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Where Do I Use Shielded Cable?

The choice of a shielded cable or non-shielded cable often comes with experience. The electrical environment around the area you are installing the cable determines what is the best choice for that job. An area such as a production/factory floor where heavy equipment is being used is a prime example of a place where you might consider a shielded cable.

Electromagnetic and Radio frequency interference (EMI/RFI) is all around us. Think of EMI/RFI as the static you might hear while talking on the phone. If the static on your phone becomes too loud, you often cannot tell what the other person is trying to say to you. When the noise becomes even stronger, the call might disconnect.

The same is true on a network. This type of noise can penetrate the cabling that carries your data on a network. If the noise is strong enough it can interfere with the actual data traffic and prevent your computers from “hearing” important data. This data is lost and your computers have to re-send it a second time. As many computers repeat this process the network slows down.

Grounding can also be a concern in some installations. If shielded cable is used to connect equipment from two different circuits, a ground loop can occur causing noise on a network line. If the ground voltage difference is great enough it may even cause damage. Installers will often leave one end of the shielded cable terminated with a non-shielded connector. This is often referred to as a "floating" shield. This too can cause a different problem. The floating shield often acts as an antenna that picks up additional noise.

Okay so now you're going to say, "What is the solution?" The answer often has little to do with the cable and a lot to do with the installation site. Proper bonding of the ground at both ends of the network will usually eliminate this issue. If this cannot be done or does not resolve the ground loop then the floating shield can be used but the grounded end must be connected to a properly bonded ground point. This ground will reduce that interference. Some installers take the extra step of grounding the shield prior to entering the network switch and use a non-shielded patch cable to make the last connection.

If none of the options mentioned are available, consider running fiber optic cable instead of copper. Ground loops do not occur over fiber cable since only light is transmitted. No electrical connection is made with fiber.
 
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