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Page 222

| Figure 12-3 Autovalve lightning arrester. (Courtesy, Westinghouse Electric Co.) |

| Thyrite Arrester |

| The thyrite arrester consists of a stack of thyrite discs in series with a number of small, interspersed air gaps (Fig. 12-4). Thyrite is a dense, homogeneous, stable, inorganic compound of a ceramic nature and mechanically strong. It possesses the characteristic of being substantially an insulator at one voltage and then changing to an excellent conductor at a higher voltage; the transition is due to voltage changes only, not to heat as in other valve materials. For each doubling of the voltage applied, the current that it will pass increases some twelve or more times. |

| These arresters are particularly suited for large high-voltage applications, such as may be found in substations of electric utility systems. They are built in modules or units, each rated at approximately 11,500 V; the modules are stacked in series to obtain the desired voltage ratings in 11,500-V steps above that rating. |

| Granulon (or Isolator) Arrester |

| The granulon differs from other valve arresters in that the valve element (also of a ceramic type) and the series gap are enclosed in clear pyrex glass, making them visible for inspection. The action of this arrester under surge voltage conditions is the same as for other valve element arresters. High-voltage surges spark across the air gap, and discharge current flows through the valve to ground. Since the valve has a low resistance under high voltage and a high resistance at normal voltage, the arc is extinguished and the arrester returns to its original state. |

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