Air dehydrating breather assembly and method for providing dehumidifying air suitable to be supplied into oil expansion vessels used in electrical devices, comprising first and second air demoisturizer units associated to two corresponding oil expansion vessels. The first air demoisturizer unit comprises a first tank and a second tank housing moisture absorbing means and alternatively providing dehumidified air; the second air demoisturizer unit comprises a third tank housing moisture absorbing means and providing dehumidified air. Sensors are associated to the first, second and third tanks in order to detect the level of saturation of their moisture absorbing means. Electronic means are operatively associated to the sensors for routing dehumidified air to the associated oil-expansion vessels from the tanks whose moisture absorbing means are not saturated.