March, 2019 Continued from previous page
be in an assembled cable? Which then begs the question: where then, should we set the pass/fail threshold when testing wire in a cable — 50 mW, 0.5, 1, 5, 10W, or something else? The answer depends primarily on how much
current the wire is expected to carry. In Figure 1, (previous page) the first circuit has no load. Because no current is flowing, there is no voltage drop across the wire resistance, and the voltage observed at the other end of the cable is unaffected by the wire resistance, regardless of the resistance
www.us-
tech.com
critical to avoid device malfunction or heating in the cable. In this case, a threshold of 200 mW or less would be prudent. The cable’s designer should specify the test parameters with the expected worst-case load in mind to avoid the expense of overtesting with unnecessarily sensitive equip- ment, or the risk of undertesting with equipment incapable of setting a sufficiently low threshold. As a rule of thumb, use a maximum resistance
threshold of 1 percent of the expected load resist- ance. For the differential receiver cable example, a 120W load from the terminating resistor will be applied to the cable in its application, so the maximum allowed cable resistance should not exceed 1W, which should be the set pass/fail threshold. For loads of higher resistance (likely for electronic circuits), the threshold can be set proportionally higher, but generally not exceeding 10W.
Crimp Faults Quality managers concern them-
Figure 2: Experiment for simulating and detecting crimp faults caused by cut strands.
value. In the second circuit, the load resistance equals the wire resistance and current flows, with an equal voltage drop across the wire and load. If the cable will drive an electronic device,
selves with various types of crimp faults that, in most cases, result in less-than- optimal surface contact between the wire and the pin. The most common faults result from an incorrectly adjusted wire stripping machine that takes off several strands of wire along with the insulation, or a crimping machine that captures part of the
outer insulation under the crimp. A visual
such as a differential receiver with a 120W termi- nating resistor, the load imposed on the cable will be minimal, and even relatively high conduction resistance values of 5 or 10W will not likely affect the signal, but still be low enough to overcome capacitive effects in the cable during high-speed digital switching. If the cable were to drive a relay coil, motor or
heating element, the load imposed on the cable will be significant, and low conduction resistance is
inspection clearly
shows these problems, however, visually inspecting crimps takes time and generally is not practical. Intuition suggests that a precise resistance test will find the problem. While theoretically true, resistance testing, unfortunately, does not provide a practical solu- tion either. The following experiment demon- strates this. Connect a 3.5 in. (8.9 cm) length of 22-gauge,
seven-strand wire, UL07-730, between two screw terminals, remove the insulation and separate the strands. This experiment was originally conducted with a CAMI CableEye® 4-wire Kelvin measure- ment system.
Table 1: Change in resistance as strands are cut.
in a laboratory setup, we cannot achieve a practi- cal test for missing strands in a manufacturing environment. While 4-wire Kelvin measurement is a more
sensitive technique than standard 2-wire resist- ance testing, it still lacks the sensitivity to diag- nose single-cut strands, single-stray strands or crimp-caught insulation. Combining continuity,
Continued on next page
Page 65 Common Misconceptions in Testing Electrical Cables The strands will be cut, one-by-one, with
resistor measurements made after each cut, to determine how the resistance varies with the num- ber of strands intact. Figure 2 shows the setup. Table 1 shows how resistance changes as
strands are cut. Interestingly, with only one strand remaining to carry the 1A test current, no heating was detectable by human touch, although clearly the resistance increased slightly with the current applied for 1s or longer, compared to the initial short 50 ms dwell. These results show that the resistance does
increase slightly with each newly cut strand. However, to find three out of the seven strands cut, we need to detect a 3 mW increase in the 3.5 in. (8.9 cm) wire. Unfortunately, variations in the contact resistance between the cable’s pins (one at each end of the wire), the corresponding pins in the mat- ing connector and the natural wire resistance of cables longer than 3.5 in. (8.9 cm), swamp the tiny differences that a few broken strands make. Variations in these pin contact resistances
may easily change over a wider range than 3 mW, without causing a cable to malfunction in its appli- cation. While we can detect these small differences
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108
