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| Induced-Voltage Test |

| The purpose of this test is to check the insulation between turns, layers, and sections of the winding. A voltage usually equal to twice the rated voltage is applied to the low-voltage windings with the high-voltage winding left open-circuited. To limit the exciting current, voltages having frequencies above the rated (60 cycle) are used. The voltage is raised gradually and held for about 1 minute and reduced gradually before the circuit is opened. The time the voltage is held is decreased as the higher frequency voltages are used. The diagram for this connection is shown in Fig. 7-12. The sphere gaps connected |

| Figure 7-12 Connection for induced voltage test. |

| across the open-circuited winding act as a safety precaution for high-voltage tests over 50 kV. For windings designed for operation with one end solidly grounded, this test is the only low-frequency insulation test that can be made. |

| Insulation-Resistance Test.. |

| This is the often mentioned Megger* test. It is meant to give some indication of the condition of the insulation, and is often used in maintenance procedures. The connections are the same as for the high-voltage test, shown in Fig. 7-9. The insulation resistance of a transformer depends largely on the temperature and cleanliness and dryness of the windings. Insulation resistance should be at least 1 megohm (1 million ohms) for each 1000 volts of test voltage. If it falls below this figure, presence of dirt or moisture may be indicated. |

| * A Megger is a small dc generator, turned by hand or motor, which is connected through a meter (mounted on the case) calibrated to read directly the megohms in the circuit connected to the terminals. See Chapter 4. |

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