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Page 133

| Types of Breakers Categorized by Insulation Medium |

| Air Circuit Breaker |

| An air circuit breaker employs air as the interrupting insulation medium. Of all the insulating media mentioned, air is the most easily ionized and, hence, arcs formed in air tend to be severe and persistent. |

| The switching elements, for an air current breaker, therefore, may often consist of main and auxiliary contacts. The auxiliary contacts open before the main contacts do, and the arc is drawn on them, thereby avoiding severe pitting of the main contacts (Fig. 8-3). As the contacts separate, a puff of air is sometimes used to blow the arc into arc chutes. Moreover, the current itself is made to flow through coils that set up magnetic fields that force the arc into these arc chutes. An electric arc is essentially an electric conductor with a surrounding magnetic field; hence it can be attracted or repulsed by another magnetic field. |

| The arc chutes are usually made of corrugated insulating material (such as porcelain or slate). Their function is to stretch out the arc until it can no longer sustain itself. In simple form, it is an enclosed channel in which the arc is dissipated and kept from striking surrounding objects. |

| Breakers of this type are usually employed in electric systems where fault |

| Figure 8-3 (A) Air circuit breaker, and (B) schematic showing main and auxiliary contacts and arcing arrangement. |

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