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Understanding Shielded Cable
Four categories of cabling and shielding.
By Chet Socha, Senior Product Engineer, Alpha Wire, Elizabeth, NJ
Industrial areas such as the factory floor are typically electrically noisy environments. Electrical noise, either radiated or conducted as electromagnetic interference (EMI), can seriously disrupt the proper operation of other equipment. Insulation protects a cable mechanically from scrapes and abrasion and environmentally from moisture and spills. But insulation is completely transparent to electromagnetic energy and offers no protection from EMI. That's where shielding is needed.
Cables can be a main source of transfer for EMI, both as a source and receiver. As a source, the cable can either conduct noise to other equipment or act as an antenna radiating noise. As a receiver, the cable can pick up EMI radiated from other sources. A shield works on both ends of the equation.
Components of a SupraShield double-shielded cable.
Consider three typical industrial electrical noise level categories. High noise levels in heavy processing plants such as steel mills and foundries have such noise sources as electrolytic processes, heavy motors, generators, transformers, induction heating, relay controls, power lines, and control wires. Medium-levels in "average" manufacturing plants have such sources as wiring near medium-size motors, and various control relays. Finally, the low noise category includes storage areas, labs, offices and light assembly operations. Here, wiring is typically located far from power lines, motors, motors under 5 hp, there is no induction heating, arcs, control or power relays nearby. It's important to note that switching heavy loads, inductive heaters, and large transformers can all present high levels of both conducted and radiated EMI. Placing signal cables next to power cables can also allow power-line noise to couple onto the signal lines.
The primary way to combat EMI in cables is through the use of shielding. The shield surrounds the inner signal- or power-carrying conductors. The shield can act on EMI in two ways. First, it can reflect the energy. Second, it can pick up the noise and conduct it to ground. In either case, the EMI has a hard time reaching the protected conductors. In either case, some energy still passes through the shield, but it is so highly attenuated that it doesn't cause interference.
Cables come with various degrees of shielding and offer varying degrees of shielding effectiveness. The amount of shielding required depends on several factors, including the electrical environment in which the cable is used, the cost of the cable — why pay for more shielding than you need? — and issues like cable diameter, weight, and flexibility.
An unshielded cable for industrial applications typically is used only in a controlled environment — such as inside a metal cabinet or a conduit — where it is protected from ambient EMI. The metal of the enclosure shields the electronics inside.
Typically, two types of shielding are used for cables: foil and braid. Foil shielding uses a thin layer of aluminum, typically attached to a carrier such as polyester to add strength and ruggedness. It provides 100 percent coverage of the conductors it surrounds, which is good. It is thin, which makes it harder to work with, especially when applying a connector. Usually, rather than attempting to ground the entire shield, the drain wire is used to terminate and ground the shield.
A braid is a woven mesh of bare or tinned copper wires. The braid provides a low-resistance path to ground and is much easier to terminate by crimping or soldering when attaching a connector. Nevertheless, braided shields do not provide 100 percent coverage, allowing small gaps in coverage. Depending on the tightness of the weave, braids typically provide between 70 percent and 95 percent coverage. When the cable is stationary, 70 percent is usually sufficient. In fact, you won't see an increase in shielding effectiveness with higher percentages of coverage. As copper has higher conductivity than aluminum and the braid has more bulk for conducting noise, the braid is more effective as a shield. Still, it adds size and cost to the cable.
A three-conductor shielded cable.
For very noisy environments, multiple shielding layers are often used. The most common is a combination of both a foil and a braid. In multiconductor cables, individual pairs are sometimes shielded with foil to provide crosstalk protection between the pairs, while the overall cable is shielded with foil, braid, or both. Cables can also use two layers of either foil or braid.
A third approach, seen in Alpha Wire's Supra-Shield
, combines both foil and braid shields in protecting the cable. Each supports the other, overcoming the limitations of one with its own compensating strengths. This presents shielding effectiveness superior to either approach used alone. Increasing the performance of the Supra-Shield design is the unique triple laminate aluminum/polyester/aluminum foil tape. This tape increases shielding effectiveness through reduced shield resistance and is in contact with a drain wire to facilitate quick and reliable termination. In practice, the purpose the shield is to conduct to ground any of the noise it has picked up. The importance of this cannot be overstated — and failure to understand the implications can mean ineffective shielding. The cable shielding and its termination must provide a low-impedance path to ground. A shielded cable that is not grounded does not work effectively. Any disruptions in the path can raise the impedance and lower the shielding effectiveness.
Make sure you have a cable with sufficient shielding for the application's needs. In moderately noisy environments, a foil alone may provide adequate protection. In noisier environments, consider braids or foil-braid combinations.
Use a cable that is suited to the application. Cables that experience repeated flexing usually use a spirally wrapped shield rather than a braid. Avoid foil-only shielding on flex cables since continuous flexing can tear the foil.
Make sure the equipment that the cable is connected to is properly grounded. Use an earth ground wherever possible and check the connection between the ground point and the equipment. Eliminating noise depends on a low resistance path to ground.
Most connector designs allow full 360° termination of the shield. Make sure the connector offers shielding effectiveness equal to that of the cable. For example, many common connectors are offered with metal-coated plastic, cast zinc, or aluminum backshells. Avoid both overspecifying and paying for more than you need or underspecifying and getting poor shielding performance.
Ground the cable at one end. This eliminates the potential for noise inducing ground loops.
A shielded system is only as good as its weakest component. A high-quality cable is defeated by a low-quality connector. Similarly, a great connector can't do anything to improve a poor cable.
Contact: Alpha Wire, 711 Lidgerwood Ave., Elizabeth, NJ 07207-0711
800-522-5742 or 908-925-8000 Web:
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