What is BTB on Boeing 737? (Bus Tie Breaker)

The Bus Tie Breaker (BTB) is an important component of the electrical system in the Boeing 737-800 aircraft. It plays a critical role in ensuring the safe and reliable operation of the aircraft’s electrical system, particularly during abnormal or emergency situations. The BTB is responsible for controlling and prioritizing the flow of electrical power between the different AC buses in the aircraft.

In this article, we will explore the functions and significance of the Bus Tie Breaker on the Boeing 737-800 and understand its role in maintaining the electrical system’s integrity.

The Role of Bus Tie Breaker

The Bus Tie Breaker is primarily designed to provide redundancy and prevent electrical failures in the event of a generator failure or other abnormal electrical conditions. It acts as a switch that connects or disconnects the AC power generators to the AC buses to ensure power distribution is maintained and critical systems are supplied with a reliable power source.

On the Boeing 737-800, there are three AC buses: the left AC bus, right AC bus, and the center AC bus. Each bus is typically powered by its respective generator, located on the left, right, and center engines. The Bus Tie Breaker provides the capability to cross-connect the AC buses, allowing power from one generator to be supplied to multiple buses.

This cross-connection feature allows for load sharing and ensures that essential systems, such as the flight controls, avionics, and communication systems, have a redundant power supply. In the event of a generator failure, the Bus Tie Breaker automatically switches to the remaining operational generator, preventing total power loss and maintaining critical systems’ functionality.

Furthermore, the Bus Tie Breaker also enables the isolation of faulty or problematic generators from the rest of the electrical system. This isolation is crucial in preventing a faulty generator from impacting the entire electrical system and ensures that the healthy generators continue to provide power to the essential systems.

How the Bus Tie Breaker Works

The Bus Tie Breaker operates based on electrical prioritization logic. It automatically senses the electrical conditions, such as generator availability, bus voltage, and frequency, and activates the appropriate switching operation.

During normal operations, the Bus Tie Breaker keeps all the buses connected to their respective generators while allowing for the sharing of excessive loads. In this state, the electrical power is efficiently distributed, and the generators are balanced.

However, in the event of a generator failure or abnormal electrical condition, the Bus Tie Breaker automatically detects the issue and disconnects the affected generator from the buses. It then connects the remaining operational generator to the affected bus(es) to restore power supply and maintain the essential systems’ functionality.

The Bus Tie Breaker’s operation is transparent to the flight crew and does not require manual intervention unless there is a need for maintenance or troubleshooting. Its automatic functionality ensures a smooth transition during abnormal electrical situations, allowing the flight crew to focus on managing the situation rather than having to manually control the power distribution.

The Significance of Bus Tie Breaker in Aviation Safety

The Bus Tie Breaker is a critical component of the Boeing 737-800’s electrical system and plays a vital role in maintaining the aircraft’s integrity and safety. Its ability to provide redundancy and automatically manage electrical power distribution ensures that essential systems receive a reliable power supply, even in abnormal or emergency situations.

The Bus Tie Breaker’s advanced electrical prioritization logic and automatic switching operations significantly reduce the risk of electrical failures leading to a total loss of power or the malfunctioning of critical systems. This enhances the aircraft’s overall safety and ensures that flight crews have the necessary resources to respond effectively to abnormal conditions.

Moreover, the Bus Tie Breaker’s isolation capability prevents a faulty or malfunctioning generator from affecting the rest of the electrical system. This ensures that a localized electrical issue does not escalate into a more severe and widespread problem, providing an added layer of protection against potential electrical failures.


The Bus Tie Breaker is an integral part of the Boeing 737-800’s electrical system, serving as a crucial link in the power distribution chain. Its ability to connect, disconnect, and prioritize electrical power between the AC buses ensures redundancy, load sharing, and the continual operation of essential systems.

Understanding the functions and significance of the Bus Tie Breaker is vital for both pilots and maintenance personnel. It enables them to have a comprehensive knowledge of the aircraft’s electrical system and ensures that they can effectively manage abnormal or emergency electrical situations, ultimately contributing to safe and reliable flight operations.

As technology continues to evolve, we can expect further advancements in electrical power distribution systems, with newer aircraft models featuring enhanced features and capabilities. The Bus Tie Breaker remains a critical component of these systems, providing reliability and redundancy to ensure the highest levels of safety and operational integrity.

For More: What is MLS on Boeing 737? (Microwave Landing System)