• Circuit breakers "break" or interrupt an electric circuit when there is a fault or an overload. This tripping action protects the circuit and connected equipment from damage because it stops the flow of electricity and cuts off any short circuits or other electric faults. In this way, circuit breaker action is similar to that of a fuse except that a circuit breaker does not have to be replaced after a fault and can be closed again or reset. Three-pole circuit breakers are used in three-phase circuits, mostly for commercial and industrial applications.
  
  

Ratings

• Three-pole circuit breakers interrupt the three live conductors of a three-phase circuit. They must be able to conduct the rated current at the rated voltage safely. Voltages range from 480 to as high as 800,000 and currents from 100 to 4000 amperes. The breakers must also be able to interrupt short-circuit currents of 10,000 amps to 200,000 amps and do so without sustaining any damage so that they can be closed again and used further. Their interrupting characteristics are detailed on curves that give the amount of time a circuit breaker will carry a particular overload or short-circuit current before tripping.
  
  

Voltage

• A circuit breaker's voltage rating is usually the maximum operating voltage that could be expected to occur in the circuit. For commercial and industrial applications, circuit breakers are usually rated at 480 volts or 600 volts for three-phase circuits. Larger factories and electrical distribution networks can have circuit breakers rated up to 30,000 volts and electrical transmission lines operate at up to 800,000 volts.
  
  

Current

• The rated current of a circuit breaker is the current it can carry continuously without tripping. If the current in the circuit increases to more than the rated current, the breaker will trip after a time given by its tripping curves. If the current is slightly more than the rated current, the breaker will typically trip after several minutes. If it is two or three times the rated current, the breaker will trip in seconds. If there is a short circuit, the breaker will trip almost instantaneously. For common commercial and industrial applications, typical ratings are 100 amps to 600 amps. For higher power applications, breakers can be rated at up to 4,000 amps.
  
  

Interrupting Capacity

• The circuit breaker must be able to break a current up to the short-circuit current that can occur in the circuit. This ability is the interrupting rating of the breaker and, for common commercial and industrial applications, it is typically 25,000 amps to 60,000 amps. The short-circuit current is determined by the characteristics of the three-phase network's connection to the local electric utility must be calculated by a qualified professional. For higher power applications, breaker interrupting ratings can be up to 200,000 amps.