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Page 138

| Construction |

| Tanks and Other Enclosures |

| The contact elements of circuit breakers are usually enclosed in metallic tanks, through which the conductors pass by means of insulating bushings; the conductors are usually made of copper rods. The tanks may contain a single contact element or two or more such elements in a common enclosure. Attached to the rods are stationary contacts that are either completely immersed or surrounded by the insulating medium. |

| The contacts of many air circuit breakers, however, are not enclosed in tanks or other receptacles. Often they are mounted on an insulating panel and exposed to the surrounding air. Sometimes insulating fireproof barriers are installed between the current-carrying breaker elements to prevent arcs from communicating between them. |

| In the case of oil circuit breakers, the tanks, are not completely filled, allowing some space in which the oil and gas generated may expand, particularly if a breaker is operating under fault conditions. The contacts of vacuum and sulphur hexafluoride breakers are contained in interrupting modules, which may or may not be enclosed within tanks or chambers (Fig. 8-8). These interrupting modules are interchangeable and may be removed for maintenance or other purposes. |

| Contacts |

| Movable contacts, operated from external mechanisms, bridge the stationary contacts and complete a circuit when its breaker is closed. These contacts are displaced from the bridging position when the breaker is opened, thus interrupting the circuit; they are usually held in the closed position by the action of a latch, lock, toggle, or similar device. When the circuit breaker has to interrupt a circuit, its mechanism usually receives a controlling impulse from a relay that is either electrically or manually operated or from a manually operated mechanical linkage. In this way, the restraining force is released, and the moving contacts recede from the stationary contacts under the action of gravity, accelerating springs, or both. |

| Types of Contacts |

| There are four principal types of circuit breaker contacts: butt, wedge, brush, and bayonet. |

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