During lightning impulse type testing of medium voltage switchgears up to two spark-overs are accepted. Consequently, when selecting conducting composites for use in screens and shields of such equipment it is important that their surfaces not degrade after a spark-over, as this normally will reduce the overall dielectric withstand level. Lightning impulse tests where one electrode is coated with polycarbonate based conducting composites have been carried out. With conductive filler of stainless steel fibers heavy surface damage occurs during spark-overs, causing fibers to stick out and thereby lowering the dielectric strength. Carbon fibers filler shows a similar behavior, but to a much lesser extent. For polycarbonate filled with carbon powder some kind of conditioning effect takes place, causing the dielectric strength of the system to increase, and no surface damage is observable even after dozens of spark-overs.