Visual critical voltage is defined as the minimum phase-neutral voltage at which corona glow appears all along the line conductors.
the corona glow does not begin at the disruptive voltage Vc, rather it occurs at a higher voltage called visual critical voltage.
Visual critical voltage is abbreviated using Vv.
The phase-neutral effective value of visual critical voltage is given mathematically by the following empirical formula:
Where in above equation:
δ = air density factor
mv = irregularity factor
value of mv = 0.72 to 0.82 for rough conductors
value of mv = 0.72 to 0.82 for polished conductors
go = breakdown strength of air at 76 cm of mercury and 25ºC = 30 kV/cm (max) or 21·2 kV/cm (r.m.s.)
r = radii of conductors
d = spacing between conductors