In Section 1.3, the specification describes three product classifications for assemblies based on their intended end use. These classifications are then used as criteria to help determine test requirements for each.
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Includes products suitable for applications where the major requirement is the function of the completed assembly.
Includes products where continued performance and extended life is required, and for which uninterrupted service is desired but not critical. Typically, the end-use environment would not cause failures.
Includes products where continued performance or performance-on-demand is critical, equipment downtime cannot be tolerated, end-use environment may be uncommonly harsh, and the equipment must function when required, such as life support systems and other critical systems.
Use of this standard requires agreement on the class to which an assembly belongs. The user has the ultimate responsibility for making this determination. However, if the user does not establish and document the acceptance class, the manufacturer may do so (A-620, section 1.3).
Cirris understands these classes to be generally defined as follows:
The Dielectric Withstand Voltage (DWV) test is designed to find sudden breakdowns in the insulation between conductors exhibited as current spikes. This test must continue for a specified minimum time. The same maximum-allowable current threshold is used throughout the test time. Longer times have a better chance of detecting faults.
The Insulation Resistance (IR) test is designed to measure a more consistent current flow between insulated conductors. The tester reports the result as a measure of resistance using Ohm’s Law (Resistance = Voltage / Current). When the voltage is first applied, the current usually peaks and then, due to humidity that might be dried out by the energy of applied voltage and something called “dielectric absorption,” it trends lower over time. This means that the measured resistance starts lower and trends higher over time. The longer voltage is applied, the better the final measured insulation resistance, so longer times allow cables with worse IR performance to pass. To shorten production test time without degrading the test, the IR test time ends as soon as the minimum resistance value is reached.
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It is true that DWV and IR testing will find errors that could be detected by low voltage isolation (shorts) testing. It is also true that many high voltage testers cannot perform shorts testing. Therefore, the A-620 standard does not require a shorts test when high voltage DWV or IR testing will be performed. However, performing low voltage isolation testing can find many common errors prior to high voltage testing, which greatly reduces the chance of damage to improperly wired assemblies. Also, because low voltage isolation testing is performed extremely fast, it does not present a significant time penalty on properly wired products. Conversely, shorts testing can save significant time when errors are detected at low voltage by (a) avoiding the comparatively long high voltage test times on products that ultimately fail and (b) eliminating rework or replacement of miswired products that have been damaged by high voltage testing. All Cirris testers, even those capable of high voltage testing, perform low voltage isolation testing and Cirris recommends the testing be performed on all products.
It is true that smaller gaps between isolated conductors result in lower breakdown voltages. They also increase the chance of an intermittent or latent defect in the form of a short. With creepage distance >2 mm, the likelihood of finding near-shorts with high voltage tests was not thought to offset the cost of the more rigorous HV tests that would be required.
In Class 2, just as in Class 3, small creepage distances can justify a reduction in the voltage applied for the high voltage test. A-620 requirements specify that electrical tests should operate at such levels as not to degrade the electrical properties. However, the need for a thorough HV test increases as the spacing gets smaller. The costs of the testing should not increase when the voltage needs to be reduced. As a guide, you can see what problems you might encounter with your creepage distance using the Arc Gap Calculator.
When testing at VDC, the gap must be larger than .15mm, (.006 inch). At VDC the gap must be larger than .48mm (.019 inch). These are rather small creepage distances. The HV test voltage is most likely problematic for very small connectors in Class 3 applications.
This question most often arises from those doing work under military contracts where interpretations for different requirements may conflict. If you have a reason to believe that testing will degrade your assembly, then you may want to perform some confirming tests and make use of a provision of the test section that requires electrical tests on good assemblies to not degrade reliability.
We are not aware of any research or proof that the insulation used in good assemblies can be degraded by high voltage during brief applications in the IR and DWV tests. Here are some sound reasons why this is not considered a risk:
If you become aware of research that supports the fear of degrading insulation in wiring or connectors with the short-term application of high voltage, we would deeply appreciate knowing of it.
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