An investigation of the electrostatic ignition risks associated with a plastic coated metal

It is often assumed that, where an electrically insulating plastic is present as a thin coating on an earthed metal substrate (as is the case with lined pipework and vessels), incendive brush discharges will not occur. In this situation, the critical thickness for determining whether a coating is “thin” is generally taken to be 2 mm. However, the experimental work detailed in this report shows that ignitions of a typical flammable gas/vapour atmosphere were obtained by brush discharges from a 0.7 mm thick perfluoroalkoxy coating. For the sample examined, the limiting charge transfer for ignitions to occur was approximately 100–150 nC. This corresponded to a surface charge potential of ∼10 kV (equivalent to a surface charge density of ∼200 μC/m2).