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As the spacing between contacts in connectors gets smaller and smaller, there comes a point where cables will not pass at higher voltages. So, what do you do about that 1000V or 1500V requirement from a specification such as the IPC/WHMA A-620 if your cable has connectors that won't pass?
Too much voltage? or Too much voltage for the creepage distance?
Creepage Distance/Air Gap, the path of least resistance
Electricity, like water, likes to travel along the path of least resistance. Looking for the best path for electricity usually means measuring the distance from one conductor to another conductor. The shortest distance may be through air (Air Gap) or along the surface of the insulation (Creepage Distance).
Connectors are the most likely place in a cable assembly where this "path of least resistance" exists. This path is across, not through, insulation and is measured in either "air gap" or "creepage distance." Ask the question: "How far does electricity need to travel, between two pins in air, or along the insulating surface between two conductors, to arc over?"
** Click Here to Learn How to Calculate Creepage Distances **
How Much Voltage to De-rate?(*NOTE creepage/air gap calculations are relevant not only to the assembly you are testing, but also to any interface cabling/test fixtures that you use to connect to the device under test.)
Once you have determined the minimum creepage, or air gap distance, what is a reasonable amount to de-rate the test voltage?
Cirris has created an arc gap calculator which can be found on our website at: http://www.cirris.com/testing/voltage/arc.html
Using the information from our Arc Gap Calculator page we can build a table of voltages and their potential arc distance. Since variations can occur in applied voltages and manufactured dimensions for creepage in connectors, we created a .002" + 10% protection factor into the Arc Gap dimensions. This results in the following recommendations which you could defend with scientific confidence.
VARIATION TO A-620
Considerations for A-620 Class 3 (High Performance Electronic Products) Class 3 requires 1500volt Dielectric Withstand Test. For Class 3, use 1500VDC as specified when creepage distance is at least .58mm (.027"). For smaller creepage distances, if the design of the product cannot be changed to improve creepage distance, then limit voltage for DWV and IR tests according to the above table.
Considerations for A-620 Class 2 (Dedicated Service Products). Occasionally someone is confused why A-620 for Class 2 skips the high voltage requirement if there are high enough creepage distances. At first read, this requirement seems backwards; high voltage testing is required for dense pin spacing but not for wide separation of pins?
The reasoning is that if connectors have large separation between conductors, there is a low risk of shorts between conductors. However, the closer the conductors are, the higher the risk of a defect such as a stray wire strand or deformed contact causing a failure. As connections get closer, there is more likelihood that shorts will occur, requiring more rigorous testing (higher voltage). The smaller the spacing between conductors, the more important it is to do high voltage testing, but only up to a point.
For Class 2 creepage distances from 2mm (.079") down to .22mm (.009") use 1000VDC as specified. For smaller creepage distances, if the design of the product cannot be changed to improve creepage distance, then limit voltage for DWV and IR tests according to the above table.
A Word of Caution if You Comply With A-620:
Until A-620 is revised to reflect this issue, remember the language;
For Class 1 and 2 assemblies with creepage distance issues, get specific, agreed on test requirements, perhaps using the information on this page to help make your case.OR
If customers have agreed to use your "documented test requirements", modify your documented requirements with something like the de-rating described on this page. This might be as simple as:
What if creepage issues only exist for "some" nets?
It is possible to have a cable that has a combination of connectors that DO and DO NOT have creepage issues. Consider, for example, a cable that has a 15 pin high-density D-Sub that contains only 15 wires out of dozens, or hundreds, of wires. In this case we recommend you test the assembly at two different voltages. A lower voltage for all wires connected to the 15 pin micro D, and a higher voltage for all other wires. Cirris' CH2 tester allows you to choose different voltages for different nets in an assembly. Other Cirris Hipot testers can accomplish this same task by utilizing our "LINK" command. You would create two separate test programs; the first one would link all of the "lower voltage" wires together during the "higher voltage" test, so they don't get voltage applied between them. In the second test you would link all of the "higher voltage" nets together (they were already tested for the higher voltage) then test the "lower voltage" nets together at the lower voltage. This can be accomplished using the Parent/Child feature so that it is still only "one" test for the operator.
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