Transfer to "Replacement" source only if this terminal contact 20-21 is closed/bridged (normal operation).
For example, this contact can be used to test the frequency of the "Replacement" source voltage.
Transfer to the "Replacement" source will only take place if the test result is within tolerances.
This condition is not taken into account for return transfer to the "Normal" source.
Please see the following diagram from the Transferpact Catalogue.
In case if the “Normal” source is lost when the terminal contact 20-21 is opened (not bridged), the “Normal” source breaker will remain closed.
The reason is that when the normal source disappears, the UA is no longer supplied. When the UA is no longer supplied, a starting Genset order is automatically sent by the UA.
Then, as soon as the UR voltage is present the UA starts to be supplied 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 closes
If the contact 20-21 is already close (for instance because of jumper presence) while the UR source is present, so the UR breaker closes.
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 shows the UR source is OFF.