errors may be dependent on the specific accident sequence displayed in the event tree, and, for that reason, may be included in the event tree. This requires the human-factors specialist to consider the context of the error in terms of stress, operator training in response to the accident, di.tgnosiic paiierns, environmental, and other performance-shaping factors. 

Literature has many theories and concepts discussing human reliability and associated human error causal factors that always trigger incidents and accidents within safety-critical systems. The main - by definition - characteristics of such safety occurrences are their randonmess, rare predictability, sophisticated, yet vague sequence of propagation. Such characteristics can basically allow for the retrospective analysis of these occurrences and their causes at various sectors and levels within industry such that re-occurrence margins are reduced if not totally eliminated. The major drawback of such reactive treatment... 

You will note that this model includes reference to the human factor involved, through the description of error. In addition, the hazard is identified and described in terms of energy exchange. This model specifically describes the factors involved in the accident sequence and, in particular, it highlights the contribution of error, as a htrman characteristic, to the seqirence which lead to injuty. 

You will need to understand which particular events led to the event scenario. For example, it is important to identify the sequence of events or failures that could lead to a loss of in-flight aircraft control. Or, for example, the event could be that a blocked valve causes system overpressure. If humans are in the process, then some sort of human factors safety analysis will help find human error causes. 

Use an advanced control room to simplify construction, maintenance and operations. Improved human factors considerations reduce the chance of operator error during an event or accident sequence compared to the large control rooms used in currently operating plants. 

In the previous chapter, a comprehensive description was provided, from four complementary perspectives, of the process of how human errors arise during the tasks typically carried out in the chemical process industry (CPI). In other words, the primary concern was with the process of error causation. In this chapter the emphasis will be on the why of error causation. In terms of the system-induced error model presented in Chapter 1, errors can be seen as arising from the conjunction of an error inducing environment, the intrinsic error tendencies of the human and some initiating event which triggers the error sequence from this imstable situation (see Figure 1.5, Chapter 1). This error sequence may then go on to lead to an accident if no barrier or recovery process intervenes. Chapter 2 describes in detail the characteristics of the basic human error tendencies. Chapter 3 describes factors which combine with these tendencies to create the error-likely situation. These factors are called performance-influencing factors or PIFs. 

Based on the collected information, a decision/action (DA) chart was developed to provide an overview of the main decisions involved in the blowdown operation and the main influential factors such as time stress, conflicting responsibilities, risk of gas ignition etc. Task Analysis and Error Analysis of the blowdown operation were subsequently carried out to obtain a description of the sequence of tasks steps and the likely human error modes which could occur. 

Once the evidence has heen collected and a timeline or sequence diagram developed, the next phase of the investigation involves identifying the causal factors. These causal factors are the negative occurrences and actions that made a major contrihution to the incident. Causal factors involve human errors and equipment failures that led to the incident, hut can also he undesirable conditions, failed harriers (layers of protection, such as process controls or operating procedures), and energy flows. Causal factors point to the key areas that need to he examined to determine what caused that factor to exist. 

Find the facts in the main sequence on the Causal Factor Chart that describe a component failure or a human error. Ensure the fact is not describing a management system failure (i.e., ensure the fact is not a root cause, near root cause, or root cause category). The identified negative events/conditions are candidate causal factors. Any candidate causal factor that is not dependent on another candidate causal factor is a valid causal factor. 

Once the timeline or sequence diagram based upon the actual scenario has been developed, the next phase of the investigation involves identifying the causal factors. Causal factors involve human errors and equipment failures that led to the incident, but can also be undesirable conditions and... 

After the task is specified, human and machine performance models can be applied to estimate task performance. The MHP and keystroke-level performance model can provide task performance estimates in terms of task completion time. THERP can be used to estimate human error probabilities for each task and task sequence. The ERM approach can be used to estimate performance along any required dimension and to compare required with available resources along any required dimension as long as the human performance data are available. The results of the ERM assessment would identify stress levels on capacities (e.g., resources stressed too long or beyond maximum capacity). These results indicate limiting factors to successful task performance. Limiting factors can be identified at elemental or intermediate performance resource levels. As such, the ERM represents a more comprehensive and internally consistent model than the others. It is more comprehensive in that it can be used to model any performance dimension. It is... 

A practice of safety based principally on the many extensions of the causation model represented by the domino sequence developed by H. W. Heinrich that focus on the so-called unsafe act or human error as the principal causal factor will be ineffective in relation to the actuality of causal factors. 

It is fundamental for assessing human error in systems analyses to identify and describe the human acts with importance for the event sequence under analysis (qualitative assessment). This corresponds to the task analyses, which are characteristic of ergonomic studies. Firstly, the important actions, the moment in time at which they are required and the time period available for their execution have to be determined. Furthermore, the requirements for the action, the information necessary, respectively available, the possibilities of correction in case of omission or faulty execution must be estabhshed. Additionally, other factors of important influence on human reliabihty such as the state of knowledge on the process in question, ergonomically favourable or disadvantageous layout of the workplace, the tools or the environment are identihed. On the basis of this task analysis reliability data (normally failure probabilities on demand) are assigned to the tasks identified. They stem from existing data collections (cf. Table 9.21). 

A local television station drafted that concise nine-sentence report. The CSB s comprehensive investi tion provided all the essential details within their 60-page report. It is named Investi tion Report - Viityl Chloride Monomer Explosion Report No. 2004-10-1-lL March 2007 [8]. If you would rather a visual look there is an excellent, free IOVa min video available from the CSB covering this incident with animation of the sequence of events. The CSB established that Human Error was clearly one of the contributing factors to this tragedy. 

---