Many articles have been written on electrical cable and harness testing. Most focus on the technical aspects of testing. This article looks at it from a different perspective – purposed-based harness testing. How does the intended purpose or end-use of a wiring harness affect the choice of the tester, from test specifications to the challenges of adapting the tester to the device under test (D.U.T)? Technically, cable and harness testers are quite similar. They all apply a stimulus to a wire or group of cables, make measurements, and report pass/ fail results based on those measurements. However, the challenges of testing simple cables vs. complex wiring harnesses are quite different: computer cables vs. automotive cables vs. medical cables vs. Mil/Aero cables vs., well, you get the point.
Not all harnesses are created equal
Cable assemblies and wiring harnesses don’t know whether they are in a plane, train, automobile, or washing machine! All wiring harnesses are basically the same: wires, connectors, ring lugs and terminals, splices, etc. Some are soldered, some are crimped, and some are terminated via IDC. However similar harnesses are, there are significant differences in the manufacturing and testing processes, depending on the intended application.
Industry classifications of wiring harnesses
Although the lines can be blurry, several industry classifications of wiring harnesses correspond with different classes of tester requirements. The Bishop Report* published in 2017 identified ten end-use categories: Computer, Business/Office equipment, Medical, Test/Instrumentation, Industrial Equipment, Automotive, Transportation non-automotive, Military/Aerospace, Telecom/Datacom, and Consumer Electronics. The WHMA-IPC A-620 Standard classifies cable assemblies into three distinctive tiers: Class 1, Class 2, and Class 3. Once again, the lines tend to blur between the three, particularly when determining which A-620 tier each of the ten end-use harness classifications falls into. It is the author’s experience that, for electrical testing, the distinction between Class 1, 2, and 3 is uncertain. I would submit the following table represents, in general terms, the matching of industry harness classifications with A-620 testing classes:
Electrical testing requirements per classification or harness type
The A-620 standard primarily focuses on build methods and includes an electrical test requirement section, 19.4. Before the publication of the A-620, there were few electrical testing standards for the cable assembly industry. The most prominent, Mil-Std 202, still is the primary test spec for Mil/Aerospace/Defense cables.
Not all testers are created equal
Plugging the cable in – test adaptation
One of the biggest challenges in testing harnesses with automatic testers is plugging them into the tester, which requires some kind of adaptation scheme (for a great look at this subject, see Mark Molnar’s article Wire Harness Testing Evolution and the Advent of Test Fixtures in the Sept/Oct issue of Wiring Harness News). Usually referred to as the “test interface,” “test adapter,” or “test fixture,” these can take the form of adapter cards that snap directly into the tester, adapter cables, or test tables with customized test modules. Regardless of the method used, the testing interface must have the requisite number of mating connectors to plug in all of the connectors of the D.U.T. Small, end-to-end cables with only a few connectors are generally the simplest to adapt to. Large, multi-legged harnesses with dozens or even hundreds of connectors are the most difficult. This is where significant differences surface when choosing the correct cable tester for the job based on the nature of the cables, i.e., how large, complex, and accessible. To reiterate, it’s not only the complexity of the testing requirements that defines the difficulty of testing different types of cables but also the difficulty of the adaptation.
“Big” vs. “Small” harnesses
Defining “small” or “big” harnesses can be misleading. It doesn’t always connote physical size. You may have an incredibly dense circuit card assembly or black box that fits on a tabletop with 10,000 interconnection points to be tested or a harness of only a few hundred wires that is hundreds of feet long and weighs hundreds or thousands of pounds.
Small cables are relatively simple to test
End-to-end cables with a few to a few hundred wires are generally tested on the bench with a smaller tester, either plugged directly into the tester or through short adapter cables.
Automobile and White Goods – Very high-volume production
Over 60 million automobiles and around 450 million significant appliances are produced globally each year, so imagine the number of wire harnesses to be tested! It is unfeasible to test these harnesses using the actual mating connector. Automotive and white goods harnesses are mostly tested using dedicated test tables with spring probe test fixtures, commonly with the wiring analyzer buried inside the test table. Although physically larger, automotive harnesses generally range from a few to only a few hundred wires, so while the test table might be sizeable, relatively small testers are all that is required.
Mil/Aero/Defense Harnesses present unique adaptation challenges
Cables in planes, trains, ships, weapons, satellites, and space vehicles tend to be the largest, most complex, and, therefore, most difficult to interface for testing. These harnesses vary greatly from small to large, both in number of wires and physical size. Electrical test specs are generally the most stringent due to A-620 Class 3 and MIL-STD 202 testing requirements. Wiring analyzers that can expand in size, i.e., the number of test points that are modular, are required for most of these large-scale testing applications. Mil/Aero harnesses generally must be tested at the point of fabrication and again after being installed in their final application, be it a missile, vehicle, aircraft, ship, satellite, or rocket. Modular analyzers generally have a central control unit and multiple switching modules that can be distributed around the U.U.T., be it on the building board or in situ, allowing for much shorter adapter cables. The switch modules are connected via single daisy chain cables that carry not only the voltage and current stimulus to each
module, but also the 110 or 220V mains power, eliminating the need to plug each module into its power supply.
Hook-up time vs. test time vs. troubleshooting time
Another critical consideration is the time to plug the harness in vs. the time it takes to do the test vs. the time to troubleshoot failures. In high-volume applications, thruput is critical. If you’re testing hundreds or thousands of cables per day, quick plugging and unplugging and fast tester cycle times are crucial, hence the discussion of adapter-based testers and spring-probe test tables above. In the case of large complex cables, it is not uncommon for the build time to be several days. Likewise, when testing these harnesses in situ, it may take hours or days to position the tester and plug all of the adapter cables into the D.U.T. In this case, the actual test time isn’t critical. A 10 or 20-minute test isn’t too inconvenient compared to 2 days to plug everything in. Providing easy-to-understand error information is essential to help technicians quickly locate and repair failures. Additionally, wiring analyzers that offer “random” or “smart” hook-up capability tremendously speed up the hook-up time. This feature utilizes smart chips embedded into each adapter cable, allowing the adapter cables to be plugged randomly into any switch module. The tester reads the intelligent chip and re-addresses the test program accordingly.
Standard test programs (TPS’s)
Another critical element of Mil/Aero harness testing is the heightened requirement for documented and verified test program sets (TPSs). While proper test programs are essential at all levels of cable testing, it becomes a whole different world when weapon systems and aircraft are involved. Government regulations often require stringent traceability and verification of TPSs. This requirement, in part, usually drives military end-users to require their cable assembly suppliers to use the same test equipment at fabrication that they use for their end-of-use testing. That way, TPS changes required for harness modifications and engineering change orders can be shared with cable vendors, assuring proper testing at all levels of the supply chain.
In Summary
Cable testers come in many sizes, shapes, and prices like the cables they test. When considering the best solution for your application, be sure to consider all aspects, and remember that one size rarely fits all.