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| each contact not only displaces the oil but decomposes it, creating gas and a carbon residue. If these carbon particles were to remain in place, as a conductor they would tend to sustain the arc formed. However, the violence of the gas and the resulting turbulence of the oil disperse these particles, and they eventually settle to the bottom of the tank. The rate of expansion and the volume of gas generated depend to a great extent on the energy to be interrupted, the speed of the separation of the contacts, the cooling effect, and the quantity of oil used as the insulating medium. |

| The use of oil as insulation also permits smaller distances between live parts and between live parts and ground (the structural and mechanical support parts of the circuit breaker) because of its better insulating qualities (higher dielectric strength) in comparison to air. The insulating oil normally used as a dielectric strength of around 30 kV per one-tenth of an inch (compared to a similar value of 1 kV for air). Oil is also an effective cooling medium. Askarels sometimes replace oil when required because of special fire hazards; refer to Chap. 2. |

| Vacuum Circuit Breaker |

| In this type of breaker, the contacts are drawn apart in a chamber from which air has been evacuated (Fig. 8-5). The electric arc is essentially an electric conductor made up of ionized air. Thus, if there is no air, theoretically the arc cannot form. In practice, however, a perfect vacuum is not likely to be obtained. The small residual amount of air that may exist permits only a small arc to be formed and one of only a very short duration. The same vacuum, however, will not dissipate the heat generated as readily as other insulating media. Although this type of breaker has certain advantages in terms of its size and simplicity, its interrupting ratings are not comparable to those of oil circuit breakers. Furthermore, maintenance procedures are more complex and, hence, more costly. |

| Figure 8-5 Vacuum circuit breaker. |

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