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Application of 17-18, 20-21 Contacts in the UA Controller (P/No. 29472)

They are the additional control contacts for the external signals, where 

20-21 Contact ("Replacement" source voltage contact) This can be controlled by a specific test on the "Replacement" source. Transfer to "Replacement" source occurs only if the the test passed and the contact is closed. 
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.

17-18 Contact: (Voluntary transfer) An external signal can be used to initiate transfer to the "Replacement" source. The load returns to the "Normal" source when the signal is cleared

Attached document for reference. 
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