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Page 219

| 4. Not be damaged by the discharge and be capable of automatically repeating the discharge action as soon and as frequently as called upon to do so. |

| Principle of Operation |

| The principle upon which lightning or surge arresters usually operate is that when a voltage surge traveling along the conductor reaches the point at which an arrester is installed (usually at some equipment or line to be protected), it finds a lower resistance path to ground than that presented by the equipment or line. The voltage surge usually breaks down the insulation of the arrester momentarily, allowing the voltage surge to travel to ground and dissipate itself. The insulation of the arrester then recovers its properties and prevents further current from flowing to ground. |

| Arrester Types. |

| The elementary arrester consists of an air gap in series with a resistive element. The high-voltage surge causes a spark to jump across the air gap and pass through the resistive element to ground (Fig. 12-1). One resistive element, sometimes referred to as a valve is usually made of a material that provides a low resistance path for the high-voltage surge but presents a high resistance to the flow of line energy at normal voltages. Arresters of this kind are referred to as valve arresters. |

| Another resistive element consists of an explusion chamber in which the surge, after traveling through the series air gap, permits current to flow through a tube in which the gases formed blow out the arc, thereby restoring the high resistance to the flow of current from normal voltage. The tube or chamber is made of, or lined with, a fiber material. When this material comes in contact with the arc, it generates gases that increase the pressure in the chamber and help blow out the arc, thereby restoring the arrester to its initial conditionready for another operation. |

| There are many different types of arresters, all of which have a resistive element and may or may not have a series gap. They differ only in mechanical construction and in the type of resistive element employed. |

| Valve Arresters |

| Pellet Arresters |

| In the pellet arrester (Fig. 12-2), the series gap is contained within a porcelain receptacle, and the valve element is made up of a column of pellets some 2 inches in diameter with a height varying according to the voltage rating |

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