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| The insulation characteristics of the equipment (its impulse level) are represented by the voltage-time curve that the equipment insulation can withstand. The characteristic of the protecting arrester is also a voltage-time curve that indicates the voltage and time at which the series gaps of the arrester spark over and begin to pass the surge to ground. The insulation characteristics of the equipment must always be at a higher voltage level than that of the sparkover characteristic of the protecting arrester, and a sufficient voltage difference must exist between the two curves. These characteristics are shown in Fig. 12-11. |

| The impulse level of equipment must be high enough for the arrester to provide adequate protection and low enough to make insulation costs economically practical. |

| Although the insulation of electrical equipment may be subject to overvoltages from system faults, switching surges, and occasional higher-than-normal line voltages, these do not usually affect the insulation but may damage the protective device. The major reason for overvoltage protection is to limit the high-voltage effects of lightning surges. Insulation coordination, therefore, consists of the proper selection of arrester protective characteristics for a particular equipment insulation characteristic so that lightning surges will be discharged to ground without damage to the equipment insulation or to the protective device. |

| Since electric systems include a number of protective devices, each of which has characteristics of its own, in addition to the circuit conductors and the equipment connected to them, it is essential that the characteristics of all |

| Figure 12-11 Insulation coordination. |

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