60-Hertz Electric-Field Exposures in Transmission Line Towers

This article reports on an investigation of 60-Hz electric-field exposures of line workers in 230- to 765-kV transmission line towers. The exposures were based on computations of the unperturbed electric field along climbing routes and at work positions on the towers and on insulated ladders suspended in towers. Computed exposures were expressed in terms of the unperturbed electric field averaged over the body as stipulated by guidelines. For the realistic on-tower positions, the worker's posture, the uniformity of the field, and the field orientation differed from the guideline exposure scenario of standing erect in a vertical uniform field. These differences suggest the need for care in comparing electric-field exposures in towers with guideline limits. The unperturbed nonuniform fields at discrete points near steel and aluminum lattice structures were computed using Monte Carlo methods that model surface and spatial electric fields on and near standard geometrical elements. To estimate a whole-body average, fields were computed at 10 discrete points positioned on segments of an articulated stick-figure model of the human body. The whole-body average field was computed from fields at all the points weighted by the fraction of body volume that the corresponding segment represented. We estimated the average unperturbed electric field, the space potential at the torso, and the induced short-circuit current for 19 climbing and work positions in six towers. The maximum average electric-field exposure during climbing ranged from 10 kV/m for a 230-kV tower to 31 kV/m for a 765-kV tower. Exposures at on-tower work positions were lower than the estimated maximum exposures during climbing. For 500- and 765-kV towers, computed exposures while climbing and at some on-tower positions exceeded the limit of 20 kV/m given in the recently adopted IEEE Standard C95.6 2002. For lower voltage towers, exposures did not exceed 20 kV/m.

Keywords:

• 60-Hz electric fields
• exposure guidelines
• occupational exposure
• power industry
• transmission lines

ACKNOWLEDGMENTS

Monty Tuominen and Danna Vermeers of the Bonneville Power Administration provided the BPA tower files and graciously shared their EF3D expertise and practical knowledge of transmission-line work practices. BPA line workers Mike Staats and Don Swanson and electrical engineer Rick Stearns of the Bonneville Power Administration were especially informative about work practices. We are grateful to Dale Williams, Jon Cronyn, and their colleagues at AEP for providing the tower drawings and essential information about AEP work practices. Dale Victor, Kurt Sussman, Michael Rasmussen, and Derek Loree of the Portland Linux/Unix Group generously contributed parts of the distributed computing platform.

This work was sponsored by EPRI under EP-P11573/C5739, Project Manager Rob Kavet.

Notes

^A Exposure limit can be doubled to 16.67 kV/m if adverse indirect effects from contact with electrically charged conductors can be excluded.

^B Intended change in 2005.

^A Position simulates belted to arm. Normal work position is sitting or prone at end of arm.

^A Position simulates belted to arm. Normal work position is sitting or prone at end of arm.