Dielectric Breakdown Voltage and Dielectric Strength

1 1 ScopeThese methods are designed for use in determin-ing the Dielectric Strength of solid electrical insulating Applicable DocumentsNone3 Test DimensionsThe specimens shall be of such a diam-eter that flashover will not occur. This usually means that thediameter should be 76 mm or Thick Solid MaterialsThe Breakdown Voltage of thicksolid materials may be so high that special test specimens cutor molded in reduced thickness may be At Various ThicknessesWhen it is desired to deter-mine the Dielectric Strength for different thicknesses of a mate-rial, it is necessary to test each different thickness, unless thevariation due to thickness is already Exceptional ConditionsThe special sizes of speci-mens required for determining Dielectric Strength underexceptional conditions shall be as specified in the TransformerThe desired test Voltage may be mostreadily obtained by a step-up transformer energized from avariable low- Voltage source.

2 The transformer and its control-ling equipment shall be of such size and design that the testspecimen in circuit, the crest factor (ratio of maximum tomean effective) of the test Voltage shall not differ by more than 5% from that of a sinusoidal wave over the upper half of therange of test Voltage . The crest factor may be checked bymeans of a sphere gap or peak-reading voltmeter in conjunc-tion with a voltmeter. For test specimens of smallcapacitance, a testing transformer as small as 500-volt-ampere rating must be used. Where the wave-form cannot bedetermined conveniently, a transformer having a rating of notless than 2 kilovolt amperes shall be used for voltages notexceeding 50,000 volts. Tests shall be made at commercialpower frequencies. When a transformer is used at voltageslower than its full rating, the current drawn from the high- Voltage winding should not exceed the full-load full-voltagecurrent Circuit BreakerThe test transformer circuit shall beprotected by an automatic circuit-breaking device designed toopen instantaneously on the current produced by breakdownof the test specimen.

3 Excessive flow of current at the time ofbreakdown causes pitting and heating of the electrodes andthereby increases the work of electrode maintenance and timeof Voltage ControlThe rate of Voltage rise shall not, forshort time tests, vary more than 25% from the specifiedrate. Control of Voltage may be secured in one of severalways:a. Variable-ratio autotransformerb. Resistance-potential dividerc. Generator-field regulationd. Induction regulatorPreference should be given to equipment having an approxi-mately straight-line Voltage -time curve over the desired oper-ating range. Motor drive with variable speed control should bepreferred to manual drive because of the difficulty in maintain-ing reasonable uniform rate of Voltage rise with the VoltmeterThe Voltage shall be measured by anapproved method, which gives root-mean-square values,preferably by means ofa. A voltmeter connected to the secondary of a separatepotential transformerb.

4 An electrostatic voltmeter in the secondary circuitc. A voltmeter connected to a well-designed tertiary coil in thetest transformer. A voltmeter connected to the primary sideof the testing transformer may be used only if the ratio oftransformer does not change appreciably with ElectrodesThe electrodes used for thin solid materials(sheets and plates) shall be metal disks 5 mm in diameter and25 mm in length, with the edges rounded to a radius of The electrodes used for thick solid materials shall bemetal disks 25 mm in diameter and 25 mm in length, withedges rounded to a radius of mm. The electrodes fortapes and sheet materials to be compared with tapes shall beThe Institute for Interconnecting and Packaging Electronic Circuits2215 Sanders Road Northbrook, IL 60062 IPC-TM-650 TEST METHODS Breakdown Voltage and DielectricStrengthDate10/86 RevisionOriginating Task GroupN/AMaterial in this Test Methods Manual was voluntarily established by Technical Committees of the IPC.

5 This material is advisory onlyand its use or adaptation is entirely voluntary. IPC disclaims all liability of any kind as to the use, application, or adaptation of thismaterial. Users are also wholly responsible for protecting themselves against all claims or liabilities for patent referenced is for the convenience of the user and does not imply endorsement by the cylindrical rods mm in diameter, with edgesrounded to a radius of mm. The upper movable elec-trodes shall weigh 2g. When mm electrodes areused, it is advisable that they be surrounded by guard elec-trodes or Dielectric Strength of an insulating material varieswith the thickness of the material and the area and geometryof the test electrodes, and these should be specified in thespecification. Tests made with different electrodes are notcomparable. Where materials are made up into forms of uni-form thickness, such as sheets and plates, tests shall bemade upon that thickness of material.

6 In other cases, a thick-ness of the test specimen and diameter and shape of theelectrode have been selected, which are compatible with con-venience of Equipment Testing For Tests Made in AirUse may be made of any welldesigned oven of sufficient size to hold the test equipment. Itshould be provided with some means of circulating air so thatapproximately constant temperature is maintained around thetest specimen and with a thermometer or thermocouple formeasuring the temperature as near the point of test as prac-ticable to the nearest 1 For Test Under OilUse may be made of an oil bath,provided with some means for circulating the oil so that thetemperature is substantially uniform around the test speci-mens and with a thermometer or thermocouple for measuringthe temperature as near the point of test as practicable to thenearest 1 Test MediumThe medium to be used in the testsshould be specified in the specification.

7 In general, it is prefer-able to test materials in the medium, whether air or oil, inwhich they are to be used. Where conditions of use are notwell defined, materials should be tested in air up to the pointwhere the Breakdown is so high that an excessive amount ofmaterial is required to prevent flashover to excessive burningof the surface. For specimens having a high Breakdown , suchas the thicker and highgrade materials, it is usually necessaryto make Dielectric Strength tests under oil: however; it shouldbe understood that Breakdown values obtained under oil arenot comparable with those obtained in Preparing the preparation of test specimens, care shall betaken to have the surfaces adjacent to the electrodes paralleland as plane and smooth as the material permits. The dielec-tric Strength of an insulating material varies with the thicknessof the test specimen. Therefore, tests on specimens of differ-ent thicknesses are not comparable.

8 The thickness used shallbe the average thickness of the sample measured as specifiedin the specification Dielectric Strength of most insulating materials var-ies with temperature and humidity. The test coiditions to beused should be specified in the specification. Usually it isdesirable to determine the Dielectric behavior of a materialover the range of temperature and humidity to which it is likelyto be subjected in use. When required, materials may be con-ditioned in a suitably controlled chamber. The test specimenshall be kept in the chamber long enough to reach a uniformtemperature and humidity prior to testing. When required, thedielectric Strength tests shall be made on the specimen whilestill in the conditioning chamber. For purpose of tests, a high- Voltage conductor may be conveniently carried into the cham-ber through an insulating Positioning and Care of ElectrodesElectrodes shallbe held truly coaxial. Where electrodes have flat test faces, thelatter shall be parallel to each other.

9 The test faces shall bekept smooth and polished and free from Application of Test for Specified Minimum RequirementThevoltage shall be applied and increased at a uniform rate fromzero to the value specified in the material specification andshall be held at the value for the specified time. Unless other-wise specified, the rate of rise per second shall be 5% of thespecified Voltage . Note that this test is to check for ability towithstand a specified Voltage and not to determine the break-down Test to Breakdown , Short-time TestThe voltageshall be increased from zero to Breakdown at a uniform rate of rise shall be or kilovolts per on the total test time required and the Voltage -timecharacteristic of the material. The rate of rise of Voltage shouldbe specified in the material Breakdown Voltage and Dielectric StrengthDate10 Test to Breakdown , Step-by-Step TestAn initialvoltage shall be applied equal to 50 of the Breakdown voltagein the short-time test, adjusted as shown in Table Voltage shall then be increased in equal increments asstated in the various material specifications, the Voltage beingheld at each step for a definite time as stated in the specifica-tions.

10 The change from each step to the next higher shall bemade as rapidly as possible and the time of change includedin the succeeding test Test to Breakdown , Slow-Rate-of-TestAn initialvoltage shall be applied equal to approximately 50% of thebreakdown Voltage in the short time test, unless otherwisespecified. The Voltage shall then be increased at a uniformrate up to the point of Breakdown . Unless otherwise specified,the rate should be chosen to give approximately the samevoltage-time exposure of the test specimen, as provided inthe step-by-step Determining Rate of Rise of VoltageThe rate ofvoltage rise may be calculated from measurements of timerequired to raise the Voltage between two prescribed motordriven regulating equipment is used, the speed-control rheostat may be calibrated in terms of Voltage rise forany particular test Number of TestsUnless otherwise specified, five testsshall be made.