Which condition would cause corona on high-voltage conductors?

• A. A very strong electric field near the conductor that ionizes surrounding air
• B. A very high humidity
• C. Low ambient temperature
• D. Low voltage

Answer: (A)

Corona occurs when the electric field right at the surface of a high-voltage conductor becomes strong enough to ionize the surrounding air. That ionization creates a faint glow, audible noise, and ozone as a partial discharge forms in the air around the conductor. The essential factor is the field strength at the conductor, not humidity, temperature, or voltage by themselves. Humidity can affect how air conducts a little, and temperature or low voltage can influence how easily ionization occurs, but they don’t initiate corona the way a sufficiently strong surface field does. A very strong electric field near the conductor that ionizes the surrounding air is the mechanism behind corona.