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The objective is to compare the electrical performance of a tubular barrier made of silicone and glass protecting an air gap with an alternating electric field. This investigation is focused on rod–rod and rod–plane electrode systems with variable interelectrode distance. An experimental device consists of wooden support in U shape. The air space disruption tests of the two systems with tubular barriers were supported by a visualization of the path of their electrical rupture. A disruption model of a non-uniform electric field air gap was developed in this study. Using the experimental results of dielectric breakdown of the considered systems, this model characterizes the flashover voltage as a function of the barrier insulation’s nature. In addition, it allows an easy comparison of its electrical insulation capacity under these conditions. Under pollution, no protection is provided by a glass barrier to the two systems due to the highly conductive behaviour of the pollution layer covering its surface. On the other hand, that offered by a tubular silicone barrier is by far the best because of the flashover discharge’s presence on its surface under these conditions. In a clean and dry atmosphere, the electrical insulation power of the systems is more efficient with a silicone barrier than with a glass one, because of its more advantageous dielectric properties and its lightness.

Isolation is required in electronic circuits to create zones for various voltage levels, where only low-voltage signals are allowed to communicate between zones. Application of such devices in circuits has challenges related to the anticipated working conditions at high voltages and its effect on design through employed materials, which need to provide the voltage isolation with good margin during the lifetime of the device. One of the technologies is provided with a capacitor-based isolators, which employ front- or back-end-of-the-line wafer-level dielectrics. Here, we propose a semiconductor packaging solution using a material, where a laminate buildup is characterized for high-voltage performance. The dielectric breakdown scaling for the material is reported for design considerations.

The results of ageing tests of biaxialy oriented impregnated capacitor polypropylene films with modified and unmodified surface structure are presented in this paper. The temperature dependence of factorD=tan δ0 (where tan δ1 and tan δ0 relate to impregnating fluid aged with and without polypropylene film, respectively), the relationship between factorD and the aromatic content of impregnating fluid and time dependence of dielectric strength for both types of film are presented. The possible destroying mechanism of the capacitor dielectric system is given.

In this paper the electrical tree growth in solid insulating materials is modeled using von Neumann's Cellular Automata (CA). The model is based upon the assumption that the electric stress at the end of a conducting tip quite often approaches the dielectric strength of the material and that progressive breakdown can occur by electrical tree formation. Because of tree advancement, the potential distribution into the insulating material changes with time and is calculated at each time step. An algorithm for the simulation of electrical tree growth in solid dielectrics based on this model has been developed. The algorithm is also used to simulate breakdown in solid dielectrics containing square or spherical voids.

In contrast to power arcs in air, for which the dielectric-recovery voltage decreases with increasing arc current, arcs in air produced by surge currents of several hundred microseconds' duration possess dielectric-recovery characteristics that vary only slightly with current magnitude and wave shape. Even when the discharge current is prolonged at low magnitude for as long as a thousand microseconds, the rate of dielectric recovery is little affected. For this condition at times when current as high as one ampere is flowing, the transient voltage required to convert to a high current arc is quite high. The mechanism for such a discharge is different than for the breakdown of virgin air, and the initial arc drop is sufficiently high to record its transient character. It is about ten times the steady-state value at the time the crest current is reached and drops about 50 per cent in the first 20 microseconds. Photographic studies show that even at short-time intervals successive discharges do not follow exactly the path blazed by the initial discharge but occur in an air column of low dielectric strength expanding radially with time. It is indicated that the high arc temperatures diffusing in this region can be the principal factor determining the rate of recovery after the first 1,000 microseconds. A comparison is made with the mechanism of natural lightning particularly from recent data obtained on the low-magnitude portions of stroke currents between successive components.

For the first time, practical use is made of the high insulating and current interrupting ability of a high vacuum. Tests show that a vacuum switch with but 1/10-inch contact travel can clear 79-amperes at 66-kv rms has a breakdown strength of 100-kv rms.

PAPER as used in the spiral laminated impregnated insulation of high-voltage power cables is available over wide ranges of density and thickness. There result important variations in the ordinary physical properties of the paper such as flexibility, tearing, tensile, and bending strength. Variations in the electrical properties of conductivity, dielectric constant, and breakdown strength also result. The peculiarly severe conditions imposed by the manufacture, transportation, installation, and operation of high-voltage cables have emphasized the importance that full attention be paid to all physical properties of the insulation, especially those relating to permanence of mechanical structure and of high insulating properties. Manufacturers of cables often utilize the several types of variation referred to in designing cable insulation to meet the particular requirements of cables for different types of service.

HIGH-voltage apparatus insulated by compressed gas requires the use of solid insulating supports. It is generally recognized that even in a uniform field the flashover voltage of such solid insulation is considerably lower than for the gas-filled gap of equal length. An increase in the allowable gradient along such insulators would in general result in a reduction in the size, weight, and cost of gas-insulated apparatus, and must therefore be considered an important and fundamental objective of high-voltage engineering. It is probable that the flashover strength of solid insulators may be brought closer to that of the gases in which they are immersed, or to their inherent volume breakdown strength, through a better understanding of the influence on insulator flashover of such factors as material, surface condition, corrugations, and gas pressure. Most of the scant literature on this subject is restricted to relatively low alternating voltages, small samples of solid dielectrics, or to special insulator shapes in non-uniform fields.1 Reher studied the flashover of plain cylinders of ebonite, paraffin, and porcelain in dry air at pressures up to 15 atmospheres, using alternating voltages up to 140 kv maximum.2 Goldman and Wul3 have investigated the flashover of plain ebonite cylinders in nitrogen with alternating voltages up to 125 kv maximum and pressures up to 20 atmospheres. The present paper reports d-c flashover values up to 250 kv across short cylindrical insulators placed in the uniform field between metallic electrodes. These studies were made on both plain and corrugated cylinders of three dissimilar insulating materials immersed in dry nitrogen at gauge pressures up to 400 lbs. per square inch.

The materials used in tank-type oil circuit breakers, the predominate outdoor type, and the effects that low ambient temperatures have upon their performance are examined. Conclusions are presented indicating that low ambient temperatures require special consideration even with modern outdoor oil circuit breakers, but further study is still needed.

For many years shellac or synthetic-resin-coated paper has been used in winding condenser bushings. Two years ago work on the development of oil-impregnated-paper condenser bushings was undertaken. When a shortage of shellac was imminent because of the war, effort to complete the development was greatly increased, resulting in its being completed well ahead of the date originally set. Manufacturing processes and impregnating treatments were developed and bushings built and tested. The processes and treatments are described, and the data obtained from tests on representative bushings are given. Complete interchangeability for transformer and circuit-breaker bushings has been obtained for current apparatus designs in the voltage ratings in which the bushings are produced. To a large extent, the new bushings are also interchangeable with bushings of older design in the same voltage rating. The entire insulating portion of the bushing is enclosed in porcelain and metal and this enables the bushing to be held in storage without special provisions for keeping it dry. The oil-impregnated-paper condenser bushings, built for voltage ratings 92 kv to 230 kv, are susceptible to careful control of quality in the processing operations. Exceptionally consistent performance and high dielectric strength are obtained. Consistently low power-factor readings on both hot and cold bushings were obtained. Radio-influence voltage measurements were below proposed standards.

Corona-starting voltages of the components of a 208-volt 400-cycle grounded-neutral electric system are reported for conditions to simulate altitudes between sea level and 50,000 feet. The corona characteristics are shown to be functions of air pressure, electric-field geometry, humidity, and the cleanliness of insulating surfaces. The effect of reducing pressure is shown to lower the corona-starting voltages of aircraft cables by as much as 60 per cent because the dielectric strength of large air gaps drops a comparable amount. Reducing the pressure has small effect on the corona-starting voltages of low-voltage capacitors, because the dielectric strength of small air gaps is nearly independent of pressure. Moisture and dust on insulating surfaces and moisture in aircraft cables produce corona-like disturbances below 208 volts. Data are presented on cables, capacitors, parallel studs, and needle gaps. Under dry conditions all corona-starting voltages were well above the operating voltages of the system.

EXPERIENCE with power circuit breakers has frequently shown that two breakers of identical design may give radically different performances, even though the operating voltages and short-circuit currents to which they are subjected are the same. Studies of the conditions surrounding these circuit breakers have traced the cause to the differences in the manner in which the recovery voltage has appeared across the contacts of the circuit breakers subsequent to the final interruption. This voltage which occurs between the final arc voltage and the 60-cycle recovery voltage is an equalizing phenomenon defined as the transient recovery voltage. To take it into account in breaker applications, studies have been made of the transient voltages which can occur on systems, and other studies have been made of the response of the circuit breakers to transient recovery voltages.

REIGNITION OF A-C. ARCS being dependent upon the arc-voltage relations during current zero, a study based on some 200 cathode-ray oscillograms has been made of the voltage and current relationships near the cyclic current zero in arcs between metallic electrodes.

Development of high-voltage dielectrics to meet the increasingly exacting demands of modern applications challenges chemical ingenuity and constitutes the broad avenue of chemical approach to the problem of high-voltage engineering

The effect that the shape of electrodes has on the dielectric strength of air when subjected to impulse and 60 cycle voltage waves is considered in this paper. Principal attention is given to rod and plate electrodes and to sphere and plate electrodes, comparative data on the different types being given. Among other factors, the effect of polarity on breakdown voltage is brought out. These data on strength of air between various electrode shapes are correlated into working principles and presented from the standpoint of the design engineer interested in their application.

Test data on impulse and 60-cycle voltage breakdown strength for compressed air with various electrode configurations are reported. The tests were made with rod-to-plane electrodes with spacings up to 8 inches and pressures to 250 psig (pounds per square inch gage). Breakdown phenomena of air insulation encountered in the development of an air insulated air-blast circuit breaker1 are reported. The methods used in locating, analyzing, and correcting the breakdown are discussed. The use of solid insulating material in conjunction with high-pressure air is discussed. Applications in barrier form, and as support members stressed in creepage, are covered briefly.

Electrically heated blankets are judged to be safe from hazards of electric shock and overtemperature if they pass standard tests for current leakage, dielectric strength, and normal and abnormal overheating. Design considerations which affect performance on these tests include the construction of the blanket wired structure, the power rating of the blanket, the type of overtemperature protection used, and the fail-safe characteristics of that protection.

THE subject of nondestructive tests on insulation has received much attention. Since a function of insulation in a given application is to provide a certain minimum dielectric strength, tests at their greatest usefulness would give a measure of dielectric strength without requiring a dielectric breakdown. Additional indications like proneness to loss of dielectric strength if subjected to certain hazards of service are valuable, and information on the deterioration or aging of insulation with service may follow from periodic tests.

A nontechnical review of the development of gaseous dielectrics. Topics discussed are history, action of gas insulators, problems in choosing a suitable gas, established uses for SF 6 , and new gaseous dielectrics.

This paper describes the behavior of water as a dielectric, in particular with reference to its use in water circuit breakers. Water is generally considered to have the properties of an electrical conductor rather than those of an insulator. There are, however, a few applications where the dielectric rather than the conductive properties of water are of importance, as, for example, in its use as the interrupting liquid in circuit breakers. Here the water between electrodes must withstand full circuit voltage during the closing operation until the electrodes are in metallic contact, and also in the opening operation following extinction of the arc, until some auxiliary switch in series can open. Since there appears to be very little published information available on the subject, further knowledge of the behavior of water as a dielectric seemed to be of considerable interest and importance. To secure this information, a study of the subject was undertaken, the results of which will be reported in this paper. Experimental data are presented which indicate that electric breakdown between electrodes immersed in water is, in general, determined by the temperature rise produced by the leakage current in the water. Breakdown appears to occur when steam generation begins. Calculation of time to cause temperature rises to 100 degrees centigrade in the water agree closely with observed time between applied voltage and breakdown.

A volt-time breakdown curve for solid insulation, constructed from test data obtained with the use of square edged electrodes, is given in this paper, considerably extending the range of information available on this subject.

1. A method has been developed for photographing repeating traces on a cathode-ray tube which measure current in liquid and solid dielectrics with slowly increasing unidirectional voltage up to within two milliseconds of breakdown. The range of current sensitivity is from 10?8 amperes up. Current values at time intervals closer to the breakdown point could be measured by increasing the speed of motion of the film and decreasing the sweep interval. The probability of catching fast transient of the order of one microsecond duration is only 2 per cent (ratio of sweep time to sweep interval). 2. In liquids, the evidence so far obtained leads to the tentative conclusion that the final breakdown current increases from a substantially steady value to failure in fractions of a microsecond. This indicates that the final breakdown mechanism is electronic. 3. In liquids, it appears also that the pre- breakdown ionic current may be unrelated to the breakdown electronic current since the breakdown gradient appears to be affected more by the electrode surface conditions than by the ionic conductivity of the liquid. 4. In polystyrene and polyvinyl formal only one record was obtained showing any pre-breakdown current greater than 10?8 amperes up to within two milliseconds of failure at breakdown gradients of the order of 2.5 × 106 volts per centimeter.

This paper presents a summary of the results of over 2,000 tests on the dielectric strength of transformer oil and insulating material for impulse, switching, and 60-cycle voltages. From these tests the voltage-time characteristics of the 2 insulations tested may be established over the entire range from a fraction of a microsecond to more than one minute.

IT HAS long been recognized by the practical engineer responsible for the proper maintenance of high-voltage oilfilled electrical equipment that water constitutes one of the chief and in some instances the only serious threat to successful service. However, consideration of the dangers presented by water during the operation of high-voltage machines must not be limited to a study of the relation between the dielectric strength and the water content of the insulating liquid used. For successful operation, the cellulosic insulation must be maintained in a dry condition.

THE superior dielectric strength of compressed gases, their low dielectric loss, their low dielectric constant, their low cost, and the fact that they are self-healing make them very promising insulating media. Compressed-gas insulation has been used in precision condensers where low dielectric loss is essential; it has been tried in other types of equipment such as transformers; it has been suggested for electric power cables where the low dielectric constant is of considerable importance; and recently it has found interesting and important application in the production of very high voltages by electrostatic means for use in X ray and nuclear physics work.

A recently developed technique known as the fluidized bed coating process makes it possible to combine the best features of plastics and metals in almost any configuration. The three coatings most frequently used in the electrical industry — cellulosic, vinyl, and epoxy — are described and compared.

The purpose of equipment dielectric test is to detect insulation faults, not to stress solid insulation. An investigation of air-gap breakdown through holes in aircraft insulation materials simulating faults in aircraft equipment demonstrated the following: 1. the 500-volt high-potential tests will not detect common faults in aircraft insulation; 2. the common concern over insulation damage from repeated high-potential tests is not justified; 3. dielectric breakdown through small diameter faults in insulation is more likely to produce immediate failure than similar discharges over surface paths. A single circuit interruption transient discharging through a 0.003-inch-diameter hole may completely fault the insulation, while a discharge of similar energy across surface gaps may require hundreds of repetitions before failure; and 4. when small-hole discharges through insulation occur between copper and iron, insulation may fail immediately even with supposedly tracking resistant materials such as mica. Insulation puncturing and tracking circuit interruption high-voltage transients reaching values of several thousand volts are discussed. The value of these transients is shown to be substantially the same for 28-volt systems as for higher voltage systems and the critical circuit factors determining the value of these transients are analyzed.

“Trees,” or microscopic channels extending partly or wholly through a wall of insulation, represent a characteristic form of latent damage. Results are given of studies made on this phase of dielectric breakdown in polyethylene.

In summing up the characteristics of fiber glass those which make it valuable for electrical insulation are: 1. Noninflammable and highly heat resistant. The operable temperature is limited only by the impregnant used, and there is a very definite temperature range between the operable temperature of organic fabrics and that of heat-resistant impregnants where glass is unique. 2. Exceptionally high tensile strength. 3. Nonhygroscopic. 4. Resists attack by moisture, acids, oils, and corrosive vapors. 5. Good thermal conductivity. 6. Excellent dielectric strength when impregnated. 7. High insulation resistance.

The purpose of this paper is to provide the engineer with co-ordinated information and data regarding the dielectric breakdown of glass which will permit him to apply this insulating material more effectively for high dielectric strength. In the discussion of the subject, particular attention has been paid to “edge effects” and other factors which cause wide variations in breakdown voltages — variations which are responsible for many of the seeming inconsistencies found in published results on the dielectric failure of glass and other materials. The author has concluded that it is possible to make a reasonable approximation of the breakdown voltage of glass, if the conditions of the test or of the application can be sufficiently well determined. The characteristic breakdown curves of this paper do not represent the results of any one series of tests, but are rationalized from data selected from many sources, including unpublished tests. The data curves included refer mainly to Pyrex∗ glass, but corresponding data have also been included on electrical porcelain because of the interesting comparisons shown.

A secondary network cable that will burn clear without producing any smoke, inflammable, explosive, or toxic gases during faults has been developed. Tight and loose faults were cleared satisfactorily in 4 series of tests in underground ducts and vertical conduits without involving adjacent unfaulted phases. Dielectric strength is high and is recovered after exposure to water. The new insulation cannot carbonize and will withstand high temperatures.

A resume of some of the important advances made during the past year in the fields of physics and chemistry as applied to the fundamental theory underlying successful electrical insulation.

Preliminary evaluations of new fluorogases indicate that the dielectric characteristics of many of the new fluorocarbons are superior to those of the halogenated gaseous compounds. However, it is often advantageous to mix these gases with nitrogen to lower the cost, reduce the condensation point, and obtain a more uniform breakdown characteristic in the nonuniform field.

Recent developments in new types of plastics and improved manufacturing techniques of older types are enabling these materials to assume an increasingly important role in the field of electrical insulation. Their electrical characteristics are reviewed here.

EXPERIMENTS to determine the influence of such parameters as electrode material, current strength, and arc length on the characteristics of the more common type of stationary arcs with molten electrodes, have given results not previously expected. Simplified calculations made by Slepian a few years ago based on the theory that ions are lost from the arc space only by direct ion recombination, predicted a manner of recovery of dielectric strength substantially the same as that actually revealed by experiments made on cold electrode arcs at about the same time. The rate of recovery of voltage across the arc space following a current zero was controlled by shunting the arc with resistance, and the maximum allowable rate of rise of voltage was determined from the maximum value of shunting resistance at which the arc would just be extinguished after one-half cycle.

A CLOSE STUDY of the type of arcs occurring with faults in low-voltage a-c. networks and the method by which they are extinguished has brought out several striking conclusions. It has been found that short arcs between copper electrodes and not adjacent to insulation are inadequate to meet the fault-clearing requirements of low-voltage a-c. networks of more than 130 volts. Actual low-voltage networks of voltage higher than 130 depend for extinction of arcs upon the strongly deionizing action of gas blasts coming from decomposing adjacent insulation. As suggested by this result, it has been found also that the extinction of arcs in cables is strongly affected by the closeness of the insulation to the arc. Also, inorganic insulating materials giving off non-combustible gases have been found to be effective in aiding arc extinction, and of the various inorganic materials tried, boric acid was the best. Further, where organic insulation has been charred, it may be expected to lose its property of aiding arc extinction.

THE INSULATION FIELD is well supplied with a very large variety of materials. When these materials are used in large quantities, such as slot insulation in a motor, the uniformity of the material over a long period of time is essential. This uniformity is assured only by the adequacy of the specification and the faithfulness of the manufacture, the inspection, and the test.

For more than a half century various improvements have been made in built-up mica insulation — splitting mica into thin films and cementing them together to form large sheets. A significant advance in micaceous insulation is the new continuous mica sheet described herein.

The development of synthetic noninflammable chlorinated liquids of the aromatic hydrocarbon type places in the hands of the engineer a chemically stable material of high dielectric strength and possessing a dielectric constant approximately equal in value to that of cellulose itself. This insures a more equitable stress distribution in the composite insulation assembly normally used in commercial practice. The result is an increase in dielectric breakdown and related phenomena. No fixed relation defining the superiority of the synthetic noninflammable liquid or liquid-treated insulation is possible, since variation in test conditions changes the relative dielectric values. In general, however, the data indicate an advantage in favor of the synthetic liquid and liquid-treated insulation equal to at least 20 per cent of the corresponding mineral-oil value.

Tests on unbleached unsized linen paper of high purity indicate that the physical defects implied by the various pyroelectric theories of insulation failure appear secondary to, and manifested as a result of, the chemical changes involved. These chemical changes result in both increased power factor and decreased dielectric strength of the insulation. The investigation shows that chemical results of overheating of cellulose insulation in service may be disastrous even within a range of temperatures not generally associated with dangerous physical effects.

THE TYPE of class B insulation referred to in this paper is primarily intended for small and medium sizes of general purpose motors, generators, and associated equipment.

The correlation of the pressure and temperature effects in determining the dielectric strength of mineral insulating oils is considered in this paper. The effect which gas dissolved in the oil has on the dielectric strength of the oil is shown by experimental results of tests on the effect of temperature and pressure. Based upon these results, simple formulas for expressing the dielectric strength of oil are obtained.