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Instrumentation

Co - Axial Cables:

These cables are used for carrying Radio and Video signals. Here the outer conductor (RebConductor) is provided in form of metai braid over theinsulationover the center conductor. T,construction is known as Co-axial cable as both the conductors have a same axis. The differEconstructions are generally known by RG numbers and are as per American Military Standards .RG 6,RGSg, etc. Any revisions there in are noted by A, B etc. i.e RGsg B/U means second revisionoriginal construction of RG 59 For co-axial cables the terms Characteristic Impedance and Attenuation are very important.Characteristic Impedance: It is the ratio of voltage to current at every point of a matched cable. n term matching conditionmeans a cable is connected across a resistance at one end that is numerical,identical with the characteristic impedance. Maximum energy transmission is possible ilthe generatoreceiver and cable's characteristic impedances are identicalFor high frequencies z= 103~UC Where is the characteristic impedance in OHMis mutual capacitance in nf/km inductance ofthe circuit in mH/km z C L

Attenuation

The attenuation Is defined as the reduction in signal amplitude OVer the length of the cable. It rises with increase in frequency

Attenuation = 8.686 Real part of ~(R+jWL)(G+jwC)dB/Km

Where

R is the resistance of the conductor in ohm/km
C is the mutual capacitance in nf/km
L is the inductance ofthe circuit in mH/km
G is the volume resistivity of the insulation

Constration

Conductor

The conductor used for these cables are invariabiy of copper. Stranded constructionwhich were used earlier are not used nowadays as it was observed that there was sudden dip in thegraph of frequency versus attenuation in case of stranded conductor.

At high frequency the current does not flow in the center of the conductor due to skin effect. Toeconomise in cost, copper coated steel wires and copper clad aluminum wires are sometimes used. Incase of high power co-axial cables hollow copper tube are used as conductor.

Co - axial Cables

Insulation

As the frequency of signals are in Megahertz the mutual capacitance between two conductors shouldbe minimum to reduce the loss of signal .Polyethylene with dielectric constant of 2.3 is the most ideallysuited for insulation for these cables. Air has got the minimum dielectric constant of 1.0 and efforts are made to approach it by making the Polyethylene porous. Earlier chemical foaming was used, wherethe chemicals added in Polyethylene evaporated out during coating due to high temp. leaving theinsulation porous. To day Nitrogen is injected in molten Polyethylene to form foam Polyethylene which has got dielectric constant of 1.70.

In case of Foam Polyethylene insulation, first a thin coating of solid Polyethylene is applied on the solid conductor (Skin coating) for proper bonding

Shielding

Aluminium foil laminated to low melting Polyethylene film(Poly-AI Tape) is wrapped around theinsulated core with Aluminum side on top as shielding. During the final sheathing, the polyethylene side of the tape melts and sticks to the insulation and thus seals the porous insulation. This prevents the moisture entering the voids in the insulation and degrading its performance Above this Aluminium alloy wire braiding with about 60 % coverage is applied. The Aluminium wires withAluminium foil under them form the return conductor

Sometimes Petroleum Jelly is applied to the wires as added protection against moisture entering the cable.

Outher

The outer sheath mainly of PVC compound is applied overtheAluminium wire braiding.In case added mechanical protection is required steel wire armouring is provided over the PVC sheath and again PVC sheath is provided over the armour.

The adjoining tables gives construction and other technical parameters of RG 6 and RG 11 cables which are extensively used in closed circuit TV and cable TV network.

	Construction	Cable type
	Perameters	RG59	RG6	RG11
•	Center Conductor	Solid bare Copper	Solid bare Copper	Solid bare Copper
•	Nominal Diameter	0.81 mm	1.02 mm	1.63 mm
•	of Conductor
•	Dielectric (Insulation)	Foam PE	Foam PE	Foam PE
•	Diameter over Insulation	3.70 mm	4.60 mm	7.10mm
•	Outer Conductor
•	1st Shield	AI-foil	AI-foil	AI-foil
•	2nd Shield	AI-alloy braidingmin 60%coverage	AI-alloy braidingmin 60 %coverage	AI-alloy braidingmin 60 %coverage
•	PVC Sheath thickness	0.90 mm	0.90 mm	0.90 mm
•	Overall Diameter	6.30 mm	7.30 mm	10.30 mm
•	Electrical Parameters
•	max. Resistance of center	3.50	2.14	0.85
•	in Ohm/100 mtr at 20°C
•	Nom. Capacitance PF/mtr	53	53	53
•	Characteristic Impedance (Ohm)	75	75	75
•	Nom. Velocity Ratio	85	85	85
•	Attenuation @ 20 C DB/100 mtr at
	5 Mhz	4.02	1.95	1.25
	55 Mhz	6.80	5.20	3.15
	211 Mhz	13.00	9.50	6.23
	250 Mhz	14.70	10.50	6.72
	300 Mhz	15.60	11.50	7.38
	350 Mhz	16.40	12.45	7.94
	400 Mhz	18.00	13.30	8.53
	450 Mhz	19.20	14.35	9.02
	550 Mhz	21.30	15.70	9.97
	600 Mhz	22.05	16.45	10.43
	750 Mhz	25.15	18.35	11.97
	870 Mhz	27.20	19.95	13.05
	1000 Mhz (1 Ghz)	47.00	21.45	14.27
Note: RG59 has become obsolete and has been replaced by RG6

Handling and Laying of Cables

The cables are costly capital items and they are arteries, veins and nerves of any process plant. Anyblockade in the same may paralyse the complete plant. Enough care should be taken in handling and installing the cables at site. If laid properly the cables will give trouble free services throughout the life of the plant.

In India Cables are packed in drums made of untreated jungle wood and are prone to termite attack.The drums must be protected from moisture and direct sunlight.At our factory we load the drums in vehicles using mechanical devices. At site the drums should beunloaded with the help of a crane of a fork lift. If it is not possible then roll the drums on a ramp having the height same as that of vehicle. Never drop the drums on ground as it is usually done. This may break the drums. Moreover the drums will become unbalanced due to impact. The unwinding of cables from unbalanced drums is very difficult. Sometimes the nails ofthe drum may also come out damaging the cable. Keep the rolling of the drums to minimum.

On receipt of the cables at site, they should be checked for continuity and should be meggared. Themeggar values decrease with increase in length, temperature and size ofthe cable. Before meggaring,both the ends of the cable should be opened out and cleaned to remove the dust and metal particles.The cables have a higher capacitance, hence a higher charging current will flow initially showing a lower meggar value which will increase subsequently when the cable is fully charged. After the meggaring the cable must be discharged by shorting the ends with an insulated wire otherwise the person handling the cable may get an electric shock, when he comes in contact with the conductors.

The minimum meggar value for PVC insulated cable is around 5 megaohms while that for Polyethylenand XLPE insulated cable is 200 megaohmsFor unreeling the cable, the drum shall be mounted on jacks or on cable wheels. Please ensure that the contractor employed for cable laying have requisite equipment like jack, cable rollers, etc. and haspast experience of cable laying. It is observed that sometimes due to non availability of jacks at site, people turn the cable drums on flange side and then unwind the cable. This gives one twist to the cable per one turn unwound and this twisting will result into caging of armour. Due to this the armour will come out damaging the outersheath and forming a bulge.

Sharp bends should be avoided while laying the cables. We recommend a minimum bending radius of 12 X D where D is the overall diameter of the cable. In case of screened (shielded) cables please ensure that all shields are grounded at one point only and that to at the instrument earth. In case of grounded thermocouples, the shields must be grounded only at the respective thermowell and the shield must be isolated from the other shields.

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