An Introduction to A-620 Cable Testing Standard
None of the information provided here is a substitute for having a copy of the A-620 standard. This article is simply intended to help readers understand what types of electrical testing are required by the standard and why differing test parameters are specified.
What is IPC/WHMA A-620?
The IPC/WHMA-A-620, Requirements and Acceptance for Cable/Wire Harness Assemblies, is a joint project of IPC (Institute of Printed Circuits) and WHMA (Wire Harness Manufacturers Association). Originally released in 2002, the IPC/WHMA-A-620 has become a widely referenced standard for workmanship and quality.
A-620 describes requirements using visual criteria. Detailed graphics communicate acceptable and unacceptable characteristics of assemblies. In this way it provides a training tool for manufacturing and quality personnel. It also helps set agreed upon standards of acceptance between customers and suppliers. The current version of the specification can be purchased on the WHMA web site at whma.org.
Defining Cable Tests and Test Level Requirements
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.
Class 1 – General Electronic Products
Includes products suitable for applications where the major requirement is the function of the completed assembly.
Class 2 – Dedicated Service Electronic Products
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.
Class 3 – High Performance Electronic Products
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:
- Class 1 – Consumer Products
- Class 2 – Communications Equipment
- Class 3 – Military, Life Support
A-620 Rev E Guidelines Summary
The table below summarizes the testing required for each product category.
|2Ω or 1Ω plus the maximum specified resistance of wire whichever is greater
|Dielectric Withstanding Voltage3
|With clearance distances (air gaps or creepage) ≥2 mm [0.079 in] and not coaxial/biaxial/triaxial assemblies:
no test required
|With clearance distance (air gaps or creepage) <2 mm [0.079 in] or coaxial/biaxial/triaxial assemblies:
Voltage Level: 1000 VDC or equivalent peak AC voltage4, Max Leakage Current: 1 mA, Dwell time: 0.1 Seconds/td>
|Voltage Level: 1500 VDC or equivalent peak AC voltage4, Max Leakage Current: 1 mA, Dwell time: 1 Second
|With clearance distances (air gaps air creepage) ≥2 mm [0.079 in]: no test required
|Class 3 and Class 2 with clearance distance (air gaps or creepage) <2 mm [0.079 in]:
Voltage Level: DC DWV Voltage or tester default
Minimum Insulation Resistance:
- Shorts Test (low voltage isolation) is not required when Dielectric Withstanding Test or Insulation Resistance Test has been performed.
- A maximum voltage and current should be specified when components within an assembly may be damaged by these tests.
- Nondestructive tests (procedure/parameters/stimuli/fixtures) shall be selected and applied in a manner that doesn’t not cause damage to the unit under test.
- Voltage Level is applicable when clearance distance tested is ≥0.58 mm (0.023 in). When clearance distances are <0.58 mm (0.023 in) an agreement between the user and manufacturer to de-rate these test levels would be expected.
- IR levels specified applicable at <80% relative humidity. When relative humidity exceeds 80%, an agreement between user and manufacturer to de-rate these tests levels would be expected.
Some Questions You Might Have About The A-620 Test Requirements
Why does A-620 specify a dwell time for DWV testing as a “minimum” and a “maximum” IR dwell time. Shouldn’t they both be minimum times?
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.
Why is shorts testing not required in some Class 2 and all Class 3 products when high voltage DWV or IR testing is being performed?
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.
In Class 2 cables, why are high voltage (HV) tests required for close spaced contacts (<2 mm [0.079 in])? This seems backwards since the closer the spacing, the lower the voltage a connector is likely to be able to withstand.
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 1000 VDC, the gap must be larger than .15mm, (.006 inch). At 1500 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.
I have insulated wires or connectors with a specified maximum working voltage that is lower than those required in the A-620 test specification. Will these test voltages damage them? Am I caught in a double bind where these required test voltages are considered destructive and render the assembly unusable for service?
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:
- Test time is very short and does not relate to the “working voltage” specifications. In addition, the energy levels are very low (test current is measured in mA) and performed under room temperature conditions.
- Tests performed at the default voltages in A-620 have become very popular. They are patterned after tests such as MIL-STD-202 and other requirements that have existed for over 50 years and used on assemblies with much lower working voltages.
- UL regularly requires 100% production tests in the thousands of volts on components and products that are rated at a working voltage of 120VAC.
- Wiring and connectors conforming to military requirements regularly require 100% testing by manufactures at the component level at thousands of volts, even when their working voltage is specified at less than 1000 volts. As an example, MIL-W-16878E TYPE EE (TFE hookup wire) rated at 1000 volts must be 100% production tested with a ball chain spark gap of 5,000 volts as part of its manufacturing process. Even wire for industrial and commercial applications is usually tested in the manufacturing process. When tested, the voltage used is in the thousands of volts.
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.