To ensure that cabling facilities can successfully
handle high-performance video streaming demands,
the LAN is expected to function like a utility-grade
infrastructure or (UTG).
However, as the telecom industry learned many years
ago, the aluminum shielding material is ineffective in
reducing lower-frequency (i.e., fundamental frequency
of the power system at either 50 or 60 Hz and lower)
induction of the magnitude that can cause equipment
malfunctions and damages from lightning or powerinduced
surges. Of course, no amount of shielding can
prevent the occurrence of GPR conditions.
Shielding effectiveness at low frequencies increases
with the use of ferromagnetic materials, such as steel,
which have much higher permeability or magnetic flux
densities than non-ferromagnetic materials, such
as aluminum and copper. This fact is well documented
in Shielding of Ground-Return Circuits at Low Frequencies
in Engineering Report #26, which was prepared in 1934
by the Joint Subcommittee for Development and Research
of the Edison Electric Institute and the Bell
Telephone System.
The most significant part of the report is in the Effect
of Added Mutual Impedance section (page 11, paragraph
2.6). It states that “any addition to the self-impedance of
the shielding circuit, which at the same time is added to
the disturbing or disturbed circuit, diminishes the shield
factor, and if the added impedance is large, the shielding
can be made almost perfect. One application of this
principle is through the insertion at intervals of mutual
inductance coils, so-called ‘neutralizing transformers.’”
Thus, shielding theory works in the same way as
a neutralizing transformer and vice versa. If one understands
how a neutralizing transformer works, it is understood
why aluminum or copper shielded cable is ineffective
in canceling or neutralizing low-frequency
induced voltages.
40 I ICT TODAY
A BREAKTHROUGH COMPUTER
NETWORKING TECHNOLOGY
IS THE SOLUTION!
On September 4, 2007, AT&T (based on the work of this
author) received the first of three U.S. patents on a Digital
Subscriber Line Induction Neutralizing Transformer Network.
The main idea was to redesign the commercially available
induction neutralizing transformer (INT) for use
on xDSL-type circuits to overcome an inherent design
problem that could potentially create crosstalk interference
issues. Instead of having the INT wound with
Cat 3 cable, as the standard and digital carrier units are,
Cat 5 or Cat 6 cable would be used to minimize this issue.
The design objective was also to obtain a smaller, less
expensive unit that was more efficient in suppressing
high-frequency impulse noise interference levels.
In December 2009, a prototype DSL INT unit was
built with Cat 5 UTP, and it was shown to effectively
suppress 98 percent of the low-frequency induced voltage
on a telephone circuit, along with achieving substantial
FIGURE 8: Before and after induced lightning surge impulses
with a DSL INT prototype.