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| Figure 8-22 Schematic diagram of insulation coordination. |

| To provide adequate insulation without resorting to excessive and expensive insulating material, the insulation requirements of the several parts must be carefully coordinated. In general, this means that the internal parts are insulated as nearly equally as practical, but usually with more strength than the bushings (including any current transformer, potential transformer, or other accessory that may be mounted on the bushing within the breaker chamber, and which may be enclosed in individual compartments to afford protection against damage during heavy interruption duties). This approach ensures that any breakdowns that may occur will be outside the tank where the damage will be comparatively light, localized, and easier to repair. |

| Furthermore, since the bushing is usually protected by an air gap or lightning arrester whose insulating value under surge conditions is even lower than its own, any failure that occurs will be caused by flashover across the bushing and the tank rather than by puncture of the bushing insulation (Fig. 8-22). Finally, the insulation of the weakest point in the breaker should be weaker by a large enough margin than that of the principal equipment it is protecting. Such a coordinated arrangement of insulation tends to restrict the damage to main pieces of equipment. |

| Insulation coordination requires that the insulation of all components of a system should exceed a minimum level and that lightning or surge arresters or other protective devices operate below that insulation level. This is known as the Basic Insulation Level (BIL) and will be discussed further in Chap. 12. |

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