Handbook for Sound Engineers

428 Chapter 14

The foil itself is much too thin and weak to even be
applied in the factory by itself. The second plastic layer
adds enough strength and flex-life (flexes until failure)
to allow the foil to be used.
The drain wire, therefore, must be in contact with the
foil. In some cables, the foil faces out, so the drain wire
must be on the outside of the foil, between the foil and
the jacket. If the foil faces in, then the drain wire must
be on the inside of the foil, adjacent to the pair (or other
components) inside the cable.
With an internal drain wire, there are a number of
additional considerations. One is SCIN, shield current
induced noise, mentioned earlier in Section 14.8.5.1.
Another is the ability to make a multipair shielded cable
where the shields are facing in and the plastic facing
out. This allows the manufacturer to color code the pairs
by coloring the plastic holding the foil.
If you have a multipair cable, with individual foil
shields, it is important that these foil shields do not
touch. If the shields touch, then any signal or noise that
is on one foil will be instantly shared by the other. You
might as well put a foil shield in common around both
pairs. Therefore, it is common to use foil shields facing
in which will help prevent them from touching. These
can then be color coded by using various colors of
plastic with each foil to help identify each pair.
However, simply coiling the foil around the pair still
leaves the very edge of the foil exposed. In a multipair
cable with many individual foils, where the cable is bent
and flexed to be installed, it would be quite easy for the
edge of one foil to touch the edge of another foil, thus
compromising shield effectiveness. The solution for this
is a Z-fold invented by Belden in 1960, shown in Fig.
14-14. This does not allow any foil edge to be exposed
no matter how the cable is flexed.

14.16.1 Ground Loops

In many installations, the ground potential between one
rack and another, or between one point in a building and
another, may be different. If the building can be installed
with a star ground, the ground potential will be identical
throughout the building. Then the connection of any two
points will have no potential difference.
When two points are connected that do have a poten-
tial difference, this causes a ground loop. A ground loop
is the flow of electricity down a ground wire from one
point to another. Any RF or other interference on a rack
or on an equipment chassis connected to ground will
now flow down this ground wire, turning that foil or
braid shield into an antenna and feeding that noise into

the twisted pair. Instead of a small area of interference,

such as where wires cross each other, a ground loop can

use the entire length of the run to introduce noise.

If one cannot afford the time or cost of a star ground

system, there are still two options. The first option is to

cut the ground at one end of the cable. This is called a

telescopic ground.

14.16.2 Telescopic Grounds

Where a cable has a ground point at each end, discon-

necting one end produces a telescopic ground. Installers

should be cautioned to disconnect only the destination

(load) end of the cable, leaving the source end

connected.

For audio applications, the effect of telescopic

grounds will eliminate a ground loop, but at a 50% reduc-

tion in shield effectiveness (one wire now connected

instead of two). If one disconnects the source end, which

in analog audio is the low-impedance end, and maintains

the destination (load) connection, this will produce a very

effective R-L-C filter at audio frequencies.

At higher frequencies, such as data cables, even a

source-only telescopic shield can have some serious

problems. Fig. 14-15 shows the effect of a telescopic

ground on a Cat 6 data cable. The left column shows the

input impedance, the impedance presented to any RF

traveling on the shield, at frequency Fk (bottom scale) in

MHz.

You will note that at every half-wavelength, the

shield acts like an open circuit. Since most audio cables

are foil shielded, and the foil is effective only at high

Figure 14-15. Effect of a telescopic ground on a Cat 6

cable.

fk

1 10 100 1000

Zink

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