Metals
We sometimes forget that many cables
are not designed to conduct electrical power or signals,
such as cables which support bridges, actuate ailerons,
and tow cars, for example. Mechanical wire & cable
is a big (but another) industry.
There are, however, similarities between
mechanical and electrical wire and cable — at
least in terms of their means of manufacture.
As strands of wire are made, they are
drawn through progressively smaller dies. This is true
of all wire. Diamond dies are used, due to their extreme
hardness, and the fact that they retain their precision
size for a long time. In fact, the American Wire Gauge
(AWG) sizing system suggests this drawing procedure.
For example, a size 22 AWG wire, smaller than 20 AWG,
is drawn, theoretically, through 22 progressively smaller
dies. Larger wire is drawn through fewer dies; hence,
the lower-number "gauge." See Table
1.
 Copper
is regarded as the standard in
electrical conductors, second only to silver in conductivity,
but far more plentiful and therefore economical.
Because soldering copper can be difficult
unless a flux is used (which can leave corrosive residues
behind), it is usually tinned or plated if it is intended
to be soldered. (This does not preclude the use of flux,
but the coating makes soldering easier, and affords
some protection against corrosion overall.)
Bare copper is perfectly suited for
pressure terminations (crimping, etc.) which break through
surface oxidation.
Aluminum' slighter weight would
suggest it being favored for the weight-conscious aircraft
industry. Its weight is about 1/3 that of copper, and
even with its poorer conductivity, it performs better
than copper on a per pound basis by a factor of almost
2:1.
So why isn't aluminum preferred? To
start with, the physical attributes of wire are only
part of the story. Years ago, when copper was in shorter
supply, aluminum was often chosen for residential wiring.
What was not fully appreciated at the time was the serious
effects of the galvanic reaction between aluminum and
the brass or copper fittings or terminals in the presence
of moisture. This would develop corrosion which would
cause failure at the connection, either in the form
of an open circuit or, worse, a high resistance, which
spawned many a fire. Aluminum proved to be galvanically
too aggressive to be placed in direct contact with copper
or brass. [Table 2 lists
a selection of metals according to their galvanic ranking.]
 The
same problem exists in other circuitry. If all terminations
were changed to aluminum, the galvanic problem might
be solved, but this would apply to all the pins, terminals,
contacts, and conducting hardware, and there are a lot
of existing systems which would need adaptation. Then,
too, aluminum develops a hard layer of oxides on its
surface, and this must be penetrated for a good electrical
connection.
Although a second-best solution, there
are bimetallic (“AL/CU”) adaptors which
interface aluminum and copper conductors where re-wiring
a home is impractical. These solve the galvanic action
problem which compromises fire safety.
One other serious deficiency of aluminum
is that it cannot be easily soldered or plated to improve
solderability.
All this may suggest there is no legitimate
use for aluminum in electrical systems, let alone on
aircraft. Not so. In truth, aluminum is approved for
airborne use in 6 AWG or larger gauges. This is aimed
at power applications, not avionics systems. At the
high currents appropriate to large conductors such as
these, the effects of possible corrosion are compensated
to some extent by the current itself.
Silver conducts better than
copper, though it is substantially more expensive. As
a result, it is often used as a coating for copper in
order to improve skin conductivity and offer some protection
against corrosion. This is of particular value at very
high frequencies, where the current is more apt to concentrate
at the "skin" of the conductor, a phenomenon
called skin effect. Silver is also readily soldered.
Tin provides corrosion protection
for a copper conductor, but does not appreciably affect
its conductivity. It is, of course, eminently solderable.
A conductor which is "tinned" may actually
be coated with a lead-tin alloy --- a solder.
Gold, though pricey, is a common
plating for brass connector pins, ARINC coax contacts,
and parts of some other connectors. Fundamentally, this
is the plating of choice because of its excellent corrosion
resistance properties in applications where there can
be great exposure. Gold is also a good conductor and
easily soldered.
Table 3
lists a selection of common conducting materials and
their properties, both absolute and relative to copper.
Jacket & Dielectric Materials
Insulation Temperature Ratings
PVC is a poor choice for wire and cable
insulation on aircraft — a position affirmed by
the FAA. Other good, and approved, choices exist and
are readily available.
Temperature ratings reflect the range
within which the integrity of the insulation will be
maintained — sufficiently flexible when cold and
free of the effects of softening or disintegrating at
the high end of the scale. It should be noted that the
upper temperature rating should take into account the
heat rise caused by power dissipation in the conductor
itself.
While most airborne wiring is not expected
to endure exposure to the rated temperature extremes,
such ratings provide a measure of "headroom"
to assure safety in the event of fire or malfunction.
Other insulation properties of concern,
depending upon the applications, include the dielectric
constant, which dictates loss, mutual capacitance (between
conductors), impedance, velocity of propagation, etc.
[See The
Velocity Factor]
The most prevalent wire & cable
insulation materials approved and generally acceptable
for aircraft are from the Teflon® family —
a familiar brand name for fluoropolymers — which
include, for example, PTFE, ETFE (also known as Tefzel®),
TFE, and FEP.
MIL-W-22759 wires are TFE- or Tefzel®-insulated.
TFE insulation is rated for upper ambient temperatures
ranging from +200°C to +260°C, depending on
thickness of the insulation and conductor materials.
Tefzel® is typically rated at +150°C. Both are
suitable to -65°C — which may be realized
in proximity to the skin at high altitudes.
Temperature/Performance Issues
There are some old "stand-by"
coaxial cables — RG58 and RG214, for example —
and some newer low-loss cables which can actually cause
serious performance problems in avionic systems. Their
usefulness is flawed by the use of polyethylene as the
dielectric material. This results in a temperature rating
of, typically, 85°C (which equals 185°F), which,
on first glance, may seem entirely adequate.
But airborne systems are much more
safely served by cables with a 200°C rating. Now,
200°C is a whopping 394°F — hot enough
to melt solder! Certainly way above human tolerance.
So, is it overkill to specify (and pay for) 200°-rated
cables? Decidedly not. And here's why.
Many avionics technicians are aware
— from experience, if not by science — that
the use of "high-temperature" cables is preferable
to the less expensive coaxes. The reason is performance
— maybe not at the outset but over time.
In a great many aircraft, cables snake
through the airframe in places that can become much
hotter than the cabin. Even though contact with or proximity
to air ducts, engine firewalls, and other hot spots
won't see temperatures even approaching 200°C, it's
not uncommon for them to experience touch-points well
above 100°C. It's there that the damage can happen.
What damage?
A little background: Coaxial cables
are, by definition, co-axial — that is to say,
the cylinder of shielding and the cross-section of the
center conductor share the same axis. The space [dielectric]
between them is the same all the way around. Ideally.
Lower temperature dielectric materials
soften at relatively low temperatures and inevitably
the center conductor migrates off-center, toward the
shield, in the direction of gravity or the inside of
a bend in the cable. In such a case, the "co-axis"
goes off axis, and the concentricity essential to maintaining
impedance is impaired. This is permanent and just part
of the damage that can happen.
The other part occurs at the box. In
the case of a receiver, changes in impedance can cause
a reduction in signal — possibly to the point
of loss of usefulness.
In the case of a transmitter, things
can get worse. The reflection of power [measured as
SWR, or Standing Wave Ratio] comes right back to the
final stage, producing heat ... and heat is the arch-enemy
of all electronic components. This is an invitation
to the bench for repairs. Do you know someone who would
rather pay for repairs than the modest extra cost of
200°C cable?
Cables using 85°C-rated polyethylene
(PE) dielectric materials become soft at temperatures
common in isolated spots in aircraft. Some low-loss
cables use foamed polyethylene which is soft to begin
with. Cable routing with careful attention to avoiding
hot spots is important in general, but crucial with
such cables.
With so much riding on the integrity
of cabling, doesn't it make sense to always use the
better choice?
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