Main-Tie-Main (3 Breaker Schemes)

Fig. 2 shows a 3-breaker scheme employed to service two motor buses from two alternate sources. Each source feeds a single motor bus through its main incoming breaker. A tie breaker is provided for coupling the two motor buses.

A typical example is that of a process industry, serviced by two separate stations SOURCES I and II, each capable to meet the load on both the Buses I & II, off the grid. The SOURCE I transformer is connected through I/C- I incoming breaker to BUS I. Similarly, SOURCE II transformer is connected through I/C - II incoming breaker to BUS II. BUS I and BUS II are connected using the TIE breaker. There are several bus transfer scenarios depending upon the choice of the normal supply to the motor buses.

  1. Normally closed TIE breaker: The entire motor bus comprising BUS I and BUS II is transferred between SOURCE I and SOURCE II.
  2. Normally open TIE breaker: Each source supplies power to a single motor bus. In case of source failure, the motor bus connected to the failed source is transferred to the source through the TIE breaker.

Since process continuity is the prime consideration in industrial plants, automatic transfers determined by different auto-initiation criteria for source contingencies as well as source equipment failure conditions are employed. Manual transfers are commonly conducted during planned start-ups and shutdowns. Typical breaker-failure logics safeguard the motor buses from a permanent paralleling position.