Abstract: An AC uninterruptible power source is disclosed that provides multi-phase wer output signals for actual use and a selectable single phase power output signal for use in testing. The AC uninterruptible power source further comprises at least one battery. The single phase output signal is connected to a dummy load by means of a power switch of the break-before-make type. The invention further includes the dummy load as preferably having metering capabilities to measure the frequency, current, and voltage parameters of the single phase output signal. The arrangement of the present invention allows for the single phase output to be mated to the dummy load without the need of removing input power to the AC uninterruptible power source so as to reduce, or even eliminate, transient caused failures to the AC uninterruptible power source and to its sensitive load equipment which the power source services.