Establishing a Bypass Route for Surge Current Using SPD and Sit Part 2

Unlike ordinary isolation transformers, SITs (Surge Isolation Transformers) have enhanced insulation capabilities against high frequencies such as surges. The IEC standard has already been published as the international standard for SIT for communication signals. The published standards are IEC61643-351: Performance requirements and test methods for telecommunications and signalling network surge isolation transformers (SITs) and IEC6143-352: Selection and application principles for telecommunications and signalling network surge isolation transformers (SITs). The IEC standard for SITs for power supplies, IEC61643-361: Surge isolation transformers (SITs) connected to low-voltage distribution system - Requirements and test methods, is also currently under discussion by the joint working group JWG5 of IECSC37A and SC37B, and is at the committee draft CD stage. The features of a power supply SIT are that it has an impulse withstand voltage of at least 30 kV against high-frequency waves such as surges, and that it can greatly attenuate incoming surges. In Japan, it has long been used as a method of protecting against lightning surge. These performances were also reported in APL 2023 and ICLP 2024. In this paper, we confirm and report the performance against switching surges that occur when the SPD and SPD separator installed together with the SIT fail.