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Until the invention of the transistor in 1948, power system control and protective relays were almost exclusively electro-mechanical. There were a few special applications, such as automatic synchronizing of ac generators, that used vacuum tubes - but these were clearly the exceptions. The physical structures of these electro-mechanical relays were robust, with current and potential coils designed to pass one minute power frequency production tests at 2000 volts. In addition, they were required to pass design (type) impulse tests at 5 kV. Insulation failures were rare. The most common failure modes in the field were dirty contacts, worn or dirty bearings (on induction cup or induction disc relays), or incorrect settings. Routine preventive maintenance was scheduled to address these problems. A few years after the invention of the transistor in 1948, some transistors designs became available that could withstand the typical extreme temperatures in electric utility substations. Their capability as solid state amplifiers was very attractive to the designers of protective relays, as the search was always on for more sensitive and faster, yet secure, products for transmission line relaying. These products -“solid state relays” (or sometimes called “static terminals”) - were often applied for EHV (345, 500 and 765 kV) transmission line protection to reduce fault clearing times. Relays for other functions such as transformer differential protection and automatic synchronizing also used transistors.