ABSTRACT:
Wire and cable can fail from a number of causes. This paper is an overview of the
various failure modes wire can exhibit, how to predict each and how to help prevent it.
Often, installers or users mistakenly suspect one failure mode when in fact another (or
even none) is to blame. This article will also present an analysis template to determine
just which failure mode might be expected.
ACRONYMS:
PE: polyethylene
PP: polypropylene
PVC: polyvinyl chloride
SRL: structural return loss
FAILURE MODES:
Wire and cable can fail in a number of ways. These include:
INSTALLATION FAILURE: The wire or cable was not installed correctly, or not installed
to correct installation practices or industry standards. This can often involve poor
connector choice or incorrect connector installation.
ENVIRONMENTAL FAILURE: The wire or cable is not intended for the place it is installed.
This may involve outdoor, direct-burial or other ruggedness requirements, or cable
installed in electronically "noisy" areas without adequate protection, or in
high or low-temperature applications where the cable is not manufactured for such
environments.
OPERATIONAL FAILURE: The inherent limitations of the wire or cable were not understood
by the designer, purchaser or installer. The cable can be run too far or a
poorly-manufactured version may be substituted for one of higher quality. This is commonly
known as the "just as good" syndrome.
MANUFACTURING FAILURE: The wire or cable was incorrectly manufactured, incorrectly
labeled, or incorrectly tested. Cable could be made on machinery out of tolerance, in need
of maintenance, or not designed for the precision required in many current designs. This
can include substandard or flawed product being sold as "normal" product. Cable
failures of this kind may or may not be apparent to the designer, purchaser or installer.
INSTALLATION FAILURE:
Installation failure can be caused by the wire or cable not being installed correctly,
or not installed to correct installation practices or industry standards. This can often
involve poor connector choice or incorrect connector installation.
General Installation Guidelines:
Here is a chart of standard practices for audio and video cable:
How to Determine Conduit Fill:
Determine area of conduit. Figure internal diameter, not external. Manufacturer of
cable should provide circular area for each cable. You can then simply add the cable areas
until you reach 40% of conduit area.
ENVIRONMENTAL FAILURE:
Environmental failure means the wire or cable is not intended for the place it is
installed. This may involve outdoor, direct-burial or other ruggedness requirements, or
cable installed in electronically "noisy" areas without adequate protection, or
in high or low-temperature applications where the cable is not manufactured for such
environments.
While there are no "standards" for analog audio, with a very low frequency
range (20 Hz to 20 KHz), the wavelengths are so long that the impedance of the cable
itself is of no consequence. What does affect performance is capacitance, resistance, and
shield effectiveness. Flexibility, flex-life and ruggedness may also be a consideration.
Here's a chart to determine the quality of baseband video cable:
MANUFACTURING FAILURE:
Manufacturing failure means the wire or cable was incorrectly manufactured, incorrectly
labeled, or incorrectly tested. This can include substandard or flawed product being sold
as "normal" product. Manufacturers may also be using old machinery, machinery in
need of maintenance or which is otherwise out of adjustment. Manufacturers may also be
using older machinery not designed to produced cutting-edge technology products. Cable
failures of this kind may or may not be apparent to the designer, purchaser or installer.
Periodicity:
If you can get a graph instead of just a number on the structural return loss (SRL) on
a cable, this can tell you a lot. Because of the complexity, most manufacturers only test
an occasional roll of cable. SRL will tell you about changes in capacitance, impedance,
conductor size, cable dimensions and, most insidious of all, periodicity.
Periodicity is the accumulated errors built up because of periodic flaws in the cable.
For instance, say you have a wheel in your extruder which is out of round. This may
stretch the center conductor making it microscopically thinner at one point. Or the
dielectric might come out slightly smaller in diameter for each turn of the wheel. While
such a change is very minor, and may not even be readable with test gear, or noticeable in
use, this flaw will be added over and over and over again each time the wheel turns.
This now adds the dimension of multiple flaws at the same spacing. If you have a flaw
every 100 cm, over and over, this distance corresponds to a wavelength of a specific
frequency. While a 100 cm flaw creates a major flaw at 3 GHz fundamental frequency, this
anomaly will also show up to a lesser extent at every harmonic of 3 GHz. It doesn't take
many divisions to get to key frequencies for cable television, digital video, digital
audio and similar applications. Also, this problem would not be a factor if the cable were
cut short, perhaps 10 meters or less. But, with increasing distance, this minor flaw could
be a major cause of SRL. Drop-outs of whole channels in CATV cable, or loss of critical
data in digital video, could result. And there is nothing in the manufacturers catalog to
tell you. You can't even tell by looking at the cable.
Compound migration.
If cables are used outdoors, or in harsh environments, the chemicals that make up the
jacket are critical. If they are low quality materials, they can leach or
"migrate" into the dielectric. This is another good reason for a foil as part of
the shield. Foil helps prevent this.
If the chemical constituents of the jacket migrate into the dielectric, it can
radically change the dielectric constant of the material. This means that capacitance and
impedance will also change. And that will cause a dramatic rise in SRL. Bad news!
Testing:
One key difference between manufacturers is how or how often they test their cables. If
it is precision video cable, for instance, every roll should be sweep tested. Ask what
frequencies it is swept to. If it is swept below 200 MHz, it is unlikely that this cable
was intended for digital video, which normally requires sweeping to 400 MHz or more.
Solutions:
Consider each company's market history. Ask other users. What is the company history?
Many companies who have made wire and cable for many years, have only recently been making
audio and video cables. What is there historic position in the audio and video industry?
Do they offer a product warranty. What does it cover? How long is it? How long have they had
a warranty?
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