Human failure

Human beings are prone to fail due to limitations in perception of risk and their ability to interpret and act on information. There is a variety of ways in which human failure may manifest itself in the workplace. On some occasions people may omit owning up to something they should have done, or taking an inappropriate course of action. These failures occur very often for complex reasons that involve individual perceptions, motivation, job design and organisational factors. 

There are two broad types of human failure - errors , where the failure to follow safety rules are made unconsciously, and violations , where there is a deliberate failure to comply. 

Human errors are occasioned by lapses of attention or mistakes 

Organisations can only reduce the incidence of human error by addressing the entire relevant job, organisational and individual factors. This will require effective systems for consultation, training, supervision and active monitoring. 

In contrast to human errors that are made unconsciously there are many occasions when humans will deliberately breach safety rules. On some occasions the breach will be of a rule that is not physically possible to follow or may just be inconvenient, for instance the removal of machinery guards to speed production and achieve enhanced productivity payments. On other occasions the deliberate breaching of safety rules will be as a result of personal antipathy for the work or the organisation. 

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Early failures may occur almost immediately, and the failure rate is determined by manufacturing faults or poor repairs. Random failures are due to mechanical or human failure, while wear failure occurs mainly due to mechanical faults as the equipment becomes old. One of the techniques used by maintenance engineers is to record the mean time to failure (MTF) of equipment items to find out in which period a piece of equipment is likely to fail. This provides some of the information required to determine an appropriate maintenance strategy tor each equipment item. 

The result of the ETA is a Hst of combiaations of equipment and human failures that ate sufficient to result ia the accident (71). These combiaations of failures are known as minimal cut sets. Each minimal cut set is the smallest set of equipment and human failures that are sufficient to cause the accident if all the failures ia that minimal set exist simultaneously. Thus a minimal cut set is logically equivalent to the undesired accident stated ia terms of equipment failures and human errors. 

Equipment Failures Safety system Ignition Sources Furnaces, Flares, Incinerators, Vehicles, Electrical switches. Static electricity, Hot surfaces. Cigarettes Human Failures Omission, Commission, Fault diagnosis. Decisions Domino Effects Other containment failures. Other material release External Conditions Meteorology, Visibility... 

The accident sequence frequencies are quantified by linking the system fault tree models together as indicated by the event trees for the accident sequence and quantified with plant-specific data to estimate initiator frequencies and component/human failure rates. The SETS code solves the fault trees for their minimal cutsets the TEMAC code quantitatively evaluates ihe cm sols and provides best estimates of component/event probabilities and frequencies. 

This model of accident causation is described further in Figure 1.3. This represents the defenses against accidents as a series of shutters (engineered safety systems, safety procedures, emergency training, etc.) When the gaps in these shutters come into coincidence then the results of earlier hardware or human failures will not be recovered and the consequences will occur. Inap-... 

The analysis of accidents and disasters in real systems makes it clear that it is not sufficient to consider error and its effects purely from the perspective of individual human failures. Major accidents are almost always the result of multiple errors or combinations of single errors with preexisting vulnerable conditions (Wagenaar et al., 1990). Another perspective from which to define errors is in terms of when in the system life cycle they occur. In the following discussion of the definitions of human error, the initial focus will be from the engineering and the accident analysis perspective. More detailed consideration of the definitions of error will be deferred to later sections in this chapter where the various error models will be described in detail (see Sections 5 and 6). 

The categorization set out in Figure 2.6 is a broad classification of the causes of human failures that can be related to the SRK concepts discussed in the last section. The issue of violations will be addressed later in Section 2.7.1.1. The distinction between slips and mistakes was first made by Norman (1981). 

The use of a model of human error allows a systematic approach to be adopted to the prediction of human failures in CPI operations. Although there are difficulties associated with predicting the precise forms of mistakes, as opposed to slips, the cognitive approach provides a framework which can be used as part of a comprehensive qualitative assessment of failure modes. This can be used during design to eliminate potential error inducing conditions. It also has applications in the context of CPQRA methods, where a comprehensive qualitative analysis is an essential precursor of quantification. The links between these approaches and CPQRA will be discussed in Chapter 5. 

The traditional approach to CPQRA only considers human failures to perform required functions (usually errors of omission). However, many critical errors arise from misdiagnoses (mistakes) leading to erroneous, inappropri-... 

Task Analysis and Error Analysis of the Blowdown Operation Task analysis was carried out in order to organize all the performance data about the way that workers process information, the nature of the emergency and the way that decisions are made. Figure 7.20 shows a tabular task analysis of the workers response to a significant unignited gas leak in MSM. The analysis was a combination of a tabular HTA and a CADET analysis (see Chapter 4). Human error analysis identified the major human failure modes which could affect time to blowdown (see Table 7.2). 

A very important contribution in this socio-technical era is made by Reason (Reason, 1990). He made a distinction between active failures, and latent conditions. The active failures are in general failures made by those at the sharp end of the accident causation (e.g. technical and human failures). Effects are felt almost immediately. Latent conditions are removed in time and space from the sharp end of the accident causation (e.g. organizational and technical failures) creating conditions for active failures to be made. A strict boundary between both concepts cannot be made and in reality can be seen as a sort of sliding transition. Here, the two concepts are separated... 

Numerous technical and human failures allowed gas to release from a salt dome storage facility. Lack of fail safe devices contributed to the explosion of the resulting vapor cloud. 

EVENT, further development into more basic faults or human failures. 

Salt-bath Explosions. The third type of disaster in which after-the-fact protection is much less important than prevention is the molten salt-bath explosion. There have been serious disasters involving such baths, because personnel involved on both the management and the operating level failed to appreciate the potential hazards of the situation. Due to mechanical failure or human failure, or a combination of both, molten salt baths have been allowed to explode. The hazards of molten salt baths may be summarized as follows ... 

Most safety, loss prevention, and security (S/LP/S) incidents result from or are intensified by some kind of human failure. Training reduces the risk of injury to people and the loss of property and technology. 

Williamson JA, Webb RK, Sellen A, Runciman WB, Van der Walt JH. Human failure analysis of 2000 incident reports. Anesth Intens Care 1993 21 678-683. 

Reason J. 1990. The contribution of latent human failures to the breakdown of complex systems. Philos Trans R Soc Lond B Biol Sci h27. 

The analyst must then define the various equipment failures and human failures that could lead up to that event. For the release of toxic gas from a reactor safety relief valve, the analyst may consider loss of cooling and the operator ignoring the high pressure alarm. Another path to this leak of toxics may be a double charge of one of the energetic reactants. 

Technical myopia bias towards hardware- rather than human failures. 

The analyst must then define the various equipment failures and human failures that could lead up to that event. For the release of toxic gas from a reactor safety relief valve. 

Organizations need to establish a policy to follow when a medication error occurs. Although pharmacists strive for 100% accuracy, human failures do not allow that level of accuracy. Mistakes will happen, and need to be dealt with appropriately and professionally. Table 16.11 outlines suggestions for dealing with a medication error. ... 

Motion sensitivity too high Power surge Incorrect code No code entered Premature notification k Motion sensor hindered Power failure Not armed (human failure) Not armed (device faiiurej Notification failure ... 

Center for Chemical Process Safety, Guidelines for Process Safety in Batch Reaction Systems, American Institute of Chemical Engineers, New York, 1999 Center for Chemical Process Safety, Process Safety Leading and Lagging Metrics, American Institute of Chemical Engineers, New York, 2007 Reason, J., The contribution of latent human failures to the breakdown of complex systems. Philosophical Transactions of the Royal Society (London), series B. 327 475-84(1990)... 

We have the power and the means to expose more heroes and to address their human failures candidly, without dissecting every last flaw. No one is perfect, no matter how much we might wish them to be. Perhaps this is the most important lesson taught by the men and women who are true heroes. They show us how to be people of courage despite the odds, and to be people of grace, kindness, and nobility despite our humanity. 

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