Tests electric strength

The efficiency of transformer oils as dielectrics is measured by electric strength tests. These give an indication of the voltage at which, under the test conditions, the oil will break down. Various national standards exist that all measure the same basic property of the oil. There is an international specification, lEC 296/1982, which may be quoted by equipment manufacturers in their oil recommendations. 

Voltage withstand. The ability of a product s insulation to withstand high voltage between circuits (i.e., 3,000 Vac prim-to-sec for 1 min). This test is sometimes referred to as hi-pot or electric strength test. 

The electric strength test (hi-pot) consists of applying to the equipment a sinusoidal AC voltage or an equivalent DC voltage (i.e., 1,5(X) Vac or 2,121 Vdc) between the primary and ground for 1 to 2 seconds, depending on the product stan-... 

Value of voltage applied during the electric strength test ... 

IEC 60243, Electrical strength of insulating materials - Test methods - Part 1 Tests at power frequencies, 1998. 

DS (ASTM D-149) is an indication of the electrical strength of an insulating material and it is dependent on the particular test conditions. 

Electric strength is usually taken as the nominal voltage gradient (applied voltage divided by test piece thickness) at which breakdown occurs under specified conditions of test. These specified conditions of test are important as the measured electric strength is not an intrinsic property of the material but depends on test piece thickness, time of electrification and the electrode geometry, as well as on conditioning of the material. 

Testing of polyurethanes for their electrical properties due to the voltages required must be carried out using properly designed equipment. The electrical tests that are normally carried out are resistivity, insulation resistance, electric strength, tracking resistance, power factor, and permittivity. 

The temperature plays, of course, a role in both mechanisms the electric strength decreases with increasing T. Moreover, though expressed in volts per meter thickness, it depends on the thickness (again non-linearity). Thin films have a considerably higher strength than most values found in tables of properties, which are results of tests on thicker samples. 

Electric strength data are meaningful only if the test conditions are adequately defined for example, if d.c. loading is employed the rate of voltage increase should be specified, if pulsed voltages are used the rise time should be specified and if a.c. loading is adopted the frequency and waveform should be specified. 

When testing electric strength there is the risk of flash-over across the specimen surface between the electrodes. This is avoided by making the measurement with the specimen immersed in an insulating liquid, such as transformer oil, which displaces air from the ceramic surface. 

Water absorption Compressive strength Bending strength Modulus of elasticity Linear thermal expansion (at 200°F) Thermal conductivity (at 200°F) Operating temperature limit Electrical resistance (tested at 3 volts and 1 mm distance)... 

The electric strength or dielectric breakdown test method (ASTM D-877) indicates the absence, or presence, of free or suspended water and other contaminant matter that will conduct electricity. A high electric strength gives no indication of the purity of an oil in the sense of degree of refinement or the absence of most types of oil-soluble contaminants. This test method is of some assistance, when applied to an otherwise satisfactory oil, to indicate that the oil is free of contaminants of the type indicated above in practice, this ensures that the oil is dry. 

Water is obviously undesirable in electrical equipment, and the water content of transformer and other insulating oils is frequently limited to a low maximum value. Traces of water that would not influence the general run of petroleum product tests could have a very significant effect on properties such as electric strength. 

The electric strength is generally highly dependent on the interelectrode distance, the surrounding medium and. to a lesser extent, the electrode area. For this reason it is preferable to report breakdown voltage, specimen dimensions, and test conditions rather than simply electric strength,... 

Evaluation of the shake-out characteristics of the sand cores made with the binder compositions was made with the AES nonstandard Retained Strength test. The standard, hardened-by-gassing. 2 x 2 in. sand specimens were soaked in an electric muffle furnace at 850°C. for 12 min in their own atmosphere, then removed from the furnace and allowed to cool to just above room temperature, and tested in the Universal Sand Strength Machine. 

Geometries for the determination of electric strength (a) Sphere on film (b) sphere recessed into sheet (c) cylinders embedded in plastic and (d) the corresponding Weibull plot of the results of many tests on LDPE at 20°C (from Seanor D. A, Ed., Elearical Properties of Polymers, Academic Press, 1982). 

Variation of electric strength of recent XLPE cable with the duration of the voltage application, tested at 90 °C, with 6h on, 18 h off. (From Ishibashi, A. et o/., IEEE Trans. Dielectrics Electrical Insuial, 5, 695, 1998) 1998 IEEE... 

Explain why the design electric field in a high voltage cable insulated with polyethylene is only lOMVm when in laboratory tests the electric strength is measured as 800 MV m. ... 

Kum] Mechanical and electrical properties tests, thermomagnetic analysis (TMA-2 apparatus) Strength, yield point, electrical resistivity... 

Miy] Mechanical and electrical properties tests, TEM Tensile strength, yield strength, elongation, conductivity, resistance to heat-softening... 

Dielectric strength (electric strength) n. A measure of the voltage required to puncture an insulating material, expressed in volts per mil of thickness (SI V/rrmi). The voltage is the root-mean-square voltage difference between the two electrodes in contact with opposite surfaces of the specimen at which electrical breakdown occurs under prescribed test conditions. Ku CC, Liepins R (1987) Electrical properties of polymers. Hanser Publishers, New York. Weast RC (ed) (1971) Handbook of chemistry and physics, 52nd edn. The Chemical Rubber Co., Boca Raton, FL. 

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