With the trend toward increasing power densities in airborne electronic systems, pressure is being placed on the system designer to package electronic components, such as wires carrying high voltages and currents, closer together, thus requiring a reduction in component size. As power densities increase, more effort is needed to select the appropriate wire as well as in designing its routing and termination in airborne and spaceborne power applications to handle the increasing power densities.

Power density increases coupled with high failure rates when wires are not properly selected precipitates the need to understand and predict the operating capacity of electrical wires. The key to this is to develop an approach to determine its capacity to withstand transient- and steadystate high voltages throughout the operating life of the system. Factor in temperature variations, moisture, vibration, and shock, and the task becomes even more daunting.

There is a distinction made between general-purpose wires used in aerospace systems and wires used continually at high voltages—for example, wires that are designed to operate above Paschen’s minimum voltage (defined, for these purposes as 240 Vrms or 340 Vdc). These wires are used in traveling wave tube amplifiers, highvoltage power supplies, and CRTs. Typical voltages for these applications start at 10 kVdc. They are also used for moderate high-voltage applications (below 10 kVdc) in devices such as miniature TWTAs, fluorescent backlighting for LCDs, and expendable decoys.

In today’s aircraft design, 270-Vdc power is being used in place of 115- Vac, 400-Hz power. Although presently there are wires and connectors that are advertised to operate at this voltage, it’s important to consider voltage transients when evaluating lifelong reliability of the part. Using 270 Vdc on an aircraft has become an issue for consideration because of possible voltage transients occurring in the range where air and other gases ionize. In these applications, wires are either terminated to a connector or soldered to a terminal and encapsulated in a dielectric potting compound.

There are a multitude of electrical wire types on the market today using many different types of insulation systems. For the purpose of this article, we’ll focus on fluoropolymers such as fluorinated ethylene propylene (FEP), which has become the de facto standard of choice in dc high-voltage systems because of its high-temperature resistance, chemical resistance, and superior dielectric strength over a wide range of temperatures.

Once the designer is aware of the natural and induced environment in which the wire will be operating, the next steps are to determine conductor size for carrying the current and the appropriate insulation system to withstand the voltage. While conductor size can be determined using available product data, figuring the wire’s voltage capacity is not as straightforward and depends on a number of factors such as the dielectric properties of the primary insulation (dielectric strength, insulation resistance, and dielectric constant), insulation thickness, abrasion resistance, chemical resistance, flammability, radiation resistance, and compatibility with encapsulants and adhesives.

Before determining the insulation thickness of a wire, some knowledge of how breakdown mechanisms work in wire insulation systems is beneficial. Breakdown voltage in a wire depends on insulation thickness, insulation material, whether the voltage is ac or dc, and environmental factors such as temperature, pressure, humidity, and how it’s mechanically attached.

Typical failure modes in wire include breakdown through the insulation wall, interface breakdown while in the encapsulant, and partial discharge (if the voltage is high enough).

The first failure mechanism can be controlled by having enough insulation to stand off the voltage as well as the environmental and mechanical stress and handling that will happen throughout the wire’s lifetime. Conventional wisdom dictates that the voltage (V) divided by the insulation thickness (t) provides the value of electrical stress induced in the wire, mathematically determined by the simple ratio: E = V / t, where E is the voltage stress in volts per mil.

However, this model for determining voltage stress is misleading and represents only an average across the wire insulation. True electrical stresses in wire insulation depend on the radius of the conductors and their closest ground plane. Electrical stress is highest next to the conductor and decreases outward.

Conductor stranding provides another complicating factor, which raises the stress even further.

This configuration is mathematically expressed as E = (K)2(V)/- (d)ln(D/k2d), where E, again, is the voltage stress, D is the outside diameter of the wire in mils, d is the average conductor diameter in mils, V is the voltage, and k, and k2, are proportionality constant that depends on conductor standing and size.

Once electrical stress is determined, it can then be compared with the dielectric strength of the insulation. Typical values for dielectric strength offered by material suppliers are normally given as ac values. These values are based on the voltage value at which the insulation breaks down.

While dielectric properties for engineering- grade materials are typically available in a manufacturer’s product literature, the values are not standardized. For one thing, they vary with thickness

For example, FEP with a wall thickness of 1/8 in. has a dielectric strength of approximately 500 Vac per mil. However, the dielectric strength is reported to be as high as 2,000 V per mil for a 10-mil thickness.

Obviously, one would want to ensure that the actual voltage stress is sufficiently below the wire’s dielectric strength over the range of operating temperatures. Determining this margin depends on the manufacturer’s literature as well as empirical data provided by accelerated life tests.

Accelerated life tests as outlined in MIL-STD-202, Method 108 and using appropriate acceleration factors can provide useful data to determine life reliability. The objective is to test a wire using higher than normal voltage at higher than ambient temperature for hundreds of hours to obtain a degree of confidence that the cable will operate for thousands of hours at the rated voltage.

Good adhesion between the wire insulation and encapsulant and its depth in the encapsulant is critical in standing off a high-dc-voltage potential. Surface treatments such as chemical etching and/or priming of the wire insulation enhances the adhesive qualities further.