Industrial applications for instance, factory floor, and presence of electronic equipment or high frequency devices cause nose in current carrying wires or cable assembly. Electrical noise is either radiated or conducted as electromagnetic interference, can seriously disrupt the proper operation of equipment. Insulation protects a cable mechanically from scraps and abrasion and environmentally from moisture or spills. But insulation is transparent to electromagnetic energy and provides no protection. Shielding is needed to nullify the effects of electromagnetic interference (EMI).
Basic way to combat with EMI in cables is through shielding. It surrounds the inner signal or power carrying conductors. The shield can effect electromagnetic interference in two ways. First, it reflects the energy and second, it pick up a noise and sink it to ground. In both cases, EMI does not reach the conductors, although some energy still passes through the shield, but it is highly attenuated and does not cause interference with primary signal. Figure shows how EMI effect is nullify when it reaches the surface of conductor carrying signal.
Cables come different types of shielding and offer variety of shielding effectiveness. The amount of shielding required depends on several factors, i.e. electrical environment in which the cable is used, cost of the cable, why pay for more shielding than need? And some other factors.
Two main types of shields in cable assemblies mainly used in industries are; Foil and Brain Shield.
Foil Shield consists of thin layer aluminum or copper, typically attached to a carrier such as polyester to add strength and ruggedness. The polyester carrier provides strength that allows the foil to be wrapped around the cable without tearing or fracturing. It completely surrounds the cable and protect it while Braid Shield is a woven mesh of plain or tinned copper wires. Braid offers a very low resistance path to ground so any noise which causes interference in grounded dynamically. But braided shields do not provide full coverage to the cable. Braid is more effective as compared to the first one but it adds size and cost to the cable.