1. Field of the Invention
The invention relates generally to electrical apparatus and more particularly to circuit interrupters utilizing a blast of arc-quenching fluid to extinguish an established arc.
2. Description of the Prior Art
As is well known by those skilled in the art, fluid blast circuit interrupters effect the interruption of alternating current circuits by establishing an arc between separating contacts located in the vicinity of a nozzle. This arc provides a high-conductance path between the contacts until the arc is extinguished by a blast of arc-quenching fluid directed against the arc. As the alternating current passes through current-zero, the arc-extinguishing fluid acts to transform the arc channel into an insulating medium. For the interruption to be successful the blast of fluid must transport energy away from the arc channel at a faster rate than the arc is delivering energy into the channel. If this is not the case the phenomenon of energy clogging occurs, delaying or perhaps preventing the successful interruption of the arc. In addition, a high-energy arc contributes to the erosion of contacts and general degradation of performance of the circuit breaker, increasing the need for maintenance. Since arc energy is directly related to arc length, it is desirable to maintain a short low-energy arc during the time preceding the first current zero.
In previous practice, long expanding nozzles were used in order to obtain desirable flow characteristics and adequate mixing of arc-quenching fluid in the nozzle. The requirement of a short low-energy arc in a circuit interrupter using a long nozzle configuration called for an arc drawn transverse to the axis of the nozzle in an area upstream from the nozzle throat. Often steady-state contact fingers, sealing valves, and other mechanisms were present in this area resulting in points of high electric field concentration. In order to assure that an arc would not be reignited following successful interruption at the first current zero and to ensure that the interrupter will pass the requirements of basic impulse level (BIL) testing, this high electric field concentration made it necessary to maintain a post-interruption nozzle separation greater than that required for optimum dynamic operation.
In order to transform the arc channel within the nozzle structure from a high conductance medium into a good insulating medium it is necessary that arc products be rapidly cleared from the chamber by the blast of arc-extinguishing fluid. The degree to which this can be accomplished determines the allowable rate of rise of recovery voltage (RRRV) which the interrupter can withstand. Since the long expanding nozzle structure results in a clearing time greater than would be required for a shorter nozzle, better rate of rise of recovery voltage performance is obtained from the latter.
Good arc control and arc extinguishing characteristics can be obtained if the arc is established and maintained axially with regard to the nozzle. It is known in the art that control of arc length during the interruption process is helpful in obtaining successful interruption. In U.S. Pat. No. 3,659,065 issued to the applicants, this was achieved in a puffer type interrupter by separately controlling fluid blast production and contact separation. It would be desirable to achieve independent control of all mechanisms involved in the interruption process.
In designing circuit interrupters for different ratings and classes of service different operating sequences for valves, nozzles, and contacts may be required. In order to achieve the lowest possible production costs interrupters made for different classes of service should have a high degree of parts commonality. Therefore it would be desirable for a circuit interrupter to achieve axial arc position and to control arc length, fluid blast initiation, contact position, and other operating parameters while providing for versatile interruption mechanism sequences with a minimum of parts changes.