The present application includes an uninterruptable power supply device for connection of a 3-wire multiphase AC source to a 3-wire multiphase load, whereby the UPS device is provided for multiphase operation, including a converter part, which is connected to at least one power source and the load, and a 3-wire bypass, which interconnects the AC source to the load, whereby the bypass includes a bypass switch, which includes an independently controlled switching unit for each phase of the AC source, and the UPS device includes a control unit, which controls the converter part and the bypass switch, whereby the control unit controls the bypass switch to power one of the three phases of the load directly via the bypass by one phase of the AC source, and the control unit controls the converter part to power the remaining two phases of the load. The application further includes an uninterruptible power supply system including multiple of the above UPS devices, wherein the UPS devices are connected in parallel to the load.