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| the arc out of the tube, returning the arrester to its original conditionready for the next operation. In later types, the external gap is sometimes omitted. |

| Ground Connection |

| The effectiveness of any arrester is dependent on a good connection to ground, and such connections must be tested frequently. This cannot be emphasized too strongly as arresters will not function without a proper ground; they are totally useless. Even worse, if they are not known to be in this state, supposedly protected equipment may be severely damaged or completely destroyed. If possible, arresters should have their own separate ground connection, usually the shortest and most direct path from the arrester to ground. They may also be connected to other common grounds. Take care, however, about the use of water pipe grounds. As mentioned before, the use of plastic water pipe has reduced the effectiveness of water systems as ground sources. |

| Location of Arresters.. |

| As a good general rule, the arrester should be placed as close as conveniently possible to the equipment, cable, or line that is to be protected. This rule may be modified for any of a number of practical considerations: construction hazards, existence of high trees or other tall obstructions, difficulty of access, and other local factors. Each installation should be considered individually since what might provide sufficient protection in some localities, on some circuits, and under certain circumstances may not prove sufficient in others. |

| Other Considerations. |

| In addition to considerations previously discussed, other points should be kept in mind: |

| 1. Lightning and other electric surges travel at practically the speed of light, and hence a few feet more or less of circuit have relatively little effect on the protective performance of an arrester. |

| 2. The characteristics of voltage surges are affected by changes in the characteristics of the circuit over which they travel. At such points of change, the surge may be reflected, sometimes with an attendant piling up of voltage. Figure 12-7 is a graphical representation of a traveling wave on a transmission line showing two possibilities of voltage surges occurring at points of discontinuity, such as an open switch, a transformer bank, or change of overhead to underground. In Fig. 12-7A, the |

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