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