Extra-large ESR refers to the ESR technology and equipment capable of melting ingots ranging from 50 to 200 tons. The application of ESR in ingot production began in the late 1950s. In 1958, the world’s first industrial-scale 0.5-ton ESR furnace was constructed at the Dnepropetrovsk Steel Plant in Ukraine. Since then, driven by the demands of industrial development, the size of ingots produced via ESR has progressively increased over the years. In 1964, China commissioned a three-phase ESR furnace at the Shanghai Heavy Machinery Plant, capable of producing 100-ton large ingots. Concurrently, the former Soviet Union installed an ESR furnace at the Novokramatorsk Heavy Machinery Plant, designed for the production of 70-ton ESR ingots. In 1971, Saarstahl AG in West Germany constructed a 165-ton ESR furnace, which could produce rotor forgings for up to a 1300 MW generator. By 1980, China had established a 200-ton class ESR furnace at the Shanghai Heavy Machinery Plant (Xiang in Large Castings and Forgings 3:49–54, 2004), capable of producing extra-large ingots weighing as much as 240 tons. At that time, it was recognized as the largest ESR furnace globally. This furnace operated successfully for three decades, supplying hundreds of high-quality ingots for critical national projects, including the Qinshan Nuclear Power Station and the Three Gorges Hydropower Station.