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How much do you know about the shield cable?

Author:yhngrfd   Post Date:09-25-2017

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, 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, people and equipment.

The best way to wire shielded cables for screening is to ground the shield at both ends of the cable. Traditionally there existed a rule of thumb to ground the shield at one end only to avoid ground loops. In airplanes, special cable is used with both an outer shield to protect against lightning and an inner shield grounded at one end to eliminate hum from the 400 Hz power system.

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. The best practice 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.

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 more quickly. 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. There is a risk of shock hazard from having only one end of the shield grounded. The maximum recommended shield potential rise is 25 volts.

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