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:

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:

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:

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:

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