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Breakdown voltage values between conductor paths on epoxy-glass printed wiring boards (PWB's) are shown to lie on the Dakin curve relating the breakdown voltage with the dielectric constant and conductor separation on the insulator. However, the insulator surface roughness is shown to affect the breakdown voltage, the smoother the surface the lower the breakdown voltage. With a roughness (peak to valley) on the order of 10 µm or greater on epoxy-glass, the breakdown voltage approaches that of an equivalent air gap. Multiple filamentary discharges were observed per flash over similar to normal air breakdown. Erosion products from flashover include small copper spheres, from the damaged printed wiring lines, scattered around the damaged area on the PWB where, in many cases, the glass fiber supports were exposed. These effects can accelerate the degradation of the PWB insulator substrate.