What is shielded cable?
Shielding is achieved by placing a conductive wrap around the inner conductors of a harness or cable assembly and under the outer protective jacket. It slightly increases the thickness of the cable, adding weight and reducing flexibility, but it protects the signals transmitted through the wires from being corrupted by EMI.
What is EMI?
Electricity and magnetism are intertwined. Move a conductor through a magnetic field, and an electric current will flow in the conductor. Likewise, the flowing current creates a magnetic field. If another conductor is close enough, the magnetic field will cause an electric current to flow in it. If that conductor is already carrying an electric current, the magnetic field can cause it to weaken or strengthen.
When the current is very small, like data signals in the milliamp range, EMI can create the illusion of signals where there are none, or cancel signals where there are. This can cause data to be corrupted and lost. In extreme cases, it can even cause actuators to operate or machines to cycle when they shouldn't.
Any device that draws or generates high, fluctuating currents can cause EMI. This includes motors, welding equipment, induction heaters, electroplating processes, and transformers.
EMI and RFI
The term EMI is sometimes used interchangeably with radio frequency interference (RFI). Both are electrically generated, but RFI refers to higher frequencies that extend to greater distances. EMI is usually generated close to the source where it has its effects, while the source of RFI may be some distance away from where the effects are produced.
Shielded wireHow does it work?
The protective metal sheath in a shielded cable works in two ways. When the cable is exposed to EMI, the grounding allows the currents that build up in the sheath to flow away safely, protecting the data/signal transmission cables inside from the EMI.
Shielded cables should not be grounded at both ends. This will create a risk of ground loops, which would negate the beneficial effects of the sheath.
Another option for shielding is twisted pair. This refers to two insulated conductors twisted around each other. Low voltage differential signaling (LVDS) is often sent over twisted pair. This arrangement cancels out electromagnetic interference between the two conductors, but has limited effectiveness against stronger signal sources.
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