The first half of 20th century witnessed Industrial Revolution, where machines powered by steam, oil or electricity took over the manual work from man, leaving him to supervise the machines. The second half of the century saw chip revolution where microprocessors and computers have relieved the man from monotonous supervisory work. Today 4-20 milliampere Distributed Control Systems (DCS) are being used in continuous process plants like chemicals, refinery, fertilizers, cement, metal, power generation, etc. Here various parameters like temperature, pressure ,flow, level etc. are converted in to milliampere signals with the help of transmitters and are sent to the control room. In control room these signals are analyzed, compared and suitable commands are sent to the field to take corrective actions. The control room has become the.brain of the plant and the instrumentation cables carrying the signals and commands are the nerves ofthe plant.

In modern days security functions are also being taken over by electronics from man. Continuous monitoring of large areas from control room with the help of Closed Circuit TV Cameras are being increasingly employed. Here the data can be stored in computers to check if required at a later date. Fire and Gas detectors are keeping a constant vigil for possible fire or gas leak. They are connected to the control room with the help of cables and any fire or gas leak is immediately reported to the control room for actions before major damage is done. .

Thermocouples and Resistance Temperature Detector (RTD) are routinely being used for Temperature measurements in Industry. Proper cables must be used to connect the above with the Temp. indicators to avoid any error in the measurements.

The signals in the above electronic circuits are in milliamperes or in micro volts and hence susceptible to out side interference. The cables carrying the signals i.e. Instrumentation Cables must be properly designed to screen off the interference over the lengths otherwise it may lead to malfunctioning of the system.

The Instrumentation Cables if properly selected and installed give trouble free service for the entire life of the plant. There is no inside heating like that in the case power cables and hence the aging of plastics is also negligible

In the following chapters we have divided the Instrumentation Cables in to

Electrostatic Interference :

Any voltage source radiates electric field and this field will induce emf in the circuit which is capacitvely coupled to the voltage source. The most effective way to protect the circuit is to place it inside a totally covered shield which is earthed.

There are two methods of providing shield as under :

1) Aluminum Polyester Tape is wrapped around the circuit with 25 % overlap.

2) By braiding Copper wires around the circuit.

To measure the effectiveness ofthe shield the following test is carried out

Fifty feet shielded single pair cable is wrapped around four inch diameter aluminum mandrel. One end ofthe pair is connected to 600 ohm resistor and other end to a voltage measuring device. The mandrel was driven with a voltage of 20 volts peak to peak at a frequency of 1000 hertz. The voltage pick up in the cable with the shield ungrounded and again with shield grounded are recorded The ratio of voltage with ungrouned shield to voltage with grounded shield is called noise rejection ratio. The higher the ratio the better is the shield effectiveness.

For Aluminum polyester tape the noise rejection ratio is more than 6000 while that with copper wire braiding with 85 % coverage is about 100%. Thus Alu-polyester tape which provides 100 % coverage is more effective than Copper braiding in combating Electrostatic Interference.

Electromagnetic Interference

Process plants are loaded with stray magnetic fields. Any time a current passes through a conductor, a magnetic field is produced radially around it. As a result, all power lines, motors, generators radiate magnetic fields in varying strengths.Any time a closed circuit cuts this magnetic field, a current is induced in the circuit to oppose the magnetic field. This current causes an interference which will get superimposed on the signal in the circuit
 

Interference (Noise) in Instrumentation Cables

The most economical way to combat this is to twist the cores in the circuit. Twisting causes the interference to be cancelled as the current induced in two wires are in opposite direction.The other way to combat this interference is to provide screening of low resistance ferro magnetic material.

Common

Different ground potential at different locations in a process plant cause common mode if the shield is grounded at different points as this will give' rise to a current in shield which will affect the signal. To avoid this the shield should be grounded at one point only and that too at the instrument earth. It should be isolated from all other grounds.

For rapid response nowadays grounded thermocouples are used, where the thermocouple junction is in physical contact with thermowell wall in which it is installed. In this case the shield should only be grounded at the thermowell itself. In case of multipair compensating cable for use with grounded thermocouples, each pair should be individually shielded and grounded at corresponding thermowell. Moreover all the shields must be isolated from each other.

Crosstalk

This occurs with AC instrument signals ,especially pulse type signals, when more than one circuit are carried in the same cable. It is the tendency for a signal to be coupled from one pair to another with in the cable, resulting the noise being superimposed on a circuit.One way to combat this is to use pairs with staggered lays in the same cable i.e. the lay lengths of two near by pairs are different.

The best way to eliminate this is to use individually shielded pairs in multipair cable.Moreover these shields must be isolated from each other. The individual pair shield protects the pair from noise picked up from the adjacent pairs ,as well as reduces the noise radiated by the pair it surrounds.

Cable Construction and Raw Materials

Conductor

The metallic part of the cable which conducts the electricity is called conductor. In case of instrumentation cables it is invariably of copper as:

Braiding

Here the shield is provided in the form of wire mesh around the cable circuits. Fine gauged soft wires of plain copper/ tinned copper/stainless steel or galvanized steel wires are woven around the circuit. The finer the mesh better is its effectiveness. Generally min. 80% coverage is specified. Due to its low resistance it provides better screening against Electromagnetic Interference but due to not full coverage, it is less effective against Electrostatic Interference when compared to Alu-polyester shielding.

It is much costly than Alu-polyester shielding and it is generally used in cables which are flexible and are moving. Apart from providing shielding it also provides mechanical protection to inside circuits.We recommend PVC inner sheath below braiding instead of plain polyester tape as it offers better mechanical protection to inside conductor against the damage caused by the wires of the braid due to constant moving.

PVC Inner Sheath

Inner sheath of extruded PVC is provided over the overall shield. It gives round shape to the cable and provides bedding for armoring. Tapped inner sheath is not recommended as it is not water tight and in case of any puncture in the outer sheath, the water reaches right up to shielding and corrodes it. Moreover it acts as an isolator between shield and armour to avoid common mode interference:

Nylon RIP Cord

Some times a nylon thread of sufficient strength along the cable is provided under the inner sheath. It helps in removal of inner sheath with out damaging the insulation at the time of termination or jointing. It is more useful in control and power cables where the inner sheath has a tendency to stick to the inside insulation due to the absence of any separator between them.

Armouring

Armouring is provided over the inner sheath for mechanical protection. Its conductance is not as important in Instrumentation cables as it is in Power cables where it has to carry heavy earth return current in case of short circuit. Generally galvanized mild steel wires of 0.90 mm diameter are used for ­armouring in cables where diameter under armour is 13.0 mm or less. Above that galvanized steel strip of size 4.0 mm x 0.80 mm is used. According to us this is more than sufficient. Steel armouring also provides protection against Electromagnetic Interference due to its magnetic nature.

It also provide protection to inside cable against Rodent attack which is quite common in tropical countries like India