Why is the More Expensive Shielded Cable Used?
Shielding in multi conductor cables combats the effects of EMI since regular insulation is transparent to electromagnetic energy, to improve connectivity reliability. Comparatively, non-shielded types offer limited protection against environmental, mechanical threats like moisture, scratches and heat.
Harmful Effects of Electromagnetic Interference (EMI)
Electrical noise, either radiated or conducted as electromagnetic interference (EMI), can seriously disrupt the proper operation of other equipment. For example in the case of data paths, the damage can range from higher error rates to complete data loses.
Cables and EMI
Cables, not only absorb EMI but they can be sources of EMI also. As a source of EMI, cables may pass noise to other equipment or function as an antenna radiating the noise.
Cross talk: Sometimes in multi conductor cables, an undesirable transfer of energy can occur between cables, which is another source of interference.
Shields help to solve these problems
A general breakdown of shielded multi conductor cables
- Cables: contain two or more signal-carrying wires, that are encased in a shield
- Shields have several layers
- Inside copper tape
- an in-between layer of polymer/ braid (made of copper or aluminum mostly)
- outer jacket.
- Connectors: used to join electrical terminations and buildout the circuit
How Shielding Works
The shield surrounds the inner signal- or power-carrying cable conductor.
Shields act on EMI in two ways.
- Either they will reflect the energy
- or Secondly, they may redirect the noise to the ground.
In either case the energy bypasses the conductors or is weakened so much so that there is little to interference caused. This is crucial for effective shielding.
There are also different shielding types: Braid, Foil and Screening
- A braid is a woven mesh of bare or tinned copper wires.
- offers good flexibility and is easier to terminate by crimping or soldering
- when the shielding is applied to individual pairs or quads
- It provides 100% coverage of the conductors it surrounds, which is good.
- Multiple Shielding Layers
- Are necessary for very noisy environments – usually foil and braid
- Also important for cross-talk protection between pairs in multi conductor cables. Individual cables inside as well as the overall cable are shielded with foil, braid or combinations of both.
- For total coverage, ensuring that every component in the cabling system is shielded against interference.
Applications of Shielded and Unshielded cables
Shielded conductors have many applications including machine controls, sound reinforcement and in communications.
While most cable types will work for specific applications, it is important to understand where and when cables will require protection from environment electric noise levels.
|Noise Levels||Shielding Recommendation||Typical Locations||Noise Source|
|High||Shielded||Heavy processing factories||Wiring near:
induction heating, transformers, , relay controls power lines and control wire in close proximity, heavy motors,
Electrolytic processes, generators, high antennas
|Medium||Shielded||Manufacturing centers||Wiring near:
motors, control relays, low powered antennas
|Low||Unshielded||Offices, Storage, Labs, light assembly operations
|Wiring far from:
motors <5 hp, power lines, control/ power relays, motors, power lines, inducting heating and arcs
Typically to be used in low-risk controlled environments, for example inside of cable shielding substitutes like metal cabinets.
Shielded multi conductor cables offer addition layers of protecting against EMI, boosting equipment connectivity, circuitry functionality and data security against internal and external interference.