The correct answer is: C. 22 kV (maximum value)/cm
Corona discharge is a form of electrical discharge that occurs in a gas when the electric field strength exceeds a critical value. This critical value is called the breakdown voltage, and it depends on the type of gas, the pressure, and the temperature.
In air, the breakdown voltage is typically about 30 kV/cm for a maximum value, but it can be much lower for high-voltage equipment. When the breakdown voltage is exceeded, a corona discharge will occur. This discharge is characterized by a bluish glow and a crackling sound.
Corona discharge can cause a number of problems, including:
- Insulation breakdown: Corona discharge can cause insulation to break down, leading to electrical failures.
- Radio interference: Corona discharge can generate radio interference, which can disrupt communications.
- Corrosion: Corona discharge can cause corrosion of metal surfaces.
To prevent corona discharge, it is important to keep the electric field strength below the breakdown voltage. This can be done by using insulators, by reducing the voltage, or by increasing the distance between the conductors.
The other options are incorrect because they are not the breakdown voltage of air.