Screening
ELECTRICAL INTERFERENCE IN CABLES.
All cables are subject to interference pickup as a result of electrostatic and electro-magnetic fields set up by
nearby electrical wiring and equipment.
There are two basic types of external noise that affect cables:
1. Electrostatic, from external high voltages
2. Electromagnetic, from external high currents
Electrostatic noise
This type of interference is reduced by adding a total coverage screen:
around the cable (core screen, CS),
around each pair of wires (element screen, ES),
or both (ESCS).
The two most important characteristics of this type of static screen are that it has total coverage and has a low
resistance path to earth. The circumferential resistance around the screen may be quite high, as electrically
induced voltages do not produce circumferential currents. This permits the use of laminate or conducting plastics
of relatively high resistance in association with drainwires to reduce the longitudinal resistance.
Electrostatic screens: earthed at one end of cable only
Types of electrostatic screens include:
Metal tape:
Aluminium or copper tapes 75µm thick applied with an overlap over the cable core and oversheathed.
Metal tape and drainwire:
Aluminium tape 75µm thick applied with an overlap in continuous contact with a tinned copper drainwire and
oversheathed.
Metal laminate & drainwire(s):
Aluminium polyester tape 23µm, 50µm, 75µm, thick applied with an overlap in continuous contact with a tinned
copper drainwire and oversheathed.
Conducting plastic & drainwire(s):
Semi-conducting PVC or polyethylene extruded over the cable core containing one or more embedded tinned
copper drain wires and oversheathed.
Metal braid:
Metal wire braid applied over the cable core. Note that open braids are not as effective as screens.
Drain wires should be tinned copper wires. In general drain wires should be solid for solid conductors and
stranded (bunched) for flexible conductor cables.
The choice of the type of material for the screen is dependent on the application:
Choose a foil shield:
• For protection against capacitive (electric field) coupling where shield coverage is more important than low dc
resistance.
• When possible sources of interference include TV signals, crosstalk from other circuits, radio transmitters,
fluorescent lights or computing equipment.
• For MATV, CATV, Video, FCC, Local Area Networks, computer I/O cables in office, industrial or commercial
environments where ambient EMI levels are low.
Choose a braid shield:
• For superior performance against diffusion coupling, where low dc resistance is important, and to a lesser
extent, capacitive and inductive coupling.
• When possible, sources of interference exhibit low impedance characteristics, such as motor control circuits
and switches, which operate inductive loads.
For computer to terminal interconnect for process, instrumentation or control applications.
Choose a spiral or lap braid shield:
• For functional shielding against diffusion and capacitive coupling at audio frequencies only.
• When possible sources of interference are power lines and fluorescent lights.
• For applications when flexibility and flex life are major concerns, such as microphone and audio cables and
retractile cords.
Choose a combination shield:
• For shielding against high frequency radiated emissions coupling and ESD. Combines the low resistance of
braid and 100% coverage of foil shields.
• When possible sources of interference include radio transmitter, TV stations, printed circuit boards, back
planes, motor control circuits and computing equipment.
For Video, CATV, MATV, Local Area Networks, computer I/O cables and computer-aided manufacturing
applications.
Magnetic noise
Any time current flows through a conductor, a magnetic field is generated around the conductor. Since power
conductors normally carry alternating current, the current flowing through the conductors normally alternates in
direction. This alternating current also produces an alternating magnetic field and an alternating interference
current is produced in the conductor loop. An aluminium/polyester laminate screen cannot block these magnetic
fields as non-ferrous materials have only a small effect on electromagnetic interference.
In a plain two-wire parallel conductor cable, magnetic fields can penetrate the area between the conductors and
induce a potential in the line. If the two wires are twisted, the voltages induced in adjacent sections (loops) are
equal and have opposite polarity.
The noise currents are still induced in the conductors, but adjacent twists cause the currents to be induced in
different directions in each conductor. Induced currents in adjacent loops of the twisted pair cancel one another
out. Reductions in magnetic noise merely by twisting the pairs is of the order of 25 times better than straight
parallel pairs or multicore construction.
The other ways to reduce magnetic noise are
(a) to increase the distance between the cable and offending power conductors (power cables, etc)
(b) Screen the cable
An electromagnetic screen is a longitudinal conducting covering having a high inductance (often by use of
ferromagnetic materials) earthed at two points to allow current to flow through the screen. The electromagnetic
screen operates by creating an opposing magnetic field produced by the current flowing in the screen so that the
net field influencing the elements of the cable is reduced.
If the cable is to be used in conjunction with power cables or railway traction systems, it is usually necessary to
calculate a system screening factor.
This is the ratio of the induced voltage on the screened cable circuit to the voltage that would be induced if the
cable had no screen at all. It depends on the cable intrinsic screening factor and the earthing system. For railway
cables and power system supervisory cables the system screening factor required is usually in the range 0.1 to
0.4.
In many cases the conditions of installation affect the selection of screens. For example, cables to be installed in
an earthed steel conduit need little if any self screening, as the conduit will provide most of the protection
required. For magnetic screens, the spacing and resistance of the earthing points have a marked effect on the
system screening factor achieved.
Magnetic screens: earthed at both ends of cable
Note: The intrinsic screening factors given are for 50Hz with a cable core diameter of 25mm.
Copper tape(s):
Copper tapes 125mm thick applied with an overlap and oversheathed. Intrinsic screening factor ∼ 0.95
Copper & steel tapes:
Copper and mild steel tape 0.125mm or more in thickness applied in alternate layers, without overlap and
oversheathed. Intrinsic screening factor ∼ 0.9
Steel tape armoured cable:
Lead or moisture barrier sheathed cable with two layers of steel armour tape 0.5mm thick applied over and
jacketed overall. Intrinsic screening factor ∼ 0.4 - 0.5.
Aluminium and steel tape screen
Moisture barrier sheathed cable covered with aluminium wire or strip and two layers of 0.5mm thick steel tape,
impregnated with anti-corrosion compound and sheathed overall. Intrinsic screening factor ∼ 0.06
Note the above examples get a better (lower) screening factor by using steel and a conducting metal layer.
Steel wire armour has the ability to screen magnetic fields but the screening factor is unknown. We would
advise a steel wire armoured B50 if power cables are in close proximity.
General Cable New Zealand
HEAD OFFICE
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Sales:
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Fax:
1300 363 382
www.generalcable.com.au
Diagrams of cables are illustrative only and are not necessarily to scale.
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