Professional Answer: Disruptive critical voltage (Vc) is the minimum phase-to-neutral voltage at which the air surrounding an overhead power line conductor begins to ionize. When this voltage is exceeded, the electric field strength at the conductor's surface becomes powerful enough to break down the air's insulating properties, leading to a phenomenon known as corona discharge. This discharge is characterized by a faint violet glow, a hissing sound, and the production of ozone gas.

The formula for disruptive critical voltage is: Vc=mo⋅δ⋅go⋅r⋅ln(d/r) Where:

• mo is the surface irregularity factor.

• δ is the air density correction factor (affected by temperature and pressure).

• go is the dielectric strength of air (approximately 30 kV/cm peak).

• r is the radius of the conductor.

• d is the distance between conductors.