Human Factors Bulletin

These human factors bulletins focus on how workers interact with their work environments. They are based on accident investigations that examine all the workplace factors that influence the decisions and actions of the workers involved in an accident. These factors help to identify the causes of an accident. Identifying these causes can help to prevent similar workplace accidents.

Human visual limitations and overhead power lines

This bulletin looks at the connection between human visual limitations and safety around overhead power lines. This is a very important area as there are many occupations in which workers are close to overhead power lines. These include construction workers, window cleaners, arborists, painters, and telephone repair workers.

What happened?

An installer and a helper were installing gutters at a residential home. They were on the roof moving long sections of gutter into position. The tip of the
gutter contacted a 14.4 kilovolt (kV) overhead electrical conductor (power line) that was running parallel to the roof edge. The installer sustained a fatal electrical shock.

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Human vision

To recognize an object, people need to see the object and to identify the object as important or relevant. So our ability to recognize an object depends on a two-step process of sensing and perceiving.

Sensing: In order to see an object there needs to be enough contrast between the object and its background. Our brain combines the two-dimensional images our eyes see with various cues in the environment to help develop a three-dimensional picture. The accuracy of these three-dimensional pictures depends on the cues available. Some of the cues we utilize are called relative cues and require other objects to be present for comparison. For example, if we see two similar vehicles at different distances away, it is the size of these vehicles or other environmental cues (coloring, size, shadows, texture, etc. of these objects or surrounding objects) that helps us predict their distances. When estimating the distance of an overhead power line from the ground or a roof top, there is usually only sky behind the wire. This backdrop gives no relative cues for predicting the distance of the lines. So judging the distance may be more difficult and may provide inaccurate estimates.

Perceiving: Once we can see an object, this information then needs to be perceived by our brains as important. Perceiving is attaining awareness or understanding of the sensory information one is exposed to. Therefore, what individuals are going to perceive is affected by their past experience, training, and knowledge. If power lines have not been part of a worker's experience, training, or knowledge, it is likely that they will not become aware of that particular piece of sensory information.

From a human factors perspective, why did it happen?

This photo shows what the sky was like on the day of the incident. There is little contrast between the grey high-voltage lines and the grey sky. Without enough contrast, the lines are difficult to see. There are also no objects surrounding the power lines, as was the case at the accident site. Without objects in the background and foreground for comparison, it is difficult to correctly predict how far away the lines are.

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It was the helper's first day on the job. He was closest to the power lines (as shown in the drawing of the incident site), but was reportedly concentrating on moving safely on the roof and was not looking at the lines.

The installer was further away from the power lines with the ridge of the roof between himself and the power lines. It's not clear if the roof ridge prevented the installer from seeing the lines, but even if he did see them, it would have been difficult for him to judge exactly where they were due to the limited relative cues (refer to Human vision section).

Understanding human factors helps avoid workplace accidents

When working near overhead power lines, workers rely on vision to keep a safe distance from the lines. However, people have limitations in their ability to perceive distance and depth of objects under various conditions. These visual limitations cannot be changed, but control measures can help compensate for these limitations. Ultimately the best measure of protection is the elimination of overhead power lines. This has started in some areas through the installation of underground power lines. Until the overhead hazard has been eliminated, other control measures to consider could include the following:

  • Methods/tools to let workers know when they or their equipment are working in dangerous proximity to power lines, and to help workers determine distances of high-voltage lines in work zones.
  • Identification of high-voltage hazards and the development of a plan on how to deal with these hazards before beginning work.
  • Awareness of the connection between length of equipment and possible hazards. The last two fatal incidences involving gutter installations and high-voltage power line contact involved gutter lengths of 50 feet and greater. Workers need to consider equipment lengths to determine if their equipment will encroach on the safe limit of approach.
  • Training and education regarding high-voltage lines and their hazards. This should include knowledge of power line voltages, the limits of approach distances, devices/tools to assist in determining distances between work area and power lines, and who to contact for information when work will be in close proximity to power lines.
  • Awareness of our natural human limitations with respect to depth perception, and how this affects our ability to properly predict the distance power lines are away from us. Understanding these limitations may in turn help workers and employers recognize the need for assistance (i.e., utilization of methods/tools) to know when workers or their equipment are within dangerous proximity to the power lines.

Reference

http://www.visualexpert.com/Resources/contrastfundamental.html

WorkSafeBC Human Factors Bulletin 2011-01
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