Evaluating occupational 60-hertz electric-field exposures for guideline compliance

This article examines determination of compliance of 60-Hz electric-field exposures with occupational guideline limits. The guidelines are expressed as a limit on the unperturbed electric field without allowance for the severity of potential spark discharges. A line worker on a 500-kV transmission-line tower provided a practical example of an occupational exposure. In this realistic case, 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. An accurate estimate of the unperturbed nonuniform fields in the climbing space of a lattice steel structure was computed using Monte Carlo methods that modeled surface and spatial electric fields on and near standard geometrical elements. Fields were computed at 20 points in a three-dimensional array, simulating the location of the human body on the tower. We estimated the average unperturbed electric field, space potential, induced short-circuit current, induced open-circuit voltage, and the stored charge and energy available for a discharge over a range of capacitances to ground. The on-tower exposure parameters were compared with those from the idealistic guideline exposure scenario. The average electric field of 24.4 kV/m for the on-tower exposure exceeded the limit of 20 kV/m stated in the recently adopted IEEE Standard C95.6 2002. However, the charge available for a spark discharge during the on-tower exposure was less than that for the guideline exposure scenario. Thus, for an exposure limit based on a constant-charge criterion for adverse reaction to spark discharges, guideline on-tower exposure would be below the limit established for the guideline exposure scenario. Evaluation of electric-field exposures in terms of the charge associated with spark discharges provides a means of comparing any electric-field exposure scenario with the ideal guideline scenario in terms of an effects-related physical quantity. This approach is consistent with the exposure limit/basic restriction methodology that employs a basic restriction on a physical quantity as the ultimate determinant of compliance.