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| Figure 7-10 Connection for impulse test. |

| value of the associated bushings. A high-voltage wave of standard values, and approximating a lightning surge, is imposed on the unit to be tested. The basic circuit is indicated in Fig. 7-10. The surge generator usually consists of a number of capacitors connected so that they be charged in parallel from a relatively low-voltage source and discharged in series to give a high voltage across the test piece. A standard impulse wave is illustrated in Fig. 7-11. |

| The standard wave reaches its peak voltage value in 1½ microseconds and reduces to half the voltage value in 40 microseconds. The value of the voltage applied depends on the rating of the insulation, and may vary from 5 to 30 times the voltage rating of the insulation. Since rather elaborate and costly equipment is needed, impulse tests are usually performed only on large station-type power transformers, and rarely made after they leave the factory. |

| Figure 7-11 Shape of standard 1½ × 40 microsecond wave. |

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