Commercial aircraft with a lot of composites will probably be equipped with metallic grids within the composite material, to lead the current towards a metallic skeleton (though light) that is located within the fuselage to duct the heavy voltage current through the fuselage (and wings TBC) without damaging the structure of the aircraft.
http://www.flightglobal.com/articles/2007/09/28/217062/metallic-strips-will-ensure-electrical-continuity-in-a350s-carbon.html