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| ment or line. The voltage momentarily breaks down the insulation or the arrester, allowing the surge to travel to ground and dissipate itself. |

| 2. The insulation which breaks down momentarily and then restores itself is sometimes called a valve, but sometimes may consist of an air gap and resistor (Fig. 12-1). |

| 3. The valve element may consist of lead oxide pellets (Fig. 12-2), ceramic material containing conducting particles, or such patented materials as thyrite (Fig. 12-4) and granulon (Fig. 12-5). |

| 4. In an expulsion arrester, the arc is formed within a fiber tube which is attacked by the arc releasing large volumes of relatively cool nonconducting gas that flows out the arc (Fig. 12-6). |

| 5. The effectiveness of any arrester is dependent on a good connection to ground, hence such connections must be tested frequently as arresters will not function and are totally useless without a proper ground. |

| 6. Arrester characteristics must be coordinated with basic characteristics of the insulation of the lines and equipment they are to protect so that the arrester operates before the insulation is damaged, and before fuses or circuit breakers operate. |

| 7. The coordination of insulation requires that the insulation of all components of a system be above minimum level and that a selected protective device operate satisfactorily below the minimum level, known as the basic insulation level, or BIL (Fig. 12-8). |

| STUDY QUESTIONS |

| 1. What is the function of a lightning or surge arrester? |

| 2. Describe the principle of operation of an arrester. |

| 3. What types of arresters are commonly used? |

| 4. What is the function of the fiber lining in an expulsion fuse? Where is this type of arrester commonly used? |

| 5. Why is a good ground of the greatest importance? |

| 6. Where should an arrester be located? |

| 7. What conditions can produce electrical surges in an electrical circuit? |

| 8. What is meant by the Basic Insulation Level? |

| 9. What is meant by a standard surge voltage wave? |

| 10. What other considerations should be taken into account in selecting and locating an arrester? |

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