Decrease In Insulation Resistance Research Articles

Analysis of the Decrease in Insulation Resistance of Pin Type Porcelain Insulation at the ULP Ampera of PT PLN (Persero)

Isolators are one of the assets at PT PLN, which serve as separators between the energized and non-energized parts. Maintenance is performed by replacing damaged isolators when there are indications of reduced insulation resistance. In this journal, the insulation resistance values were measured using a Kyoritsu Insulation Tester and six porcelain isolator samples in various conditions were used. The aim was to determine the extent of the decrease in insulation resistance values, as well as the leakage current and touch voltage values based on Ohm's law formula. Based on the measurement results, the sixth isolator in partially damaged condition is the worst, supported by an insulation resistance value of 800 MΩ for a test voltage of 10kV and 400 MΩ for a test voltage of 5kV. In good condition, the measured insulation resistance value of the isolator is 1 TΩ. Based on the calculation results, in the sixth condition, the leakage current values were 6.25 mA and 25 mA, and the touch voltage values, assuming an average human insulation resistance value of 1000 Ω, were 6.25 Volts and 25 Volts. According to 0520-2.K.DIR.2014 standards, the insulation resistance values meet the requirement that 1kV insulation resistance can be sustained by 1MΩ and are still below the maximum touch voltage limit, which is < 50V effective.

Neutron irradiation impact on structural and electrical properties of polycrystalline Al2O3

High energy neutron irradiations impact on structural and electrical properties of alumina are studied with particular emphasis on real time in-situ radiation induced conductivity measurement in low flux region. Polycrystalline Al2O3 samples are subjected to high energy neutrons produced from D-T neutron generator and Am–Be neutron source. 14 MeV neutrons from D-T generator are chosen to study the role of fast neutron irradiation in the structural modification of samples. Real time in-situ electrical measurement is performed to investigate the change in insulation resistance of Al2O3 due to radiation induced conductivity at low flux regime. During neutron irradiation, a significant transient decrease in insulation resistance is observed which recovers relative higher value just after neutron exposure is switched off. XRD results of 14 MeV neutron irradiated samples suggest annealing effect. Impact of relatively low energy neutrons on the structural properties is also studied using Am–Be neutrons. In this case, clustering is observed on the sample surface after prolonged neutron exposure. The structural characterizations of pristine and irradiated Al2O3 samples are performed using XRD, SEM, and EDX. The results from these characterizations are analysed and interpreted in the manuscript.

Isolation of Responsive Elements of Planar Multi-Element Photodiodes

In the mass production of multi-element silicon p-i-n photodiodes, the problem of systematic rejection of products due to a decrease in the insulation resistance between the active elements of photodetectors has been revealed. The purpose of this work is to study the causes of insulation resistance degradation and to establish optimal methods for avoiding this phenomenon. A comparative analysis of three insulation methods was carried out: classical insulation by the surface of a non-conductive substrate and a dielectric layer; insulation by means of mesaprofile grooves with a dielectric film; insulation by means of areas of limitation of surface leakage channels isotypic with the substrate material (in this case, p+-type) formed in the gaps between active elements. The study found that the reason for the deterioration of the insulation resistance between the active elements of photodiodes is the presence of conductive inversion channels at the Si-SiO2 interface due to the use of silicon with high resistivity. One mechanism for the formation of inversion channels is the redistribution of impurities in the masking oxide (in particular, phosphorus) and their diffusion to the interface during thermal operations. Another mechanism for the formation of inversion layers is the diffusion of boron from silicon into SiO2 during heat treatment due to the fact that the boron segregation coefficient is less than one. In the manufacture of samples with insulation using non-conductive areas of the substrate, a decrease in insulation resistance was observed as the technological route was performed (after each subsequent operation, the resistance degraded). The degree of degradation can be reduced by reducing the duration of thermal operations. It has been shown that reducing the thickness of the masking oxide causes a decrease in insulation resistance. When using mesa-technology, it is possible to increase the insulation resistance by eliminating the high-temperature oxidation operation and, in fact, due to the absence of a masking coating during phosphorus deposition. Insulation by means of p+-type areas in the gaps between the active elements allows to obtain the highest insulation resistance values. The formation of these regions with a width of 100 μm in the gaps with a width of 200 μm allowed us to obtain an insulation resistance of 25-30 MΩ. To ensure the insulation of the active elements of photodiodes by this method, two thermal operations are added to the technological route. The number of thermal operations can be reduced by doping the entire silicon surface with a low boron concentration before forming a masking coating.

Problems of Masking and Anti-Reflective SiO2 in Silicon Technology

The article examines the problems of thermal oxidation of silicon. Oxidation plays an important role in planar technology, which in turn is the basis of the technology of silicon integrated circuits, photodetectors and other solid-state electronics. During our production of silicon p-i-n photodiodes, a number of systematic types of defects and deterioration of product parameters caused by the degradation of masking or anti-reflective coatings during the manufacturing process were observed. A decrease in the insulation resistance of responsive elements in multi-element photodiodes was observed, which contributed to the increase of dark currents. A decrease in the responsivity of the products due to the degradation of the thickness or structure of the anti-reflective coating during technological operations, etc., was also revealed. It was established that the reason for the decrease in insulation resistance is the formation of inversion layers at the Si-SiO2 interface, the presence of which can be detected when measuring CV-characteristics. It was also established that chemical treatment of substrates with SiO2 in boiling acid solutions helps to reduce the thickness of the oxide. To avoid deviation of the thickness of the film from the condition of minimum reflection, it is necessary to grow a thicker layer of anti-reflective coating. It is noted that when etching the oxide during photolithography or when removing the PSG/BSG in hydrofluoric acid, it is not permissible to remove the cassette with plates from the solution for a long time, as this leads to uneven etching of the film due to the flow of the herb on the surface of the substrate. The causes of defect formation in Si and SiO2 during oxidation are given. Thus, with improper mechanical and chemical processing of the plates, cristobalite inclusions may form in the film during oxidation. Cristobalite has a higher density than quartz glass, and the boundaries between amorphous regions and denser crystalline regions represent voids, which can be filled both by impurities from the surface and by the diffusant in the diffusion process. Also, during oxidation in silicon, packing defects are often formed. Centers of defect genesis can be mechanical damage to the plate surface or growth defects.

DETERMINATION OF EMERGENCY REGIMES OF ELECTRIC MOTORS BY MEANS OF MODERN SYSTEMS OF DIAGNOSING THEIR CONDITION

Electric motors are usually designed for a service life of 15… 20 years without major repairs, provided they are properly operated. Proper operation means operation in accordance with the nominal parameters specified in the passport data of the motor. However, in practice there is a significant deviation from the nominal modes of operation. These are, first of all, low quality of supply voltage and violation of rules of technical operation: technological overloads, environmental conditions (high humidity, temperature), decrease in insulation resistance, cooling disorders.
 The consequence of such deviations are emergency modes of operation of electric motors. In some subsectors of the industry, the accident rate of electric motors ranges from 20 to 50% per year. Failure of the motor leads to severe accidents and significant material damage due to downtime, the cost of troubleshooting and repair of the failed motor. Repair of an electric car with a capacity of 1 kW costs 550-700 UAH. To estimate the cost of repairing a more powerful machine, you need to multiply this figure by the engine power. In addition, operation in emergency modes leads to increased energy consumption and increased reactive power consumption.
 One of the promising types of protection against emergencies is the use of modern diagnostic systems with the function of monitoring the signs of emergencies. Therefore, the urgent task is to eliminate the shortcomings of protection devices through the use of modern intelligent systems for diagnosing their condition.
 The task is to determine the set of diagnostic features of emergency electric modes of induction motors for the application of these features in the systems of functional diagnostics of energy and technical condition of the engine and increase the reliability of establishing the characteristic types of damage.
 The article considers a set of signs of emergency electrical modes for different types of damage to induction motors. These features can be used in modern systems for diagnosing energy efficiency and technical condition of electromechanical systems, which can also perform the function of protecting electric motors from emergency modes.

Measurement and automatic monitoring insulation resistance of AC/DC mixed unearthed networks

Where the danger of electric shock is high, for example underground mines or medical equipment directly connected to a patient, special ungrounded power systems (unearthed networks) may be used to minimize possible leakage current to the ground. In mixed unearthed networks (AC/DC-IT) comprising alternating and direct current circuits, AC part of system is connected with DC part through rectifyer valves. Commutation of the valves causes cyclic variation of configuration of the entire galvanically connected network. A distinct feature of AC/DC-IT systems is that voltages between all points of AC side and ground may have mean value different from zero. This characteristic parameter can be used as a signal to determine fault and position to ground or decrease in insulation resistance. However, it also affects the results of the network equivalent insulation resistance measurements when a DC source is used to check insulation. The paper presents a solution to eliminate this effect and proposes a device model to measure and monitor insulation resistance. The results of this study can be applied in the design and manufacture of automatic resistance measuring and monitoring devices for AC/DC mixed unearthed networks.

NEURAL NETWORKS AS A MEANS TO PREDICTING THE STATE OF ISOLATION IN NETWORKS WITH ISOLATED NEUTRAL

Electrical insulation is an important parameter of the structural element of electrical equipment, provides reliability and durability of electrical networks with an isolated neutral and safe use of electricity in underground conditions. This article solves the problem of predicting changes in insulation resistance of electrical equipment, taking into account changes in external factors such as humidity and air temperature. An example of using a conventional neural network such as MLP did not give an acceptable result. To solve this problem, a recurrent neural network with longterm memory type LSTM was proposed. Its structure was developed. The simulation results showed that the neural network LSTM successfully coped with the task of predicting changes in insulation resistance, taking into account changing environmental factors. The prediction error for the one-hour interval was 0.052. The proposed idea of continuous monitoring of the insulation resistance with the prediction of its changes for a certain period will avoid sudden failures of electrical components, electric shock and fire caused by a decrease in insulation resistance below the critical value. The forecast of insulation resistance will allow in cases of delay of actuation of the device leakage at high rate of change of the insulation resistance, allows you to increase the efficiency of mining equipment through the reduction of damage from sudden shutdowns of the equipment underground mining machines in the course of their normal work.

Preparation and characterization of ultra-low-temperature of (BiAg)0.5MoO4 dielectric ceramic doped with Y2O3

This paper reports on the use of solid-phase sintering to prepare (BiAg)0.5MoO4 (BAMO) ceramic doped with Yttrium oxide (Y2O3), which is less expensive than the dopants currently used in the commercial formulation of powders for the fabrication of multilayer ceramic capacitors. Experiments were conducted to determine the means by which Y2O3 doping concentration and sintering temperature affect the phase evolution, microstructure, and dielectric properties of BAMO ceramic. Our results reveal the following important findings. Sintering Y2O3-doped BAMO ceramic at 600 °C resulted in the formation of secondary phases (Bi2Mo2O9 and γ"-Bi2MoO6) and constrained grain growth. Sintering at 600 °C promoted the precipitation of silver ions (Ag+) from the starting material (Ag2O) to agglomerate at grain boundaries, leading to a decrease in insulation resistance and breakdown voltage. Interestingly, 5 wt% Y2O3-doped BAMO ceramic sintered at 600 °C presented good dielectric characteristics: εr = 26.6, Q × f value = 22194 GHz, and temperature coefficient of resonant frequency = +14 ppm/°C. Our findings demonstrate that single-phase BAMO ceramic with good density characteristics can be produced via sintering at 600 °C for 1 h. The ultra-low-temperature production of this dielectric material would allow for co-firing with aluminum electrode to further reduce manufacturing costs.

Analysis of Mechanism of Improvement in Highly Accelerated Lifetime via Measurement of Vanadium Valence in Multilayer Ceramic Capacitors

The valence of vanadium was measured by electron paramagnetic resonance in order to explain the decrease in insulation resistance (IR) and the improvement in highly accelerated lifetime that resulted from the addition of vanadium. V4+ was detected in specimens with vanadium contents of 0.20 and 0.30 mol %, while no V4+ was detected in a specimen with a vanadium content of 0.06 mol %. It was also revealed that the content of the vanadium except for V4+ are the main factor responsible for the decrease in IR and the improvement in lifetime. The impedance of BaTiO3-based materials in multilayer ceramic capacitors with various vanadium contents was investigated in order to determine the mechanism of improving the highly accelerated lifetime using a four resistance and capacitor section electrical equivalent circuit. All four resistance components (R components) decreased with an increase in vanadium content. During the lifetime test, all four R components were degraded. In particular, the R component corresponding to the ceramic/internal electrode interface regions was more strongly degraded than the other three R components, and it was found that this component was the main factor responsible for the degradation of IR during the test. The resistance degradation of this component tended to occur slowly when the vanadium content increased, which resulted in the improvement in lifetime. The primary part of this degradation was implied to be controlled by diffusion.

Feasibility study of non-conductive film (NCF) for plasma display panel (PDP) application

In this study, attempts were made to develop an electrically reliable and low cost bonding process for plasma display panels (PDPs) by using non-conductive film (NCF). The adhesive joints were constructed with Ag electrodes on a glass substrate and Au finished Cu electrodes on a flexible printed circuit (FPC). To clarify the feasibility of NCF application, all experimental results of NCF were compared with those of anisotropic conductive film (ACF) which is currently used for PDP application. The reliability of adhesive interconnections using NCF and ACF was evaluated by measuring daisy resistance during high temperature and humidity storage test (85 °C, 85% RH for 1000 h). Ag electrochemical migration, which leads to the degradation of surface insulation resistance, was also investigated by measuring insulation resistance during the storage test (1000 h at 85 °C and 85% RH). The initial resistance of NCF joints was lower than that of ACF joints due to larger contact area between Ag and Au finished Cu electrodes in the NCF joints. During the reliability test, the daisy resistances of both joints increased slightly due to Z-directional swelling of the adhesives, but NCF joints showed more stable resistance than ACF joints. In addition, upon application of high voltage of 100 V (DC current), severe degradation of insulation resistance due to the Ag migration was observed in ACF joints, whereas no remarkable decrease of insulation resistance was observed in NCF joints. Conclusively, the application of NCF to PDP interconnection process has good potential from a practical point of view.

Water-induced dc and ac degradations in TiO 2-based ceramic capacitors

Water-induced degradations of TiO 2-based ceramic capacitors in the presence of dc and ac voltages are reported in this paper. Atomic hydrogen generated by electrolysis of water using dc voltages reduced TiO 2-based ceramics at ambient temperature. The resulting degradation was characterized by an increase in capacitance, a large dielectric loss and a dramatic decrease in insulation resistance. Hydrogen and oxygen generated by the electrolysis of water using ac voltages reacted with TiO 2-based ceramics. The resulting degradation was characterized by a decrease in capacitance, a large dielectric loss, but no noticeable changes in insulation resistance. Water played a vital role in both dc and ac degradations of TiO 2-based capacitors and an effort should be made to prevent water-induced degradations.

Soil Burial Tests: Effect of Soil Burial Exposure on the Properties of Electrical Grade Reinforced Plastic Laminates

Twenty-three types of unclad laminates based on phenolic, melamine, silicone, epoxy, and polyester resins were examined after exposure in alkaline soil at Roswell, New Mexico, and in acid soil at Bainbridge, Georgia. The laminates have initial insulation resistance values ranging from 1.4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> to less than 30 megohms. Each suffers a large decrease in insulation resistance upon exposure. The percentage decrease can be in excess of 90 percent after four years underground exposure in New Mexico or Georgia. Moderate decreases in flexural strengths also were observed.

The Effects of γ-Ray Radiation on the Electric Properties of Epoxy Resin

The radiation damage to materials insulating electric current presents serious problems affecting the delicate functions of such electric apparatuses and measuring instruments as are constantly exposed to electronic radiation. One of the problems concerns epoxy resin which is appropriate material of wide application for manufacture of electric apparatuses by reason of its insulating capacity. Studies have been made, accordingly, of the effect of γ-ray radiation on epoxy resin with this excellent property. Our experimental studies will best be stated in three respects, first regarding the insulation, secondly regarding the test material, and thirdly regarding the radiation. Regarding electric insulation, the resistance against electric insulation, the strength against dielectric breakdown and the resistance against arc were respectively examined. Regarding the test material three kinds of epoxy resins were used, that of the bisphenol type, that of the novolac type and that of the cycloaliphatic type. Regarding radiation the dose of γ was prescribed between 106∼109.The results obtained from the experiment are briefly as follows.(1) With increase of γ-ray radiation in dose there is general decrease in insulation resistance. It is observed, however, at the same time that the maximum value of γ-ray radiation so far produced has been about 4×106∼5×107, and it is found also that there can sometimes be restoration of insulation resistance by virtue of its temperature dependence.(2) The strength in voltage against dielectric breakdotwn decrease slowly with increase of γ-ray radiation in dose.(3) Regarding the amout of γ-ray radiation that can have an effect on arc resistance of epoxy resin, as far as such an effect is observable, its arc resistance begins to drop at exposure to radiation of ca. 108-γ and over, though there can sometimes be epoxy resin that is intact in arc resistance despite exposure to radiation even of ca. 109γ.(4) When aromatic amine and acid anhydride are used as curing agents for epoxy resin there is but little effect γ-ray radiation to be observed on its arc resistance.(5) The amout of curing agents consumed by epoxy resin when it shows its maximum arc resistance in value, and their amount when epoxy resin shows its minimum in value, both under the influence of γ-ray radiation to which the material is respectively exposed, vary from the amount respectively when the material shows its maximum arc resistance in value, and that when it shows its minimum in value accordingly, both free from such influence. This is attributed to the relative variation in the corresponding quantity of epoxy resin itself owing to the molecular split and cross-linking.

Studies on Electrode Connector Materials in the Electron Tube

For the purpose of selecting materials for manufacture of electrode connector in the electron tube, Ni, Ni-Mn, Ni-Cr, Ni-Fe and Ni plated Fe were examined with respect to their embrittlement by annealing them in city coal gas, to the embrittlement of the stem lead wire in the stem making process, to the decrease in insulation resistance of inter-electrodes during their life, and to the fatigue of the cathode tab in the inverted pinched cathode due to the on-off cycling test.The results are summarized as follows;(1) Embrittlement by city coal gas firing can be evaluated by measurement of tensile strength and elongation. Embrittled Ni has low tensile strength and elongation, and its metallographic structure shows micro cracks formed along the grain boundaries.(2) Embrittlement and fracture of Ni stem lead and Ni cathode tab was caused by city coal gas firing in stem making process and in glass envelope sealing-in process. Metallographic structures of the embrittled lead wire and tab showed also micro cracks along the grain boundaries.(3) Ni-Mn, when used as cathode tab, decreased in insulation resistance of inter-electrodes during its life. But it was not the case with Ni, Ni-Cr, Ni-Fe and Ni plated Fe.(4) Ni plated Ni-Fe is recommended for the stem lead wire, because it is not embrittled and hardened by city coal gas firing, nor is it oxidized in the stem making process, nor does it wet glass.(5) 42% Ni-Fe is recommended for the cathode tab material, because it is not embrittled by city coal gas firing, nor does it decrease in insulation resistance of inter-electrodes, and it is strong resistant to fatigue.

The effects of fungi on the direct current surface conductance of electrical insulating materials.

The extent of visible fungus growth on pressuresensitive electrical insulating tapes, as well as the effects of fungi on the mechanical strength of the tapes, was reported previously (Berk and Teitell, 1951). seriously decreased the tensile strength of tapes with cellulosic backings but had no more than slight effects on the tensile strength of tapes with elastomeric backings. This report deals with the second phase of the investigation, the effects of fungi on the electrical properties of the pressure-sensitive insulating tapes and, particularly, the effects on surface conductance. The literature is somewhat controversial concerning the relative importance of moisture and fungus growth on the failure of electrical equipment in service (Leutritz and Hermann, 1946; Leutritz, 1948). The Summary Technical Report of the National Defense Research Council (1946) on Tropical Deterioration of Electric and Electronic Equipment states, Fungi are also important agents of deterioration in electric and electronic equipment. Hyphal strands of surfacegrowing fungi can introduce leakage paths which reduce insulation resistance and establish couples which promote electrolytic corrosion. Titus (1945) considered fungi to be secondary causes of operational failures but stated that fungus filaments, when bridging terminals, might cause flash-overs. Proskauer and Smith (1945) stated that microorganisms contribute to the retention of moisture on surfaces that would normally dry out quickly, and the presence of these microorganisms might produce paths of low electrical resistance. Leutritz and Hermann (1946) searched the literature from 1820, the year the galvanometer was invented, to 1945 and found no experiments that showed the effects of fungi on insulation resistance. They then studied the effects of humidity and fungus on unfilled and variously filled plastics and concluded that the decrease in insulation resistance was due to water absorption or adsorption and not due to fungi. There have been several additional investigations of the problem since 1945. Witt, Chapman, and Raskin (1952) tested various plastics exposed to fungus and humidity; they used nitrogen atmospheres to prevent fungus growth where only humidity was desired; and they reported that the presence of moisture overshadowed any effects that fungi might have produced. Luce and Mathes (1951) and Gauvey (1951) worked with plastic hook-up wire insulation and, using a technique of fumigation to prevent fungus growth, were able to separate the effects of humidity and fungi. They found that for many good plastic insulating materials there was a definite decrease in the direct current surface resistance caused by fungi and separate from any lowering caused by moisture. This decrease in resistance was detected electrically before any visual growth of fungi took place. Leonard and Patouillet (1951) reported on a laboratory method that compared creepage resistance results from sterilized and viable inocula, and they were able to show that viable fungi lowered the resistance. Dubois and Herou (1952) investigated the effects of mold on various plastic materials. They used as criteria the time required for travel of high voltage sparks and the number of drops of water or ammonium chloride solution required for a short circuit. They found that with some materials there is a degrading effect from the molds, which they attributed to the secretion of an electrolytic solution capable of penetrating the material. In studying a slightly different aspect of the problem, Blake, Kitchin, and Pratt (1950) and Blake and Kitchin (1949) reported that soil microorganisms can cause failure of rubber insulation. The decreases in volume resistance were so great that sensitive measuring methods were not necessary.

The Effects of Fungi on the Direct Current Conductance of Electrical Insulating Materials

The extent of visible fungus growth on pressuresensitive electrical insulating tapes, as well as the effects of fungi on the mechanical strength of the tapes, was reported previously (Berk and Teitell, 1951). Fungi seriously decreased the tensile strength of tapes with cellulosic backings but had no more than slight effects on the tensile strength of tapes with elastomeric backings. This report deals with the second phase of the investigation, the effects of fungi on the electrical properties of the pressure-sensitive insulating tapes and, particularly, the effects on surface conductance. The literature is somewhat controversial concerning the relative importance of moisture and fungus growth on the failure of electrical equipment in service (Leutritz and Hermann, 1946; Leutritz, 1948). The Summary Technical Report of the National Defense Research Council (1946) on "Tropical Deterioration of Electric and Electronic Equipment" states, "Fungi are also important agents of deterioration in electric and electronic equipment. Hyphal strands of surfacegrowing fungi can introduce leakage paths which reduce insulation resistance and establish couples which promote electrolytic corrosion." Titus (1945) considered fungi to be secondary causes of operational failures but stated that fungus filaments, when bridging terminals, might cause flash-overs. Proskauer and Smith (1945) stated that microorganisms contribute to the retention of moisture on surfaces that would normally dry out quickly, and the presence of these microorganisms might produce paths of low electrical resistance. Leutritz and Hermann (1946) searched the literature from 1820, the year the galvanometer was invented, to 1945 and found no experiments that showed the effects of fungi on insulation resistance. They then studied the effects of humidity and fungus on unfilled and variously filled plastics and concluded that the decrease in insulation resistance was due to water absorption or adsorption and not due to fungi. There have been several additional investigations of the problem since 1945. Witt, Chapman, and Raskin (1952) tested various plastics exposed to fungus and humidity; they used nitrogen atmospheres to prevent fungus growth where only humidity was desired; and they reported that the presence of moisture overshadowed any effects that fungi might have produced. Luce and Mathes (1951) and Gauvey (1951) worked with plastic hook-up wire insulation and, using a technique of fumigation to prevent fungus growth, were able to separate the effects of humidity and fungi. They found that for many good plastic insulating materials there was a definite decrease in the direct current surface resistance caused by fungi and separate from any lowering caused by moisture. This decrease in resistance was detected electrically before any visual growth of fungi took place. Leonard and Patouillet (1951) reported on a laboratory method that compared creepage resistance results from sterilized and viable inocula, and they were able to show that viable fungi lowered the resistance. Dubois and Herou (1952) investigated the effects of mold on various plastic materials. They used as criteria the time required for travel of high voltage sparks and the number of drops of water or ammonium chloride solution required for a short circuit. They found that with some materials there is a degrading effect from the molds, which they attributed to the secretion of an electrolytic solution capable of penetrating the material. In studying a slightly different aspect of the problem, Blake, Kitchin, and Pratt (1950) and Blake and Kitchin (1949) reported that soil microorganisms can cause failure of rubber insulation. The decreases in volume resistance were so great that sensitive measuring methods were not necessary.

Effect of Soil Microörganisms on Rubber Insulation

Abstract Evidence is presented that some of the rare failures of nonleaded rubber-insulated underground cables were caused by soil microörganisms. Laboratory conditions deliberately chosen to favor microbial attack made possible greatly accelerated tests in which failures like those found after ten years or more in service have been produced in a few weeks or months. These failures are characterized by discrete, minute low resistance spots. Drastic decrease of insulation resistance was used as the criterion of failure. The exact distribution of low-resistance spots has been shown by an electrolytic test. Parallel tests in sterile and in active soil proved that the failures in soil were caused by living microorganisms. Soil exposure tests have been made on natural and synthetic rubbers and several other plastic insulations. Natural rubber compounds were found to be the most susceptible. In some soils GR-S compounds proved stable up to two years, but failed in a more active soil. The exact mechanism of attack is not known. In some cases there is drastic electrical failure without visible surface effect. In actual operation of underground cables the exact combination of conditions necessary for producing failure by microbial attack can occur only rarely. The purpose of the severe laboratory tests is to make possible the development of insulations stable under all soil conditions.