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Thursday, 9 February 2012

CABLE Types & Selection

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

            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.

            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 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. 

            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)

            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.

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)

            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 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 30mW. The SNR is 20 dB. See the table in EMC notes for acceptable ratio for various circuits.

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