The extinction of an a-c. arc is analyzed as depending on two factors, the rate of recovery of dielectric strength of the arc space after current zero, and the rate at which voltage tending to reignite the arc is applied by the external circuit. In the short arc, most of the recovered dielectric strength resides in a deionized layer next to the cathode but in the long arc the rest of the arc space contributes largely to the dielectric strength. The breakdown gradient of the still ionized arc space is defined, and using a thermal ionization theory, a formula for growth of breakdown gradient is derived. The extinction of long a-c. arcs in the open is greatly influenced by the sectional area which the arc stream has at current zero. By confining arcs to slots and holes, the rate of deionization at current zero is greatly increased, and so large voltages per cm. of arc can be interrupted. A gas blast passing turbulently through an arc stream greatly accelerates deionization at current zero and so is effective in increasing the capacity of the a-c. arc to interrupt high-voltage circuits. The expulsion fuse is an example of a gas blast circuit interrupter, the gas blast resulting from the decomposition of the fiber fuse case. The oil circuit breaker is also a gas blast circuit interrupter, the blast arising from the gases produced by the decomposition of the oil.

Extinction of a Long A-C. Arc

Various DC power supply systems such as photovoltaic power generation, fuel cell etc. have become gradually popular. Under such circumstances switches for electric appliances have to interrupt DC current in DC power systems (for example, DC 300–400V) DC current is much more difficult to be interrupted than AC current which has current-zero, so magnetic blow-out is usually used to extinguish arc discharge. In this paper a model switch is developed with three levels of magnetic blow-out, and gap width and arc length at arc extinction are obtained by a high-speed camera in range of current less than 5 A and voltage lower than 500V. These results are compared with those without magnetic blow-out and the followings can be made clear. When the contacts open, the gap width and arc length at extinction are much shorter than those without magnetic blow-out. As the interrupted current increases, the gap width at arc extinction approaches almost the same value, which is independent of source voltage and separation velocity. However, the arc length at extinction has a minimum value in case of source voltages of 300 and 500V, while the arc length becomes longer with interrupted current increase in case of source voltage of 100V.

Fundamental characteristics of arc extinction by magnetic blow-out at DC voltages (<500V) II

An interrupter has been developed for a 50-ka (kiloamperes) 600-volt industrial circuit breaker, capable of being mounted, four-high, in a switchboard with a height of 90 in. (inches), and of meeting unusually onerous interrupting-test duty requirements. The results are summarized of short-circuit tests conducted on experimetal arc chutes containing a variety of metallic and nonmetallic baffles and cooling rods. Their performance characteristics are illustrated by oscillograms correlated with excerpts from high-speed cine film records.

Observations on the Development of Arc Interrupters of Small Pysical Volume for Industrial Circuit Breakers with Large Interrupting Ratings

Power circuit breakers for voltage services of 2,300 volts and above use either of two basic principles to complete circuit interruption. The more familiar types, oil breakers or compressed air breakers, complete their operation by maintaining a low resistance arc until the final current zero where interruption is completed. Magnetic air circuit breakers establish a high resistance arc prior to circuit interruptions which modifies the circuit constants before recovery voltage is established. The ``Magne-blast'' breaker falls in this second class of breakers. The treatment of the arc in this breaker to obtain a high are resistance is explained in some detail. Also, design practices and operating characteristics of the final magnetic type breaker are reviewed.

The Development, Design, and Performance of Magnetic-Type Power Circuit Breakers

A new approach to the analysis of the performance of d-c interrupters has been made in which the arc voltage of the interrupter is considered on an equal footing with the d-c generated voltage. The effect of variations in the circuit constants as well as circuit breaker speed and arc voltage wave shape and magnitude are evaluated on a quantitative basis. The equations and curves are presented on a per-unit basis which makes the work applicable to any d-c interruption problem. The work sheds light on design and application criteria of d-c interrupters. The presentation is arranged to be easily digested by the student of electrical engineering.

Performance Criteria of D-C Interrupters

Magnetic blow-outs have been used in contactors, circuit breakers and controllers for many years for rupturing both a-c. and d-c. power circuits, but their commercial use, particularly on alternating current has been largely confined to relatively low voltages. Oil circuit breakers and switches have been generally used for rupturing high-voltage a-c. power circuits, and their development has reached a high state of perfection. The air break has the advantage of avoiding the possibilities which attend the use of any inflammable material?like oil, with its possible gasification and explosion on heavy short circuits. While there are many different types of magnetic blow-outs this paper deals largely with the ``individual'' type, in which a blow-out coil is connected in series with each pair of current-rupturing contacts, since it is with this type that most of the progress and studies have been made in recent years. Contactors and circuit breakers with the ``individual'' type of blow-out are now used almost exclusively in the main d-c. power circuits of the 1500 and 3000-volt d-c. railway systems. Oil circuit breakers have been tried for this service, but they are rather unsatisfactory because there is no periodic zero point in the current wave at which the oil can form an insulating seal between contacts. The oil under d-c. arc conditions carbonizes rapidly and involves the possible danger from explosive gases. Recently the use of magnetic blow-out contactors on a-c. circuits has been extended to moderately high voltage and capacity. Short-circuit tests on a 6600-volt, 26,700-kv-a.

J. F. Tritle

Papers

Air-Break Magnetic Blow-Outs For Contactors and Circuit Breakers Both A-C. and D-C