Shielded cables
Shielded cables are also called screened cables. Shield cables are electrical cables made up of an insulating conductor surrounded by a common conductor layer. Shielded cables are thicker and more rigid than shielded cables. They need more attention when working with them. The shield can be made of braided copper (or similar metal), spiral copper tape or any other conductive polymer.
Protected cables are commonly found in industrial settings and installations, where other devices may be exposed to electromagnetic interference (EMI). It is impossible to overestimate the importance of shielded cables.
Applications
- Protected cables are used to protect data transmitted over cables in data centers, industrial settings, offices, computer technology, electrical equipment, or other areas where electrical equipment is widely used, as well as damage caused by common EMI exposure.
- An external source generates EMI, which affects an electrical circuit by electrostatic coupling, conduction or electromagnetic induction.
- Variable currents and voltages can be generated by natural and artificial (man-made) sources, resulting in EMI. Vehicle manufacturing and mobile phones are two prime examples of man-made EMI sources. Electric storms and the sun are natural sources of EMI. EMIs are usually blocked by AM radios, televisions, and cell phones.
Types of shielded cables
There are many types of cable shields available commercially, they are.
- Combination shields
- Foil Shields
- Metallic Braid Shields
- Spiral Shields or Serve Shields
- Tape Shields
Combination shields
Combination shields are formed by two or more shields joined together in the same cable. The most common combinations are the braid shield over the foil shield or the braid shield over the braid shield.
The main factors to consider when using combination shields include:
- Maximum shield effectiveness
- Good physical care
- 100% foil coverage
- Good flexibility
- Low DC resistance
- Good flex life
- Good mechanical strength
- High Cost
Foil Shields
Foil shields are usually made of an aluminium and mylar composite tape. The shape is like foil on a gum wrapper. Foil shields are lightweight, cheap and very easy to use. The foil can be faced inside or out. The drain wire should be in contact with the metal part of the shield.
Foil shields provide 100% shield coverage, usually terminated with a drain wire (grounded). Drain wire provides easy termination. Foil shields, especially multi-pair constructions, can be color coded. The foil offers good flexibility, but limited flex life.
One side of a foil shield is metal, the other side is not conductive. The drain wire must be connected to the aluminium (metallic) side of the shield to provide proper grounding of the shield. The drain wire is usually one size smaller than the shield end conductors. The shield may face the conductors or face the jacket.
Main factors to consider when using foil shields are:
- Lightweight
- Thin
- Good flexibility
- Easy to produce
- Lower cost than braids
- For maximum performance, Braids can be combined with foil.
- No mechanical strength
- Less physical care
- A drain wire is required
Metallic braid shields
Metal braid shields are usually made of bare, tin- or silver-plated copper strings. Physical protection can be provided by steel and other metals. In the same way that a textile braid is woven over a conductor or conductors, they are woven over a conductor or conductors.
The effectiveness of braid shields is proportional to the amount of braid coverage above the wire. 75-85% coverage is standard and 100% coverage is not technically available. The higher the braid coverage, the slower it will produce, the higher the cost, and the lower it will be. Braids provide high mechanical strength, flexibility, and most effective at low frequencies too.
A braid shield is frequently attached to a foil shield to increase shield coverage while maintaining the benefits of a braid shield. Braid shields are specified in all Mill Spec applications.
When using metallic braid shields, there are a few things to keep in mind.
- Great flex life and flexibility
- Increased mechanical strength
- Provides physical protection
- Effective at all frequencies
- Braids can be combined with foil for maximum performance
- 40% to 96% coverage
- It’s hard to end
- Slow and expensive to produce
- More expensive than foil
Spiral or service shields
Spiral or serve shields are usually made of bare, tinned or silver-plated copper strings. Physical protection can be provided by steel and other metals.
The main advantage of a spiral or serve shield over braid shields is that they are more flexible and can be easily terminated. These shields are made by wrapping the shield material in one direction around the conductors. Shielding materials in both directions are wrapped in separate layers without the intervention of wires to form a reverse spiral shield.
Tape shields
Tape shields are made from a variety of materials, the most common being copper, aluminum, and bronze. Tape shields are applied to the cabling and wrapped around the shielded conductors. A drain wire is required for easy termination. Copper and aluminum are usually combined with foil shields with backing such as mylar or polyester. Tape Shields get 100% coverage. The properties are similar to foil shields.