Hipot Test Theory

In order to understand what a Hot Hipot test is and how it is performed, it is first necessary to discuss the theory of the Hipot test itself. The Hipot test, sometimes called a Dielectric Withstand test, is used to verify the strength of the insulation between a product’s current-carrying components and its chassis or enclosure. This is done by applying a high voltage from the mains-input lines to the chassis of the product and measuring the resulting leakage current flowing through its insulation. The theory: if a voltage much higher than the product would normally see is applied across the insulation without a breakdown (which results in an excessive amount of leakage current flow), the product will be able to operate safely when run under nominal operating conditions.

The Hipot tester is used to indicate whether or not a dielectric breakdown of the insulation has occurred by monitoring the leakage current resulting from the applied test voltage. Even under normal operating conditions, some leakage current will be present in any device under test (DUT), but at minute and safe levels; however, when the insulation breaks down or is damaged an excessive amount flows to the chassis. This can present a substantial shock hazard to anyone that comes into contact with the product.

The Hipot test is so crucial because it is the best way to uncover workmanship and assembly defects in an electrical product that can lead to insulation breakdown. Mistakes during assembly or faulty/damaged components exist to an extent in any manufacturing environment, and the Hipot test can uncover units that are unfit and dangerous to sell. Some of the defects which could result in insulation breakdown include: pinched insulation, pinholes, and poorly crimped wiring. In order to detect for breakdown in electrical products, this test is usually performed during the manufacturing process on 100% of all manufactured units, as well as during routine repair and maintenance.

Hipot Test Specifications

Hipot tests can be performed using either an AC or a DC voltage. Manufacturers may or may not be required to perform a specific type of Hipot test depending on the product and the standard to which it is being tested. Both AC and DC Hipot tests have inherent advantages and disadvantages that become evident depending on the characteristics of the DUT. Basic Advantages of each type of test:

AC Hipot Advantages

• Slow ramping of the test voltage isn’t necessary due to the changing polarity of the applied waveform.
• It is unnecessary to discharge the DUT after AC testing
• AC testing stresses the insulation alternately in both polarities.

DC Hipot Advantages

• The test can be performed at a much lower current level, saving power and with less risk to the test operator.
• Leakage current measurement is a more accurate representation of the real current
• DC testing is the only option for some circuit components: diodes, capacitors, ect.

These differences between AC and DC waveforms necessitate a variation in Hipot test procedures. Although the test is basically the same, the test operator needs to take into account the relationship between a DC waveform and its equivalent AC waveform. AC waveforms are often listed as RMS (root mean squared). This RMS value, known as the
effective value, provides a load with the same amount of energy as a DC waveform of the same voltage: a 25 volt DC source provides the same amount of effective energy as a 25 volt rms AC source.

The actual quantitative value of the RMS AC waveform is much higher at the peaks of the sine wave. In fact, the difference between a peak AC waveform measurement and the RMS measured value is 1.414. The calculation is as follows:

Volts rms * 1.414 = Volts peak

Since a Hipot test stresses the insulation of a DUT with a high voltage, the applied test voltage must be the same value whether it is AC or DC. It is unnecessary to worry about the effective RMS value since the energy delivered to the DUT is of no importance; the peak (maximum) voltage is what we are concerned with.

A good rule of thumb for determining the test voltage during an AC Hipot test is to multiply the nominal input voltage (usually from a wall outlet given as an RMS voltage) by 2 and add 1000 volts.

AC Hipot test voltage = Nominal input voltage * 2 + 1000

For a DC test use the following procedure to assure that the DC voltage is the same value as the peak of the AC waveform: multiply the calculated AC voltage by 1.414.

DC Hipot test voltage = AC Hipot test voltage * 1.414

By performing this operation, the DC voltage is applied at the same level as the peak of the AC voltage waveform.

The amount of time high voltage must be applied during testing is also specified in many safety agency standards. The most common test durations are 1 second for production tests and 1 minute for design tests. Further, agencies such as UL require that Hipot testers meet certain output voltage regulation specifications to ensure that the DUT is stressed at the correct voltage. Contact your local safety agency for more information about test duration and voltage requirements.

The Hipot test is set up by connecting the two output leads of the tester to the device under test. Follow the steps below to ensure that your tester is properly connected.

1. For products terminated in a three-pronged line cord (known as Class I products) or a two-pronged line cord (known as Class II products), connect the hot lead of the Hipot tester to both the line and the neutral inputs to the DUT.
2. Place the DUT’s power switch to the ON position.
3. Connect the return lead of the Hipot tester to the metal chassis or enclosure of a Class I DUT.
4. For a Class II product, connect the return lead of the tester to a piece of aluminum foil that is wrapped around the chassis of the DUT. The aluminum foil is necessary to create a conductive material around the insulation which comprises the chassis of a Class II product.

*By connecting the tester in this way, all of the internal current-carrying conductors are raised to the same potential with respect to the chassis. This connection scheme ensures that the high voltage waveform is applied directly across the insulation of the product.

Hipot Test Shortcomings

The Hipot test has long been considered the most important electrical safety test; as such it is usually specified by safety agencies to be performed on all consumer and industrial products terminated in three- or two-pronged line cords. Historically this test has been effective on the gamut of electrical products due to a dependence on single-pole
relays and mechanical switches. Yet products that operate off of a 220 volt input often incorporate double-pole relays that open both sides (line and neutral) of the input line. Further, with the dawn of the digital age we now find that many products incorporate electronic switches. Often these switches and relays cannot be closed manually without
powering-up the product under test. In these cases a standard Hipot test becomes ineffective.

With both sides of the line open the Hipot tester cannot energize all the current-carrying conductors within the DUT and the test results become invalid. The only way to perform a valid Hipot test on products that contain these types of relays or electronic switches is to energize the product while the Hipot test is being performed. Yet in order to Hipot test a powered product, special steps must be taken since under normal conditions the line and neutral inputs of the DUT would be shorted together. This modified setup is commonly called a “Hot Hipot test.”

Hot Hipot Test Procedure

A Hot Hipot test is performed in the same fashion as a standard Hipot test. The primary difference is the addition of 1 piece of equipment, an isolation transformer. This transformer is used to isolate the input power to the DUT from earth ground. Without the use of this type of transformer, the chassis of the DUT, which is usually grounded, would be directly connected to the return of the Hipot tester (which is also referenced at or near ground potential). The return of the Hipot tester usually sees current in the milliamp range; however, without an isolation transformer the Hipot tester could be exposed to several amps of line current flowing back through its return. This could cause damage to
the tester as well as create a possible shock or fire hazard during a Hot Hipot test.

The isolation transformer creates the necessary isolation between the input lines of the DUT and the Hipot tester. Of course, an AC test voltage is necessary for this test since DC waveforms don’t work with transformers. It is also important to verify that the isolation transformer is rated to handle the applied Hipot test voltage; this will prevent
damage to the transformer.

1. In order to set up the test correctly, the primary side of the isolation transformer should be plugged into the power source used to provide the input power to the device under test.
2. The secondary side of the transformer should then be connected to the input of the DUT.
3. Once connected, the Hipot tester may then be plugged into a standard wall outlet.
4. The hot lead of the Hipot tester should then be connected to the output of the secondary side of the isolation transformer. By doing this, you are connecting the hot lead of the Hipot in between the isolation transformer output and the line side input of the DUT.
5. The return lead of the Hipot tester should then be connected to the chassis of the DUT.
6. Once the setup is completed, you may turn on the Hipot tester and the DUT.
7. Perform the test as you would a standard Hipot test.

Summary

With the advancement of the electronics industry Hot Hipot testing is becoming more and more common during routine production line testing. Products that were once operated solely through the use of mechanical relays and switches are now being controlled via electronic circuits that can only be energized while the product is running. Still other products that use 220 volt inputs contain relays that open both sides of the line, rendering a standard Hipot test ineffective. Whatever the reason, a working knowledge of the Hot Hipot test makes good sense of anyone working in the quality assurance or safety testing fields.

Although the Hot Hipot test has long been considered a mysterious and complex safety test, in actuality it isn’t much more difficult to perform than a standard Hipot test. With an understanding of the basic test procedure involved in performing a Hipot test and possession of the right equipment, a Hot Hipot test can be performed safely and efficiently. Paying attention to careful setup and implementation, a test operator, quality assurance supervisor, or engineer alike can feel comfortable performing a Hot Hipot test on a variety of products.

Reference: www.asresearch.com

The test applies high voltage to the product to check the insulation between the live conductors and exposed metal surfaces. For Class I equipment, the high voltage is applied between conductors and earth. For Class II equipment, the high voltage is applied between the conductors and the outer surface of the product.

Developments

Although there have been few noticeable alterations in the hipot testing requirements of most standards in recent years, many changes have been made to the technical specifications of electrical and electronic products. Such modifications have been prompted by technical standards such as the Electromagnetic Compatibility (EMC) Directive. For example, EMC considerations have required the introduction of circuit devices on the supply input to prevent emissions back into the mains.

These devices often take the form of resistive and capacitive circuits. When tested using an ac hipot test, these circuits often prove problematic because the capacitive part can induce leakage currents in excess of the capacity of the test instrument. Situations such as these have led to a substantial increase in the use of dc hipot testers that are unaffected by this capacitive effect.

The test is designed to ensure the safety of the product and to ensure that manufacturers meet legislative demands in relation to product liability and due diligence. Hipot testing, ground-bond testing, and insulation-resistance measurement are probably the three core tests for electrical safety testing.

Identification and Traceability

The hipot test can be regarded as a negative test on the basis that a good product most likely does not provide a measurable flow of leakage. With the development of modern instrumentation using microprocessor control and data-logging software, it is now possible to produce fully traceable records of testing undertaken.

Knowing why flash testing is necessary is important, but being able to prove that electrical or electronic products comply with the various standards is also vital—particularly if a subsequent failure or fault is identified.

The only effective means of demonstrating that a product has been tested properly is through sufficient documentation. The latest available testers automate the testing process on the production line and retain results in an internal memory for later download or print out. Documentation through automated hipot testing minimizes liability and provides effective proof at the end of the manufacturing process that a product is safe.

Test Conditions

Two distinct forms of testing are usually recognized: type testing and production line testing.

Type-testing levels vary according to the relevant product-specific standard. Class I equipment normally requires between 1000 and 1500 V applied for 1 minute with a trip level of up to 100 mA. Some standards specify that “no flashover shall occur,” suggesting that the trip level should be 0 mA. But, because all devices have a small amount of leakage, 0 mA is not practical. A low level of between 3 and 5 mA is usually selected. A higher level should be used if the device under test (DUT) has large capacitive leakage.

For Class II equipment, voltages are usually between 2500 and 4200 V, but with timing and trip settings similar to Class I equipment. The same criteria for the fault-trip levels are applied to both Class I and Class II products.

Production line hipot testing requires special considerations in terms of conditions such as duration and voltage levels. On the production line, the need to have faster, but equally rigorous, tests is addressed by applying 10% overvoltage, but reducing the test duration to a few seconds. Therefore, a type test with a voltage rating of 1250 V would be carried out at 1375 V on the production line, with trip levels of 5 mA. Often, the annex section of a standard advises routine testing. When a recommendation is not available, the manufacturer must apply a suitable test-voltage level to ensure that the device is safe to be offered for sale.

One note of caution is that it is possible to get an apparently satisfactory result when the equipment under test is switched off or not properly connected. It is imperative to ensure that the equipment is switched on and properly connected. Even for experienced operators, this can be a challenge. In a production line situation, such problems are greater because of the greater throughput of products. Solutions include:

• A simple continuity test, applied on live and neutral, built in to the test program prior to the flash test.

• The detection of capacitive leakage that occurs whenever an ac hipot is applied. If no leakage is detected, a warning is initiated.

• Regular fault simulation at the test connection point.

Pass-Fail Criteria. The test itself is not quantitative, and “fail” is recorded if a breakdown of insulation or a flashover between components occurs. Most testers indicate pass or fail via a warning light or sound that activates when 5 mA of leakage occurs.

Hipot and Insulation Testing. On first examination, these two tests appear very similar. However, hipot testing is designed simply to detect gaps or clearances between conductive parts and earth, pinholes in insulation, and other degradation. Insulation resistance testing is designed to provide an actual quantitative measurement of the insulation quality.

If a wire were positioned 1/2 mm from exposed metal, an insulation test—conducted in dry air—might easily provide a pass reading. A hipot test, however, is more likely to detect this situation as dangerous. Similarly, if insulation were somehow contaminated, a hipot test would produce a pass, whereas an insulation test would highlight a deficiency. For example, the normal minimum insulation resistance value for Class I appliances is 2 M(omega). With a 1500-V hipot test, the current would be 0.75 mA and would not be detected by the 5-mA trip that must accommodate the capacitive losses that occur. Obviously a dc hipot test with a leakage meter can provide insulation resistance monitoring because the capacitive component is overlooked after the initial inrush.

Test Duration. The best way to maximize productivity is to minimize the time taken to apply all of the safety and functionality tests. By using an integrated test station and enclosure connection to the DUT, only one test sequence is required. Establishing a connection is often the most time-consuming part of production line testing, so combining four or five tests at an integrated test station can significantly reduce test times.

Production Line Safety. Type tests often call for high levels of high voltages to be applied for up to 1 minute. This is not practical in production facilities. In many facilities, a 1-minute test would adversely affect productivity. The call for a 100-mA trip level can be potentially lethal. In addition, voltage levels and test procedures realistically demand a skilled operator.

Because production line hipot testing is conducted with reduced test times, it reduces the risk to operators. Effectively designed test instruments mean that the required operator skill level can be reduced, and the use of high trip levels can be protected by a key system for which only qualified operators have access.

Safe Test Areas. With the integration of electrical safety testing standards in EN 50191, specific safety conditions have been specified for all locations where electrical testing is carried out.1 For example, the use of test enclosures on the production line is advisable to maximize the safe working area around the points where flash tests are to be applied.

The type test, with its high-level leakage limits at 100 mA, is potentially lethal to the human body. Hence, type testing is carried out in a laboratory and not on a production line. The test is also only to be carried out by a skilled person who is aware of the potential hazards and who is following procedures clearly defined before any test is applied. The ideal situation is for the DUT to be enclosed in a safety-test enclosure with automatic isolation of the test points on the enclosure opening. This enclosure protects the test technician from electrical sources as well as from airborne particles caused by an unforeseen failure of the DUT that terminates in an explosion.

Class II Equipment

In Class II equipment, the absence of an earth requires protection via primary and secondary insulation. Hipot testing of Class II equipment involves much higher voltage levels, typically between 2500 and 4200 V. A common problem, particularly on new equipment, is that failure can be detected on the primary insulation that is undetectable by a hipot test on the outer surface, which tests the secondary insulation only. Testing programs must include both tests.

To test the primary protection, the selected method must access the primary insulation. This is essentially a contradiction in terms, because this connection needs to be inaccessible metal. However, experience shows the following options are feasible:

• Test the primary insulation prior to final assembly. Be sure to check that on assembly no degrading of this protection takes place (e.g., screws penetrating the insulation).

• Design the product with an access that can be permanently sealed after testing. This is often an element that product designers fail to anticipate.

• Design test jigs and probes that allow access through the enclosure, ensuring that the integrity of the product (in terms of the relevant standard finger tests) is maintained.

Testing also needs to be carried out on the secondary protection. Standards generally require that the product be wrapped in aluminum foil so that high voltages can be applied to all outer surfaces. This test may be practical for laboratory situations, but it is impractical for production testing, because of both the complexity in test setup and time required and because the outer surfaces of the product must be easily marked. The use of conductive foam in a special jig creates a nest or envelope around the outer surface of the product; test voltages can then be applied. Although this method is not quoted in standards, the standards authorities recommend this procedure.

Does Hipot Testing Degrade Insulation? The view that hipot testing is essentially a destructive test is often an area of discussion. This view originates from the use of flash in type testing where the long time period required provides potential for the degradation of insulation. However, in terms of production line testing, the reduced time period and the 5-mA trip setting significantly reduce this risk. The fact remains that many manufacturers successfully conduct the test without witnessing any degradation. Under certain circumstances, an ac flash test could corrupt sensitive electronic components.

Sensitive Equipment

In situations in which ac hipot testing could corrupt sensitive electronic components, the following solutions are possible:

• Use a dc hipot test. The voltage must match the specified peak ac voltage, which is achieved by multiplying the specified ac voltage by 1.414. A discharged facility following application ensures that no residual voltage remains.

• Use a soft dc hipot test. This test requires ramping up to the required voltage. In some instances, this test can benefit from ramping down as well. This process involves a slow ramp up from zero to the required value, and then holds for a timed period before ramping back down to zero and discharging the unit under test.

Suppression Devices

The advent of measures to control EMC has increased the use of suppression devices, but these can cause problems for hipot testing. It should be noted that most designers of such components have upgraded their products to meet the specified tests. However, in some cases the following solutions may be necessary:

• Disconnect such components. Some standards allow disconnection for safety testing. However, it is important to note that disconnection is often impractical for production line testing.

• Set higher trip levels (e.g., 10 or 15 mA). The use of this option should always be accompanied by the use of safety precautions such as key-locked switches, thereby ensuring proper authorization to conduct the test. The safest solution in such circumstances is to conduct the test with the test item housed in an enclosure and with appropriate interlocks. Such precautions do not need to be complicated or expensive to provide maximum operator safety.

• Apply the high-voltage test from a dc voltage source. Almost all standards now provide for this option. The test voltage is the ac level times 1.414 to provide the suitable dc test voltage. A dc voltage will ignore the capacitive leakage levels.

Conclusion

The hipot test is designed to indicate whether a product remains safe when subjected to high voltage and whether the user is protected from danger. Being able to prove that electrical or electronic products comply with the various standards is particularly important if a subsequent failure or fault is identified.

To demonstrate that a product has been tested properly, manufacturers must provide sufficient documentation. Many testers now automate the testing process on the production line and retain the results. Documentation through automated hipot testing maximizes protection from liability and provides effective proof at the end of the manufacturing process that a product is safe.

Reference: CE Compliance Engineering