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Electromagnetic interference (EMI) is prevalent throughout the factory floor. This is why data and signal cables are usually protected with insulated conductors and wrapped with a conductive layer. Shielding reduces electrical noise and reduces its impact on signals and also lowers electromagnetic radiation. Shielding prevents crosstalk between cables near each other. Shielding not only protects cable but it can also protect machinery and people as well.
Power cables are constructed to be electromagnetic compatible (EMC) to minimize noise generation, which affects many other systems like radio and data communication.
Communication cables are shielded to prevent the effects on the data transmitted from EMI. To further prevent cross talk and coupling, communication cables are also paired and individually shielded.
In some applications, such as those needing servo cables, double or even triple shielding is required: around individual conductors, around twisted pairs, and around the entire cable.
Some applications do not require shielded cables. For example, if a cable will be used in a cabinet or otherwise away from other sources of noise, it does not need to be shielded, as it will be protected from noise and EMI already.
Cable shielding uses either a braided, spiral design or metal-coated Mylar or foil shield. The shielding wraps around each conductor to mitigate noise by 85% to 100%, depending on the configuration. The maximum shielding a braided shield can have is 90%. Spiral shields can offer 98%, while metal-coated Mylar can deflect 100% of EMI.
http://www.avanzados.net/shielding-material/
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