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Page 165

| 1. Contacts and associated arc-quenching devices |

| 2. Insulation, including oil, insulating partitions or spacers, and bushings |

| 3. The mechanisms, including closing and opening devices and control circuitry. |

| Contacts and Arc Quenchers |

| Contacts should be properly aligned and adjusted, if necessary, so that contact surfaces bear with firm, uniform pressure. One of the best ways to check contact impression is to close the contacts on a piece of thin tissue paper and a piece of thin carbon paper, with the carbon next to the tissue. When the breaker is closed and opened, it is a simple matter to determine the amount of impression made on the paper (Fig. 9-7). |

| Silver-to-silver contacts normally require little maintenance but should be resilvered and dressed to maintain a line contact. Badly pitted or burned contacts should be replaced before causing damage to other parts. If the contact surfaces are only roughened, they may be smoothed down with clean fine files or sandpaper. Arcing tips are normally replaced while the main contacts are being maintained. Arc-chutes, explosion chambers, and de-ion grids should be cleaned. Badly projecting tips and scale (the result of pitting) should be filed or scraped off. |

| Figure 9-7 Checking contact impression: (A) This contact needs no adjustment since the impression of each bar is satisfactory (75 percent or more of contact length); (B) adjustment needed since impression shows that contacts are not closing properly. |

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