【正文】 (Fig. 75) provide another scheme for circuit interruption. In all our discussions so far we have considered the behavior of the gas between the parting contacts. This gas is ionized by several different processes and so provides free electron, which move to the anode, and positive ions, which move to the cathode. The positive ions ing to the cathode provide a mechanism by which electrons e out of the metal either thermionically or by high field emission. Practically all these phenomena disappear if the gas is removed from the space between the electrodes, The arc is interrupted at the first current zero and dose not reignite, Vacuum breakers fail if leaks permit air to enter the interrupting chamber. The moving contacts are sealed by airtight metal bellow. Fig. 76 (a) and (b) illustrate the construction of 3AF and 3AG vacuum bre3kers, respectively. The 3AF and 3AG vacuum breakers consist of three vacuum interrupters, their supports and the operating mechanism. The slim1ine design of the vacuum interrupters makes it possible to install the breakers in space saving switchingboards. When the contact separates, the current to be interrupted initiates a metalvapour arc discharge and flows Through this plasma until the next current zero. The arc is then extinguished and the conductive metal vapour condenses within a matter of microseconds. As a result. the dielectric strength in the break bui1ds up very rapidly. The contact pieces are designed to generate a magic field which causes the arc root to rotate. This prevents their local overheating when interrupting large currents. In breakers for very high fault currents (50/63kA) ,an axial magic field ensures that the arc burns diffusely, at every current level, throughout the whole current range. Circuit Breaker Characteristics At a given voltage each insulation class requires minimum clearance and creepage paths. The minimum clearance path is the minimum distance that must be provided between conducting surfaces which differ in voltage。 it is primarily dependent on the dielectric strength of the insulating material. Creepage denotes the undesirable flow of current between conductors , along the insulation surface the minimum creepage paths are thus dependent on the environmental contamination. Besides clearance and creepage restrictions, the insulation classes impose diverse temperature limitations which must he respected at all times by the circuit breaker during its operation. The main contacts of a circuit breaker are monly made of copper, silver, or silver alloy, and are subject to diverse temperature limitations. Copper contacts are not normally fit for continuous operation。 this is called the rated maxmum voltage. The rated short circuit current of a breaker is the highest symmetrical current that it is capable of interrupting when used in a system carrying rated maximum voltage. At this point it is appropriate to introduce an additional restriction known as the K factor of a breaker. The relation above for the current interrupting capability holds provided the resulting value does not exceed K times the rated short circuit current, that is, as long as the system voltage is not more than K times smaller than the rated maximum voltage. Thus, under no circumstance does the currentinterrupting capability exceed K times rated short circuit current no matter how low the system voltage is. As an example, let a circult breaker have a rated maximum voltage of 4. 8kV, a rated sbortcircuir current of 6000A, and a K factor of . This breaker is capable of interrupting a current whose sym metrical ponent is 6000A when used in a system whose voltage is 4800V. For a system voltage of 4000V the current interrupting capability is 6000 4800/4000 = 7200A The highest currentinterrupting capability is 6000 =13200A which is available when the system voltage is 4800/=2180V For a system voltage smaller than 2180V, say 1800V, the current interrupting capability remains at 13, 200A. A circuit breaker is also capable of closing and latching a faulted circuit provided the resulting current does not exceed times the rated short circuit current, where K is the breaker Kfactor. In other words ,the maximum peak should not exceed 2. K times the rated short circuit current. This capability is often referred to as the momentary rating of the breaker. The latter is , of course, expected to interrupt tile circuit a short time later. The momentary rating of the breaker discussed above is 13,200 l. 6 = 21,100 Arms or 13,200 =35,600 A peak The rated continuous current of a breaker is that value of rms current at rated frequency which