A shielded cable is an electrical cable of one or more insulated conductors enclosed by a shield that 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. Any conductor will pick up current due to mutual inductance from nearby conductors having a "high" alternating current, or a "high" current spike. Cabling or twisting rejects some of the inductance.

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 doesn't carry circuit current.

Signal cables

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Twisted pairs reject some noise; Coaxial cable is better at higher frequencies (although more expensive). Sheilding each twisted pair is better.

Since optical fibers are not susceptible to electromagnetic interference, they are not usually covered with a shield, though they may have layers of material applied for mechanical protection and ease of handling.

Power cables

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Medium and high-voltage power cables, in circuits over 1000 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 and may over-stress the insulation, resulting in corona discharge, and eventual deteroration of the insulation layer. As well, leakage current and capacitive current through the insulation layer presents a danger of electrical shock on contact. The grounded shield equalizes electrical stress around the conductor and collects and diverts any leakage current to ground.

Shields must be connected to earth at each splice and termination for redundancy for personnel safety. Thus, the shield currents must be taken into account during circuit design. The shielding must have enough ampacity to handle possible fault currents, yet small to prevent unnecessary mutual inductance and cost.

The shield must be flaired in a logrithmic pattern at each end and on both ends of a splice to prevent flashover due to the high voltage stress at each shield end. by Hassan Mohammadzadeh

Public address audio cables

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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.

Cables