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ev is the "visual critical corona voltage" or "corona inception voltage" (CIV), the voltage required to initiate a visible corona discharge between the wires.
mv is an irregularity factor to account for the condition of the wires. For smooth, polished wires, mv = 1. For roughened, dirty or weathered wires, 0.98 to 0.93, and for cables, 0.87 to 0.83, namely the surface irregularities result in diminishing the corona threshold voltage.
r is the radius of the wires in cm.
S is the distance between the center of the wires.
gv is the "visual critical" electric field, and is given by:
g0 is the "disruptive electric field."
c is an empirical dimensional constant.