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Concept
Brush discharge
Summary
A brush discharge is an electrical disruptive discharge similar to a corona discharge that takes place at an electrode with a high voltage applied to it, embedded in a nonconducting fluid, usually air. It is characterized by multiple luminous writhing sparks, plasma streamers composed of ionized air molecules, which repeatedly strike out from the electrode into the air, often with a crackling sound. The streamers spread out in a fan shape, giving it the appearance of a "brush".
Corona and brush discharges are sometimes called one-electrode discharges because they occur in the vicinity of a single electrode, and don't extend as far as the electrode carrying opposite polarity voltage in the circuit, as an electric arc (a two-electrode discharge) does.
Corona discharge — occurs at sharp points and edges (radius < 1 mm). It is a uniform ionization (glow discharge) visible as a dim stationary blue glow, fading out as it extends from the conductor.
Brush discharge — occurs at a curved electrode (radius between 5 and 50 mm) in the vicinity of a flat electrode. It consists of a short ionization channel which breaks up into a fan of multiple moving streamers which strike toward the other electrode. If the electrode is too sharp, a corona discharge will usually occur instead of a brush discharge.
Arc or spark discharge — A "two electrode" discharge that occurs when an ionized channel extends all the way from one electrode to the other. This allows a large current to flow, releasing a large amount of energy.
Both brush and corona discharges represent local regions next to conductors where due to the high voltage the air has undergone electrical breakdown: it has ionized and become conductive, allowing current to leak into the air. They occur when the electric field at the conductor exceeds the dielectric strength of the air, the "disruptive potential gradient", roughly 30 kilovolts per centimeter.
Official source
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