Different measures are known and recommended [6,7] to limit the incendivity of Brush discharges from plastic surfaces. Besides the limitation of the surface area and the decrease of the surface resistivity, the limitation of the thickness (2mm for explosion groups IIA and IIB, 0.2 mm for explosion groupIIC) of non-conductive layers and coatings on top of earthed conductive surfaces is recommended. The physical basis of this recommendation is the orientation of the electrical field and the quenching of a starting ignition. Due to the special field orientation the discharge only occurs over a very small distance, when the approaching electrode is close to the charged surface. Recent results [8] show, however, that these limits have to be applied with caution. Ignition of a towns gas atmosphere, adjusted to 50% ignition probability when tested with 0.2mJ sparks, occurred with PFA (perfluoroalkoxy
fluoropolymer resin) coatings of a thickness 0.7 mm. In these experiments the surface charge density was below the threshold for propagating brush discharges [9]. In accordance with the effect of flame quenching the charge transfer required for ignition was much (100-150nC) higher compared to an insulating plastic sheet that is not in contact with a conductive surface (30nC).