At the beginning of the electrical era, in order to complete the switching switch on the alternating current, the use of electrical circuits and large resistors have been used to control the large contactor, which occupies a large space, the cost is also high. Relays, on the other hand, are generally smaller, and then subsequent to the circuit the greater the demand, the industry and functional components continue to improve. Especially in the field of photovoltaics and energy storage, for this reason, electrical research and development from large machines into small machines. That is, a compressed version of the contactor, which was later called a relay. Because of the development of miniaturisation, in the ranks of the original engineers are the name of the contactor also inherited, and subsequent circuits have developed a temperature-controlled relays, timing contactors, etc., it is difficult to unify as well as distinguish for a while.