Shielded cable
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A shielded cable is an electrical cable of one or more insulated conductors enclosed by a common conductive layer. The shield may be composed of braided strands of copper (or other metal), a non-braided spiral winding of copper tape, or a layer of conducting polymer. Usually, this shield is covered with a jacket. The shield acts as a Faraday cage to reduce electrical noise from affecting the signals, and to reduce electromagnetic radiation that may interfere with other devices. The shield minimizes capacitively coupled noise from other electrical sources.
In single conductor signal cables the shield may act as the return path for the signal and is usually connected only at the signal source. In multiconductor cables the shield should be grounded only at the source end, and will not carry circuit current.
High voltage power cables with solid insulation are shielded to protect the cable insulation and also people and equipment.
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[edit] Signal cables
By twisting two conductors of a signal circuit, some cancellation of inductively coupled noise is obtained. However, a metallic shield layer over the twisted pair provides better suppression of noise. Coaxial cable is used at higher frequencies to provide controlled circuit impedance, but the outer tubular conductor is also effective at reducing coupling of noise into a circuit.
[edit] Applications
The use of shielded cables in security systems protects them from power frequency and radio frequency interference, reducing the number of false alarms being generated.
Microphone or "signal" cable used in setting up PA is usually shielded twisted pair cable, terminated in XLR connectors. The twisted pair carries the signal in a balanced audio configuration.
The cable laid from the stage to the mixer is often multicore cable carrying several pairs of conductors.
Also see: High-end audio cables
[edit] Power cables
Medium and high-voltage power cables, in circuits over 2000 volts, will have a shield layer of copper or aluminum tape or conducting polymer. If an unshielded insulated cable is in contact with earth or a grounded object, the electrostatic field around the conductor will be concentrated, resulting in corona discharge, and eventual destruction of the insulation. As well, leakage current and capacitive current through the insulation presents a danger of electrical shock on contact. The grounded shield equalizes electrical stress around the conductor, diverts any leakage current to ground, and blocks capacitive current from flowing into objects (or persons) touching the exterior of the cable.
Shields on power cables are connected to earth ground. On high current circuits the shields must be connected only at one end, since induced current will flow in the shield. This current will produce losses and heating and will reduce the maximum current rating of the circuit. On very long high-voltage circuits, the shield may be broken into several sections since a long shield run may rise to dangerous voltages during a circuit fault.
