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withstand voltage capability was still 15 times greater than the wire insulation-rated value.
Another significant result from the report is that the withstand voltage began to level off between
250 and 512 hours of vibration conditioning. It is unknown whether the withstand voltage would
continue to be unchanged after 512 hours of vibration. Visual observations from the study
indicate that the reduction in withstand voltage capability was probably due to wire-to-wire wear.
3.5 DOCUMENTATION REVIEW SUMMARY.
There is limited documentation in the commercial industry (OAMs, wire manufacturers, etc.),
academia, military, and government agencies that define the current state of knowledge of the
effects of mixed wire in the aircraft EWIS. Maintenance and service (accident/incident) reports
examined for specific effects of the mixing of wire types yielded no data. Overall, the
documentation indicates that it is standard practice to mix wire types in new aircraft, aged
aircraft, and in repair actions, although the general consensus (ATSRAC WG-6 report [3]) is to
refrain from mixing different wire types unless necessary. The review also indicates that only
limited tests were performed to determine the long-term impact of mixing wire types in an
aircraft.
4. TEST PROGRAM.
A limited test program was designed to expand on previous testing performed in this area and to
answer a number of questions related to the effects of mixing wire types within the EWIS in an
aircraft environment.
4
The test program was developed from previous Raytheon testing experiences and a review of the
test procedures from the NAC TR-2333 report and Douglas Corporation report MDC J1530/01
[4]. Some of the fixtures and equipment from the NAC testing program were used for the test
program. The basis of the tests was that physical interactions are accelerated by vibration of
mixed wire bundles. The vibration causes the motion of the wires against each other to occur at
a quick rate, accelerating the physical contact and degradation mechanisms. The mixed wire
bundles were baselined to bundles with only one wire type. Foreign object debris and a known
chemical were also used to simulate other aircraft environments.
4.1 TEST PLAN.
A test plan was established to evaluate the baseline wire bundles (one wire type) and the mixed
wire combinations at specified intervals. The following summarizes the test plan:
• Groups I and II
− Baseline dry insulation resistance (500 Vdc, 1000 megohms pass/fail threshold, 1
second dwell time)
− Baseline dry dielectric withstand voltage (1000 Vac, 1 milliamp (mA) leakage
current, 1 second dwell time)
− Random vibration exposure (23.88 grms; see figure B-1 in appendix B), two or
three subcycles:
• Visual inspection on mounted bundles after each subcycle
• Dry insulation resistance after each subcycle
• Dry dielectric withstand voltage after each subcycle
− Wet insulation resistance (500 Vdc, 1000 megohms pass/fail threshold, 1 minute
dwell time)
− Wet dielectric withstand voltage (1000 Vac, 1 mA leakage current, 1 minute
dwell time)
− Visual inspection of unassembled tested bundles
• Group III
− Crush test
Group I consisted of 48 wire combinations that were subjected to a total of 500 hours of
vibration conditioning, with the conditioning paused after 100 and 250 hours for the performance
of visual and electrical evaluations.
Group II was similar to group I, except only 24 of the original combinations were selected. One
set of the 24 wire combinations was soaked in hydraulic fluid prior to vibration conditioning
5
(group IIF), and a similar set was contaminated with metal shavings (group IIM). Since these
bundles were contaminated, the vibration exposure was reduced to 250 hours, and a visual
inspection was performed after 100 hours.
Group III consisted of a crush test on the same 24 combinations as tested in group II. Single
wires were placed perpendicular to each other, and a load was applied to determine which
insulation exhibited the most damage. For each combination of wire selected, four tests were
performed with a load of 80 pounds, and four tests with a load of 120 pounds. The wires were
evaluated by quantifying the degree of permanent deformation to the insulation of each wire type
in the pairs tested. (The complete details of the test plan are in appendix B.)
4.2 SPECIMEN DESCRIPTION.
The wire types chosen for this test program represent typical wire families used in aged aircraft,
though there are various construction types within a wire family. For example, PVC/glass/nylon
differs from PVC/nylon in that it uses glass fiber braid between the layers of insulation. Since
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