Electrical Hazard -- Power Press

KP had worked for many years at a plant that manufactured prefabricated structural components for buildings. One assembly line made use of a power press that fastened the wood sections together. Power for the press was delivered by way of several electrical conduits that ran underneath the assembly table. The electrical conduits were solid bars surrounded with insulation except for a groove where a brush made contact with the conductor.

Periodically, a brush assembly would come detached from the conduit and the press would come to a halt. When this happened, the main disconnect switch, which was located in a nearby shed, was turned off ; and the switch on the power press was turned off.

On the day of the accident, the above procedure was followed. The power light on the power press was off when KP's supervisor told him to go underneath the table and check the brushes. A few moments later, KP indicated that, indeed, there was a problem with one of the brushes. A scream was then heard. KP had been electrocuted . Subsequently, it was determined that the main power switch, which was a rotary knob, had failed to break the circuit when it was rotated to the "off" position. Considerable effort was allocated to determining the mechanical causes of the switch's failure.

HF Issues: This graphic shows the method used to identify some of the issues in this case.

 

 

HF Investigation and Analysis: Material reviewed included accident reports, video of the accident site; equipment manuals; and depositions of all the parties involved.

There were several human factors issues. The employee who rotated the main disconnect knob to the left towards the "off" position did not rotate it far enough to break the circuit, though he believed that he had. Human factor considerations included the location of the control's legend, which was obscured by the operator's hand; changes in control resistance with knob rotation; and other sources of feedback as to the position of the knob.

In addition, consideration was given to whether the electrical conduits were provided adequate guarding in terms of both proximity to the operators (guarding through distance) and the dimensions of the gap in the insulation..

Another human factor's issue concerned the status of the indicator light on the power press's control panel. KP's supervisor maintained that he had turned off the power press, and that the indicator light was dark. An engineer examined diagrams of the circuitry of the power press and opined that power to the indicator light would be interrupted if the brush assembly for one, and only one, of the conduits was not properly aligned. In other words, the conduits could still be "hot" even if the indicator light on the power press indicated that no power was being delivered to the machine.

To illustrate the interaction between the main power disconnect switch, the alignment of the electrical conduit brushes, and the status of the indicator light on the power press, I constructed an interactive computer model in Working Model 2D (Knowledge Revolution). The model of the main power disconnect switch could be rotated and end up in one of three positions. The far right position resulted in the power being disconnected, while the circuit remained intact in the remaining two positions. The model permitted manipulation of brush contact with the conductor for three conduits. As shown in the following two graphics, if, for example, the middle brush was not making contact with the conductor, the power indicator light on the power press would correctly indicate that power was still being delivered (provided the main disconnect switch was not rotated all the way to the right).

However, if under the same conditions, the brush of the conduit on the far right did not make contact with the conductor, the indicator light on the power press failed to remain on. (See figure below.) In my opinion as a human factors specialist, this accident could not be solely attributed to defects in the main power disconnect switch. Further, it illustrates the importance of adopting a systems approach to identifying what can go wrong in equipment operation.
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