23.3 SHIELDED (SCREENED) CABLES
Any signal or control cable screens and drain wires should be terminated onto a low-impedance earth
(ground), preferably using a 360° shield (screen) clamp. If pigtails are used, where the screen is brought down to
a single wire and connected to the earth point, then these must be as short as possible; otherwise the inductance
of the pigtail renders it ineffective at high frequencies.
Note: Pigtails just 25mm long have been seen to completely ruin any shielding effect in a 3-metre-
long cable at frequencies above 30 MHz
• For a circuit with an ungrounded source the screen should be terminated at the input end, whereas
if the input is floating and the source is grounded then the screen should be terminated at the
source end.
• If both the signal source and signal inputs are both grounded, terminating the screen at both ends
may reduce the performance of the system.
• To shield against low-frequency electric fields terminate the screen at one end only.
• To shield against low-frequency magnetic fields terminate both ends of the screen.
Using a shielded twisted pair cable with the screen terminated at only one end gives a good
compromise as the twisting minimizes the magnetic coupling, and the screen reduces external
capacitive coupling. The circuit loop area should be kept small, and the screen should not form part of
the circuit.
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