Talk:Shielded cable
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I am an electrician at a major steel company. I occasionally work with high voltage shielded cable and am curious as to why a semiconductor material is usually used in between the cable insulator and the cable shielding.
The Semiconductor (Semicon) is used between the Shield and Insulation on Low Voltage cables and between the Conductor and Insulation and between the Shield and Insulation to help "smooth out" the voltage stress caused by higher voltages. High voltages will damage the insulation by creating stress in the insulation - basically ripping the insulation at the molecular level - the stress is greatest in Corners and sharp radius turns - the Semiconductor provides a slight buffer - rounding out the area of the stress.. The reason this is applied on the shield side in Low Voltage cables - is that the shield typically sees higher voltages than the conductor- these are created by gound currents and voltages as well as disturbances such a lightning.
Paul.J.Moore 21:41, 15 August 2006 (UTC)Paul.J.Moore
[edit] Grounding at both ends
The article mentions that the shield of signal cable should only be grounded at the source end. I know this has been common practice for years for many years, but there has been some new thought in this area.
The original reason for only grounding on one end was to eliminate "ground loops" or common mode hum, on audio circuits and the like. Line frequency would get onto the audio (and now network) circuit and screw up the signal. That is a good fix for that problem and it works. However, the better solution is to fix the grounding of the overall installation so that nothing is leakingto ground in the first place.
The cable shield works best when the signal lines are completely surrounded by a conductive "tunnel" that is completely at ground potential. That means it should be grounded at BOTH ends (grounded at one end means the other end is similar to an antenna at some frequencies). But to prevent ground loops, both devices have to be at the same ground potential (otherwise current will flow on the shield). That means lots of high quality ground straps between both ends. Think of it this way, if everything in the installation were sitting on a 12 inch slab of pure copper, and bonded to it, how could a ground loop arise? It couldn't, as long as all the components in the installation are connected correctly and have not failed. In a real world application, the real solution is to reduce the slab of copper and the bonding to something practical (lots of conductive straps between all components), and to watch that all components are installed correctly and to the same standards. This is the infamous Multipoint Ground.
Cutting the shield at one end works when you have an installation with uncorrected problems, but it is not the best way to shield a signal cable. --Wolfram.Tungsten 18:47, 16 October 2006 (UTC)
