A power system comprises a number of components, such as lines, cables, transformers, and disconnectors. Every component in a power system has an inherent risk of failure. Electric power components are typically assigned a capacity rating and an estimated life. The negative effect on a transformer is mainly produced by the excess heat caused by overloading. The principal component of transmission and distribution lines is the conductor: a wire or bundle of wires made of aluminum, copper, or steel. Transmission cables for underground circuits consist of copper conductors surrounded by paper or polyethylene insulation, and jacketed by layers of polyvinyl chloride or metal cladding. Aging electrical equipment in the aviation industry poses an actual danger to aircraft, since even minor defects in the electrical system may threaten the lives of the passengers and crew.