A cap system for enclosing a metallic fastener assembly extending through a structure, which includes a first securement mechanism positioned about a periphery of a nut member of the metallic fastener assembly, wherein the first securement mechanism comprises a plurality of continuous grooves which extend about the periphery of the nut member. The cap system further includes a cap member which includes a sidewall having an inner surface which defines a cavity dimensioned to receive the nut member and defines a second securement mechanism complementary configured to engage the first securement mechanism positioned about the periphery of the nut member. The cap system also includes an end of the sidewall of the cap member defines an opening which provides the nut member to have access into the cavity.