Bus Tie Breaker
A bus tie breaker is a circuit breaker within switchgear that connects two separate busbars or bus sections of the same voltage, frequency, and phase sequence. It allows them to be operated together or independently for reliability, flexibility, and maintenance. Typically normally open in double-ended systems, it provides redundancy, supports maintenance bypass, and enables fault isolation by segmenting the switchboard.
On a manufacturing shop floor with double-ended switchboards or medium-voltage switchgear, a bus tie breaker enhances reliability by connecting two bus sections fed from separate sources. Normally open, it isolates the sections so a fault or loss of one source does not affect the other. If one source fails, control logic can trip the failed incoming breaker and close the tie to transfer load to the healthy source, keeping critical processes like motors and compressors running. During maintenance, the tie can temporarily substitute for a feeder breaker, allowing reconfiguration without downtime. It also segments the switchboard to contain faults, preventing propagation between production lines. Interlocks and undervoltage relays ensure safe automatic transfer without paralleling off-site sources, making the tie a key engineered safeguard for continuity and safety.
- Incomplete isolation: Only isolating one bus while the other remains energized through a closed or bypassed bus tie, leading to live bus exposure and shock risk, violating lockout/tagout requirements.
- Using disconnect switches instead of proper bus tie breakers: Disconnects lack fault interruption capability and protective relaying integration, causing equipment damage and arc flash hazards under fault conditions.
- Operating status and protection miscoordination: Closing the tie without proper interlocks or protection settings can create unexpected fault paths, parallel energization of separate sources, and increased arc flash incident energy.
How should a bus tie breaker be treated in lockout/tagout procedures?
Under OSHA 1910.147 and WorkSafeBC Part 10, the bus tie breaker must be identified as an energy-isolating device on single-line diagrams and in isolation procedures. It must be locked out and tested for absence of voltage on both sides whenever either connected bus is worked on, as it can back-feed energy. Group lockout must include the tie breaker unless engineering assessment confirms it cannot energize the work area.
What protective relaying is typically associated with a bus tie breaker?
Bus differential protection must recognize when the tie is closed to treat both buses as a single zone. Undervoltage relays and transfer logic trip the incoming breaker of a failed source and close the tie for load transfer. When used as a maintenance substitute, relays must be reconfigured to ensure fault coverage. Misconfigured relaying can cause slow fault clearing, increasing arc flash risk.
When should a bus tie breaker be normally open vs. normally closed?
In double-ended, separate-source systems, the bus tie breaker is normally open to prevent parallel operation of off-site sources and limit fault propagation. In transfer bus schemes, it may be normally closed for alternate paths. The chosen status must be reflected in SOPs, arc-flash studies, and training, as closing the tie can create higher fault levels and new parallel sources.