Propagating Brush Discharge PBD

PLATE 5. Propagating brush discharge (PBD) on charged layer initiated by grounded electrode. 

The layer thickness must be less than about 8 mm or air breakdown will occur above the charged layer before the critical surface charge density of 2.5 x 10- C/m can be attained. 

Sufficient charge must be available to supply the required critical surface charge density. Calculations show that 50-pound plastic bags and plastic lined 55-gallon drums are too small to represent a credible risk. 

PBDs may occur on plastic surfaces with no metal substrate, for example the wall of a plastic pipe conveying charged material, in this case the double layer forms between the inner charged wall of the pipe and a countercharge which accumulates on the outer wall via conduction or via corona discharge. in the latter case both layers of charge reside on nonconductive sur- 

For a pipe C = 2jrfe /InCr /rj) where / = length and r, Tj = inner and outer radii. 

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