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jmjones2000(at)mindspring
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 Posted: Fri May 09, 2014 8:02 pm Post subject: When to use Shielded Wire? |
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I am getting ready to purchase the wire for my project and need to know what wire to purchase. I have read Bob’s articles and plan to purchase 18 and 22 gauge wire. I have an EFIS, Garmin 430, PS Engineering Audio Panel, and a KT76 transponder. I will have electronic ignition and LED lights. My question is what wire needs to be shielded and what wire doesn’t need shielding? Is it better to just use shielded wire on everything? Seems like extra weight. Is it better to use shielded wire with numerous separate wires inside? or shielded wire with a single wire inside the shielding?
Thanks for the help!
Justin
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nuckolls.bob(at)aeroelect
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| Posted: Sat May 10, 2014 5:42 am Post subject: When to use Shielded Wire? |
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At 10:58 PM 5/9/2014, you wrote:
<jmjones2000(at)mindspring.com>
I am getting ready to purchase the wire for my project and need to
know what wire to purchase. I have read Bob's articles and plan to
purchase 18 and 22 gauge wire. I have an EFIS, Garmin 430, PS
Engineering Audio Panel, and a KT76 transponder. I will have
electronic ignition and LED lights. My question is what wire needs
to be shielded and what wire doesn't need shielding? Is it better to
just use shielded wire on everything? Seems like extra weight. Is it
better to use shielded wire with numerous separate wires inside? or
shielded wire with a single wire inside the shielding?
Shielding of wires is the weakest of prophylactics
against propagation of noise . . . and then effective
on a very narrow range of antagonists.
If you run two wires side-by-side in the same bundle
wherein one wire carries something like millivolt levels
of audio from one box to the other (potential victim
wire) and the other wire carries some form of
electronic violence (potential antagonist wire) then
there is a potential for undesirable coupling of
energy from the former into the later.
Coupling in wire bundles predominates in two forms.
Magnetic and electro-static. If the electronic violence
is in the form of a high current where the antagonist
wire presents a strong local magnetic field, then
the effects of that field will be impressed upon the
victim wire "longitudinally".
The fix for this propagation mode is to break the
common longitudinal path (read ground loop) by means
of techniques like running potential victim signals
on twisted pairs . . .perhaps even transformer coupled
at one end. In other words, this propagation mode
is best broken by judicious architecture of the
victim's i/o ports.
If the electronic violence is in the form of high
voltage, fast rise-time wave-forms (like magneto
p-leads and strobe trigger pulses) then the
propagation mode becomes electro-static. The
insulation around antagonist and victim wires
become 'dielectric' components of a capacitor
Albeit a tiny capacitor, effects of the signals
on one wire can be impressed on top of the tiny
signals in the other.
This propagation mode can be broken by
shielding one or the other of the two wires.
It's best to shield the single antagonist (like the
p-lead) so that you don't have to shield a
hand-full of potential victims.
MIL-STD-704/DO-160 design goals dictate that a
product intended for installation on airplanes
demonstrate minimum levels of immunity to
external stresses along with limits to emitted
violence. Products qualified to these design
goals are expected to function in communal
harmony WITHOUT special attention from the
designers or installers of the system. I.e.
no automatic inclusion of EXTERNAL prophylactics
against propagation of noise.
In some few cases (like p-leads and strobe wiring)
electronic violence in the wires is a fundamental
component of that system's functionality. I.e.
it cannot be avoided. Hence, the learned designers
and installers of such systems will take judicious
steps to contain the noise. I.e. shielded wires
and/or p-lead filters installed at the magnetos.
Outside these special cases the system integrator.
YOU, is held harmless from the need to worry about
piling on more prophylactics against noise.
The numbers and brutishness of such systems
has been declining. "P-leads" from electronic
ignitions don't carry magneto-like trash.
LED strobes don't utilize fast-rise, HV
trigger pulses. Given these advances in
system features, I'd venture an assertion
that the value of shielding of any wires
in an airplane is minimal if not zero.
The short answer to you question is: Unless
called out on the installation drawings for
a device, you don't need to shield the wires.
Legacy practices for shielding wires in the
communication and navigation systems is a
hedge against poor practice in the design
or wiring of a potentially antagonistic
system that shares the wire bundle.
If it's not on the drawing, don't shield it.
Bob . . .
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