A shielded or screened 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, such as aluminium), 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. The shield must be applied across cable splices.
In shielded signal cables the shield may act as the return path for the signal, or may act as screening only.
High voltage power cables with solid insulation are shielded to protect the cable insulation and also people and equipment.
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By twisting two conductors of a balanced-line signal circuit into a twisted pair, some cancellation of inductively coupled noise is obtained. However, a metallic shield layer over the twisted pair provides better suppression of noise. Be sure to continue the shield within control panels. 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.
The common method to wire shielded cables is to ground only the source end of the shield to avoid ground loops. However, in airplanes special cable is used with both an outer shield to protect for lightning and an inner shield grounded at one end to eliminate hum from the 400 Hz power system.[1]
The use of shielded cables in security systems provides some protection from power frequency and radio frequency interference, reducing the number of false alarms being generated. Best is to keep data or signal cables physically separated by at least 3 inches (75mm) from 'heavy' power circuits which are in parallel.
Microphone or "signal" cable used in setting up PA and recording studios 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.
Consumer grade microphones use screened copper wire with one central conductor in an unbalanced configuration.
Also see: High-end audio cables
Medium and high-voltage power cables, in circuits over 2000 volts, usually 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 at the contact point, 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. The grounded shield equalizes electrical stress around the conductor, diverts any leakage current to ground. Be sure to apply stress relief/ cones at the shield ends, especially for cables operating at more than 2kV to earth.
Shields on power cables may be connected to earth ground at each shield end and at splices for redundancy to prevent shock even though induced current will flow in the shield. This current will produce losses and heating and will reduce the maximum current rating of the circuit. Tests show that having a bare grounding conductor adjacent to the insulated wires will conduct the fault current to earth quicker. On high current circuits the shields might be connected only at one end. 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. However, the shock hazard of having only one end of the shield grounded must be evaluated for the risk. The maximum recommended shield potential rise is 25 volts. IEEE 422 and 525 lists the cable lengths that would limit shield potential to 25 volts for a single point ground application.[2]
| Size Conductor | One Cable per Duct (ft) | Three Cables per Duct (ft) |
|---|---|---|
| 1/0 AWG | 1250 | 4500 |
| 2/0 AWG | 1110 | 3970 |
| 4/0 AWG | 865 | 3000 |
| 250 kcmil | 815 | 2730 |
| 350 kcmil | 710 | 2260 |
| 400 kcmil | 655 | 2100 |
| 500 kcmil | 580 | 1870 |
| 750 kcmil | 510 | 1500 |
| 1000 kcmil | 450 | - |
| 2000 kcmil | 340 | - |
[3] IEEE 422: IEEE Guide for the Design and Installation of Cable Systems in Power Generating Stations IEEE 525: IEEE Guide for the Design and Installation of Cable Systems in Substations [4]