Calculating the Current Limits to Use in Production LineDielectric Withstand TestsFor Dielectric Withstand tests, also known as Hipot tests, test standards specify thevoltage and the time requirement. In regard to current limits, however, the standardsmust be vague. Each product has unique impedance. Therefore, in determining whether aDUT passes or fails, a standard can say only, "each product shall withstand withoutelectrical breakdown." Two questions must be addressed by the manufacturer performingthis safety test. 1) What does the term "breakdown" mean? 2) How to calculate thesettings for the current limits on a programmable tester?

This article includes the result of a survey as to what the term "breakdown" means, andprovides a working definition of the term. Next, the article will investigate variousmethods for determining the impedance of a product to assist in setting the current limits.

What the Standards Say

Most standards state similar requirements for the Dielectric Withstand test. They specifya voltage and test time, and require that no breakdown shall occur while the voltage isbeing applied. Passages from three standards show the various approaches:

61010 -1 (Electrical Equipment for Measurement, Control, and Laboratory Use)6.8.4 No breakdown or repeated flashover shall occur. Corona effects and similarphenomena are disregarded.

60601-1 ((Medical)8.8.3 During the test, breakdown is considered a failure. Insulation breakdown isconsidered to have occurred when the current which flows as a result of theapplication of the test voltage rapidly increases in an uncontrolled manner, that isthe insulation does not restrict the flow of current. Corona discharge or a singlemomentary flashover is not regarded as insulation breakdown.

UL 507 (Electric Fans)34.1 a) One thousand volts for an appliance employing a motor rated 372.85 watts(1/2 horsepower) or less and rated 250 volts or less

34.2.1 The test equipment for conducting the dielectric voltage-withstand test is tobe adjusted for sensitivity such that when a resistor of 120,000 ohms is connectedacross the output, the test equipment does not indicate unacceptable performancefor any output voltage less than the specified test voltage, and the test equipmentdoes indicate unacceptable performance for any output voltage equal to or greaterthan the specified test value. The resistance of the calibrating resistor is to beadjusted as close to 120,000 ohms as instrument accuracy can provide, and notmore than 120,000 ohms.

1

Looking at the 120kohm rule of thumb, when testing is performed at 1 OOOV and the testeris adjusted so that breakdown occurs if the resistance of the product is fewer than120kohms, then the maximum allowable leakage current would be 8.333mA(lOOOV 120000ohms = 8.333mA).

What Is Breakdown?

Those who responded to our survey said the following about "breakdown:".Dielectric Withstand (Breakdown Voltage) is the maximum voltage an insulatorof electricity can endure without voltage electrical conduction through thematerial. (Raytek website)

"Electrical breakdown involves a path over the surface of insulating material."(Medical Engineer)"Breakdown is uncontrolled current flow (inability of the insulation toholdback)." (Senior Compliance Engineer for Power Supply Mfg)

"Breakdown is the deterioration of the insulation as a result of applying a highvoltage." (President of Compliance Testing Service)

Breakdown occurs when the insulation within a product fails to prevent the flow ofcurrent. When a voltage is gradually applied, breakdown occurs when the current flow isnot proportional to the voltage. If breakdown occurs, a short circuit is often present;therefore when the product is retested it will fail at a very low voltage.When breakdown occurs the current flow is no longer controlled; therefore, regardless ofthe tester being used and its current capability, the tester shuts down and, per SafetyStandard requirements, emits an audible alarm. One may conclude that setting theleakage current to the maximum capability of the tester is sufficient to program the highlimit. There are two problems with this assumption. First is safety. Most testers thesedays are programmed from 2OmA up to 1 OOmA. If the operator performing the testcomes in contact with an accessible part during test, the result could be lethal. Regardlessof the programmed limit, best safety practices should be used when performing DielectricWithstand tests. Second, even when we remove safety reasons from the argument,programming to the maximum capability is still not a recommended method inprogramming the high limit; the insulation within a product could be extremely poor andstill pass.

Determining High Limit

Dielectric Withstand testers today provide quite a bit of flexibility for programming thelimits to determine pass/fail. A question often asked is "What do I set the High Limitto?" As shown in Figure 1, when the leakage current of the product under test exceedsthe high limit, the product under test is considered to be a failure.

2

w 1 1 111111FAIL HIGH NG

n _

Figure 1: Pass/Fail Criteria for High limit

Using, the insulation requirements from the standards

Some standards state minimum insulation resistance requirements for the product. Forexample Class I appliances typically have a minimum insulation requirement of 2Mohm.If the Dielectric Withstand test is performed at 1 500V, the expected leakage based on theminimum insulation requirement would be 0.75mA. One may consider setting a limitslightly higher than the calculated value, say at 0.80mA. Practically speaking, however,this may not work. Leakage from an AC test is comprised of resistive and reactivecurrent. If the product has any capacitance within it, reactive current will be present inaddition to the resistive current, resulting in a leakage current not correlating to thecalculated value from the insulation. In products where there a large amounts ofcapacitance the reactive current could mask the real current, making it impossible tomeasure the actual leakage through insulation.

Figure 2 shows that both total and real (neutralized) leakage can be used if testing AC.The real limit would be set based on the calculation using the insulation requirement. Thelimit that corresponds to the high limit can then be set higher to accommodate theadditional reactive current. (Because the capacitance of the product is negligible once thecapacitors have been charged, using a DC voltage can provide a leakage that correspondsdirectly to the insulation.)

0~~~~~~~~~~~~~~~~~~~~~~

Total Current "A"Total Current "B"

ReactiveToaCurn(Capacitive)

CurrentComponent C

I:B =1 10%ChnReal(Resistive)

"A""B" ~CurrentComponent

Real "B" - 200 > 1000 hagReal "A" 100 /CagTotal "B" = 101 > 1% ChangeTotal "A" 100

Reactive "B"'A 100 > 0% ChangeReactive "A" 100

Figure 2: The Masking Effect Due To High Capacitance

Line Leakage current

Line Leakage current is the current that flows from the mains part of the product undertest through or across the insulation into protective earth. Line Leakage current ismeasured with the product operating at its nominal line voltage (in some cases 10% >line voltage). This current is a good representation of the insulation and impedance of theproduct. Using the impedance calculation obtained from the line voltage and leakagecurrent, one can calculate an approximate Dielectric Withstand current.

Product Impedance = (Line voltage/Line Leakage Current)Hipot Limit = (Hipot voltage/Impedance)

For example, for a product with a nominal line voltage of 240V, an acceptable lineleakage current for a grounded product may be 3mA. Applying Ohms law we candetermine the product impedance to be 80kohm. If the voltage requirement is 1500V andwe know the impedance is 80kohm, we can expect a leakage of 18.75mA.

Even when the Earth Leakage current of the product is unknown, the Safety Standard hasactual limits based on the class of the product under test. For instance a Class I handheldproduct requires the Leakage Current to be fewer than 0.75mA. To meet thisrequirement, if the product is being powered by 240V, the impedance of this productmust be at least 32OkOhm. If tested at 1500V the current would be 4.6875mA, and ahigh limit of 5mA may be reasonable.Sample and StatisticsIn a production environment the opportunity to test multiple products may be present.This is a very good way to determine the high limit, to ensure the product is safe and the

4

process is consistent. Using a Variable Sampling Plan, known good product is selectedfrom each lot. The leakage is measured and the normal distribution among the samples isdetermined. Table 1 shows the measurements of 15 samples

Sample Measured (A)

1 1.673E-04

2 1.673E-04

3 1.442E-04

4 1.442E-04

5 1.442E-04

6 1.442E-04

7 1.442E-04

8 1.442E-04

9 1.442E-04

10 1.442E-04

11 1.673E-04

12 1.442E-04

13 1.442E-04

14 1.673E-04

15 1.442E-04

Table 1: Leakage Reading of 15 SamplesThe average of these measurements is 1 .5038E-4A (0.1 5038mA), and the standarddeviation is 1.058E-5A (0.01058mA). Taking a Six Sigma approach, we can calculatewith 95% confidence that all good product will fall within this limit.

High Limit = 3S +X where S is the Standard Deviation, and X is the average of thesamples taken.

High Limit = 3*(1.058E-5A) + 1.5038E-4A

High Limit = 1.8212E-4A (0.18212mA)Adding some headroom, a recommended high limit in this case is 0.2000mA.Of course, following this method could result in some false failures, in other words, whenthe product is not showing signs of breakdown but is inconsistent with what is typicallyproduced. If a product does fail, increasing the high limit and watching the leakagecurrent as the test progresses may reveal the reason for the failure.

Testers these days generally indicate the reason for the failure; e.g., if the leakageexceeds the high limit, the tester will display HI as the fail reason and set an audiblealarm to alert the operator. If there is indeed breakdown the tester will show a currentoverload, because the current has exceeded the tester's capability.

5

In summary, while there is no set standard for determining the high limits for theDielectric Withstand test, the methods discussed in this article get you off to a good start.To ensure the tester is within the correct measurement range, begin with Line Leakagecurrent. If samples are available, add the sample method to tighten down the process andensure the products are safe and the production is consistent.

About the Author

Shari Richardson joined QuadTech, Inc. of Maynard MA in 2001 as an ApplicationsEngineer. She has since been promoted to Product Manager of the Electrical Safety Testand Cable Test product lines. Prior to joining QuadTech, Inc. she was a Senior ProcessEngineer at Biz2Net Technologies.

6