Electrical breakdown tests

In the first method a series of electrical breakdown tests is carried out at the maximum design service temperature and these data are fitted to a 3 parameter Weibull distribution... 

Typical electrodes for electric breakdown tests are described in ASTM D 149, lEC Publ. 243, and material specifications. Different procedures are used in the United States and Europe. It should be emphasized that electric breakdown values are influenced greatly by electrode geometry. For research and development, electrodes may be recessed in the material to be tested. Although tests are most often made in air, it is common to immerse the test specimens in oil or to use a pressurized gas, such as SFe to prevent flashover. 

Test results Any disruptive discharge or electrical breakdown during the application of high voltage should be considered as a dielectric failure. 

Electrical Stability of Emuisions. The electrical stability test indicates the stability of emulsions of water in oil. The emulsion tester consists of a reliable circuit using a source of variable AC current (or DC current in portable units) connected to strip electrodes. The voltage imposed across the electrodes can be increased until a predetermined amount of current flows through the mud emulsion-breakdown point. Relative stability is indicated as the voltage at the breakdown point. 

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. 

There are many published discussions of both the effects of polymeric structure and testing conditions on the dielectric strength (for example, see [23,26,27]) and the underlying mechanisms of electrical breakdown (for example, see [23,26-28]). At the risk of greatly oversimplifying these complicated effects and mechanisms, they will now be reviewed briefly. 

The average voltage gradient at which electric breakdown occurs under specific conditions of test. 

Electrical breakdown is associated with the growth of trees., named after the structures that grow from charged metal needles in laboratory tests. Bow tie shaped trees grow in both directions from voids in the XLPE of high voltage DC cables (Fig. 12.7). The void acts as an electrical stress concentration, which initiates the electrical or electrochemical breakdown process. 

The dielectric strength of PES, as measured in oil by the short term tests ASTM D 149, is 800 KV/em for a 0.23 mm section and 158 KV/cm for 3 2.3 mm. The tracking behaviour of PES when subjected to high voltages is very complex and this phen< nenon is currently under investigation. There are indications that electrical breakdown associated with carbonisation tends to occur across the surface of the material. However, results obtained are similar to those found for other amorphous aromatic polymers like polycarbonate. 

The interface layer formed in the CVD process between the silicon substrate and the SIPOS film can lead to unstable I-V characteristics or to an electric breakdown in high-voltage planar devices. The modified ATR method was applied to study this interfacial layer between a Si substrate and polycrystalline Si [77], IR reflectance spectra were recorded in p- and 5-polarized radiation at 80°. The normalized reflectance spectra R = Rp/Rs were compared to the spectra of a test (reference) structure with a known thickness of Si02 and were calcnlated by the three-phase model, with an absorbing layer characterized by a complex dielectric constant and two transparent silicon layers with real dielectric constants. The analysis of the vlo band at 1210 cm revealed that the interface layer is comprised of silicon oxides, mainly Si02, with a thickness of 0.7 nm [77]. 

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. 

The effects of irradiation on both the electrical and the mechanical properties of the specimens were measured. Electrical tests comprised breakdown and in situ resistivity and postirradiation resistivity. Breakdown tests were performed after irradiation at room temperature. The need for tests in situ is recognized and being studied. Equipment originally designed for ASTM D 877-67 (breakdown testing of transformer coils) was modified to use solid... 

Here we shall concentrate on experiments designed to test certain models of the electric breakdown process. In these experiments certain fundamental mechanisms are allowed to dominate the breakdown process. 

The fidelity of Equation 32 was tested with respect to the data of Kao and Higham on the temperature and pressure dependence of the electric breakdown of n-hexane (Kao and Higham, 1951). Some of the fits obtained are shown in Figures 5 and 6. 

The electric breakdown of a dielectric liquid under high electric stress is a complex phenomenon where many elementary processes contribute to the change of the electrical current through the test gap from values of pico- to nano-amperes to values of kilo-amperes on a time scale of nanoseconds. Electronic processes are always involved in the initiation of the electric breakdown of nonpolar dielectric liquids. Unambiguous experimental evidence is scarce in the literature since a multitude of other effects obscures the electronic contribution of the breakdown process. In addition, many breakdown tests were performed on industrial-grade liquids, such as transformer oil etc., which are not pure liquids but rather mixtures of several components. More unambiguous information on electronic processes can be obtained from breakdown measurements with impulse voltages of nanosecond to microsecond duration. Complementary are studies of the laser-induced breakdown. 

The arc resistance of a material is the time (in seconds) that its surface may be exposed to an arc before electrical breakdown occurs. A high-voltage, low-current arc is used to simulate the conditions likely to be found in service. A standard test procedure used in the United States is described in ASTM-D-495 and a number of arc testers are commercially available that comply with this specification. The specimen to be examined is placed between the electrodes and an arc generated at specified intervals and currents. 

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