A brief introduction on electromagnetic shielding and shielding cables.

Abstract

The development of a sophisticated machine, artificial intelligence, and robotics society is still ongoing. Automation is the way of the future. There is a vast amount of data all around us that is being transferred via wires and wireless, in this age of robotics and humanoids. These data are merely the energy-carrying information that is being transmitted via electromagnetic waves. Since electromagnetic waves are present all around us, we must make sure that our cables are secure in order to protect our targeted data and messages from stray electromagnetic waves. This paper will provide a brief overview of shielding cables' operation, applications, and usage.

Full text

An Introduction to Electromagnetic Shielding
and Shielding Cables
Abhisek Praharaj; abhisekp19@iiserbpr.ac.in
Indian Institute of Science Education and Research, Berhampur, Odisha, India
Abstract
The development of a sophisticated machine, artificial intelligence, and
robotics society is still ongoing. Automation is the way of the future.
There is a vast amount of data all around us that is being transferred
via wires and wireless, in this age of robotics and humanoids. These data
are merely the energy-carrying information that is being transmitted via
electromagnetic waves. Since electromagnetic waves are present all around
us, we must make sure that our cables are secure in order to protect our
targeted data and messages from stray electromagnetic waves. This paper
will provide a brief overview of shielding cables’ operation, applications,
and usage.
1 Introduction
Let us discuss a basic electrical circuit made from a wire, battery, and light bulb.
If we attempt to design a lovely circuit with a switch and turn it on. Then, we
can see that the turn-on time is almost one second or less (instantaneously).
But what do you think caused it to occur so quickly. Do the electrons move so
quickly and supply the light bulb with energy. No, the electron never transfers
from one location to another. It simply oscillates, acting as a conduit for the
movement of energy. Using a pointing vector, we can determine the direction of
the flow of energy, and it is simple to see how energy is transported from one area
to another locally. Due to the oscillating behaviour of electrons, this energy is
being transmitted through the electromagnetic field that is being formed outside
the wire. Therefore, we must shield our cables to shield the data and signals from
electromagnetic radiation, which are typically present all around us. Shielding
of wires was developed as a result.
1

Figure 1: A picture of a circuit
.[5]
Electromagnetic shielding is a technique used in electrical engineering to
lower or block the electromagnetic field (EMF) in a place using barriers formed
of conductive or magnetic materials. It is frequently used on cables to isolate
wires from the environment through which the cable flows and on enclosures
to separate electrical devices from their surroundings (see Shielded cable). RF
shielding is another name for electromagnetic shielding that prevents radio fre-
quency (RF) electromagnetic radiation. The purpose of EMF shielding is to
reduce electromagnetic interference. Radio waves, electromagnetic fields, and
electrostatic fields can all be coupled less when there is shielding. A Faraday
cage is a conductive container that is used to suppress electrostatic fields. The
shielding acts as a Faraday’s’s cage. It reflects the electromagnetic radiation.
This reduces interference coming from outside noise onto the signals and the
signals from radiating out.
2 Shielded Cable and Its Features
The shielded cable shown in figure 2 can act both as a source of EMI and
receiver. As a source they can radiate signals to other cables and even act as
an antenna that radiates noise. As a receiver, it can receive signals from other
cables and generate EMI. The shielding of an electrical cable has a common
conductive layer around conductors. The shield usually covered by an outer
most layer of the cable.
2

Figure 2: Shielded Wire.
[7]
Figure 3: Shielded Wire Scientific Symbol.
[10]
3 Types of Shielding
3.1 Foil Shielding
Foil shielding or metal shielding usually compromised of tape and aluminium.
Such type of shielding is cheap and cost effective and efficient. Because of
such traits, they are so much used in case of business, Offices, and Industrial
applications. The shielding capacity of such shielding can reach up to 100
percentage. The cons of such shielding are less physical protection and Zero
Mechanical Strength.
Figure 4: foil shielding
[10]
3

3.2 Braided Shielding
This type of shielding is the oldest one. A woven pattern of small gauge wires
defines the braided shielding. These braids are usually made up of copper,
aluminium, steel, and tinned copper. They are most effective and useful for low
frequencies. They are not as powerful as foil shielding. The coverage of such
shielding can go up to 95 percentage. Because of their woven pattern they are
hard to terminate and that is why they are commonly used in mill operating
applications. While foil shielding fails at mechanical strength, such types of
shielding can give enhanced mechanical power and strength, ensure long lasting
protection and greater longevity and flexibility of a cable though they are not
cost effective and takes time for manufacturing as compared to foil shielding.
Figure 5: Braided shielding
[1]
3.3 Combination Shielding
This type of shields is formed of 2 or more shield s combined in the same cable.
The most common combination is braided over foil or foil over braided. Combi-
nation Shield will give maximum shield effectiveness, good physical protection,
easy to terminate and 100 percentage foil coverage with good flexibility and
mechanical strength and a little bit costly bit produce.
Figure 6: Combination Shielding
[6]
3.4 Spiral Shielding
This type of shielding is very much like Braided Shielding. The difference is the
wrapping around a conductor or cable’s core, including spirally wound single
strand. Such shielding is much more flexible than braided shielding, having a
coverage of 90 percentage to 99 percentage. Easier to terminate that braided
shields with more control over the EMIs and RMIs.
4

Figure 7: Spiral Shielding
[8]
4 Types of Radiation and EMI
EM Shielding used to block radiation which is basically harmful to electronic de-
vices, environment, and human body. The electro magnetic radiation spectrum
can be divided into different categories such as Ionizing, Nonionizing, Thermal
and Optical etc. Frequencies above 1016 hz are called ionizing frequencies, con-
sisting of alpha, beta, gamma, X, and UV. However, Non-ionizing frequencies
are considered below 1016 hz and relates to microwave, infrared, visible and
radio frequencies.
EMI is further divided into 2 types-
•Narrow-band EMI- It happens typically in radios, TV stations and mobile
phones. It has a discrete frequency. We can tune out the disruption that
will not cause damage to equipment’s. Example- radio, tv or mobile phone
•Broadband EMI- such type of EMI occurs over a broadband spectrum
since it occupies a large part of the electromagnetic spectrum. It can
cause damage to our devices. Example- Electric Power Transmission line,
Noise form fluorescent lights and Auto ignitions.
Figure 8: Electro Magnetic Waves
[4]
5

Figure 9: Electro Magnetic spectrum
[2]
5 Faraday’s Cage
Shielded Cables retain the signal wire or current wire inside of them like a
tubular faraday’s cage. Therefore, we shall examine how Faraday’s cage operates
and how its interior components can be protected from external electric storms.
A Faraday cage or shield is a barrier used to block electromagnetic fields. A
continuous covering of conductive materials can be utilised to make a Faraday
shield in the case of a Faraday cage. Faraday cages are named for the scientist
Michael Faraday who invented them in 1836.
Figure 10: Faraday’s Cage
[9]
5.1 The Working of Faraday’s Cage
An external electrical field induces the distribution of electric charges within a
conducting material, which negates the field’s impact inside the cage. This is
6

how a Faraday cage functions. This phenomenon is frequently used to protect
fragile electronic equipment from outside radio frequency interference when a
device is being tested or aligned (RFI). They are also used to protect people and
equipment from actual electric currents like lightning strikes and electrostatic
discharges since the enclosing cage conducts current around the exterior of the
enclosed space but none through the interior. In essence, when an electromag-
netic field is applied to a faraday cage, the distribution of charges changes and
an opposing electric field that cancels out the electric field is produced.
Figure 11: Effects of Electric Field on a Faraday’s cage which contains a mea-
surement cell
[3]
As previously mentioned, electromagnetic waves also cause current to flow
in the opposite direction to an incoming magnetic field. The so-called eddy
currents create a second magnetic field (blue), which opposes and reroutes the
first magnetic field (red). The cell inside the Faraday cage thus experiences a
weaker net magnetic field.
Figure 12: Effects of Magnetic Field on a Faraday’s cage which contains a
measurement cell
[3]
Such action of Faraday’s’s cage largely depends upon the how fast the charges
rearrange themselves (Charge Relaxation) in contrast to the outer electromag-
netic field so that it will cancel out the external electromagnetic field inside a
conductor. Also, we cannot neglect the fact that attenuation which is related
to skin-depth also happens when the wave collides into the metallic cage.
Where the skin depth defined as the distance below the surface of a conductor
where the current density has diminished to 1/e of its value at the surface.Here
7

is a graphical representation of Skin depth vs. frequency for some materials at
room temperature
Figure 13: Skin-Depth vs Frequency relation
[9]
𝛿=𝑠𝑘 𝑖𝑛𝑑𝑒 𝑝𝑡 ℎ
𝑓=𝑓 𝑟 𝑒𝑞𝑢𝑒 𝑛𝑐𝑦
•Mn–Zn – magnetically soft ferrite
•Al – metallic aluminium
•Cu – metallic copper
•steel 410 – magnetic stainless steel
•Fe–Si – grain-oriented electrical steel
•Fe–Ni – high-permeability Permalloy (80% Ni and 20% Fe)
6 Charge Relaxation: (Zangwill, 2013)
Since free electric charge despises a conducting medium, which dominates quasi-
static behaviour in metals or conductors, the process of charge relaxation elim-
inates the charge from the volume of any system whose current density obeys
Ohm’s law. Or simply the extent to which the charges rearrange themselves in
relation to any outside field.[11]
𝑗𝑓=𝜎𝑒 (1)
8

Now we know that,
𝜖∇𝐸=𝜌𝑓(2)
Substitution these above 2 equations into the continuity equation, we will get,
∇𝑗𝑓+𝜕 𝜌 𝑓
𝜕𝑡
=0 (3)
The result is a partial differential equation for the time evolution of charge
density.
𝜕 𝜌 𝑓
𝜕𝑡 +𝜎
𝜖𝜌𝑓=0 (4)
If the conductivity 𝜎is strictly constant, then the solution for equation 4 will
be
𝜌𝑓(𝑟, 𝑡 )=𝜌𝑓(𝑟, 0)𝑒
−𝑡
𝜏𝐸(5)
where 𝜏𝐸=𝜖
𝜎
This formula depicts that the volume charge disappears from the interior of
a conductor on a time scale set by an electric time constant 𝜏𝐸. Thus we can
clearly state that the greater the conductivity, the faster this process occurs.
Since charge is conserved, the disappearance of bulk charge is accompanied by
the appearance of charge on the surface of the conductor.
7 Screening and shielding: (Zangwill, 2013)
Conductors have the unique ability to shield or screen a sample from an electric
field’s effects. In this way, we attempt to imply that the presence of a conductor
generally reduces or, in the ideal case, removes the electric field at the location
of the sample. By the definition of a conductor, the electric field is zero at every
point in the conductor body. The key to shielding is that 𝐸(𝑟)=0 inside the
cavity.
Let us try to prove this by taking an assumption that the electric field inside
the cavity is not zero. Every line of 𝐸𝑐 𝑎𝑣 𝑖𝑡 𝑦 (r) must begin on positive charge
at some point A on the cavity boundary and end on the negative charge at a
distinct point B on the cavity boundary. Now taking a line integral around the
dashed closed path which begins at A, follows the path of the presumptive field
line inside the cavity to B, then returns to A by a path that lies entirely inside
the body of the conductor. This line integral is zero for any electrostatic field.
Since the field is zero inside the body of the conductor.
9

Figure 14: A perfect conductor with a cavity: (a) The electric field inside an
empty cavity is always zero. If there is source charge outside the conductor, the
outer surface of the conductor develops a surface charge density. (b) The electric
field outside the conductor is not zero when there is source charge inside the
cavity. The conductor develops a surface charge density on the cavity surface
and on the outer surface of the conductor
.[11]
0=∮𝑑𝑙 ·𝐸=
𝐵
∫
𝐴
𝑑𝑙 · | 𝐸𝑐 𝑎𝑣 𝑖𝑡 𝑦 | +
𝐵
∫
𝐴
𝑑𝑙 · | 𝐸𝑐 𝑜𝑛 𝑑𝑢𝑐𝑡 𝑜𝑟 |=
𝐵
∫
𝐴
𝑑𝑙 · | 𝐸𝑐 𝑎𝑣 𝑖𝑡 𝑦 |(6)
From this equation we can say that the 𝐸𝑐𝑎 𝑣𝑖𝑡 𝑦 = 0 because the integrand on
the far-right side of the above equation is positive definite. The fact 𝐸𝑐𝑎 𝑣 𝑖𝑡 𝑦 =0
implies that any object inside the cavity is completely shielded from the elec-
trostatics effects of the exterior point charge q. The field responsible for the
shielding is produced by a surface charge density which develops on the outer
surface of the conductor by electrostatic induction.
In contrast, a conductor does not protect the area outside of itself from
charge introduced into a cavity that has been scooped out. Figure shows how
this happens, showing how the point charge in the cavity produces a surface
charge density on both the cavity’s walls and the conductor’s outside surface.
The latter is important because any Gaussian surface that encloses the conduc-
tor must allow non-zero electric flux to pass through it for Gauss’ law to hold.
The precise location of the point charge inside the cavity has no bearing on the
field outside the conductor, despite what might seem to be the case.
The Faraday cage works flawlessly to block both static and dynamic electric
fields, but not static magnetic fields. As you can see, the compass functions
perfectly inside a Faraday’s cage because there is not a magnetic charge density
that is analogous to an electric charge density that would act like electric charges
to cancel out the magnetic field. However, blocking a time-varying magnetic field
makes more sense than blocking a static magnetic field because time-varying
magnetic fields come with associated variations in electric fields.
10

8 Application Shielded Cables
There are several ways to request insulated cables. The quantity of power your
factory produces each day considers protecting your electrical wires. Addition-
ally, it is based on the kind of factory you have. Electronic interference can be
produced at different levels by various industries and industrial facilities. Some
of them are-
•High level Electrical Noise: e.g., Heavy motors, electrolytic process, con-
trol lines, power lines.
•Medium level of electrical noise: e.g., Average manufacturing plants, op-
eration of heavy control relays.
•Low level of electrical Interference: e.g., This type of disturbances usually
come from the power sources that is a little bit far from the observer. Gen-
erally, Big store areas, offices, Laboratories, small scale operating plants.
Now the important fact we can stop generating such stray EMIs by installing
shielded cables.
9 Conclusion
Thus, shielded wires are very practical and ensure security. With insulated
wires, operating large, powerful electrical motors and equipment is made simple.
Every business must modify and tailor the shielding that is necessary. Ensure
a safer and more sustainable work environment for the entire firm.
10 Bibliography
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11

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