Brush discharge occurs from a nonconductive charged surface or highly charged nonconductive liquids. The energy is limited to roughly 4 millijoules. To be hazardous, the voltage involved must be above about 20 kilovolts, the surface polarity is negative, flammable atmosphere is present at the point of discharge, and the discharge energy is sufficient for ignition. Due to maximum charge density on surface, an area of at least 100 cm2 has to be involved. Not observed as a hazard for dust clouds.
Propagating brush discharge is high in energy and dangerous. Occurs when an insulating surface of up to 8 mm thick (e.g. a teflon or glass lining of a grounded metal pipe or a reactor) is subjected to a large charge buildup between the opposite surfaces, acting as a large-area capacitor.
Cone discharge, also called bulking brush discharge, occurs over surfaces of charged powders with resistivity above 1010 ohms, or also deep through the powder mass. Cone discharges aren't usually observed in dust volumes below 1 m3. The energy involved depends on the grain size of the powder and the charge magnitude, and can reach up to 20 mJ. Larger dust volumes produce higher energies.