A fuse is an electrical safety device that provides overcurrent protection to an electrical circuit. Its main component is a metal wire or strip that melts when an extremely high current flows through it, thereby stopping the current flow. It is a sacrificial device; once a fuse has operated it is an open circuit, it must be replaced or rewired, depending on the type. Fuses have been used as essential safety devices from the early days of electrical engineering. Today there are thousands of different fuse designs that have specific current and voltage ratings, breaking capacity and response times, depending on the application. The time and current operating characteristics of fuses are chosen to provide adequate protection without needless interruption.