Types of Cable Shielding
A shield is a material that is electrically conductive and surrounds a wire or cable. The shield's primary function is to limit any signal trying to escape from the wire or cable. It also inhibits external signals or interference from flowing into the wire or cable. Braids, armor and corrugated metal can also provide mechanical protection for the cable.
Now that you know what a shield is, we will discuss the various types of interference:
- Electromagnetic interference (EMI): Any electromagnetic disturbance that interrupts or limits the performance of electrical or electronic devices. Examples of electromagnetic interference: Electric Motors or Ballasts in fluorescent light fixtures.
- Electrostatic interference: Any electrostatic interference such as motor spiking.
There are a few basic types of shields used in the wire and cable industry today. The most popular ones are listed below:
Braid shields: This is the oldest form of shielding still used today. Braids can be traced back to the shoelace industry since the machines used to make the laces were later used to make metal braids. A braid shield is a woven pattern made up of small gauge wires. Most braids are made up of either bare copper, tinned copper, silver plated copper, aluminum or steel. Rayon and fiberglass are also used in some instances as strength members but not for shielding, since these materials are non-conductive. There are other types of braiding material, but they are less popular in our industry. Braid shields typically have a nominal coverage of 55% to 95%. Braids are typically more expensive than a tape shield, due to the slow speed of the braider, the percent coverage desired and the material used to construct the braid. They are also more difficult to terminate.
Spiral shields: This is similar to a braid except that it is made up of single strands of wires spirally wrapped around a conductor or cable core. This type of braid is more flexible than a woven braid. It is also easier to terminate than the woven braid. Spiral shields typically have a nominal coverage of 95% to 98%. Spiral shields are most effective in the audio frequency range and are typically found in microphone and audio cables.
Tape shields: These are made from various materials. They normally consist of a laminate of aluminum and polyester layers joined together by adhesive. These types of shields are fragile and provide little in the way of mechanical strength. The adhesive is typically a pressure sensitive solvent release adhesive. Listed below are some of the more popular ones used today:
- Aluminum/Polypropylene (AP)
- Aluminum/Polyester/Aluminum (APA)
- Foil Free Edge
Spiral wrap: This is where a tape is spirally wrapped around the core.
Tri-Shield: This is where the shielding combination is foil/braid/foil.
Quad Shield: This is foil/braid/foil/braid and is typically found in coaxial cables.