AT20 building PC grade automatic transfer switch products are composed of automatic transfer switch, special controller. and excitation transfer mechanism. The structure part adopts GEIS advanced manufacturing technology and production experience, delivering fast conversion speed and high reliability. Reliable driving mechanism and high-power silver alloy contacts enable long-term work and make it not easy to oxidize, to ensure that the electrical performance meets the needs of the system. The product has mechanical and electrical interlocking, and the controller has logical locking function to ensure that two power sources are not closed at the same time. AT20 structure adopts modular design, controller and conversion unit. So the body failure can be quickly replaced or maintained. The controller is connected with the switch by data cable, and can be replaced without turning off the power.