Until now, replacing existing equipment with new without interruption has been technically possible because the power supply has been DC, and we have only had to control voltage. Increasingly customers are using AC power supply, and their need for uninterrupted power to their equipment is also increasing. However in the case of AC power supply, replacing equipment without interruption is difficult because we have to control voltage, phase, phase rotation, and phase difference. This paper presents a portable-transfer switch that we developed to enable UPS replacement for AC equipment without such interruption. It allows electrical workers to replace an existing UPS to a new one safely without interrupting the supply of power.