Abstract: An automatic bus transfer device is provided for single phase and multiphase power systems. The automatic bus transfer device provides infinite isolation and automatic switching from a primary power bus to a secondary power bus when the power (or voltage) being provided by the primary power bus falls below a predetermined level. The automatic bus transfer device includes a relay and a signal detector. The relay includes a master portion and a slave portion. The slave portion has a first input terminal, a second input terminal and an output terminal. The slave portion is switchable from a primary state to a secondary state and vice versa in a manner responsive to the master portion. The output terminal of the slave portion is electrically connected to a primary one of the input terminals when the slave portion is in the primary state and is electrically connected to a secondary one of the input terminals when the slave portion is in the secondary state.