What causes the BUS TIE annunciator to illuminate?

• A. Bus tie switch closed
• B. Bus tie switch open or inoperative
• C. Bus tie switch operational
• D. Bus tie functioning normally

Answer: (B)

The illumination of the BUS TIE annunciator indicates that there is an issue with the bus tie system. Specifically, this occurs when the bus tie switch is open or inoperative. In a typical electrical system, the bus tie switch is responsible for allowing or preventing the electrical buses from connecting to one another. If the switch fails to function properly or remains open when it should be closed, it prevents the transfer of power between buses, which is critical for maintaining electrical stability and redundancy in the aircraft. Therefore, when the BUS TIE annunciator lights up, it serves as a warning signal to the crew that the bus tie system is not properly configured or is malfunctioning, potentially leading to a loss of electrical power to certain components.

In contrast, the other scenarios mention the bus tie switch being closed, operational, or functioning normally, which would not trigger the annunciator. When the switch is closed and operational, the buses are connected properly, allowing power to be shared as necessary, and thus the warning light would not activate.