How does the "Replacement" source voltage contacts terminals "20-21" on the UA Controller work in Auto Transfer Switch?


02 December 2021


When the normal source disappears, the UA is no longer supplied. When the UA is no longer supplied, automatically a starting Genset order is sent by the UA. Then as soon as the UR voltage is present the UA get the supply and checks the status of the contact "20-21"


If the contact 20-21 is open, the UR breaker stays open. When the contact 20-21 closes, the UN breaker opens and the UR breaker close.

If the contact 20-21 is already closed (for instance because of jumper presence) while the UR source is present, the UN Breaker opens and the UR breaker close.


When the UR breaker is closed (load energized by UR source), if the contact 20-21 opens, the UR breaker remains closed. But the UR led in front of the UA is turned OFF shows the UR source is OFF.


So Will it transfer back to Normal "N" as 20-21 opens? No if UN is not available, UR breaker remains closed irrespective of the state of 20-21 and only go back to Normal "N" when UN is present i.e. Mains power is back.