A Bus Coupler or tie breaker is just another breaker used to connect two lineups of switchgear that would normally be not connected. For example you might have two transformers feeding two sections of separated swithgear but on occasions want to feed both sections with the same transformer. In this case you could close the bus coupler, paralleling the two transformers and then isolate one transfomer for a "bumpless" transfer.
I'm not getting into the pros and cons of paralleling transfomers (phase shift, voltage, short circuit capability etc,
Over here 'bus coupler' describes a circuit breaker linking the main and reserve busbars of a dual bus system. By contrast a bus section switch or bus sectioniser links two halves of one busbar. The relative lack of dual-bus equipment outside of major transmission substations has led to a lot of people only familiar with single busbar equipment using the term bus coupler in lieu of bus sectioniser or bus section switch.
----------------------------------
If we learn from our mistakes I'm getting a great education!
