Corona is a luminous, audible discharge that occurs when there is an excessive localized electric field gradient upon an object that causes the ionization and possible electrical breakdown of the air adjacent to this point. Corona is characterized by a colored glow frequently visible in a darkened environment. The audible discharge, usually a subtle hissing sound, increases in intensity with increasing output voltage. Ozone, an odorous, unstable form of oxygen is frequently generated during this process. Rubber is destroyed by ozone, and nitric acid can be created if sufficient moisture is present. These items have detrimental affects on materials, inclusive of electrical insulators.

A good high voltage design takes corona generation into account and provides design countermeasures to limit the possibility of problems developing. Spellman engineers use sophisticated e-field modeling software and a Biddle Partial Discharge Detector to ensure that each high voltage design does not have excessive field gradients, preventing partial discharge and corona generation.