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Page 228

| 6. An arrester is only one of many devices for the protection of a circuit or equipment; other devices include circuit breakers, reclosers, fuses, and the like. For the proper protection of the circuit or equipment, all of these devices must have their characteristics coordinated so that each of them may operate correctly and positively. |

| Insulation and Arrester Coordination |

| Schemes for the protection of lines and equipment are usually based on the protection of insulation against failure. |

| The failure of insulation may result from overheating, which first causes the insulation to deteriorate and ultimately to fail. Overheating is usually allied to the magnitude and duration of the current flowing in the conductors. Protection against it is achieved through the speedy operation of fuses and circuit breakers that cut off the damaging current. |

| Insulation may also fail from punctures caused by the application of voltages of a magnitude and duration greater than those for which the insulation was designed (including the factors of safety and manufacturing tolerances). The level of insulation is not based on the supply voltage, which it must withstand under normal operations, but on the peak voltages that might be expected from surges capable of occurring on the electric supply circuits. Such surges usually are caused by lightning or improper switching. Protection is achieved by draining the surge to ground fast enough to prevent damage to the insulation of the equipment or line being protected. Hence, the characteristics of the arrester must be such that it operates well below the basic insulation level that has to withstand any surges that may be imposed on the equipment or line being protected. |

| Basic Insulation Level (BIL) |

| The coordination of insulation requires that the insulation of all components of a system be above a minimum level and that a selected protective device operate satisfactorily below that minimum level, known as the basic insulation level, or BIL (Fig. 12-8). |

| The minimum level of insulation must withstand not only the normal operating voltage applied continuously, but also the surge voltages that may exist for a comparatively very short time but can nevertheless cause damage. To select the value of this surge voltage, it is necessary to define its rise to its peak value and its fall back to lower values in terms of time, that is, its duration, for, although the peak voltage may rise considerably above the normal voltage, the insulation need sustain its stress only for an extremely short period of time. Put another way, the insulation need not be such as to |

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