The Dielectric Voltage Withstand Test
The Dielectric VoltageWithstand TestBenefits and limitations
The Dielectric Voltage Withstand TestThe Dielectric Voltage Withstand TestBenefits and limitationsThe dielectric voltage withstand test is an integral part of the product safety evaluationof electrical and electronic devices, and provides manufacturers with importantinformation regarding the quality and appropriateness of the chosen insulation system.The test involves placing an extra-high voltage across the insulation barrier of thedevice for one minute. If the insulation holds the voltage, the device is deemed to havepassed the test. However, if the applied voltage leads to the sudden breakdown ofthe insulation material and allows current to flow, the insulation is determined to beinsufficient since it might pose a shock hazard to users.While the dielectric voltage withstand test is widely used, the real objective of the testis often misunderstood, which may lead to incomplete testing or misleading test results.This white paper seeks to clarify the theory of dielectric breakdown and the objective ofthe dielectric voltage withstand test. It explores the applications and limitations of thetest in order to better ensure its appropriate use in the product safety approval process.The Mechanism of Insulating MaterialsCurrent flows in a material when an electromotive force is applied that is strong enoughto force the movement of electric charge. This charge movement is carried by electronsin the material, and can be measured as electric current. Metals, such as copper, havemany free electrons available to transfer electric charge. This makes copper a goodconductor because there is little resistance to the flow of charge and the energy lost asheat, due to current is minimal.The performance of conductive materials is in marked contrast to that of insulatingmaterials, which have a physical structure that prevents the easy movement ofelectrons. Since the electrons cannot move freely, they cannot effectively carry chargethrough the material. However, it is always possible to force the material to conduct byexposing it to sufficiently high voltage.page 2
The Dielectric Voltage Withstand TestThe Theory ofDielectric Breakdownconduction band, dramatically increasingthe conductivity of the material. Thistransformation is called dielectricThe dielectric breakdown of aninsulating material is a complex physicalphenomenon but it may be generallycharacterized as a sudden change in theresistance of the insulation under test duecertain voltage is reached.When a voltage is applied to anelectrical or electronic device, the naturalbreakdown and the electric fieldcapacitance of the insulation charges,necessary to start the breakdown isresulting in the flow of current. If a DCcalled the dielectric strength, orvoltage is applied, the charging occursbreakdown strength.only when voltage is first applied. Theto the applied voltage. Simply stated, theThus, dielectric breakdown is a dramaticDC voltage charges the capacitance andtest voltage breaks down the insulatingand sudden increase in the conductivitythe capacitive charging current is thenproperties of the material.of a material due to an applied voltage.reduced to near zero.The mechanism of dielectric breakdownLeakage Current vs. DielectricCurrentFor 60 Hz AC voltages, the chargingbegins with the application of a strongelectric field to the insulating material bya high voltage. Different materials requiredifferent levels of electric field in orderfor dielectric breakdown to occur. Metalsand other conductors have free electronswithout the application of any electricfield, but insulating materials typicallyWhen the electric field is sufficient,it energizes the insulator’s electronsuntil they gain enough energy tocross the bandgap and move into themodel which applies to every electricaland electronic device.The resistor accounts for resistive currentthrough the insulation and the capacitoraccounts for capacitively coupled currents.require a very high electric field to allowelectrical current to flow.Figure 1 illustrates a simple insulationFor modern insulation material, theinsulation resistance is very large, soresistive current is typically several ordersof magnitude lower than the capacitivelycoupled current. The spark gap representsthe avalanche (breakdown) phenomenon,where a good insulator turns bad after aSpark Gapand discharging of this capacitanceoccurs 60 times each second, so thecapacitive charging current remains ata steady-state level and never goes tozero. This is known as capacitive leakagecurrent. For modern electronic devices,the resistive leakage current is normallyvery small, so the overall leakage currentof such devices is dominated by thecapacitive effect. To protect users frominjury, many product safety standards setlimits for the allowable leakage current.A typical leakage current limit is 0.5mA at60 Hz and at the rated voltage.ResistorCapacitorFigure 1: Basic insulation scheme in electrical productspage 3
The Dielectric Voltage Withstand TestElectrical and electronic devices alsoinvoluntary startle reaction and/orthe device to accommodate transientcurrent during dielectric testing. However,that pass through parts of the body. Theseswitching transients. This provides aexhibit current analogous to leakagesince the dielectric test voltage is muchhigher, the current will be higher as wellbecause the capacitive charging currentincreases in proportion to the increasedvoltage. Using the example from above,a device with a leakage current of 0.5mAat 120 Vac would be expected to haveapproximately 5.2mA dielectric currentduring a 1240 Vac dielectric test.muscle contraction in reaction to currentsreactions can lead to hazards from theuncontrolled body movements than canoccur when a person is subjected to theunpleasant nerve and muscle stimulationof electric current. Current passingthrough the body may also cause musclecontractions that make it impossible tolet go of the electrical source.overvoltages, such as voltage surges andsafety margin for production variationsand the aging of the material.The objective of the dielectric voltagewithstand test is to establish theminimum level of electrical insulationnecessary to prevent human contactwith a potentially harmful voltageand resulting current. In addition, theProtecting humans from hazardousdielectric voltage withstand test mayterm “leakage current” when describingpreventing contact with any electricalinsulation or the presence of a foreignrated voltage and “dielectric current”current to shock the user. To isolate thesebridge the insulation. This test iswhen the circuit is exposed to theand electronic devices employ electricaltemperature tests to confirm that thebe sufficient for the working voltageinsulating capabilities.currents is typically accomplished byreveal faults in mechanically damagedleakage current when the circuit is atcircuits that carry sufficient voltage andmaterial (such as water) which maywhen describing the analogous currentcircuits from human contact, electricaloften used after mechanical abuse ordielectric test voltage.insulation, the quality of which mustproduct has maintained itsthroughout the expected life of theThe dielectric voltage withstand test is alsoTo avoid confusion, this paper uses theThe dielectric strength test is notintended to evaluate the amount ofleakage current, as these two tests havefundamentally different objectives andcompliance criteria. The leakage currentlimit is only applicable to the leakagedevice. The dielectric voltage withstandtest is performed in order to verify thecapability of the insulation.Air is the most readily available electricalcurrent test, and only at the rated voltage.insulator, and through-air spacingObjective of the DielectricVoltage Withstand Testare defined in many product safetyThe human heart is controlled by thebody’s nervous system using electricalimpulses. Even small currents passingthrough the heart can interrupt thiscontrol, leading to ventricular fibrillationand in the most severe instances, to death.Thus, the most fundamental requirementof electrical safety is to provide sufficientseparation between the human body andhazardous currents to avoid interruptingthe heart’s normal function.In addition to ventricular fibrillation,the human body may also exhibit anpage 4requirements (also known as “clearance”)standards in order to maintain voltageseparation. Spacings across the surfaceof an insulating material are known asover-surface spacings or “creepage” andare often defined in applicable standards.However, even in cases where a devicecomplies with the defined through-air andover-surface spacing requirements, theoverall insulation system must be testedby the application of the dielectric voltagewithstand test to the insulation system.The applied voltage for the dielectricvoltage withstand test is usually muchhigher than the working voltage ofused on the manufacturing productionline to identify material and workmanshipdefects in assembled devices.Trip Current During theDielectric Voltage Withstand TestWhen dielectric breakdown occurs,the loss of the insulation material’sresistance causes the current to go froma low dielectric current level to a muchhigher breakdown current. This changein the level of dielectric current hastraditionally been monitored by a currentsensitive circuit, which trips-out the testequipment when the current exceedsthe test equipment’s trip sensitivitysetting. This approach has the significantadvantages of being simple andinexpensive and has been used for manyyears. However, although this methodworks reasonably well a closer analysisreveals potential limits to its use.
The Dielectric Voltage Withstand TestThe Problem of False FailuresThe use of a trip current setting to determine dielectric breakdown is based on the assumption that current above the trip setting isindicative of dielectric breakdown. While this is true in many cases, there are instances in which a current above the trip-out thresholdis not indicative of a breakdown at all. It may actually be reflective of the natural dielectric current of the tested device. Thus, thefollowing dilemma: Trip-out might indicate dielectric breakdown (a failure condition) or it might merely indicate dielectric currentabove the trip- current setting (not a failure condition). This uncertainty may lead to test results deemed unacceptable, when theresults actually mask a favorable outcome (See Figures 2 and 3).Trip-out givesa false failureDielectric CurrentImproper Trip Current Setting120 VApplied Voltage1240 VFigure 2Trip Current SettingDielectric CurrentTrip-out indicatesbreakdownBreakdown120 VFigure 3page 5Applied Voltage1240 V
The Dielectric Voltage Withstand TestBecause the dielectric test is intendedconducted. This further evaluation mayvoltage ramp-up must be examined tolimit is specified for dielectric current.paths, including DC dielectric testing,is normal charging current. Repeatingto evaluate the insulation system, noinclude several different investigativeThus, there is no correct trip-out currentuse of dielectric testers with arc detection,practice, the only valid determiningreapplication of the dielectric withstandprecise value of dielectric current whichthe breakdown.setting for determining failure. Inspecialized diagnostic testers andcriterion for failure is “breakdown.” Thetest voltage to locate the site offlows is unimportant; what is importantis the sudden, dramatic increase inthe dielectric current, indicating thatinsulation breakdown has occurred. It isfor this reason that most product safetystandards do not specify a trip currentthreshold for the dielectric test.However, if the expected leakage currentis known, then a common procedure isto set the trip-out threshold at 20 timesthe maximum leakage current limit. Thismultiplier includes a factor of 10 for theapproximate ratio of the dielectric testvoltage divided by the normal operatingvoltage, and a factor of 2 for variability. Ifthe current exceeds this higher trip-outlevel, it is likely the result of a dielectricfailure, and not the expected currentflowing in the leakage current pathfrom the application of the dielectrictest voltage.To summarize, tripping of the dielectrictester only signifies that the tripcurrent setting of the tester has beenexceeded. Therefore, this result shouldAC vs. DC Dielectric Testingprovide a simple testing solution.Some products contain electromagneticinterference filters, which may addcapacitance. To accommodate thesefilters, the leakage current limit in someproduct safety standards is 3.5 mA. TheIn fact, many product safety standardsmight be as high as 3.5 mA times 1240 Vdielectric test methods. When testingloading, some dielectric testers are notmust be set for a voltage equal to 1.414terminals to perform the test. However, ifvoltage measurement is an RMS value.just the one-time charging current. Oncewave are 1.414 times the metered AC RMStester requires only a small currentbe met with DC as well as AC voltages.expected current during a dielectric testalready include DC voltages in their/ 120V 36 mA ac rms. With this heavywith DC voltage, the metered voltageable to deliver enough voltage across thetimes the AC test voltage, since the ACa DC test is used, the capacitive current isThe actual peak voltages from the ACthe capacitance is charged, the dielectricvalue. Thus, the DC test voltage is equal toload that the tester can easily supply. Inthe highest peak AC voltage.the case of high leakage current limits,With a DC test, steady-state dielectricreplacing the AC test with a DC test mayproduct’s capacitance is charged onlytester has enough capacity to deliver thevoltage application. If the test voltage isThere is one disadvantage to DC testing,current will be much lower because thebe necessary, unless the AC dielectriconce. This occurs at the beginning of thetest voltage under heavy current load.raised slowly, the charging current will below and, once the test voltage stabilizes,there will be very little current flow. Thisis an advantage because the trip-out limitof the dielectric tester can be set lowerwithout causing false alarms.If the voltage is ramped up too quickly,further testing or evaluation should becreate false failures. Trip-out during DCpage 6the test with a doubled ramp time mayThe objective of dielectric testing cannot be automatically interpreted tomean that breakdown has occurred anddetermine if the cause of the trip-outthe capacitive charging current maywhich becomes evident at the end ofthe test. Once DC testing has beencompleted, the inherent capacitanceof the device under test has been fullycharged and now poses a potential shockhazard. Accordingly, certain products mayneed to be discharged via a short with ableeder resistor, adding another step andcomplexity to the DC dielectric test.
The Dielectric Voltage Withstand TestSummaryThe dielectric voltage withstand test can provide important insight into the capabilitiesand limits of a device’s insulation system. However, it is important to remember thatthe objective of the test is only to evaluate the insulation’s electrical strength. Thecurrent during the dielectric test should not be evaluated against the device’s leakagecurrent limits, since the dielectric test is performed at a much higher voltage.It is common for dielectric test equipment to use a current trip-out limit to indicatedielectric failure. When the tripping device is properly set to allow the expected currentto flow through an acceptable dielectric current path, the dielectric test equipment willgive trustworthy results. However, in cases where the expected current is not knownor the trip-out threshold is selected generically, the trip- out may produce a falsefailure result. Thus, trip-out by itself should not be assumed to indicate a truedielectric breakdown.An indication of trip-out should always be followed by an investigation into the rootcause of the trip-out. Such an investigation will then lead to an invaluable discussionas to the fundamental quality of the device’s electrical insulation system.For additional information about The Dielectric Voltage Withstand Test — Benefitsand Limitations white paper, please contact Paul W. Brazis Jr., PhD, Research Manager,Electrical Hazards at paul.brazis@us.ul.com.UL and the UL logo are trademarks of UL LLC 2012. No part of this document may be copied or distributed without the prior writtenconsent of UL LLC 2012.page 7
Capacitor. The Dielectric Voltage Withstand Test page 4 Electrical and electronic devices also . It may actually be reflective of the natural dielectric current of the tested device. Thus, the following dilemma: Trip-out might indicate dielectric breakdown (a failure cond
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Dielectric strength . All insulting materials fail t some level of applied voltage, and „dielectric strength is the voltage material can withstand before breakdown occurs [1]. Dielectric strength is measured through the thickness of the material (taking care to void surface defects) and is normally expressed s a voltage gradient
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