The case of the worn wire

April 21, 2014
Sometimes catastrophic fires and other problems arise from simply neglecting to make sure electrical wires don’t chaff against their surroundings. The problem is particularly worrisome in motion applications.

An important part of any design that involves electrical wiring is to make sure wire insulation doesn’t fail because of chafing. Chafing can arise when wire insulation rubs repeatedly against a surface. The classic example of chafing arises because of vibratory wear: The designer lets the wire rest on a surface that vibrates, compromising the insulation and eventually causing an electrical fault (assuming the surface is at a different electrical potential, which is likely the case). Wires routed over an abrading surface can chafe from thermal expansion and contraction because of environmental thermal effects. And of course, pinching of the wire can cause insulation breakdown often accelerated by high temperatures and tensile loads in the wire.

The accompanying examples show real-world situations where wire chafing caused problems severe enough to merit bringing in engineering experts who could pinpoint the root cause. They bring some design-related tenants to mind:

  1. In the absence of a vibratory environment, cyclic movement from thermal expansion and contraction can still arise which can cause chafing. Leave an appropriate amount of wire slack to avoid wire tension during thermal contraction.
  2. One big cause of chafing is interference of electrical wiring with mechanical apparatus. Get the wire routing right.
  3. In vibratory environments, consider using cable clamps and other restraining devices as part of the design. Avoid laying cables randomly over a machine and relying on the quality of the insulation to prevent a malfunction.
  4. Pay attention to wire bundles that contain a mixture of low-current and high-current wiring. Thermal cycling can cause relative movement between wires in a bundle or harness. Also, different types of wire insulation can aggravate chafing from relative movements. Finally, routing wiring horizontally, rather than vertically, may limit damage to equipment as a result of a fault. Vertical wiring harnesses burn more readily than horizontal wiring harnesses because convective forces drive combustion gases vertically, easily boosting the chance you’ll need a fire department.
  5. Avoid routing wires in high personnel traffic areas such as under carpets. Avoid having wiring harnesses routed in an area where the harness could be a convenient handheld device during maintenance or other activities.
  6. When the design includes standoff clamps or other means of restraint, installers need to take care not to pinch individual wires in a bundle, when securing the clamp.
  7. Particulates, such as metallic debris, should be kept well clear of wiring harnesses because it can accelerate chafing from thermal expansion and vibrations.
  8. There are many references available on the physical design of wiring. Some well-known examples include Aircraft Electrical Wiring Interconnect System Best Practices, Federal Aviation Administration, Revision 2; Society of Automotive Engineers Standards on Wiring; and the National Electric Code.

Charles C. Roberts, Jr., Ph.D., P.E. is an engineering consultant in the areas of accident reconstruction, failure analysis, structural analysis, heat transfer, fire origin analysis, computer analysis, mechanics, and biomechanics. He can be reached at [email protected].

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