Analysis, design, and testing of mechanical switch for the backup protection of switching network unit in fusion device

•A mechanical switch has been designed for SNU backup protection, which can timely close and short-circuit other equipment in case of faults to cope with the problem of partial device damage affecting the operation status of all semiconductor devices in multi parallel/series semiconductor device switches, leading to further expansion of faults.•The contact system of the mechanical switch is designed as a split module structure, which can be targeted to search for and replace damaged contact modules, improving maintainability, indirectly improving the working life of the contact system, and thus improving the overall operation life of the switch.•The actuator of this mechanical switch is a combination of electromagnetic repulsion actuator and permanent magnet actuator to achieve fast and reliable closing.•Preliminary performance testing and design verification on the prototype were conducted. The results showed that the current design meets the relevant design requirements. Switching network unit (SNU) is one of the important systems in the power supply system of fusion devices, which excites and establishes plasma current. A mechanical switch has been designed for SNU backup protection, which can short-circuit other components in case of SNU failure to prevent further deterioration of the situation. An optimized contact system is used in this switch to improve the maintainability of the contacts. A detailed structural design was carried out for the switch, and the final prototype test results verified that it meets the relevant design requirements. This backup protection scheme can provide a certain reference for the engineering design and reliable operation of SNU.