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March, 2019
Common Misconceptions in Testing Electrical Cables
By Christopher E. Strangio, President and Founder, CAMI Research, Inc. W
ith the exception of high-volume consumer products and automobiles, cable and wire harness assembly remains largely
unyield ing to automation. In a surprisingly large number of applications, the wiring of connectors to multi-conductor cable demands the acuity of human vision and the dexterity of human hands. Some medical cables use wire so thin that it
is almost invisible and must be joined to connec- tors under a microscope, while other cables designed to carry very high-frequency signals consist of multiple miniature coaxial conduc- tors that must be wired to suitable connec- tors with great care to avoid introducing dis- tortion or signal reflections. While today’s technology has come to
the aid of assembly technicians tasked with this work by offering video guidance, light- fiber targeting of connector cavities and immediate electrical feedback upon complet- ing a good connection, fully automating the task continues to be beyond the reach of cur- rent robotics and software. Wherever an assembly task depends on
human skill, assembly error remains a dis- tinct possibility — people are not machines. No matter how well-trained a technician may be, occasional faults will creep in. Good assembly practice requires expecting this and planning ahead. Detecting assembly errors is possible
through testing. All testing begins with meas- urement. Once test results are obtained, com- paring measured results against expected stan- dards reveals faults. The most likely fault in a cable assembly is the
easiest and fastest to check: basic continuity. To do so, pass a current through one connector pin, and
ensure that it reaches only the intended pins on other connectors. This reveals missing connections, short circuits and miswired pins. However, this test alone does not provide the
assurance that the wiring will operate correctly in it its expected application. Critically important are the quality of the connection between pins and the isolation of conductors from each other. We can easily agree that these factors need to be proven in a thorough test.
Three Misconceptions There are three commonly held misconceptions
in continuity and hipot testing: “It’s not necessary to check resistance if there are no resistors in my cables,” “Crimp faults with a few cut strands or par- tial insulation capture will be detected by a milliohm resistance test,” and “Nicks and pinholes in wire insulation and conductors that are too close together will always be caught by a high-voltage test.”
Connection Resistance Resistance measurement tells us how
Figure 1: How low should the connection resistance be in an assembled cable?
Unfortunately, our intuition may fail us when
we try to set specific resistance thresholds for resistance or leakage limits for isolation. Several experiments will help to better understand the limits of fault detection in cables.
natural resistance of about 16 mW/ft (52 mW/m). A gold-plated connector pin crimped to the wire will add a few milliohms to this, depending on the contact area and crimp tightness. Alternatively, soldering the pin to the wire greatly increases the surface contact between the wire and pin, offering negligible additional resistance on the order of a similar length of the wire itself. Does soldering wires to pins (usually an
competently a wire will carry its signal from one end to the other. The wire itself has an unavoidable natural resistance as a function of length and diameter. During assembly, the goal is to eliminate additional resistance introduced by poor crimps, cold solder joints, or broken strands. Resistance testing tells us how successful we have been. Typical 22-gauge electrical wire has a
expensive manual operation) provide a meas- urably better quality cable than crimping the
pins (a frequently automated operation)? While it depends on the application, the answer is usually “No.” So, how low should the connection resistance
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