An on-load tap changer for changing taps in a plurality of transformer windings, the tap changer comprising: a tank defining an interior space and having a first side with an access opening and a second side for mounting against a transformer, the tank being adapted to hold a volume of dielectric fluid; a door mounted to the tank and movable between an open position, wherein the door does not cover the access opening, and a closed position, wherein the door covers the access opening; a plurality of modules disposed in the interior space and arranged in a side-by-side manner, each module being operable to change taps in one of the transformer windings, the modules each comprising: a board having opposing first and second sides, the first side facing the door when the door is in the closed position; a selector switch assembly comprising first and second movable selector switches for connection to taps in the transformer winding; first and second branch circuits (44, 46) for connecting the first and second movable selector switches to a common terminal; a bypass switch assembly mounted to the first side of the board, the bypass switch assembly comprising first and second bypass switches (66, 68) connected into the first and second branch circuits, respectively, and being actuated by first and second linkages (132, 134) that are moved by the rotation of a bypass cam; a vacuum interrupter mounted to the first side of the board, the vacuum interrupter being connected between the first and second branch circuits and having contacts that can be opened and closed; an actuation assembly mounted to the first side of the board, the actuation assembly being operable to open and close the contacts of the vacuum interrupter, the actuation assembly comprising a rotatable interrupter cam; and a transmission system mounted to the second side of the board and operable to rotate both the interrupter cam and the bypass cam. a tank defining an interior space and having a first side with an access opening and a second side for mounting against a transformer, the tank being adapted to hold a volume of dielectric fluid; a door mounted to the tank and movable between an open position, wherein the door does not cover the access opening, and a closed position, wherein the door covers the access opening; a plurality of modules disposed in the interior space and arranged in a side-by-side manner, each module being operable to change taps in one of the transformer windings, the modules each comprising: a board having opposing first and second sides, the first side facing the door when the door is in the closed position; a selector switch assembly comprising first and second movable selector switches for connection to taps in the transformer winding; first and second branch circuits (44, 46) for connecting the first and second movable selector switches to a common terminal; a bypass switch assembly mounted to the first side of the board, the bypass switch assembly comprising first and second bypass switches (66, 68) connected into the first and second branch circuits, respectively, and being actuated by first and second linkages (132, 134) that are moved by the rotation of a bypass cam; a vacuum interrupter mounted to the first side of the board, the vacuum interrupter being connected between the first and second branch circuits and having contacts that can be opened and closed; an actuation assembly mounted to the first side of the board, the actuation assembly being operable to open and close the contacts of the vacuum interrupter, the actuation assembly comprising a rotatable interrupter cam; and a transmission system mounted to the second side of the board and operable to rotate both the interrupter cam and the bypass cam. a board having opposing first and second sides, the first side facing the door when the door is in the closed position; a selector switch assembly comprising first and second movable selector switches for connection to taps in the transformer winding; first and second branch circuits (44, 46) for connecting the first and second movable selector switches to a common terminal; a bypass switch assembly mounted to the first side of the board, the bypass switch assembly comprising first and second bypass switches (66, 68) connected into the first and second branch circuits, respectively, and being actuated by first and second linkages (132, 134) that are moved by the rotation of a bypass cam; a vacuum interrupter mounted to the first side of the board, the vacuum interrupter being connected between the first and second branch circuits and having contacts that can be opened and closed; an actuation assembly mounted to the first side of the board, the actuation assembly being operable to open and close the contacts of the vacuum interrupter, the actuation assembly comprising a rotatable interrupter cam; and a transmission system mounted to the second side of the board and operable to rotate both the interrupter cam and the bypass cam.