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Part II : Shield cable

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Shield cable : Section for details

Applications of Faraday CagesCommon types of shield cableA third approach combines both foil and braidHow shielding helps reduce EMI problemsTypical shielding configurations

Lightning spectrumShielded cable for zone1 environmentsInterconnecting two LPZ1 and two LPZ2 refer to IEC62305

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Applications of Faraday Cages

This Faraday cage effect causes Faraday cages to act as shields for strong electric fields or other electrical effects. In addition electromagnetic waves consist ofoscillating electric and magnetic fields. Therefore Faraday cages effectively shield electromagnetic waves or electromagnetic radiation as long as the holes in the wire

mesh are significantly smaller than the wavelength of the electromagnetic waves. For this reason Faraday cages are sometimes called Faraday shields.

Applications of Faraday CagesPerhaps the most dramatic application of Faraday cages is lightning safety. If lightning strikes a closed metal aircraft or car, the occupants are safe as long as theyare not in electrical contact with the outside metallic surface. The enclosed metallic car or aircraft acts as a Faraday cage and shields the interior from the strongelectric field of the lightning strike.

AC currents induce changing magnetic fields which in turn induce electric fields. These electric fields can disrupt sensitive electronic devices, so they use Faradaycages to shield crucial electronic components from stray electromagnetic fields. Coaxial cables are surrounded by a conducting shell to provide Faraday shields.

Read more at Suite101: Understanding the Faraday Cage: What Is the Physics of Shields for Electromagnetic Fields and Waves? | Suite101.comhttp://www.suite101.com/content/understanding-the-faraday-cage-a53389#ixzz1RmqAFrOY

Shielding Cable:Coaxial cables are surrounded by a conducting shell to provide Faraday shields

Safety against lightening:

Cars and aircraft act as Faraday cages/ shields to protect people whenthe vehicle is struck by lightening. The cage protects the interior of thevehicle from the strong, electric fields of the lightening.

Microwave:

the microwaves inside the oven are trapped and used forthe purpose of cooking in a microwave where the metalshell of the microwave acts as a Faraday cage.

Protections for electronic goods:Electronic equipment can be shielded and protected from strayelectromagnetic fields by using coaxial cables that contain a conductingshell that acts as a Faraday cage.

The cooking chamber itself is a Faraday cageenclosure which prevents the microwaves fromescaping into the environment. The oven door isusually a glass panel for easy viewing, but has alayer of conductive mesh to maintain the shielding.Because the size of the perforations in the mesh ismuch less than the wavelength of 12 cm, most ofthe microwave radiation cannot pass through thedoor, while visible light (with a much shorterwavelength) can.

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Common types of shield cable

The two most common types of shield are foil and braid; cable can be wrapped in a conductive foil, most often aluminum, or it can bewrapped in a braided mesh of tiny wires. Foil and braid have very different characteristics, which, as we will see, accounts for the factthat many cables have both types of shield.

Braid cannot provide the 100% coverage of foil; when one weaves wire into a braid, inevitably there are holes. When the cable is flexed,these holes open up wider. The larger the holes, the less effective the shield, and the best braids offer only about 95% coverage. Butbraid does have advantages in precisely the areas where foil is lacking: conductivity and connectorization. The mass of a braid is muchhigher than any foil shield, and its conductivity is therefore better; and the sheath of braided wires makes an ideal material to attach aconnector to, whether by soldering or crimping, and will endure flexing at the connection point much better than foil can.

Foil offers the obvious advantage of complete coverage; it is a very easy matter to apply foil to a cable in such a way as to cover every lastbit of the dielectric, and by binding the foil to tape (usually mylar), one can make sure that the foil will endure flexing without significantdamage. But foil fails as a good shield on the two other counts; it offers rather high resistance, so doesn't provide the best path to ground,and it is not easy to reliably attach to connectors, so the connection to ground at each end of the cable is compromised in a foil-only shield.

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A third approach combines both foil and braid

Combination Foil/Braid Shields: A twisted pair cable with a combination foil/braid shield provides maximum shield effectiveness acrossthe frequency spectrum. The combination foil/braid combines the advantages of 100 percent foil coverage with the strength and low DCresistance of a braid.

The two types of shielding most often used are metallic foil and braided wire. Foil shielding uses a thin layer of aluminum or copper backed by

a polyester liner for strength. This technique supplies excellent coverage for the conductors, though foil is rather fragile.Attaching a connectorand grounding can prove difficult.A drain wire, which is an uninsulated wire, is sometimes used to terminate and ground a foil shield.

A braided shield is a mesh woven from copper wire. This does not provide as effective coverage as foil. Copper's conductivity and the greaterbulk of the wire mesh work together to make the braid an effective choice. It is also much easier to attach a connector and terminate toground. This type of cable is more expensive than foil and increases the likelihood of size, weight, and flexibility issues.

In areas of extremely high EMI, a double shielding approach has been used. The shielded cable has an inner foil shield and an outer layer ofbraided copper. This approach takes advantage of the greater coverage of foil and the superior conductivity of the copper braid.

http://www.wisegeek.com/what-is-copper.htmhttp://www.wisegeek.com/what-is-copper.htm

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A third approach combines both foil and braid

Transfer impedance is a function of frequency, so to gauge the relative effectiveness of shields, one has to know what the frequency band inquestion is, but at all frequencies, a precision video cable with a 95% braid and foil out performs quad shield cable. The following table istaken (with permission of the author) from theAudio/Video Cable Installer's Pocket Guide, McGraw-Hill 2002, by Stephen Lampen, anengineer with Belden Wire and Cable. It shows the transfer impedance for various shield configurations on RG-6 type cables at variousfrequencies; the lower the number, the better .

Video Cable Shielding

As can be seen, at baseband video frequencies (applicable to component video, composite video, s-video, and rgb, and represented here by therange of 5, 10 and 50 MHz), the precision video cable with 95% copper braid outperforms quad shielded cable by about 2 to 1.As we move into

the higher RF range (ironically, the very range for which quad-shield is primarily used), the performance gap widens substantially, with theprecision video cable outperforming quad shield at 10 to 1 at 500 MHz.

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How shielding helps reduce EMI problemsStrong electrical and magnetic fields can cause one electrical device to affect another. When this is unintentional, it is known as electromagneticinterference, commonly abbreviated as EMI.

Reflection and absorption are the primary ways in which shielding significantly reducesthe EMI strength on the signal carrying conductors inside a shielded twisted pair cable.Figure 4 is a representation of how the cable can reflect the high frequency EMI suchas from a radio transmitter. The same shield will also absorb some of the energy of theradio transmitter EMI, further reducing the amount of EMI that makes it to the innertwisted pair conductors. When properly installed with shielded connectors to groundedequipment, the shielding redirects a small amount of the electromagnetic energy.These effects of reflection, absorption, and redirection make shielding very effective atreducing problems from high frequency interference.

The best way to protect against EMI from low frequency magnetic fields, such as thosefrom a motor or a large transformer, is to provide sufficient distance between the cableand the source of the interference field. Cable shielding has only limited effectiveness at

preventing interference problems from lower frequency magnetic fields. Instead ofreducing a magnetic field by reflection or absorption, the cable shielding produces amagnetic field in opposition to the interfering lower frequency magnetic field. This hasthe result of reducing the intensity of the changing magnetic field that reaches thetwisted pair conductors. Figure 5 is a representation of how shielding can reduce thestrength of this type of interference from reaching the internal twisted pair conductors.

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Typical shielding configurations

Lightning Strikes create powerful radio waves in the frequency range of 3 KHz (audio, VLF) through 10 MHz (shortwave radio). The VLF (3000 Hz to 30000 Hz) "lightningsignatures" can travel around the world, allowing monitoring of world-wide lightning. The shortwave "lightning signatures can travel half-way around the Earth (the night-time side of the Earth). The best region to listen for distant shortwave lightning signatures is from 2 MHz through 8 MHz.

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Lightning spectrum

Lightning Strikes create powerful radio waves in the frequency range of 3 KHz (audio, VLF) through 10 MHz (shortwave radio). The VLF (3000 Hz to30000 Hz) "lightning signatures" can travel around the world, allowing monitoring of world-wide lightning. The shortwave "lightning signatures cantravel half-way around the Earth (the night-time side of the Earth). The best region to listen for distant shortwave lightning signatures is from 2 MHzthrough 8 MHz.

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Shielded cable for zone1 environments

Variable frequency AC motor drive cable (VFD cable)

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Interconnecting two LPZ1 and two LPZ2 refer to IEC62305

Figure 3a shows two LPZ 1 connected by electrical or signal lines. Special care should be taken,if both LPZ 1 represent separate structures with separate earthing systems, spaced tens orhundreds of meters from each other. In this case a large part of the lightning current can flowalong the connecting lines, which are not protected.

Figure 3b shows, that this problem can be solved using shielded cables or shielded cable ductsto interconnect both LPZ1, provided that the shields are able to carry the partial lightningcurrent. The SPD can be omitted, if the voltage drop along the shield is not too high.

Figure 3c shows two LPZ 2 connected by electrical or signal lines. Because the lines areexposed to the threat level of LPZ 1, SPD at the entry into each LPZ 2 are required.

Figure 3d shows that such interference can be avoided and the SPD can be omitted, if shieldedcables or shielded cable ducts are used to interconnect both LPZ 2.