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Page 229

| Figure 12-8 The Basic Insulation Level (BIL). |

| withstand a continuous stress from the high peak voltage. A committee representing engineers, utilities, and manufacturers, has therefore, established standards for the characteristics of a voltage surge and recommended insulation levels for equipment of various voltage classes. |

| The surge voltage is defined as one that rises to its maximum or crest value in 1.5 microseconds and falls to one-half that value in 40 microseconds (a microsecond is one one-thousandth1/1,000of a second). This wave is illustrated in Fig. 12-9. For ease of reference, it is designated as a 1.5/40 wave. The steep rising portion of the wave is called the wave front and the receding portion, the wave tail. |

| Recommended insulation levels for a number of voltage classes are listed in Table 12-1. Note that the ratio of the BIL (capability to withstand peak |

| Figure 12-9 1.5/40-microsecond voltage wave. |

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