Understanding Shielded Cable

Industrial applications 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 scraps and abrasion and environmentally from moisture and spills. But insulation is transparent to electromagnetic energy and offers no protection. Shielding is needed to combat the effects of EMI.

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.

There are two types of shielding typically used for cables: foil and braid.Foil shielding used a thin layer of aluminum, typically attached to a carrier such as polyester to add strength and ruggedness. It provides 100% 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 termination by crimping or soldering when attaching a connector. But braided shields do not provide 100% coverage. They allow small gaps in coverage. Depending on the tightness of the weave, braids typically provide between 70% and 95% coverage. When the cable is stationary, 70% is usually sufficient. In fact, you won’t see an increase in shielding effectiveness with higher percentages of coverage. Because copper has higher conductivity than aluminum and the braid has more bulk for conducting noise, the braid is more effective as a shield. But it adds size and cost to the cable.For very noisy environments, multiple shielding layers are often used. Most common is using both a foil and a braid. In multi conductor cables, individual pairs are sometimes shielded with foil to provide cross talk protection between the pairs, while the overall cable is shielded with foil, braid, or both. Cables also use two layers of foil or braid.

A third approach, 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 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.

Practical Guidelines for Effective Shielding

1. 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.

2. Use a cable 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.

3. Make sure the equipment that the cable is connected 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.

4. 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.

5. 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.