Corona discharge is the most common form of gas discharge, which refers to the partial self-sustaining discharge of gas medium in the non-uniform electric field. In the vicinity of the tip electrode with a small radius of curvature, the local electric field intensity exceeds the ionization field intensity of the gas, which causes ionization and excitation of the gas, and thus the corona discharge occurs. When corona occurs, light is visible around the electrode, accompanied by a hissing sound.

Corona discharge can damage insulators and may produce breakdown of products, which can lead to catastrophic insulation failure.