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It is often observed that the power interruption in the distribution system is approximately 70% for single-phase faults, while two other phases are in normal conditions. Therefore, in any commercial or domestic power supply system in which phase 3 is available, it is advisable to have an automatic switching system for uninterrupted power at critical loads in case of phase failure.
In this system, automatic selection is achieved by using a set of interconnected relays such that if one of the relays that feed the load remains energized at all times. Under the condition of phase failure, the secondary of the reducing transformer supplies zero voltage that is duly rectified to CC and then fed to the logic gates comprising Y and O to turn on the next relay that delivers the power to the load. It also has a provision for connection to an inverter source that delivers uninterrupted power to the load in case the 3 phases disappear.
Block diagram