Page 391 - Handbook of Electrical Engineering

EARTHING AND SCREENING 379

If the cable core terminates at a high impedance device such as the input channel of an
operational amplifier, then R o is large when compared with R c , R in , R e and R s .
Therefore R cc R ss unless the cable is extremely long.
Let the voltage appearing across the load be V os :-

−jωMR ss I 3
V os = I c R o = volts
R ss + jωL s
−jωMI 3
= volts
L s
1 + jω
R ss
And revising the expression for V oo :-

−jωMR cc I 3
V oo = volts
R cc + jωL c
−jωMI 3
= volts
L c
1 + jω
R cc
It can be seen from these two expressions that the screening effectiveness is mainly determined
by the separation of the signal cable from the interference cable, which is not surprising and supports
the standard practice of laying these cables. It is also seen that at low frequencies the screen and
the core have the same magnitude of induced current and load voltage. Attenuation begins at a high
frequency for both the screen and the core. The cut-off (or 3 db) frequency is typically in the range
0.5 kHz to 2.0 kHz for coaxial and twisted pair screened cables.
The overall armouring of a typical offshore signal cable is phosphor-bronze, copper or gal-
vanised steel braid. Steel wire armouring is used where extra mechanical protection is required. The
armouring provides some of the screening effect. An inner overall tinned copper, copper or alu-
minium tape is also frequently used. Paired, tripled and quadrupled conductors are often screened
with similar tapes. However, all these various layers of screening are not very effective against low
frequency interference from sources such as adjacent power cables.

13.5.3 Earthing of Screens

In some situations the core of a coaxial cable and the screen are used as a two-wire circuit, e.g.
antenna cables, computer cables. In this case the signal current flows in one direction along the inner
core and returns in the opposite direction in the screen. In this way the induced noise is reduced.
It is often necessary to earth one end of the screen for practical reasons. If the end at A e in
Figure 13.15 is earthed then the earth path resistance R e shunts the screen completely and some of the
screen current will flow along the earth path. This will unbalance the core and screen currents and so
noise cancellation will not occur. A noise voltage will appear in the core circuit. Earthing the screen
at the end B e overcomes this difficulty because the positive channel of the amplifier is a virtual earth.
In some cases the connection at B e is made at a ‘clean’ or ‘instrument’ earth if the receiving device
has only one channel or input terminal (the chassis or framework would be the second channel or