The Metal Shell Circular Plastic Connector, or CMC, provides the reliability of an all-metal connector at a much lower cost. It consists of a black thermoplastic insert that houses the contacts, and a nickel plated zinc alloy shell. CMCs are available in Series 1, 2, 3, or 4 in limited sizes. CMCs can come with optional TETRASEALS rings which create a splash-proof seal between the mating halves.