CABLE Types & Selection


Use wise, the
selection of cable can be divided into two categories as follows
1)    
Power
cables
2)    
Signal
cables

POWER
CABLES
The power cables main characteristics
required for selection can be listed as
1)    
High
current carrying capacity
2)    
High
line voltages (24 to 220 V dc and 230 V, 415 V to about voltages in kV for ac.)
3)    
Normally,
low and fixed operating frequency like 50 Hz or 400 Hz. The conductors are
copper and aluminum. Aluminum cables are used in house wiring for saving cost.
However due to low ductility while bending, it develops cracks and hence it is
not a preferred material. Aluminum bus bars are used as conductors inside
panels.
The three main parameters for
selection are
    1)    
Line
voltage
    2)    
Line
current
    3)    
Line
drop from source to equipment
According to Indian standards for
voltage less than 415 V ac, the insulation rating of the cable is 1.1 kV.
Beyond this, the test voltage is 2 *rated voltage + 1.0 kV.
The manufacturer
specifies the maximum current capacity of the cable and this depends upon number
of cores, insulation used, type of cooling (the cables used in air have less
rating than when used underground). This rating is much less than its fusing
current. When the conductor cross-section of the cables is the same, multi
strand cable rating is higher than single core cable. This is due to skin
effect in single strand cable. Normally the cables current are much below this
rating because of the voltage drop consideration across the line and these form
the main consideration in its selection. For example, the drop expected between
the neutral cable used in single-phase supply, and the power ground wire should
not exceed about 3 Volts. While supplying three phase 415 V ac, 50 Hz supply
the cable used is three and half. Half specifies that the ground cable size
used has half the cross section that of phase. The manufacturer specifies the
resistance/ km for the cable. The power cables sometimes are armored for
protecting it from mechanical damage and gives added strength to the wire.
The termination of
these cables is normally on bus bars through lugs crimped on the cable and lug
is fixed on bus bar by nut bolt. The colors of wires are red, yellow, blue and
black (black for neutral). For control power wiring 1, 1.5, 2.5 square mm wire
of 1.1 kV is used. Gray color is the preferred color.

SIGNAL CABLES
            The
main characteristics can be listed as low voltage, low current. The frequency
can be from dc to MHz. Frequency is one major parameter in selection of signal
cable. It is necessary to match the characteristic impedance of the cable with
that of source. The receiving equipment also should have same terminating input
impedance. For low frequency, twisted cables are used. The twisted pair helps
in reducing the magnetic and RF pickup. For high frequency, coaxial cables are
used. The use of these cables reduces capacitance between the two signal
carrying wires. Shielded cables are used for RF signals and for signals with
high output impedance, for example, pH electrode-probes and oscilloscopes.

INHERENT NOISE IN CIRCUITS
            There
are two types of noise. One is external interference and the second is the
inherent noise of the circuit itself. Inherent noise cannot be eliminated
because it is caused by components in the actual circuit, such as resistors
within the circuit. The best that can be done is to minimize the noise in a
specific bandwidth of interest. The inherent noise in the circuit can be
divided into four types of noise commonly encountered as
1)
Popcorn noise                                               3)
Schottky (shot) noise
2)
Flicker (1/f) noise                                           4)
Johnson noise
POPCORN
NOISE
            Popcorn
noise is so called because when played through audio system it sounds like
cooking popcorn. It consists of random step changes of offset voltage that
takes place at random intervals in the 10+ ms timeframe. Such noise results
from high level of contamination and crystal lattice dislocation at the surface
of the silicon chip, which results from poor processing techniques and poor
quality of material. Today because of proper manufacturing techniques this problem
has reduced substantially. 

FLICKER
(1/f) NOISE
            This noise is dominant at low
frequencies. It has a power spectral density that is inversely proportional to
frequency (hence the term 1/f noise). The noise voltage spectral density is
therefore inversely proportional to the square root of the frequency. The noise
spectral density drops at 10 dB/decade with rising frequency. Noise with such a
spectrum is called “pink” noise. Today it is rarely significant above 50 Hz and
in special amplifiers it is below 2 Hz (as in OP-27)

JOHNSON
NOISE
            Thermal excitation of the electrons
in the conductor cause random movement of the charge. In a resistance, this
random current causes a noise voltage, known as Johnson noise, whose amplitude
is given by the formula

En
=
Ö 4k*T*R*B
Where
k = Boltzmann constant
            T = temperature °K
            R = resistance in ohms
            B = bandwidth in Hz
Reduction
in temperature is not a practical solution because at liquid nitrogen
temperature it will reduce by 42%. Reducing the value of the resistors and
bandwidth to a minimum are the real solutions. These are more important when
the input signal value is small. For example, in audio amplifiers.

EXAMPLE
The
bandwidth of an amplifier is 15kHz..The input resistance is 1k ohm. The source
voltage is 1mV and source output impedance is 1k ohm. The temperature is 20
°
C. The Boltzmann constant is 1.38 *10-23 J/K. Calculate the thermal
noise generated by the source. Calculate the input noise power and the input
SNR.

En
=
Ö
4k*T*R*B
                                  T=20 + 273=293°K
      =Ö (4*1.38*10-23*293*1000*15000)
            =0.49mV

SHOT
NOISE
            Current in a conductor consists of a
flow of electrons, each of which has a discreet charge. Current in a
semiconductor consists of a flow of electrons or holes. The statistical
variation in the rate of electron flow results in this noise. This noise is
significant only when the bandwidth is large.

SIGNAL
TO NOISE RATIO (SNR OR S/N)
            Signal to noise ratio is a method
that compares the relative (power) magnitude of the two at the frequency of
interest. It is indicated in decibels.
                                    SNR = 10 *
log10 (S/N) in dB
For
example, the signal input power to an amplifier is 3mW and noise is 30
mW.
The SNR is 20 dB. See the table in EMC notes for acceptable ratio for various
circuits.

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