Showing papers in "Transactions of The American Institute of Electrical Engineers in 1931"

Calculation of Synchronous Machine Constants- Reactances and Time Constants Affecting Transient Characteristics

Abstract: Recent advances in the theory of synchronous machines have introduced a large number of new constants. The method of symmetrical components requires sequence reactances, and an accurate theory of transients requires transient and subtransient reactances and time constants. Most of the published discussion on the constants has been concerned with the application, rather than the calculation of values. In this paper, all of the most significant constants are calculated, except the subtransient time constant. A general method of calculation is discussed in which the reactances are accurately resolved into components. Comparisons of test and calculated values are given. The formulas for salient-pole machines and turbine generators are given in Appendixes A and B. The principle of superposition is applied to resolve accurately the reactances into components which can be readily calculated. The induced currents in the field and additional damping circuits are accounted for simply by applying the constant interlinkage theorem.

Determination of Synchronous Machine Constants by Test Reactances, Resistances, and Time Constants

Abstract: In recent years there have been many advancements made in the art of analysis of three-phase synchronous machine operation. Most treatments to date have very properly stressed the theoretical point of view, the practical point of view receiving less attention due in part to the complications involved, particularly those due to saturation effects. In this paper the practical stand-point is stressed and discussed on the basis of observations made from many tests; an important feature of these tests is that saturation effects under short circuit were studied by applying short circuits to machines operating at rated voltage. Due to serious saturation effects, the so-called constants of synchronous machines may have different values under different conditions. What are considered to be the most important of these values are discussed, and by defining certain saturated and other values, an attempt is made to provide a definite basis for the specification of machine constants. Part I reviews certain basic concepts and discusses practical considerations of test methods. Part II describes methods of determining the more important constants by test. Part III illustrates the application of these methods to actual power machinery, and presents tabulated test results as well as a table of typical constants. The Appendixes are devoted to certain additional tests and conclusions, and to development of formulas.

Induction Motor Slot Combinations Rules to Predetermine Crawling, Vibration, Noise and Hooks in the Speed-Torque Curve

Abstract: A theory of coexisting stationary and rotating slot openings is developed with the aid of ``revolving permeances,'' and the magnitude and speed of the parasitic fluxes due to the slot openings and the fundamental current density wave are found. It is shown that the interaction of the fundamental flux and a parasitic flux of the same order of magnitude as the fundamental flux causes the vibrations, objectionable noises, and crawlings of induction motors. The existence of these ruinous irregularities depends only in the difference of slots, the number of poles, and in some cases on the critical speeds of the rotor for circular or tortional vibrations, and is independent of the number of phases or the type of winding. Three rules are developed for the determination of destructive vibrations and noise, eight rules for establishing crawling speeds, and eight rules for finding hooks in the speed-torque curves of induction motors.

Circuit Breaker Recovery Voltages Magnitudes and Rates of Rise

Abstract: This paper shows the conditions affecting the magnitude and rate of rise of recovery voltage at the terminals of an oil circuit breaker upon interruption of a short circuit, and explains their effect in quantitative terms. Factors are presented whose numerical value takes into account the number of phases involved in the short circuit and the ground connections of short circuit and generator, the decrement of short-circuit current previous to interruption, and the effect of unequal reactances in the direct and quadrature axes of synchronous machines. It is also indicated how to take into account the effect of displacement and of initial load current, and the effect of saturation is discussed very briefly. Magnetic oscillograms are presented which verify the most important of the points brought out. The most common locations and a rough idea of the magnitude of the capacitances affecting the rate of rise of recovery voltage for various types of short circuit are indicated. Calculations of the recovery voltage curve for several representative cases, at both low and high voltages, are presented, and in some cases cathode ray oscillograms are presented for comparison with the calculated curves. While no systematic data appear to be available at present regarding the effect of the rate of recovery voltage rise upon circuit breaker operation, several tests showing a very pronounced effect, some of them made by the authors and some by others, are reported briefly.

Forces in Turbine Generator Stator Windings

Abstract: Experience has shown that the forces on turbine generator armature windings during short-circuit conditions are large. The windings may be seriously damaged if adequate consideration is not given to the design of the coil bracing and support. In this paper the forces are calculated for a particular case by the use of images, by the change of flux interlinkages, and by the ? H2 d s over some surface in air which surrounds the conductor under consideration. A new and relatively simple proof is given for the validity of the latter method. The limitations and desirable applications of each method are considered. Forces on conductors in armature slots are computed when saturation is considered, and when neglected. Approximate formulas are derived for the forces on coil sides within one layer of the conical end winding, and for the straight coil extensions from the slots. A numerical example is given in each of those cases. The applicability of certain of these formulas to the phase connecting rings is pointed out. The force on the entire conical surface of the end winding is discussed qualitatively. Certain desirable features of end winding bracing are mentioned. In most of these cases, the instant considered is that following a line-to-neutral short circuit when the maximum possible instantaneous current is flowing.

The Pulling into Step of a Salient-Pole Synchronous Motor

Abstract: A synchronous motor started as an induction motor must pass through an electro-mechanical transient after the field circuit is supplied with electrical excitation in order to change its action from that of an induction motor to that of a synchronous motor. The calculation of this problem has been difficult because the differential equation of motion is of a non-linear type of which no analytical solution has been made. The integraph at the Massachusetts Institute of Technology has given a new and direct method of solving equations of the type encountered in these pulling-into-step transients. In this paper are compiled the results of many integraph solutions which were made after combining the factors that influence the performance of such machines into ratios and relative factors so that the results will apply to all practical cases. From the solutions several generalizations are made which can be directly applied to determine whether or not any type of salient-pole synchronous motor will synchronize properly. These solutions take into account inertia, saliency, incompleteness of amortisseur windings, load, switching angle, and field current, and assume negligible armature resistance, negligible rotor leakage reactance, no saturation, constant load torque near synchronous speed, and negligible electrical transients. Oscillograph tests of a 160-hp. motor and a sample calculation are given.

Steady Flow of Heat in Large Turbine-Generators

Abstract: The present paper represents an attempt at a reasonably complete solution of the heat flow in large turbine generators, where the temperature drops through the various iron paths are of consequence. The solution of two-dimensional problems of heat flow is obtained by means of ``equivalent thermal circuits.'' The approximations involved in this method are discussed in two appendixes where exact solutions are developed. The theory has been applied to a number of large turbine generators with satisfactory agreement between measured and calculated temperatures. The application of the results to actual machines will be made the subject of a subsequent paper.

Critique of Ground Wire Theory

Abstract: The paper consists of three parts, I?Induced Potentials, II?Direct Hits, and III?Other Effects. The work of previous investigators is briefly reviewed, and the limitations of their premises pointed out. Under Part I a generalized theory of ideal ground wires is offered, which takes into account the law of cloud discharge, the distribution of bound charge, and the formation of traveling waves. It is found that the protective ratio is independent of these factors. A more extensive theory taking the additional factors of successive reflections and tower resistance into account is then developed. Part II discusses the probability of a line's being hit, and applies a method for computing the effect of successive reflections to the calculation of potentials on the line and ground wires. Curves of these potentials at successive towers, and as functions of tower resistances and of time, are given. Part III discusses the effect of ground wires on attenuation, telephone interference, zero phase sequence reactance, corona, and the reduction in surge impedance due to the introduction of extra ground wires. There are three mathematical appendixes. In Appendix I, Maxwell's electrostatic and electromagnetic coeficients are reviewed, and the theory of traveling waves on any number of parallel wires is developed, including the behavior of these waves at rather general transition points. While this extension to the theory of traveling waves was developed incidental to the study of ground wire theory, it is believed to be of considerable interest and value on its own account.

The Theory of Thermal Breakdown of Solid Dielectrics

Abstract: The purpose of the paper is to correlate the work which has been done on thermal breakdown and to put it in a form in which it can be used by the electrical engineer in the calculation of breakdown voltage. Besides a treatment of the Fock theory, the paper includes the derivation of new formulas for breakdown of very thin and very thick samples and for internal temperature rise and current. A startling fact brought out by this analysis is that before breakdown the internal temperature never rises more than about 10 deg. cent. above ambient temperature. The Fock tables of breakdown voltage are extended to cover a wider range of temperature and thickness, and the results are put in several convenient forms for numerical calculation. Experimental data are also presented to verify the theory.

Experimental Studies in the Propagation of Lightning Surges on Transmission Lines

Abstract: A discussion is given of cathode-ray oscillograms showing the attenuation and distortion of artificial lightning surges on a power transmission line, the voltages induced by such surges in parallel wires and the effect on tower footing impedance of earth wires or counterpoises.

The Conductivity of Insulating Oils

Abstract: The paper describes experiments on the charging current and other associated phenomena in high grade transformer oil. The charging currents remaining after the elimination of the initial transient were studied to within a few hundredths second of application of continuous voltage. Two samples of the same oil obtained at different times differed radically. In one case the charging current fell off from the start, while in the other it remained constant for an appreciable time or even temporarily increased. On reversal of polarity after a long charge the initial current with both specimens was the same as before reversal, but with one a large momentary increase occurred a few seconds later. Evidence is given on the time of recovery of the initial condition. The existence of space charges in the charged oil is shown, and the time of formation of these charges together with the resulting non-uniform distribution of voltage is measured. The importance of the charging current in its early stages is emphasized, since it is the initial conductivity which determines the a-c. loss. In an appendix a theoretical discussion is given on the influence of the space charges. It is shown that at least qualitatively these account for the observed phenomena.

Lightning Investigation on a Wood Pole Transmission Line

Abstract: The purpose of this paper is to report the results of an investigation made on a wood pole line. The entire line was originally very highly insulated. The splintering of the structures by lightning indicated that excessively high voltages were appearing quite frequently. Because of the high insulation a cathode ray oscillograph station was installed on the line. A total of 38 significant surges was recorded, two of these being a five million and a four and one-half million-volt surge. Analysis shows that these high surges were the results of lightning bolts striking the lines causing faults between conductors but not to ground. The data also indicate that harmful effects are caused only by direct strokes. These and current measuring records indicate that currents varying from very low values to 100,000 amperes may readily be obtained. Two methods of protection are dealt with, one utilizes a fuse, the other an expulsion lightning discharge device. The new device operated eight times, all of these operations being successful.

Reignition of Metallic A-C. Arcs in Air

Abstract: Developments made in circuit breakers in the last two years have emphasized the necessity for obtaining experimental evidence of the current-voltage-time relationships that exist during the period when the a-c. arc between metallic electrodes passes through its cyclic current zero. Twenty-nine cathode-ray oscillograms of these relationships are presented. During the current zero period the arc electrode voltage is determined by the circuit constants and rises until the electrode voltage reaches the breakdown or reignition value, which is determined by the deionizing influences at work while the arc is extinguished. Alteration of the circuit constants permits a variation in the rate of voltage rise with a consequent change in the reignition voltage. Permanent extinguishment of the arc occurs when the gap breakdown voltage has risen, due to deionizing influences, to a value that cannot be reached by the electrode voltage controlled by circuit constants. The action of a circuit breaker in extinguishing an arc is greatly influenced by the presence of adjacent load circuits and by the presence of distributed inductance and capacity in the connecting lines.

Arcs in Low-Voltage A-C. Networks

Abstract: The extinction of a-c. arcs at current zero is reviewed, and arc reignition characteristic and circuit reignition characteristic are defined. From a study of arc reignition characteristics of short arcs remote from insulation it is concluded that such arcs are incapable of interrupting practical low-voltage a-c. network circuits. The extinction of arcs in practical network cables is then ascribed to the deionizing action of gas blasts coming from decomposing adjacent insulation. Experiments with arcs in cables, and arcs between parallel plates remote from insulation, and closely bounded by insulation, confirm this view. Inorganic insulating materials may also assist in arc extinction by generating gas blasts by their decomposition. Of the various inorganic materials tried, boric acid was the most effective. Charring of organic insulation may be expected to cause it to lose its arc extinction aiding characteristic.

Simultaneous Faults on Three-Phase Systems

Abstract: The method of symmetrical components now so extensively used to determine short-circuit currents and stability limits during transient conditions for three-phase transmission systems when a fault involving one or more of the three conductors occurs at any one point of the system, has been extended to apply to three-phase systems during simultaneous faults at two or more points of the system. A general equivalent circuit is developed to replace, in the positive phase diagram, two simultaneous faults involving any combination of the six conductors. An approximate equivalent circuit to be used with the d-c. calculating table when resistance is neglected is also given. Special equivalent circuits are employed to rep, lace two simultaneous faults and the lines upon which they occur, when the lines are unloaded feeders radiating from a common point or lines of equal impedances bussed at both ends. The methods and formulas given in this paper were developed in answer to such questions as the following: 1. Which is a more severe shock to a system, a double line-to-ground fault on one circuit or two single line-to-ground faults occurring simultaneously on two separate circuits? 2. Do simultaneous double line-to-ground faults which involve the same phases, a and b, on two circuits, result in more or less ground current than faults which involve phases a and b on one cir-cuit and phases b and c on the other?

Lightning Discharges and Line Protective Measures

Abstract: This paper describes the mechanics of the development of a lightning stroke in a cloud and the formation of a surge on a line. It shows that the energy of a stroke can vary between wide limits so that the actual surge on the line may be of practically any magnitude. Fundamental types of high-tension line construction are analyzed to show the performance that would be obtained with them. Experience obtained on the 220-kv. lines of the Public Service Electric and Gas Company is offered to substantiate the belief that induced surges are of relatively small importance and that the direct stroke is the criterion for good design. The effects of line construction on the character of surges impressed on the terminal substation apparatus is discussed to indicate the degree of protection that would be required.

The Oil-Blast Circuit Breaker

Abstract: The principle of operation of oil circuit breakers has been conceived to the present time as deionization of the arc stream during the time required for the recovery voltage to appear across the circuit breaker terminals. An alternative principle is here developed in which the arc products are swept away by the scavenging action of an oil blast. As a logical development from this conception a new type of oil circuit breaker is described which owes its efficiency to careful provision for a scavenging blast of oil which is driven by the gas pressure from one break and which displaces the arc in the space between the electrodes of another break.

Temperatures in Electric Power Cables Under Variable Loading

Abstract: Considerable need has been felt for accurate methods of soluttion of the problem of temperature rise under variable loading in order that the maximum use may be made of the large investment in power cables. In this paper a rigorous solution of the problem of temperature rise from sheath surface to conductor is attempted, making use of Bessel functions. The heat flow cycle is resolved into harmonics, and each harmonic solved separately for temperature at the conductor. The various harmonics of temperature are then combined in their proper phase relation to obtain the temperature cycle. For purposes of assigning emergency ratings a solution is arrived at for suddenly applied steady loads, making use of the Fourier integral. The problem is solved rigorously for single-conductor cables and three-conductor cables of shielded (type ``H'') construction. Modifications of the constants of the cables are described which will allow the theory to be applied to cables of standard belted construction with reasonable accuracy. The probable errors involved in the assumptions necessary in the solution are discussed. It is believed that knowing the temperature of the air at the sheath surface, the temperature of the conductor can be calculated within 4 or 5 per cent of the correct value if the constants of the cable are known within this accuracy. The method can also be applied to solution of the temperature rise of the sheath surface provided the constants of the duct bank are known with sufficient accuracy.

Lightning Investigation on the 220-Kv. System of the Pennsylvania Power & Light Company (1930)

Abstract: This paper describes the 1930 results (fifth consecutive year) of a lightning investigation conducted on 220-kv. lines located in eastern Pennsylvania and where lightning storms are prevalent. During 1930 the investigation was expanded; two new devices, the surge (insulator assembly flashover) indicator and the lightning (storm) severity meter being successfully applied. Unique and comprehensive data were secured on the magnitudes and wave shapes of natural lightning surges both at and remote from the point of origin on the transmission line; numbers of phases faulted during trip-outs; numbers, magnitudes and effects of direct lightning strokes; some data on the relative importance of induced strokes; results of measurements of atmospheric voltage gradients; and the indicated effect of overhead ground wires and tower footing resistances.

Studies in Non-Linear Circuits

Abstract: The series circuit consisting of an inductance L, capacitance C, resistance R, and a source of alternating electromotive force, where the circuit elements are all linear, is regarded as a special case of a more general type of circuit in which L, C, and R may be functions of the current or other variables. The practical importance of these non-linear circuits is noted and some of their properties are reported. In the series circuit with a linear resistance, capacitance, and an iron core inductance it is shown that the abrupt rise in current at a certain critical voltage is characterized by resonance in the sense that L di/dt = - 1/c ? i d t. In this non-linear resonance however the above condition is satisfied in but a limited portion of the cycle. During this portion of the cycle the peak current may be calculated from Ohm's law. The series circuit with linear resistance, capacitance, and an iron core reactor with superimposed steady magnetizing force, is shown to exhibit a pulsation oscillation which is explained on the basis of the positive impedance-current characteristic of such reactors. Parallel resonance for the non-linear case of an iron core reactor is reported, and it is shown that in conjunction with series resonance the unique characteristic is obtained wherein the r. m. s. current decreases with increasing applied voltage.

The Expulsion Fuse

Abstract: The general theory of extinction of a-c. arcs is reviewed. The expulsion fuse is stated to depend upon the gas blast produced by rapid decomposition of fuse tube material under the heat of the arc. Comparison of interrupting capacities of a soapstone fuse tube and a fiber fuse tube supports this view. The effectiveness of the gas blast is stated to be due to the high degree of turbulence it introduces into the confined arc space. The theory of the action of such turbulence is discussed. Data are given as to the composition and volume of gas ejected from a fiber fuse tube. The voltage interrupting capacity vs. ampere characteristics as obtained experimentally, are given for a fiber tube, and a boric acid lined tube. The curves for boric acid lie much higher than for fiber. The characteristics were found to depen dvery materially on the number of half cycles of arcing. Reasons are given for the shape of the characteristics, and their dependence on length of arcing time. Data are given showing the effect of variation in size and shape of the tube section. The design problem is discussed of obtaining sufficiently large voltage interrupting capacity at smaller currents without the development of excessive pressures or unmanageably large volumes of flame at large currents. The use of flame suppressors for deionizing the issuing flame is mentioned.

Physical Nature of Neutral Instability

Abstract: Experience has established the fact that the neutral of a three-phase system may become subject to certain strange phenomena of instability under apparently normal conditions. The disturbances are of two distinct classes: (1) persistent shift or inversion of the neutral, resulting in unequal leg voltages; and, (2) persistent oscillation of the neutral (with equal effective voltage in all three legs) at approximately one-half, double, or triple frequency. While in the ultimate analysis saturation is at the basis of the phenomena, a more definite explanation is given as follows: 1. Neutral shift or inversion is a fundamental frequency phenomenon, and is due to the fact that the volt-ampere curve of the combination of an iron core reactor (transformer magnetizing current) in shunt with a suitable capacitor has one zone which is lagging and one which is leading. In a Y-Y bank of transformers, with suitable balanced line capacitances to neutral following a switching disturbance, one leg may act leading, the others lagging, and thus invert the neutral. 2. Oscillations of the neutral'tend to take place at its natural frequency, but since, due to inevitable losses, not all oscillations can persist, in course of the starting transient the oscillation is resolved to the nearest lower frequency which is able to draw energy from the circuit by approximating harmonic relationship to it. The even harmonics are accounted for by the persistence of residual in the core, whether left from previous excitation or brought about by the direct current component of starting transient.

Transient Oscillations in Distributed Circuits With Special Reference to Transformer Windings

Abstract: This paper is essentially a mathematical study of transient voltages in transformer and other distributed windings, and is primarily concerned with showing the effects of wave shape, circuit constants and neutral impedances on the voltage distributions, and how these distributions may be controlled or changed. In the appendix there is derived the general differential equation for a circuit consisting of distributed self and mutual inductance, series and shunt capacitance, series resistance, and conductance along the stack and to ground. This equation is solved for the following conditions: (1) initial and final conditions with generalized impedance in the neutral; (2) grounded neutral; (3) isolated neutral for zero losses; and (4) capacitance in the neutral when the losses and mutual inductance are neglected. Equations are given corresponding to a number of different applied waves, showing the effects of wavelength, wave front, damped and sustained oscillations, and typical lightning waves, and these effects are illustrated by curves and oscillograms. Equations are also given for the potential difference between any two points on the winding and for the voltage gradients. Tables are included showing the influence of the various circuit constants on the amplitude and frequency of oscillation, linear velocity and surge impedance of harmonic waves, and the type of propagation. These effects are illustrated by curves of the potential distribution at different instants of time. Methods of controlling the transient so as to prevent or alleviate abnormal voltage distributions are discussed at length.

Backfires in Mercury Arc Rectifiers

Abstract: Mercury arc rectifier design of today was arrived at in an endeavor to reduce the frequency of backfire. Efficiency and cost are sacrificed to obtain sufficient backfire immunity. Study of backfires in actual rectifiers and laboratory tubes indicates that this occurrence is a random one. Backfires differ in kind from other breakdown phenomena because they do not occur with certainty after some definite limitation has been reached, and in fact no limit can be chosen above which or below which backfires will or will not take place. The state of a rectifier in relation to backfiring must be described in terms of a backfire probability, or average rate of backfire. There are no commercial tank rectifiers for which the mean frequency of backfire is absolutely zero, but some probably reach the low value of one per several years. The significance of the random nature of backfire on the rating and testing of rectifiers is discussed, and suggestions are made as to means for testing which lead to the determination of the true quality of a rectifier. Data are given showing the variation of the backfire rate with current and voltage. The application of these ideas to research and development is illustrated by several examples. The theory of the formation of a backfire is described, and backfires are attributed to ``causes'' which lead to the formation of a cathode spot on an anode, but which do not correspond to any fundamental limitation of the rectifying process.

Governor Performance During System Disturbances

Abstract: This paper describes short-circuit tests which were made on a large power system under normal load conditions, in order to ascertain the operating characteristics of the system and the turbine governors during system disturbances The introduction gives a brief description of the main features of the station and the connected System, a statement of the tests, and a list of the major conclusions Part I covers the short-circuit tests on the system to determine the stability limits Part II describes the load transfer and short-circuit tests to produce ``governor pumping'' Part, III deals with the short circuits which were made to investigate the effect of a special valve mechanism on the stability limits Part IV contains a discussion of miscellaneous tests, data, and general comments In appendixes A and B, a detailed description of the system station and test equipment is given with estimates of the stability limits and short-circuit currents to be expected Appendix C deals with the theory of the change in speed of a rotating body for a given energy differential

Experimental Studies of Arcing Faults on a 75-Kv. Transmission System

Abstract: Qualitative results of an investigation, in which high-voltage impulses were applied to an energized transmission line, indicate that the probability of a lightning flashover developing into a power arc is increased when current is flowing past the flashover point, and when the normal frequency voltage is increased or the insulation decreased. An oscillographic study of power arcs ranging in magnitude from 8 to 800 amperes indicated that the resistance of an arc varies over the cycle and over the duration of the arc. The effective arc resistance is approximated by the ratio of the peak voltage to the peak current. The apparent power factor of the arc current and voltage is practically unity in most cases. The tests indicated that arcs formed in a capacitive circuit are more persistent than those in inductive or resistive circuits, and that the average peak volts per foot of arc length, for currents above 100 amperes peak, is about 300. The maximum overvoltage recorded on an isolated neutral 75-kv. transmission system, 134 miles in length, as a result of 63 arcing ground tests was 3.0 times normal on a sound phase. The overvoltage is essentially a fundamental frequency phenomenon in which the sound phase voltage takes its new value without going through an oscillation of appreciable magnitude.

Characteristics of Oil-Filled Cable

Abstract: This paper deals briefly with the general theory and some of the outstanding characteristics of oil-filled cable practise. Oil-filled cable has had a perfect operating record during the past four years and during this time many simplifications and economies have been introduced. Further gains in this direction are indicated. Today, this type of cable is well in line with ordinary paper-insulated cable from an economic standpoint. The limiting features of oil demand and oil supply are explained and typical methods of oil feed with expansible reservoirs described. Space does not allow the inclusion of many additional things of interest such as a description and drawings of the complete line of cable and accessories that have been developed.

Corona Loss Measurements on a 220-Kv. 60-Cycle Three-Phase Experimental Line

Abstract: Corona loss measurements were made on seven different conductor specimens. The losses were measured directly, on a three-phase line 700 ft. long, by means of three single-phase high-voltage wattmeters. The object of the test was to obtain data for the choice of conductor to be used on a 220-kv. 60-cycle line. The results show the effect of weathering of conductors, the comparison of conductors of two different diameters, the effect of size of strands and the method of stranding and also the effect on new conductors of the change in surface conditions caused by rain and washing.

Extinction of Short A-C. Arcs

Abstract: This paper presents the results of a study made with short, stationary a-c. arcs between metallic electrodes, in order to determine the effect of arc length, electrode material, and current magnitude on the rate of recovery of dielectric strength of the arc space following a current zero. The experimental results show (1) that the arc space recovers the ability to withstand a hundred volts or more within a few microseconds after a current zero, as predicted by Slepian's theory, (2) that the maximum allowable rate of rise of voltage across the arc space which will just permit arc extinction is much greater for very short arcs (1 cm. or less) than for longer arcs, (3) that the critical rate of rise of voltage depends largely on the electrode material used and tends to vary as a decreasing linear function of the boiling point, and (4) that the higher current arcs have a much lower critical rate of rise of voltage than the lower current arcs.