Judging Clearance Distances near Overhead Power Lines

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A field study was conducted to determine the accuracy of clearance judgments involving lifting equipment near overhead power lines. Results showed generally unreliable performance in making visual judgments and significant differences by observer station. Background and implications are presented.

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Estimating the height of a wire above the ground is a task that may be required to avoid contacting electrical distribution lines with ladders, antennae, long poles, or other reach extending devices. It has been suggested that this task is difficult because wire is often presented against the homogeneous field of the sky. The accuracy of estimations under this condition may be poor when made on an absolute basis, that is using an internal frame of reference. However, accuracy should improve when the estimation is made on a relative basis, using some external frame of reference. In order to assess the accuracy of such estimations, forty subjects were each asked to give a verbal estimate of the height of a suspended wire, as well as to match the wire height using a horizontal reference pole and a vertical reference pole. As expected, the verbal, or absolute, estimates had a higher average percent error (17.1%) than the relative horizontal estimates (7.7%). Relative vertical estimates produced the most accurate values, within about 2.0% of the actual wire height. The degree of accuracy observed in subjects using external references suggests that the ability to avoid contact with overhead electrical distribution lines may be high, given appropriate knowledge, attention, and motivation.
This study examined the effect that an auditory collision warning signal's pulse rate, pulse pattern, and onset distance had on subject braking responses in a simulated crane/overhead power line collision scenario. The experimental warning signal alerted subjects of their proximity to overhead power lines. The experimental taks required subjects to monitor a simulated auditory collision warning signal while simultaneously operating a single-axis driving simulation task. The driving task simulated an actual crane operator's mental workload required to navigate a crane and manipulate various loads. Subjects were required to initiate braking responses based on the information conveyed solely through the auditory collision warning system. No visual information was provided to the subjects to isolate the effects of the warning signal. Subjective ratings of the auditory warning signals were obtained to compare subjects' actual performance using the warning signal versus their subjective preferences. Results indicate that subjects were able to initiate appropriate braking responses while using an auditory collision warning signal with moderate onset distances and low pulse rates. The auditory pulse pattern did not have a large impact on subjects' braking responses except at extremely short onset distances. Overall, it was concluded that a pulsing auditory warning signal comprised of a moderate onset distance and low pulse rate could work effectively as a proximity warning device for mobile cranes.
Investigation of accidents involving lifting equipment has revealed that ineffective accident reporting systems are presently obscuring identification of human factors considerations omitted in initial design. Hazards created by man/machine mismatches can be identified by using REME, a simplified analysis system for design safety.
Conference Paper
The convex mirror with 0. 9m-1. 2m (35in-47in) radius of curvature is preferable for visibility requirements. FOV on the driver's side must be such that the driver sees at least a 2. 15-m(7. 1ft)-wide extending horizontal plane to the rear of the driver's side of the car at the widest point, said point located 4. 60m (15. 1ft) behind the driver's eyes. The minimum horizontal mirror sizes were determined based upon this required FOV. As a result, using a plane mirror is not a practical way to meet the required FOV because it must be at least 250mm (9. 8in) wide even mounted on a door. Using convex mirrors with a 0. 9m-1. 2m (35in-47in) radius of curvature is a practical way because they must be at least 120mm-144mm (4. 7in-5. 7in) wide even if mounted on a fender. Outside rearview mirrors must be placed inside of the binocular fixation field, which can be performed on the right and left fenders.
Ten years ago the authors carried out a survey on tower cranes in the Birmingham area, and recommended a number of improvements to control layout, warning devices, cabin design and operator training. A later survey enables a comparison to be made, with a review of the application in the intervening decade of the earlier recommendations.
Development of a Methodology for Evaluating Road Signs. EES-315B
  • Rockwell T.
  • Bhise V.
  • Safford R.