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Metal-enclosed circuit-breakers with all three phases in a common enclosure are used when a compact and economical substation lay-out is required. The three interrupters influence each other during the interrupting process. According to the latest IEC-Standard, three phase testing is necessary to cover all possible interactions between the interrupters. Several test methods and test circuits have been proposed and used in the past. This paper describes a synthetic test circuit for three phase tests, which has been used with success since 1981. It can be used to test metal-enclosed circuit-breakers with a rated voltage up to 300 kV. The results of tests on a 145 kV - 40 kA three phase GIS are presented.