Extinction of a violent, high energy arc drawn between circuit interrupting contacts opening in air to interrupt the flow of large fault currents is an extremely formidable task. Such an arc must be extinguished quickly to prevent damaging consequences to the electrical wiring and load, as well as to the circuit interrupting device itself. As the current availables are increased by the electrical utilities, the potential magnitudes of fault currents correspondingly increase, and thus the current interrupting capacities of circuit interrupting devices must be upgraded commensurately.
For low voltage alternating current circuit interrupting devices, i.e., up to 600 volts AC, it is common practice to utilize an arc chute composed of a stack of closely spaced metallic plates positioned along the path of an arc drawn between the contacts as they are opened to interrupt current flow in a circuit. The magnetic field associated with the arc current interacts with the arc plates to draw the arc into the chute. As the arc encounters the arc plates, it is chopped up into a series of arclets drawn between adjacent plates, each arclet having its own anode-cathode voltage drop contributing to an increase in the arc voltage. Moreover, the arc plates function to cool the arc, thus enhancing deionization with consequent increase in the arc path resistance. The arc plates also cause a turbulent mixing of the hot arc gases and the relatively cool air in the spaces between the plates through which the arclets move, thus further promoting deionization.
For a successful interruption, all of these factors must have the cumulative effect of not only extinguishing the arc, but also of sufficiently deionizing the gases in the gap between the separated contacts so as to prevent reignition or restrike of the arc into the next current half cycle after a current zero. In other words, the circuit interrupting device must operate to raise the voltage gradient in its contact gap to a level permanently in excess of the system recovery voltage to prevent arc restrike.
It is believed, in the case of extremely high fault current interruption, that the arc is so large that the arc plates are ineffective in chopping the arc into arclets. That is, ionization of the gases is so complete that, when the arc encounters the front edges of the plates, it does not root on the plates, but simply travels around the front edges of the plates. The arc thus assumes a serpentine path as it bulges outwardly in the spaces between arc plates. This serves to elongate the arc and thus increase path resistance, but the benefit of the anode-cathode voltage drops associated with the creation of arclets is not thought to be achieved in the case of extremely high energy arcs. Consequently, the major contribution of the arc plates in this situation is in achieving deionization of the arc path through cooling of the arc.
Regardless of whether or not arclets are formed, the extreme heat of the arc vaporizes the metal of the arc plates. Ideally this metal vapor along with the other arc gases would be exhausted out of the back of the arc chute before condensing. Unfortunately, this is not the case in practice. The condensed vapor forms molten metal spheres which can and in fact do weld as metal particles to the arc plates. Further condensation causes a build-up in the sizes of these welded particles to the extent that, on occasion, they become so enlarged as to actually bridge the gaps between adjacent arc plates. The existence of these welded particles saps the dielectric strength of the arc chute, and, if extensive, will jeopardize the gap dielectric strength to the extent that arc restrike may not be prevented. Even if restrike is avoided, the condition of the arc plates will be such that a subsequent attempt to interrupt a large fault current will so exacerbate the number and sizes of the welded metal particles that arc restrike can not be avoided.
It is accordingly an object of the present invention to provide an improved arc chute for circuit interrupting devices.
A further object is to provide an arc chute of the above-character which is capable of extinguishing a violent, high energy arc while maintaining sufficient dielectric strength to prevent arc restrike.
Yet another object of the present invention is to provide an arc chute of the above-character which is capable of maintaining its dielectric integrity over plural high fault current interruptions.
A still further object is to provide an arc chute of the above-character which is equipped to discourage the welding of condensed metal particles to its arc plates.
Other objects of the invention will in part be obvious and in part appear hereinafter.