Electrical enclosure is a cabinet for electrical equipment to mount switches, knobs and displays and to prevent electrical shock while protecting the contents from the environment. The National Electrical Manufacturers Association publishes standards for the performance of various classes of electrical enclosures. The NEMA standards cover corrosion resistance, ability to protect from rain and submersion. Electrical enclosures are made from plastic, steel, aluminum and wood. Enclosures often have partially punched openings (knockouts) meant for optional later removal to accommodate standard cables or conduits. An example of an electrical enclosure is the metal box of a radio receiver.