The Physical Designer’s Nightmare: Chaffing of Electrical Wire Insulation
Charles C. Roberts, Jr., Ph. D., P.E.
The design of electronic equipment takes place through the efforts of circuit designers who develop the parameters for an electronic system, and physical designers who deal with the layout of the electronic equipment. The physical design of wiring systems is an important aspect of their development. Choice of wire type, wire layout, wire supporting means, and wire restraints are typical design parameters used in the physical design of electrical equipment. An important part of this design process is the phenomenon of electrical wire insulation failure, as a result of chaffing. Chaffing is the wearing of electrical wire insulation by a variety of means, as shown in Figure 1.
Figure 1A shows classical vibratory wear of wire. Here, the designer has allowed the wire to rest on a surface that vibrates, causing insulation breakdown and eventually an electrical fault; assuming that the surface is at a different electrical potential, which is likely the case. In Figure 1B, a wire is routed over an abrading surface and is chaffed from thermal expansion and contraction as a result of environmental thermal effects as well as resistive heating when the electrical system is active. Figure 1C shows a mechanical device whose surface moves relative to the wire insulation, causing chaffing. Figure 1D shows pinching of the wire, causing insulation breakdown often accelerated by high temperatures and tensile loads in the wire.
In order to illustrate typical chaffing problems, the following is a sampling of wiring malfunctions brought on by chaffing.
Figure 2 shows a fan system with a fire origin (arrow) at control wiring lying against a sheet metal support bracket. Wire chaffing has occurred, developing an electrical fault and causing the fire. In many instances, the fuse or breaker would interrupt the circuit, stopping any further damage to the device. In other instances, the electrical fault may have sufficient resistance to limit current and not blow a fuse or trip a breaker. It should be noted that energy dissipation density at the fault area causes the fire, not only the amount of electrical current. A relatively small current driving a fault through a very small area can cause high temperatures and ignite combustible material, resulting in a fire and significant damage.
Figure 3 depicts an unfused battery wire that was chaffing against an hydraulic line in a large vehicle. The wire chaffed, faulted against the hydraulic tube and formed a penetration through the tubing wall, in a manner similar to electric discharge machining (EDM). The resulting hydraulic fluid pressure release caused a spray of hydraulic fluid, which was ignited, destroying the vehicle.
Figure 4 shows the remains of an engine block heater cord located at the front bumper of a pickup truck. It is not unusual for vehicle owners to back out of the garage without unplugging the cord. Chaffing occurs quite readily, which caused an electrical fault, a fire and total destruction of the vehicle and garage.
Figure 5 is a view of a battery wire that faulted against an engine mount near the exhaust pipe on a large truck. Vibration and softening of the electrical insulation from exhaust pipe heating resulted in an electrical fault, which ignited combustible material and destroyed the vehicle.
Figure 6 is an example of mechanical interference with wiring. A bell crank for a heater vent in an automobile was moving back and forth under normal usage, chaffing the wiring harness that eventually faulted, causing a fire and destruction of the vehicle.
When analyzing the remains of equipment, an item that tends to attract one’s interest is the existence of non-factory related modifications. Figure 7 depicts a non-factory related connection to a battery post. Following this add on wire, which lead to the point of origin of the fire, verified an unsecured condition that was subject to vibration and consequent chaffing.
Wiring bundles or harnesses have their own set of problems. In Figure 8, a power wire in the wiring harness faulted, causing severe damage to a piece of equipment. Thermal expansion and contraction from electric current in the power wire inside the harness caused chaffing and an electrical fault.
In Figure 9, power wires (arrows) are draped over various parts of a vehicle, a poor physical wiring design. One of the wires chaffed, faulted, and the vehicle was destroyed by fire. A more organized physical design of wiring, along with appropriate stand off clamps, would have prevented this failure.
These case studies serve to illustrate many physical wiring design deficiencies that caused severe damage to equipment. Some design related tenants come to mind:
1. In the absence of a vibratory environment, cyclic movement from thermal expansion and contraction can still occur, causing chaffing. Leave an appropriate amount of wire slack to avoid wire tension during thermal contraction (Figure 1B).
2. Interference of electrical wiring with mechanical apparatus is a cause of chaffing requiring proper routing and restraint (Figure 1C).
3. In vibratory environments, usage of cable clamps and other restraining devices should be considered as part of the design. Avoid laying cables randomly over a machine (Figure 9) and relying on the quality of the insulation to prevent a malfunction.
4. Attention should be paid 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 chaffing 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 since convective forces drive combustion gases vertically, easily increasing the chance of fire development.
5. Avoid routing wires in high personnel traffic areas such as under carpets. Avoid having wiring harnesses routed in an area where it is convenient to be used as a hand hold device during maintenance or other activity.
6. When using stand off clamps or other means of restraint, care should be taken not to pinch individual wires in a bundle, when securing the clamp.
7. Particulates, such as metallic debris, should not be in the vicinity of wiring harnesses since the debris can accelerate chaffing as a result of thermal expansion and vibratory means.
Many references are available on physical design of wiring, examples of which are:
1. Aircraft Electrical Wiring Interconnect System Best Practices, Federal Aviation Administration, Revision 2.
2. Society of Automotive Engineers Standards on Wiring.
3. National Electric Code.
Published in Machine Design 2014