What is the purpose of a bus tie switch or bus tie relay in an aircraft electrical system?

• A. It allows tying two buses together to share power or to isolate them for safety or fault isolation, enabling continued operation if one bus fails
• B. It steps voltage up to 120V
• C. It provides a backup generator
• D. It disconnects all buses during maintenance

Answer: (A)

The purpose of a bus tie switch or bus tie relay is to connect two electrical distribution buses so they can share power or be kept separate for safety or fault isolation. When activated, it links the buses, allowing loads on either side to be supplied from the other power source if one bus or its generator goes offline, which helps keep critical systems powered and improves fault tolerance. In normal operation, the tie can be open to keep the buses isolated; in a fault or during generator switching, it can close to cross-feed power and maintain operation. This is especially important in aircraft with multiple buses (essential, main, nonessential) and multiple power sources, where cross-connection provides redundancy by enabling power transfer or isolation as needed.

It does not step voltage up; that function belongs to transformers or power converters. It is not a backup generator, nor does it create power. It also isn’t intended to disconnect all buses during maintenance; rather, it can be opened or closed to manage power to a specific bus while maintenance is performed or to isolate faults.