Under what condition does the MAIN TIE close?

The MAIN TIE closes when there is an issue with either TRU 1 or TRU 2, allowing critical electrical loads to be powered through the remaining operable TRU. This function is essential for maintaining electrical system redundancy and stability in a situation where one of the TRU units fails. When one TRU is not functioning, the MAIN TIE closing ensures that the aircraft can still operate normally by redistributing power to the essential systems.

In contrast, during normal operations, the MAIN TIE typically remains open to minimize the possibility of fault propagation between the two TRUs and to facilitate load management. The MAIN TIE does not automatically close simply based on the functionality of both TRUs being intact. Lastly, the closing of the MAIN TIE is not time-dependent, such as immediately after landing, but rather dependent on the system state regarding TRU failures.