What triggers the ESS TIE to close?

The closing of the ESS TIE is triggered by a failure in either ESS TRU 1 or ESS TRU 2. This mechanism is in place to ensure that when one of these essential power sources fails, the system responds by isolating the failed component to maintain the integrity and reliability of the remaining power system. Therefore, this action helps to prevent potential damage and ensures that the necessary power continues to be supplied to essential systems and components, allowing for safe operation of the aircraft.

The other options, while they involve important operational factors, do not specifically trigger the closure of the ESS TIE. For example, a total power loss would result in different responses and strategies rather than specifically closing an ESS TIE. Similarly, the activation of a maintenance switch or starting the APU are also unrelated to the triggering mechanism for closing the ESS TIE under the outlined conditions.