Power line induced current in the earth determined by magnetotelluric techniques
2004
Pellerin, L. | Alumbaugh, D.L. | Reinemann, D.J. | Thompson, P.D.
Power frequency (60 Hz and harmonics) AC currents were measured on four Wisconsin dairy farms using resistivity and potential gradient measurements by techniques adapted from magnetotelluric (MT) geophysical exploration. Several measurement sites were located on each farm in addition to a remote site as far away from the nearest power line as could reasonably be achieved. Electric field intensity and earth resistivity measurements were used to calculate the near-surface current density for 60 Hz and the 12 next order harmonics. Earth currents and the corresponding electrical voltages at 60 Hz and higher harmonics were inconsistent from farm to farm. Electric fields at the farms ranged from roughly 1 to 10 mV/m RMS at 60 Hz, and the current densities ranged from about 5 to 50 microampere/m2, roughly 10 to 1000 times stronger than at the corresponding remote sites. As expected, the presence of 60-Hz power frequency current and its harmonics was stronger near points of electrical use and distribution. The resulting step potentials, measured with high impedance equipment, were 26 to 8300 times lower than the maximum cow contact voltages prescribed by the Public Service Commission of Wisconsin (PSCW). Assuming a typical reduction of half in the contact voltage with a 500-ohm body impedance the resulting current flow through a cow attributable to earth currents in this study was 50 to 16,000 times below the PSCW action level. Considering the substantial body of literature supporting the PSCW level of concern, the extremely low exposure levels resulting from earth currents would not be expected to cause harm to dairy cows.
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