Safety illusion and error trap in a collectively-operated machine accident

This brief paper re-evaluated an accident that occurred where an operator’s arm was amputated during operation of a brake-clutch type mechanical press.

The authors took a more systemic approach, unpacking cognitive traps and design/organisational factors of the event that were seen to be largely absent from the original investigation.

Providing a bit of context:

·?????? They argue that, in Brazil, “accidents with machines tend to be explained in a manner that attributes blame to the victim or emphasize technical aspects of the system”

·?????? This perspective can be enhanced with a view that visualises work as part of a sociotechnical system. In this case, they draw on a perspective that incorporates a bow-tie, a description of normal work, and analysis of barriers and changes

·?????? In the event (schematic below), one of the operators activated the command to lower the hammer, while a colleague was still present in the interior of the machine

·?????? In the original investigation, “the behavior of this worker [press operator] was emphasized as the principal cause of the event”

Results

Overall, they argued that by drawing on a more nuanced perspective of the event, one issue that was present was an “illusion of safety”, which permitted activation of the machine when a worker was still found within the operational zone.

According to the accident as viewed by the company, the accident was characterised by three workers at the press, with one operator in control. That operator then commanded the closing of the pressing zone while one colleague was still in its interior.

Even though the investigation recognised the “non-functioning status of the barriers installed in the machine”, the investigation nevertheless “concluded that this condition resulted from the unsafe act of the colleague that had activated the descent of the hammer”.

From the alternate perspective, the authors then provide a description of the context surrounding the event (normal work). The press was programmed to be operated by 3 workers, two who were present in the frontal area, while the third worker was behind the press and out of sight and in a poorly lit area.

Changes in the composition of the team for operating the machine was identified as a “habitual variability in the system”, and this wasn’t recognised as an indication of a threat by management requiring additional safety precautions.

That is, the “inclusion of newcomers on the team was not viewed as differing from the other changes”.

Problems also existed with the design of the press and was also delivered in that state and placed in operation with no indications by safety or engineering professionals that this was the case.

Other failures related to barriers. The vertically installed light curtain could be traversed, leaving the operator in a “death zone”, meaning an area where the operator was in the firing line but unable to be detected by the sensors.

Moreover, the curtains and two-handed press control were connected to automatic relays and not to a safety monitoring relay, increasing the chances of an accident due to failure of functioning. The two-hand control was operating with a defect where the activation movement wasn’t obeyed; requiring the operator to repeat their action. The study says that “As the problem did not impede production and its isolated occurrence was not indicated as a threat to safety, the machine was left in operation”.

Note that the authors are clear that these findings “do not negate the fact that operator 2 commanded the closing while his colleague was still within the machine”, but the original investigation and indeed this re-evaluation “do not explain the reasons that a part of the worker’s body was present in the operation zone”.

Explaining why the operator was in that zone, the authors argue that the injured operator’s expectation that the light curtain was in place and functioning “led to [feeling] a sense of safety”.

Moreover, the press provided no indication around the state of its controls, hindering the operator’s comprehension “because the system does not offer signals that help them clearly interpret which of the two situations is indicated by the machine’s behavior” [*** not discussed in the paper, but this is called mode confusion].

They note that this error trap “is loaded and ready to strike”.

Also, the operators worked on the press “without knowing the characteristics of the functioning of the barriers installed in the machine” and especially the fact that the light curtain had been previously programmed in a way that was unknown to the operators.

Incompatibilities and masking of the underlying logics of the system versus operator knowledge of those assumptions “configures a situation of cognitive vulnerability aggravated by an interface that did not provide adequate feedback [7] and, due to failures in safety management, stimulated, ?in the workers, an illusion or false sense of protection” (emphasis added).

The paper then discusses matters of expertise and new workers, but I’ve skipped.

In all, they argue that this additional conceptual enlargement of the event provide another mode of explanation. Moreover, with this new perspective “the behaviors that fail are taken as a point of departure, as factors whose origins must be clarified in reasons that are not those intrinsic to the personality of the workers involved”.

Link in comments.

Authors: de Almeida, I. M., Nobre Jr, H., do Amaral Dias, M. D., & Vilela, R. A. G. (2012). Safety illusion and error trap in a collectively-operated machine accident. Work, 41(Supplement 1), 3202-3206.