Discussion on reliability and test number of switches in Tokamak fast discharge units

•The reliability of high-power switches in Fast Discharge Unit (FDU) is very important for the safety operation of superconducting magnets. In this paper, the reliability and test number of FDU switches are discussed.•The FDU configuration characteristics of different types of superconducting magnets are introduced, and some explanation and calculation on reliability are provided.•The reliability interval estimation after a certain test number is discussed by using the binomial distribution theory.•The reliability conformance test based on the claimed reliability is discussed to derive the test number. High-power switches, as the important active part of Fast Discharge Unit (FDU) in Tokamak, are designed to interrupt the circuit current and transfer the quench energy for protecting expensive superconducting magnets. Hence, their reliability has always been highly underlined. In this paper, the reliability and test number of FDU switches are discussed, mainly including three issues. First, the FDU configuration characteristics of different types of superconducting magnets are introduced, and explanations and calculations on reliability are provided. Second, the reliability interval estimation after a certain test number is discussed by using the binomial distribution theory. Three, the compliance test based on the claimed reliability is studied, and two types of test plans are discussed and compared to derive the test number. The discussion in this paper focuses on the FDU switches and provides valuable guidance for their reliability test and assessment.