A breaker is a circuit interrupt device that will stop the flow of current and voltage when the breaker exceeds its amperage value labeled on the switch itself. For instance if the breaker is listed as 20 amps and more than 20 amps is being drawn from the circuit, it will trip and disconnect the power. This prevents the wires from overheating and melting, causing shorts or fire.
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Two Pole Breakers
A two pole breaker takes up two spaces in the electrical panel. It feeds power to two wires connected to the breaker from each phase. Individually the wires carry 110 volts each. If measured together they carry 220 volts. There are two kinds of double pole breakers, internal and external trip. Both breaker types are considered common trip breakers because if one leg trips the other is forced off as well.
Two Pole Breakers With One Switch
A two pole breaker that has only one switch is called an internal trip breaker. The two phases disconnects are tied together inside the breaker itself so that if one phase trips it kills power to both. Because the two phases are tripped inside the breaker, it's considered an internal trip. Resetting the single switch will reset both phases inside of the breaker.
Two Pole Breaker With Two Switches
A two pole breaker with two switches is an external trip breaker. There is a clip binding the two switches together, either plastic or metal. If one of the phases trips, the switch on that phase will move toward the off position of the switch and force the second breaker off via the clip connecting them. Because the action of turning off the second phase happens outside of the breaker via the connection clip, it is considered an external trip.
QO/QC and Homeline Models
A two pole one switch breaker is the more expensive QO/QC model. The two pole two switch breaker is the Homeline model. Both will do the same job and have the similar specs. The Homeline model was Square D's attempt to compete with such brands as Siemens and Murry's interchangeable breakers. Some breakers will fit in other panel brands and models. However, according to the panel-board manufacturer's printed labels, your only supposed to replace breakers with the listed breaker types on the label.