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Dielectric strength is defined as the maximum voltage required to produce a dielectric breakdown through the material and is expressed as Volts per unit thickness. A higher dielectric strength represents a better quality of insulator. Test Procedure: There are three basic procedures that can be used to determine the dielectric strength of an insulator. These procedures are the short-time method, the slow rate-of-rise method and the step-by-step method. Each of these three methods has the same basic set-up, which consists of the test specimen placed between two electrodes in air or oil.
For the most common test, the short-time method, voltage is applied across the two electrodes and raised from zero to dielectric breakdown at a uniform rate. Breakdown is when an electrical burn-through punctures the sample, or decomposition occurs in the specimen.
The rate of voltage rise is determined by the time it takes the sample to reach dielectric breakdown. The test is sometimes performed in oil to prevent arcing from the electrode to the ground.
Specimen size: The recommended specimen type for this test is a 4 inch plaque or larger. Rates Electrostatic voltmeters, voltage divid- ers, or potential transformers having comparable accuracy may be used for calibration measurement. Conditioning material, the same rate-of-rise shall be used with preference Mate- and 20 s.
If the time to breakdown cannot be adhered to, the rials so affected should be brought to equilibrium with an time shall be made a part of the report.
For such materials, the conditioning should be test electrodes at the preferred starting voltage and in steps and included in the standard referencing this test method. Condition- ing times for these materials should be sufficiently long to permit the specimens to reach moisture equilibrium as well as temperature equilibrium.
This will usually reduce the probability of surface flashover. Use the short-time test unless Vs kV A is kV otherwise specified. Similar requirements shall apply to the procedure used to increase the voltage between successive steps. If breakdown occurs prior to the 50 end of the holding period at any step, the dielectric withstand The voltage at breakdown, Vbd, is to be used to calculate dielectric breakdown strength. The dielectric with- See Fig. This phenomenon is most commonly evidenced steps, but in not less than s.
If failure occurs at the third during the test by an abrupt visible and audible rupture through step or less, or in less than s, whichever is greater, on more the thickness of the specimen, resulting in a visible puncture than one specimen in a group, the tests should be repeated with and decomposition of the specimen in the breakdown area. If failure does not occur before the This form of breakdown is generally irreversible. Repeated twelfth step or greater than s, increase the initial voltage.
Such repeated applica- voltage was held. If failure occurred while the voltage was tions of voltage may be used to give positive evidence of being increased to the starting voltage the failure time shall be breakdown and to make the breakdown path more visible.
Commonly specimen. This type of failure, usually associated with slow- used lengths are 20 s and s 5 min. For research purposes, rise tests at elevated temperatures, may in some cases be it may be of value to conduct tests using more than one time reversible, that is, recovery of the dielectric strength may occur interval on a given material.
The voltage source must trip the test electrodes, from the starting voltage and at the rate rapidly at relatively low current for this type of failure to occur. The initial voltage shall be reached as high capacitance specimen, or to malfunctioning of the breaker.
Such interruptions of the test do not constitute breakdown Ordi- satisfactory test. Calculation the initial value. If breakdown repeatedly occurs at a value Precision and Bias dard deviation, or other measure of variability. Report laboratories and eight materials are summarized in Table 2. This study made use of one electrode system and one test When making Polyethylene 0.
Using identical step test specimens only, types of equipment and controlling specimen preparation, electrodes and testing procedures closely, the single-operator When making a direct comparison Standards which refer to this method should deter- Refer to 5. Request RR:D Keywords having a thickness of 0. The electrodes must not Accurately determine the breakdown; dielectric breakdown voltage; dielectric failure; voltage at breakdown.
The test values are dependent upon process-control testing; proof testing; quality-control testing; specimen geometry, electrodes, and other variable factors, in rapid rise; research testing; sampling; slow rate-of-rise; step- addition to the properties of the sample, so that it is not by-step; surrounding medium; voltage withstand possible to make a statement of bias. Weak spots within the volume X1. The breakdown mechanisms usually operate in combina- being more pronounced with thin test specimens.
Test results tion rather than singly. The following discussion applies only to are also affected by the electrode geometry. Results may be solid and semisolid materials. Generally many tests on commercial materials, breakdown is caused by speaking, the effect of the electrode material is difficult to electrical discharges, which produce high local fields.
With establish because of the scatter of experimental data. Discharges may occur in any dependent upon the specimen thickness. Experience has shown internal voids or bubbles that are present or may develop. These may cause local erosion or chemical decomposition. In the case of solids that can be melted and poured conduction losses which generate heat more rapidly than can to solidify between fixed electrodes, the effect of electrode be dissipated.
Breakdown may then occur because of thermal separation is less clearly defined. Since the electrode separation instability of the material. Since the when the field intensity becomes sufficient to accelerate elec- dielectric strength is so dependent upon thickness it is mean- trons through the material.
This critical field intensity is called ingless to report dielectric strength data for a material without the intrinsic dielectric strength. It cannot be determined by this stating the thickness of the test specimens used. In general, the dielectric generally nonhomogeneous and may contain dielectric defects strength will decrease with increasing temperatures, but the of various kinds. Dielectric breakdown often occurs in an area extent to which this is true depends upon the material under of the test specimen other than that where the field intensity is test.
When it is known that a material will be required to greatest and sometimes in an area remote from the material function at other than normal room temperature, it is essential.
In general, the breakdown voltage will Es 5 S 4. In cases of large thickness of specimen and low permittivity This is to be expected because the thermal breakdown mecha- of specimen, the term containing ts becomes relatively insig- nism is time-dependent and the discharge mechanism is usually nificant and the product of permittivity and electric strength is time-dependent, although in some cases the latter mechanism approximately a constant.
Unless the breakdown path X1. Within the medium cannot be compared with those in a different medium. Hence, its impor- X1. However, even materials that absorb ing the test specimens in a liquid dielectric such as transformer little or no moisture may be affected because of greatly oil, silicone oil, or chlorofluorocarbons, in order to minimize increased chemical effects of discharge in the presence of the effects of surface discharges prior to breakdown.
It has been moisture. Except in cases where the effect of exposure on shown by S. Whitehead12 that in order to avoid discharges in dielectric strength is being investigated, it is customary to the surrounding medium prior to reaching the breakdown control or limit the relative humidity effects by standard voltage of the solid test specimen, in alternating voltage tests it conditioning procedures.
Therefore there is a great need for a test to evaluate the performance of particular materials at high voltage Eme8m. The dielectric breakdown voltage test represents a and if the liquid immersion medium is a semiconducting convenient preliminary test to determine whether a material material the criterion becomes merits further consideration, but it falls short of a complete E msm. Some of these effects Subscripts: can be incorporated in laboratory tests, and a better estimate of m refers to immersion medium, the material will result, but the final consideration must always r refers to relative, be that of the performance of the material in actual service.
Transformer oil is usually specified and its dielectric 13 Starr, R. In these uses of the test it is the relative value of the breakdown voltage that is important rather than the absolute value.
Do not use another details used to determine this property. While every effort has document, including those listed in this appendix, as a model been made to include as many as possible of the standards for reference to this test method unless there is conformity with referring to Test Method D , the list may not be complete, 5. ASTM International takes no position respecting the validity of any patent rights asserted in connection with any item mentioned in this standard.
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Open navigation menu. We have no amendments or corrections for this standard. Terminology safety concerns, if any, associated with its use. It is the responsibility ast the user of this d to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use. Link to Active This link will always route to the current Active version of the standard. Continuous light with equilibrium uninsulated black 8.
This unexposed surface must not be cerned parties must agree on the control material used. Enclosed carbon-arc exposures of plastics are described in Practice D, and in G, which gives requirements for exposing nonmetallic materials in enclosed carbon-arc devices. The exposure used in this practice is not intended to simulate the deterioration caused by localized weather phenomena such as atmospheric pollution, biological attack, and saltwater exposure.
The type and rate of degradation and the performance rankings produced by exposures to filtered open-flame carbon-arcs can be much different from those produced by exposures to other types of laboratory light sources.
Significant factors include regulation of line voltage, freedom from salt or other deposits from water, temperature and humidity control, and conditions of the electrodes. Please first log in with a verified email before subscribing to alerts.
It is recommended that at least three replicates of each material be exposed to allow for statistical evaluation of results. Conduct these lengths in which the measurements were made.
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