Critical Disruptive Voltage is defined as the voltage level at which complete disruption of dielectric strength of air surrounding conductor occurs. If the voltage further rise above critical disruptive voltage Corona starts occurring.

The air around the conductor has free electrons due to cosmic rays. As the potential between two conductors increases, the potential gradient near the conductor also starts increasing. When the voltage becomes greater than the critical disruptive voltage called visual critical voltage corona starts occurring.

Corona creates hissing and cracking noise with violet grow aground conductor and ozone gas emission.

Consider two parallel conductors having radius r cm and placed at d cm from each other.

If V is the phase to neutral voltage then the potential gradient near conductor surface is,

The value of g must be equal to the breakdown strength of air. At normal temperature and pressure breakdown strength of air is 30KV/cm.

If Vc is the phase to the neutral voltage required under these conditions then,

Where,

g0 is the breakdown strength of air

δ = Air density factor

r = Radius of the conductor

d = Spacing between the conductors.

Hence critical disruptive voltage

The above equation is at standard atmospheric condition and temperature.

However, if these standard conditions of the atmosphere change the air density also changes thus the value of g₀ is dependent upon the air density and directly proportional to the air density.

The breakdown strength of air at a barometric pressure of b cm of mercury and temperature t c becomes

Under standard condition δ=1#### g₀𝛿

In the above derivation, an assumption is made that the conductor is solid and the surface of the conductor is smooth and polished.

Correction must also be made for the surface condition of the power conductor

**Critical disruptive voltage**=Vc

Where Value of m0

m0= 1 for smooth and polished conductor

m0= 0.92 to 0.98 for rough conductor

m0= 0.87 for standard condcuctor