NEPP - Wire and Cable

General.
The type of wire insulation to be selected for a design depends on application constraints. (An outline of the benefits and advantages of various insulation materials is available in section W1, Table 1C of GSFC EEE-INST-002).

In the vacuum of space, the ability to cool wires is more difficult as cooling by convection is lost since there is no surrounding atmosphere. Derating of wire currents must be performed in order to minimize heat rise in cable harnesses. Section W1, Table 4A of GSFC EEE-INST-002 provides for GSFC preferred wire derating. These levels of current derating are conservative, and any deviation from the established levels on GSFC projects requires project approval.

Design considerations for hookup wire used in cable harnesses.
If continuous flexing of cable harnesses is necessary in the application, such as with solar arrays that must be continuously repositioned to follow the sun, wire insulations should be selected that are robust from cracking or abrasion. Insulations that are robust in terms of creep resistance (cold flow) and abrasion resistance are more difficult to work with as the insulation is not as flexible and is more difficult to strip.

Small wire size can save weight but requires special handling in order to prevent breakage. Conductors smaller than 26 AWG are discouraged for use in cable assemblies. When conductors smaller than 24 AWG must be used, conductor material must be high strength copper alloy.

Silver plated conductors may be used in crimp or solder applications. Tin plated conductors should only be used in solder applications.

When wire harnesses are required to be formed so that they must tightly "snake" between boards and units, PTFE insulation should be used with caution due to cold flow concerns. However, where high or low temperature extremes are involved (above 150 °C or below -100 °C), PTFE insulation withstands temperature extremes better than other insulation types.

Magnet wire.
The majority of magnet wire available on the U.S. market is made in accordance with in NEMA MW-1000 (Standard for Magnet Wire, published by the National Electrical Manufacturers Association; NEMA). This standard includes test methods and wire detail specifications for different types of magnet wire. It is available for purchase by contacting: http://www.nema.org/index_nema.cfm/1427/.

Magnet wire coatings are available in many thermal classes, ranging from 90°C to 240°C. Use of magnet wires having a thermal class of 180°C and above are preferred. Use of heavy coating (double coating) is preferred for increased insulation coating robustness, but results in lower flux density and larger magnetic devices.

Pre established levels of current derating are not set for magnet wire. A listing and discussion of commonly applied magnet wire current densities can be found in section W1, Table 4B of GSFC EEE-INST-002.

Wire reliability.
Proper selection of wire is important to avoid problems. Most wire failures result from damaged insulation due to mishandling, heat, or harsh chemicals. Insulation material outgassing results in a source of contamination. Some of these outgassed materials can condense resulting in clouded optical surfaces and degraded performance, while other outgassed materials may be corrosive or conductive in nature.

Excessive current can cause excessive temperature rise, which can shorten life by causing pre-mature chemical decomposition of wire insulation and degraded insulating properties. Excessive voltage can induce failure by causing dielectric breakdown through insulating materials, resulting in high leakage currents or short circuits.

Moisture, oil or contamination trapped under insulation during the manufacturing stranding and extrusion process can result in corroded wire coating. (Corroded silver wire strand coating is known as Red plague.) Therefore, supplier selection is critical in selecting wire and cable for high reliable space applications. Use of wire and cable made by well established military qualified suppliers is recommended.

It is important to store bulk wire and cable in stable indoor environments, away from excess humidity and temperature fluctuations.