Page 389 - Handbook of Electrical Engineering

EARTHING AND SCREENING 377

Find I c in terms of I 3 . Assume V in = 0 in order to determine the amount of current induced from I 3 .
Let this amount be called I co .

−jωM 3c · I 3
I co = amps (13.7)
R in + R c + R o + R e + jωL c
Let the voltage appearing across the load be V oo :-

−jωM 3c − R o · I 3
V oo = volts (13.8)
R in + R c + R o + R e + jωL c

Let the core loop resistance be called R cc :-

R cc = R in + R c + R o + R e ohms (13.9)

Note that the components R 3 and L 3 of the source are not included in this result because the
interference is a current source, which is known.

Case B. Signal cable with a screen.
Assume that the screen is earthed at both ends at the same points as the signal source and the output
load. Hence the common earth resistance R e is shared by both the screen and the core circuits. Again
assume that V in is zero for the same reason as above.

The summation of voltages and emfs in the core loop of the signal cable is:-

V in = (R in + R c + jωL c )I c + (R o + R e )I c
+ jωM 3c I 3 − jωM sc I s = 0, which upon rearranging is,

− jωM 3c I 3 = (R cc + jωL c )I c − jωM sc I s
This has the form:-
(13.10)
y 1 = a 11 I c + a 12 I s
Where

y 1 =−jωM 3c I 3
a 11 =+(R cc + jωL c )

a 12 =−jωM sc
The summation of voltages and emfs in the screen loop of the signal cable is:-

0 = (R s + jωL s )I s − jωM 3s I 3 − jωM sc I c + R e I s

which upon rearranging is,

−jωM 3s I 3 =+jωM sc I c − (R ss + jωL s )I s