Impulse breakdown

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. 

In pulse breakdown experiments, usually an electrode geometry is employed which confines the high stress region to a small volume (see Section 2.9). In the case of two spherical electrodes, the locus of highest electric field strength in the gap is in the line connecting the two centers of the spheres. In other arrangements an etched 

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Figure 6. Switching impulse breakdown voltage for H/D = 1, which is average...

Side-flashing can also occur below the ground to buried metal pipes or wires and care must be taken in the design and positioning of the grounding electrodes. Typical values of impulse breakdown in soil are 2 to 5kV/cm, which leads to side-flashes of several meters. In air the value is 9kV/cm and brick and concrete has a slightly lower breakdown strength. 

In another study, Sukhushin and coworkers [35] studied the impulse breakdown strength of lead azide pellets of different thicknesses and densities. The effect of changing electrode polarity was also investigated. The specimen was a wafer pressed on a polished, hardened steel cylinder into a polyvinyl chloride sheath. The outer spherical electrode was of 10 mm diam and positioned against the outer surface of the specimen. Square-wave pulses, 1.5-40 //sec, were used to determine the breakdown voltages. 

Eleetrieal properties of biodegradable polylactic acid films were measured and compared with those of erosslinked polyetlylene (XLPE) cimently used as insulation for cables and electric wire [10]. The volume resistivity, dielectric constant and dielectric loss tangent of PLLA were found to be almost the same as those of XLPE. However, the impulse breakdown strength of PLLA was 1.3 times that of XLPE. 

In addition to the dielectric-strength test at power frequency, which remains the basic test, other criteria have been adopted, such as impulse breakdown voltage, partial-discharge voltage and electrical stability under partial discharges as characterized by gassing. 

Figure 10 Impulse breakdown strength as a function of pulse duration, d = 63.5 im. (Redrawn from the data of Crowe, R.W., /. Appl. Phys., 17, 156, 1956.)...

Kitani, I., and Arii, K., 1980, Impulse breakdown of polymer dielectrics in the ns range in divergent fields, IEEE Trans. Elec. Ins., EI-15, 2 134. 

FIG. 16 Relationships between density and ac ( ) or impulse ( ) breakdown stresses of XLPE. (Adapted from Ref 52.)... 

The procedure for testing will be the same as that discussed for switchgear assemblies in Section 14.3.4. The impulse test voltage is applied as in Table 32.1(A) for series I and Table 32.2, for series II voltage systems with a full wave standard lightning impulse of 1.2/50 (Section 17.6.1). There should be no disruptive discharge or insulation breakdown. 

ASTM D3426, 1997 (2004). Standard test method for dielectric breakdown voltage and dielectric strength of electrical insulating materials using impulse waves. 

In a reanalysis of these data, Raine et al. found that the reduction in frontal activation was much more pronounced in murderers whose crimes had an affective rather than predatory basis (Raine et al., 1998). Affectively motivated crimes are generally considered to have a more impulsive quality than predatory murders that are, by definition, planned. Davidson et al. (2000) have postulated that impulsive violence results from a breakdown in the brain s ability to regulate negative affect. Thus, affective crimes may provide a window into the brain mechanisms underlying impulsive aggression. 

Similar results have been obtained in the study of foam breakdown by alcohols using a special device for determination of the rate of contact foam breakdown [69]. The foam was supplied at constant rate over the surface of organic solvent. The rate of foam breakdown was determined in two regimes impulse and continuous. In the impulse regime, after contacting the organic solvent, the foam expanded as a consequence of its breakdown in the contact zone and detached itself from the solvent surface. Since the foam was constantly supplied, after a... 

Unlike the nonspecific effects and uncommon occurrence of direct mortality observed in wildlife exposed to chlorinated hydrocarbon pesticides, several studies have documented direct mortality from exposure to OP and carbamate insecticides. The method by which the OPs and carbamate insecticides affect wildlife is quite different from the method by which the chlorinated hydrocarbon insecticides effect wildlife. The OPs and carbamates inhibit cholinesterase, primarily acetylcholinesterase (AChE), which is an enzyme that functions in the breakdown of the neurotransmitter acetylcholine. Acetylcholine functions in the transmission of nerve impulses. Therefore, when AChE is inhibited by an OP or carbamate insecticide, it can no longer breakdown acetylcholine and there is continued transmission of nerve impulses that eventually leads to nerve and muscle exhaustion. The respiratory muscles are a critical muscle group that is affected, often leading to respiratory paralysis as the immediate cause of death. A major difference in the mode of action between OPs and carbamates is that the inhibition of AChE by OPs is, from a biological standpoint, irreversible, while the inhibition from exposure to carbamates is reversible in a biologically relevant time frame. There... 

Potassium is an electrolyte (cation) inside cells and is used to transmit and conduct neuro-impulses and maintain cardiac rhythms. Potassium regulates intracellular osmolality. Potassium also promotes cell growth by moving into cells of new tissues and leaves cells during the breakdown of dead tissues. Potassium is used to contract skeletal and smooth muscles. 

In normal transmission of a nervous impulse from nerve to nerve, acetylcholine is released into the synapse in order to excite the receiving neuron (Figure 5.10). Unless acetylcholine is rapidly broken down, the receiving nerve is constantly fired, resulting in uncoordinated muscle movement, nausea, dizziness, and eventually seizures and unconsciousness. The serine enzyme acetylcholinesterase is responsible for the expedient breakdown of the neurotransmitter acetylcholinesterase. 

He discovered and measured heat production associated with nerve impulses and analyzed physical and chemical changes associated with nerve excitation, among other studies. In 1922 he won the Nobel Prize in physiology or medicine (with otto meyerhof) for work on chemical and mechanical events in muscle contraction, such as the production of heat in muscles. This research helped establish the origin of muscular force in the breakdown of carbohydrates while forming lactic acid in the muscle. 

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