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at these reduced levels the wire voltage is still 15 time greater than its rated value. The most
significant result is that, as the vibration time is continued, the withstand voltage begins to level
off after 250 hours. It is unknown whether the voltage would continue to stay the same after 512
hours. Visual observations indicate the reduction in voltage capability is probably due to wireto-
wire wear.
8. RAYTHEON INDIANAPOLIS DESIGN AND AIRCRAFT INSPECTION
EXPERIENCES
Comment: Based on Raytheon Indianapolis and NAC wiring designs as well as previous
military aircraft inspections, it is not uncommon to find two different wire types in the same
bundles or adjacent bundles. The wire types are necessary as indicated in the NAC TR-2333
report to meet all the extreme environmental zones in the aircraft. The designer has the option of
using one wire type by protecting it more in the most extreme environmental zones, but this is
seldom done for labor cost and weight reasons.
A-9/A-10
APPENDIX B—TEST PROGRAM
B.1. BACKGROUND.
Questions have been raised related to the degradation effects of the insulations of different wire
types in contact with each other. Chemicals, debris, and other environmental contamination are
also expected to accelerate degradation on certain insulation surfaces, as well as increase the
coefficient of friction and cause surface wear. Since many of these wires are constructed with
very stable polymer insulations, such as polytetrafluoroethylene, the actual chemical interaction
between them is expected to be negligible. There are potential issues related to the mixing of
wires with different conductor types as well. A stiffer wire and a harder subsurface to the
insulations may accelerate the physical interaction of the different wire types. Raytheon has
experience testing wire with all of these possible interactions. Previously generated test data has
validated the possibility that these factors could affect the wire-to-wire interaction.
B.2. TEST DESIGN.
Physically conditioning wire bundles with vibration accelerates the interactions between the
wires, such as abrasion and chaffing, and evaluates their characteristic behavior in mixed
bundles. The magnitude depends on environmental factors such as temperature and humidity;
chemical stresses with the presence of aircraft fluids; and physical stresses such as the presence
of foreign matter, vibration, and shock. Three primary mechanisms by which the wire is
damaged in a mixed state include:
a. Abrasion and Chaffing. Abrasion is a primary action that will degrade the wire when a
“sawing” motion is involved. Factors that have been speculated as having influence on
this degradation mechanism include the hardness of one insulation compared to another,
tape edges which bite into the another wire, and pigment and mineral fillers that will
abrade another insulation.
b. Cut Through. Cut through of the insulation, to expose the underlying conductor, is a
mechanism that affects the wire integrity. This is the mechanism of one wire pushing
aside or cutting into another wire by pressure or by the rigidity of the surrounding wires
that forces the wire into a structural member or other sharp object. Cold flow is a
mechanism closely related to cut through whereby the insulation will flow, leaving the
wire less protected with insulation.
c. Chemical Degradation. The presence of chemicals or debris could be the mechanism that
is degrading the wire insulation, and the abrasion against another wire may continually
expose fresh wire surfaces to the damaging effects.
In addition to those listed, combinations of the above stresses may occur.
B-1
Several specific conditioning stresses may affect the wire samples based on the above
degradation mechanisms. These stresses include:
a. Vibration. Vibration is a major issue in that it increases the overall exposure of the wire
to cycles of wear. This stress will increase the abrading or sawing mechanisms that may
be occurring between wires. Operation of aircraft engines and equipment subjects all
electrical wiring interconnect systems (EWIS) components to some level of vibration.
Damage to the support system (clamps and straps) or improper replacement of supports
following maintenance actions can prevent vibration levels from being dampened
properly and permit increased chafing due to undesirable contact between wire types.
The introduction of chemicals, dirt, and debris can cause further damage to the EWIS
when vibration is not restricted properly.
b. Wire Flexure. Flexure of the wires, especially when twisted, such as in twisted
conductors, may occur during maintenance and operation. The flexing will cause a force
　
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