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| Expulsion Arrester |

| In the expulsion arrester, there are two series gaps, one of them being the usual gap in series with the valve element (Fig. 12-6). The valve element is essentially another gap in series with the first, although it is contained inside a fiber tube, with an electrode placed at each end of the tube. It is so designed that the voltage breakdown through the tube will be less than that in the insulation of the line or equipment to be protected. When a voltage surge occurs that is sufficient to spark over the series gap and the gap in the fiber tube, discharge current flows to ground. The arc in the tube attacks the fiber, releasing large volumes of a relatively cool, nonconducting gas. The gas produced acts not only to extinguish the arc but also builds up pressure that blows |

| Figure 12-6 Expulsion lightning arrester. |

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