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Common Misconceptions in Testing Electrical Cables Continued from previous page


resistance and intermittance testing with the use of appropriate test set- tings can provide confidence in the electrical integrity of a part.


Isolation Resistance A typical cable or wire harness


has multiple insulated conductors bundled tightly together, possibly including a woven copper shield and an outer insulating jacket. Ideally, current flowing through one wire should have no electrical effect on adjacent wires — no current would directly leak from one wire to anoth- er and no induced current would result from capacitive coupling or electromagnetic interference. Limiting induced current comes


largely through the design of the bulk cable by controlling insulation thickness and shielding and remains outside the control of the quality engineer. However, direct leakage between adjacent wires may well be caused by assembly defects in the connectors, or insulation containing pinholes, cuts, or other damage. Applying voltage across uncon-


nected wires will produce detectable current flow when various insulation faults exist. Such faults include moisture that has penetrated the connector body or insulation, excess solder flux or other contamination introduced during assembly, unter- minated wire strands inside a con- nector from an improper crimp, con- ductive debris left inside a connector following assembly, or other defects resulting from uninsulated termina- tions being too close together inside a connector. High voltage testing may reveal defects of excess leakage or insuffi-


Figure 3: Experiment for simulating and


detecting air gap violations.


lowing experiment was conducted to test this and produced results that may be surprising. A micrometer is securely mount-


ed in a vice. A 30-gauge wire (0.25 mm in diameter) is attached to the fixed end of the micrometer and held in place by insulated double-stick tape. This exposed wire remains fixed and completely flat against the microme- ter’s anvil. The positive side of the high voltage output drives this wire. The frame of the micrometer includ- ing the spindle (the movable part) is attached to ground. As the spindle turns, the gap


narrows, and the ground point moves closer to the positive wire. Each test starts with the spindle open between 0.04 and 0.08 in. (1 and 2 mm) and is gradually closed until a discharge occurs. We note the size of the gap at


cient air gap between conductors. We must ask, though, how much voltage should be applied and how big an air gap violation will it detect? The fol-


the moment of discharge. The exper- imental setup appears in Figure 3. The wire represents a conductor with insulation damage, and the flat


ity of wire insulation, and detect moisture penetration or contamina- tion, it cannot detect gap violations in excess of 0.006 in. (0.15 mm) at 1,500 VDC. To detect a pinhole open- ing in wire insulation, two wires would need to have overlapping pin- holes and wire insulation in the thou- sandths of an inch (not likely). A loose strand flailing around inside a connectors would need to be 0.006 in. (0.15 mm) away from another termi- nal to be detected by hipot (possible but unlikely) and would better be found by an intermittent connection test, while the cable is flexed.


Table 2: Air gap versus leakage current for a 30-gauge wire.


end of the spindle represents a grounded connector shell or another wire. At the time these measure- ments were made, the relative humidity was about 50 percent. In drier conditions, the discharge will occur at a slightly greater distance. Table 2 summarizes the results. While high-voltage testing pro- vides a good test to confirm the qual-


While our intuition would lead


us to believe that a high-voltage test will detect clearance violations and nicks in insulation, the detectable gaps using voltages of 2,100 VDC or less are far smaller than one might think. Using higher voltages to bridge wider gaps could pose an unsafe situation for the operator and


Continued on page 70


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