As technology advances, communication and control systems have become more complex and are expected to travel distances farther than ever before. This can lead to an increased chance of electrical interference related failures.

Depending on the application, cables can be adversely affected by electromagnetic interference (EMI), radio frequency interference (RFI), and electrostatic interference (ESI). These interferences, also known as “signal interference” or simply “noise,” can not be blocked by insulation alone, making proper shielding vital for most cables. However, before discussing the properties of different shielding, knowledge of the different types of interference is needed.

Types of Signal Interference

1. Static Noise: This is a distortion caused by an electrical field that has been created by another voltage source and has coupled into the signal-bearing circuit. Static noise can be prevented by shielding the full circuit. Foil shields are the most effective in protecting against this type of noise but must cover both the transmitting and receiving ends of the circuit in order to reduce high levels of interference. The shield must also be grounded to deliver the best results.
2. Magnetic Noise: Magnetic fields radiated by power wiring used in different mechanical applications cause this type of interference. The magnetic fields can create opposing current flows, which disrupts the regular current flow of a wire. Twisting the cable elements is the simplest and most effective way to combat magnetic noise.
3. Common Mode Noise: Currents flowing from different potential grounds at different points within a system cause this type of interference. Receivers with high common-mode rejection ratios can reduce the effects of common-mode noise.
4. Crosstalk: When signals (pulsed DC or standard AC) from one wire are superimposed onto another wire nearby, it is called crosstalk. Individual cable pair shielding coupled with pair twists is the best way to prevent crosstalk, though pair twists alone can reduce this type of interference.

Once the type of noise is identified, appropriate materials and styles of the shield can be chosen.

Common Shield Materials

Shielding refers to the metallic layer surrounding a cable’s conductor, created to limit signal interference between the wire and external fields. Shielding plays an essential role in maximizing the effectiveness of cable systems and is designed to minimize signal leakage and the reception of signals produced by external sources.

Shields are available in a variety of conductive materials:

• Bare copper
• Tinned copper
• Galvanized steel
• Stainless steel
• Aluminum foil
• Fiberglass
• Kevlar

Of these materials, aluminum and copper are the most common. Conductive nylon tapes, plastics, and textiles can also be used to minimize signal interference, but are less effective and less common than most other shielding materials.

Shields are also available in a number of styles that can be chosen to accommodate specific environments, desired wiring characteristics, and other shielding needs.

Shield Types

Foil: This shield consists of aluminum foil laminated to either a polyester or polypropylene film, giving it mechanical strength and extra insulation. Foil provides 100% cable coverage, making it an ideal electrostatic shield. Foil shields are also lightweight, less expensive, and more flexible than braid or spiral shields, but have a shorter flex life and lower mechanical strength. Many times, the foil is used to shield pairs of multi-pair data cables to limit crosstalk.

Braid: Woven copper or aluminum strands form braid shields. The braided structure keeps the shield structurally sound and flexible while maintaining a long flex life. Usually, braid shields cover 80% to 95% of a wire. Braids can not provide 100% coverage. Other disadvantages of braids include bulkiness and weight. They are also more difficult to terminate than other shields because they must be combed out. Braid shields are most commonly used to minimize low-frequency noise.

Spiral: Conductive wire wraps around and up the central cable to form this shield. Benefits of spirals include flexibility, long flex life, and up to 97% coverage. Spirals are most commonly used in audio applications. Unfortunately, they are generally ineffective above the audio frequency range.

Combination: Some cables have both foil and braid shielding, referred to as combination shielding. Combination shielding is used to provide the benefits of both types of the shield, specifically the 100% coverage of the foil shield and the physical strength and low-frequency resistance of the braid.

Other terms used in the discussion of shielding:

Shield Coverage: The percentage of wire that is physically covered by metallic shielding

Shield Effectiveness: The ability of a shield to prevent signal interference

Choosing the correct shield type, material, and amount of coverage is important to maximize the productivity of cable systems. The environment in which the cable will be used, the potential sources of interference around the cable, and the mechanical characteristics that the cable or wire must maintain are all important elements to consider when designing a shield. The appropriate shield will minimize interference and ensure productive signal communication within your cable systems.

Choosing the Proper Shielding for Your Application

• Low-Frequency Applications
  • Braid or spiral wire shields are often used for low-frequency applications (up to about 1MHz). For low-frequencies, the end-to-end electrical resistance of a shield is a crucial factor in its effectiveness. For example, microphone cables are often made with a spiral wire shield, because of its effectiveness at audio frequencies.
• Medium-Frequency Applications

• • Braid shields are frequently used for medium-frequency applications (1 MHz to 100 MHz). Braid shield effectiveness depends on the coverage it provides-- the tightness of the weave. Coverage from a braided shield ranges from 65 to 98 percent. Higher braid coverage results in better shield performance but costs more.

• High-Frequency Applications

• • Combination shields are the best option for high-frequency applications (above 100HMz). By combining a braided shield with a foil shield, any energy leaks that would normally come from a braided shield alone are blocked.

The purpose of the shield is to ground any of the noise a cable has picked up. The cable shielding and its termination must provide a low-impedance path to the ground. A shielded cable that is not grounded allows disruptions that can raise impedance and lower the effectiveness of the cable.