To preserve the power supply to the DC power buses when a TRU fails, ________ causing power to be routed to the affected buses.

When a Transformer Rectifier Unit (TRU) fails, the proper response to maintain power supply to the DC power buses is to close the ties. This action allows alternate power sources or working TRUs to redirect power to the affected buses, ensuring that essential systems remain operational. The closure of ties effectively combines the power outputs of redundant systems to provide continuity of service.

In this situation, the operation of ties is crucial as it mitigates the risk of loss of power and maintains the system’s reliability. Other responses, such as activating switches or isolating power, would not facilitate the necessary redirection of current to keep the buses alive. Similarly, system resets may not address the immediate need to re-route power. Closing the ties is designed specifically for such scenarios, thereby preserving functionality during a TRU failure.