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Page 170

| Oil Test. |

| The condition of the oil may be determined in the field by two simple tests. These are similar to those for transformers and are described in Chap. 2. |

| Megohmmeter Test |

| This test is also known as a Megger test. A Megger (trade name) is a small dc generator, turned by hand or motor, that is connected to the terminals of the circuit through a meter calibrated to read directly the megohms (millions of ohms) in the circuit. See Chap. 2. |

| In testing the bushings of a breaker, the breaker contacts are in the open position, and the Megger is connected between the terminal of one bushing at a time and the tank. A comparison of readings of the several bushings over a period of time will give an indication of the condition of the bushings. |

| The Megger can also be used for an overall check of the breaker. Again, with the breaker in the open position, the Megger is connected across the breaker pole, that is, one lead of the Megger is connected to the incoming bushing terminal and the other lead to the outgoing bushing terminal. The reading of the Megger will then indicate the insulating condition of the two bushings and the condition of the oil in the breaker (Fig. 9-10). |

| Figure 9-10 Megger testing of circuit breaker insulation. |

| Power-Factor Test |

| The condition of a bushing may be determined by this test. The bushing may or may not be removed from the circuit breaker, but is must be disconnected from the line and the breaker must be in the open position. The power factor is obtained by reading volts, amperes, and watts in the circuit (Fig. 9-11). The instruments and instrument transformers are sometimes contained in one package, with only a power factor indicating scale and voltage-control device mounted on the package. Power-factor values cover a range of a few percent, perhaps 10 percent. A single reading does not necessarily determine the condition of the bushing insulation. As it deteriorates, more current will |

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