3.2. Lessons learned from incidents
Propagating brush discharges typically may occur along the surface of insulating layers and coatings of metal surfaces, if two requirements are fulfilled: (i) high dielectric strength (break down voltage) of the coating and (ii) strong charge build- up process leading to a very high charge density on the insulating surface.
Up to now it was commonly agreed that propagating brush discharges would hardly occur when powders are transferred through pipes and valves coated with insulating material by gravity only. A recent analysis of two incidents and subsequent conclusions suggest, however, that incendive propagating brush discharges may occur under these circumstances, particularly if large amounts of powders are transferred within a short period of time across a height of several meters [10]. The consequence of these findings is to use conductive coatings under these circumstances or to perform such operations within an inert atmosphere.