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| forces created may be so high as to damage or destroy the breaker. Hence, the unit must be built ruggedly enough to withstand these forces, the limits of which are included in the rating of the breakers and are referred to as the interrupting capacity or interrupting duty of the breaker. |

| 3. Ratings of circuit breakers specify the operating voltage of the circuit in which they may be connected, the normal operating or maximum load current that will not result in heat values damaging to the breaker, and maximum abnormal or fault current that may be safely interrupted. |

| 4. Circuit breakers are generally filled with oil, not only as an insulating medium, but also to aid in quenching arcs and in cooling the unit (Fig. 8-4). Other media are askarels (where fire hazards may be high), air (Fig. 8-3), vacuum (Fig. 8-5), and sulphur hexafluoride (Fig. 8-6). Arcs resulting from opening breakers while current is flowing may be broken into a series of small arcs, called deionization, that may be more easily quenched. |

| 5. The material, shape, method, and speed of operation of the contacts are very important as all these factors determine the rating of breakers, including their interrupting duty. Contacts may be of the butt, (Fig. 8-9), wedge (Fig. 8-10), brush (Fig. 8-11), or bayonet types (Fig. 8-12). |

| 6. Tanks are usually of steel and conductors are carried through by means of bushings, insulation surrounding the conductors. The insulation of the internal conductors and of the bushings is coordinated so that failure may be more apt to occur outside the tank where repairs or replacement can be more easily accomplished. All insulation must withstand line or lightning surges of voltage. For greater safety, tanks and supporting structures should be connected to one or more grounds. |

| STUDY QUESTIONS |

| 1. What is the function of a circuit breaker? |

| 2. What is meant by a short circuit? |

| 3. What happens when short-circuit current flows through a circuit breaker? |

| 4. What is meant by the interrupting duty of a circuit breaker? |

| 5. How are circuit breakers rated? Why? |

| 6. What is the function of the oil in an oil circuit breaker? |

| 7. What other types of breakers are there besides oil circuit breakers? What are some of their advantages? Disadvantages? |

| 8. What are the principal types of circuit breaker contacts? |

| 9. What are explosion chambers? De-ion arc quenchers? How does each function? |

| 10. What is an oil circuit recloser? How does it operate? |

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