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Page 146

| External Current-Limiting Methods |

| If the fault current to be interrupted can be reduced, the strength and duration of the arcs and the stresses applied to the breaker and its parts can likewise be reduced. Resistances and reactors are sometimes installed in a circuit to reduce current flow. Under normal conditions, the voltage drop produced by these means is relatively small and has little effect on the operating voltage of the circuit at the point of utilization. With extremely large currents flowing, however, the voltage drop becomes considerable, thereby limiting the current flow. Moreover, the lowered voltage across the breaker contacts tends to prevent arcs from persisting. These methods will be discussed more fully in Chap. 16. |

| Insulation Requirements |

| In addition to design considerations providing for mechanical and structural stresses in circuit breakers, it is essential that their current-carrying parts be properly insulated from their operating and structural parts. |

| For smaller air circuit breakers, the principal insulation is the panel upon which the main contacts are mounted. Slate, marble, and bakelite have been employed, as well as plastics that have been especially developed for this purpose. |

| In other circuit breakers, care must be taken to provide sufficient insulation between tank, moving contact or lift rod, and energized parts, as well as between the bushings that bring the energized conductors through the tank wall. |

| Tanks |

| Design of tanks for a circuit breaker must take into account insulation clearances, the depth in the case of oil, and the mounting of bushings. Although the main purpose of the tank is to contain the conductors, contacts, and insulation under both normal and abnormal circuit interruptions, it must also dissipate the heat produced by the currents flowing through the current-carrying parts as well as that produced by arcs. The distance between contacts, length of the stroke of moving contacts, insulation, mechanical struts, reinforcement, clearances between parts-all these factors contribute to the dimensions of the tank, which must be ample enough to provide for the large volumes of gas that may form and effectively reduce clearance distances. |

| The shape of the tank is governed by the circuit breaker service requirements. For moderate duty (to about 2500 amps and 15,000 V), all poles of a circuit breaker may be in one tank, and the tank may be rectangular in shape. Medium-capacity breakers (about 10,000 amps and 15,000 V) may be of |

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