Action errors

Operations The QRA team will need specific data on how the system is actually operated. For example, are the bypass valves normally left open to increase throughput, what happens when the high level alarm sounds, or do operators bypass interlocks to continue production Human actions/errors are usually dominant contributors to the real-world risks, and truthful data on actual process operations are vital to credible QRA results. Expect to commit one full-time equivalent for the life of the project. 

Figure 4.4 gives an example of an OAET for events that might follow release of gas from a furnace. In this example a gas leak is the initiating event and an explosion is the final hazard. Each task in the sequence is represented by a node in the tree structure. The possible outcomes of the task are depicted as "success" or "failure" paths leading out of the node. This method of task representation does not consider how alternative actions (errors of commission) could give rise to other critical situations. To overcome such problems, separate OAETs must be constructed to model each particular error of commission. 

The whole range of error types that could occur at each task step are described in Figure 4.16. The terms action errors and checking errors are self-explanatory. Retrieval errors refer to the retrieval of infonnation either from an external source (e.g., a chart recorder or a procedure) or from memory. Transmission/ communication errors refer to communications among individuals either directly or via written communications. Selection/choice errors refer to making incorrect choices among alternative operations, for example, manual instead of automatic. 

The solution is considered valid by the program if both the RMS mass balance error and the RMS mass action error are less than 3 x 10 , and the Valid Solution message... 

An indication of the size of the errors is given by the variable IOPT, which is set by ARITH and printed by ERRORS. Instead of printing out the errors, one can simply test the value of IOPT. If the RMS mass balance and mass action errors are both less than 3 x 10 , then IOPT is set to 1 and the solution is called valid. Otherwise IOPT is set to zero and the solution is called invalid. 

Errors. Two types of errors are computed by the program the mass balance errors and the mass action errors. 

T0 reduce the quantity of sodium leakage, the drain system was found to require two improvements. One is the addition of a new drain line pipe at the inlet of the secondary pump. The other is the replacement of all drain lines by pipes of larger diameter. In the existing system, each drain line has two drain valves in series to prevent accidental draining by single valve action error. In future so as to assure the draining, each drain line will have double drain valves in parallel. After these improvements, the drain time will be shortened from approximately 50 minutes to 20 minutes. It is estimated that it takes 40 minutes from the occurrence of sodium leakage to finish the drain for the secondary circuit. The modified drain system is shown in Fig.9. 

The methodology should be computerized to minimize the possibility that important aspects (e.g, dependencies) of the identified human actions/errors are lost. This computerization scheme should ensure that all information necessary for documentation purposes is captured and that documentation of any or all human actions could be performed by simply selecting the appropriate human action and producing a report. 

The wrong action error mode features an inadvertent contact with peripheral components/devices while working at a local place. It also includes an unintended contact between a work apparatus or tool and peripheral components/devices of the workplace. Analyses of the work process, paths, specific actions, and tools/apparatus to be used with peripheral components/devices are required to predict the possibility of an occurrence of a wrong action . 

Human activity Hazards caused by insufficient control or incorrect warning display, life support, probable action error, hazardous location Injury or even death caused by abrasion, cuts, bruises, bums, falls, etc. Sensory impairment or loss. Equipment/control system damage caused by maloperation, handling, or nonuse of protection... 

Alarm Saturation. Errors in analysis can result when an operator is asked to cope with too many alarms. Operators often must scan a series of alarms as they occur, deduce what is happening within the process, draw conclusions, and take the appropriate action. Errors of analysis occur when, after properly observing a valid set of data, an operator draws a wrong conclusion or responds inappropriately. These errors can be avoided by assigning priorities to alarms in order to guide the operator. 

Looked But Failed To See Situation Awareness Failure Action Errors... 

On the basis of Nagel s (1988) information processing stages, 71 per cent of the 284 accidents studied were considered to involve human error, whereby 22 per cent were eoded as information errors, 35 per cent as decision errors, and 43 per cent as action errors (O Hare et al., 1994). However, a different picture emerged when the mishaps were divided according to accident severity. Of the 34 accidents in which there was a fatality or serious injury, 62.5 per cent were attributable to decision errors, while only 25 per eent involved the action stage (O Hare et al., 1994). Of the 169 cases involving minor/non-injury occurrences, only 30.5 per cent were the result of decision errors, while 45.6 per eent were attributed to action errors. These results closely mirror those reported by Jensen and Benel (1977) in their examination of NTSB reeords between 1970 and 1974 in whieh the majority of non-fatal incidents appeared to involve perceptual-motor factors, whereas most fatal accidents were associated with decision-related factors (Jensen and Benel, 1977). 

At the most basic level of error classification, a distinction between errors of omission and errors of conunission is proposed. Errors of omission are those instances where an actor fails to act at all, such as failing or forgetting to perform a particular action. Errors of commission are those instances where an actor performs an action... 

Action errors (e.g. pressing a button or pulling a switch) Action too late/early action too fast/ slow action in wrong direction action too little/much wrong action on right object, action omitted, action incomplete... 

Totally 67 errors were identified in all permit to work system tasks including 28 action error, 23 checking error, 8 retrieval error, 5 selection error and 3 information communication error. 

In many cases the consequences of human errors will be similar to the consequences of failures of components. Human errors may range from faulty or incomplete maintenance operations, to incorrect setting of control equipment limits or wrong or omitted operator actions (errors of commission and errors of omission). 

Action error Overview of Tasks Breakdown of Probability and... 

Action error analysis Fault tree analysis Event tree analysis Comparlsan analysis CRIOP... 

Action error analysis analyses interactions between machine and humans. It is used to study the consequences of potential human errors in task execution related to directing automated functions. 

The cybernetic model of system control is based on the assumption that one can achieve at best only some approximation of what he aims at. Therefore, salvation lies in rapid detection of error and adjustment to correct for that error. this cybernetic model... makes it natural to think of the second-order consequences of one s actions. Error is not a rare phenomenon that occurs because of bad planning and inept control. It is the natural and inevitable outcome of all purposive action. ... 

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