A global leader in overvoltage protection devices, ASwich offers a comprehensive line of Surge Protective Devices (SPD) , components, and systems.Aswich Surge Protective Device (SPD) is a no-fuse, fail-safe surge suppressor featuring . UL 1449 4th Edition approved, it is DIN-rail mountable featuring a fail-safe self-protected design, visual indicator, small footprint, and a 40kA surge capacity.