Instrumentation
 
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







Core and Paired Copper conductor cables for PLC and DCS Control
Resistance Temperature Detector (RTD) Cables
Thermo couple Extension and Compensating Cables
Co-axial Cables

Interference (Noise) in Instrumentation Cables

The signals in Instrumentation cables are in milliampere and in microvolts and hence can get easily distorted by out side interference.

The interference can be due to

External source like near by Power lines, big motors, generators etc.
Internal source like voltage or current in the other circuit ofthe multipair cable.

External Interference can further be subdivided in to

Electro Static and
Electro Magnetic While Internal Interference can be subdivided in to
 
a.
Common Mode and
 
b.
Cross talk

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:

It has got better conductivity (Next to Silver)
It is free from oxide film in normal atmosphere.
It is ductile and does not easily break when bent sharply .Itcan be drawn in to fine wires (45swg)
It can be soldered easily

Copper has got affinity for sulphur and hence where sulphur is present it should be protected by tinning. All regular rubber cables have sulphur added in rubber for vulcanization and hence tinned copper conductors are always used in rubber cables. I n bigger sized cables a separator tape is applied on bare copper conductor to prevent it from coming in to contact with sulphur in rubber.

Conductor for high temp. rating Le PTFE and FEP Insulated cables is of silvered copper to protect the copper from oxidation at higher process and operating temp.(About 200 C).In case of compensating cables the conductor material depends upon its type. This is discussed in details in chapterThemocouple Extension Cables & Compensating cables.


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 :

a) It has got better conductivity (Next to Silver)
b) It is free from oxide film in normal atmosphere.
c) It is ductile and does not easily break when bent sharply .It can be drawn in to fine wires (45 swg) d) It can be soldered easily

Copper has got affinity for sulphur and hence where sulphur is present it should be protected by tinning. All regular rubber cables have sulphur added in rubber for vulcanization and hence tinned copper conductors are always used in rubber cables. I n bigger sized cables a separator tape is applied on bare copper conductor to prevent it from coming in to contact with sulphur in rubber.

Conductor for high temp. rating i.e PTFE and FEP Insulated cables is of silvered copper to protect the copper from oxidation at higher process and operating temp.(About 200 C).In case of compensating cables the conductor material depends upon its type. This is discussed in details in chapter Themocouple Extension Cables & Compensating cables.


Conductor Construction

SOLID The conductor is made of one solid wire Le for 1.50 sq.mm conductor there will be one Wire of 1.38 mm diameter and it is denoted as 1/1.38 mm

1.5 SQ. MM SOLID
STRANDED b)The conductor is made of seven wires (strands). Here six strands are laid around one central strand. For 1.50 sq.mm conductor there will be 7 strands of 0.53 mm.dia.each and it is denoted as 7/0.53 mm

1.5 SQ. MM STRANDED
BUNCHED The conductor is made of several wires twisted together in one direction Lefor 1.50 sq.mmtherewill be 30 wires of 0.25 mm dia. each and it is denoted as 30/0.25 mm.

1.5 SQ. MM BUNCHED FLEXIBLE


Cable Construction and Raw Materials

The relative merits ofthe above constructions are as under:

CONSTRUCTION BENDING CAPACITY CRIMPING OF LUGS SUITABLE FOR TERMINAL COST FACTOR
SOLID POOR NOT RECOMMENDED SCREW ECONOMICAL
STRANDED GOOD RECOMMENDED ALL TYPES COSTLIER
BUNCHED BEST RECOMMENDED ALL TYPES COSTLIEST

Insulation

The coating over the conductor, which insulates it from other conductor and earth is called Insulation and it must have

Good electrical properties
Better heat stability
Should not be affected by normal environmental conditions
lexibility
Reasonable price

Considering the above following Thermoplastic and Thermosetting materials have found wide application as insulating material in cables.

Thermoplastic Material

These are the plastics which when heated their linear molecule chains drift apart and they become soft and regain their original form on cooling. PVC, POLYETHYLENE, NYLON are thermoplastics and can not be used at higher temperatures. They can be recycled number of times.These are plastics and rubbers when initially heated, their linear molecule chains get interlinked and this is not reversible. Due to this they can be operated at higher temperature. VULCANIZED RUBBER, CROSSLINK POLYETHYLENE are thermosetting material and they can not be recycled.

Polyvinyl Ccloride (PVC) Compound


This is the most widely used Plastic due to its versatility. Unlike other plastics this can not be used alone. It is required to mix with other ingredients like plasticisers, fillers, stabilizers, lubricants etc. to make it workable. By varying the ingredients and their proportions in the compound the end properties can be varied to a great extent to meet the specific requirements.

The normal PVC insulated cables are suitable for continuous 70°C operation. By changing the plastisier and stabilizer we can have H R PVC which is suitable for continuous 8SoC operation.Due to higher dielectric constant and hence higher mutual capacitance it is not suitable for operation above audio frequency Le. 20 kilohertz.

Low Density Polyethylene (LOPE)

This has got excellent electrical properties but is suitable for continuous 60°C operation and hence it is not used in Power Cables. Due to its Ipw dielectric constant it is widely used for telephone and radio frequency cables.

Cross Linked Polyethylene (XLPE)

As discussed earlier this modified Polyethylene can with stand higher temperature (8SoC) due to interlinking of ethylene chains and hence it is widely used for L T and HT power cables

Florinated Ethylene Propylene (FEP) And Polytetraflouroethylene (PTFE)


These are other plastics, which are used as insulating material for higher temp. application (2000C).They have not found wide application due to their prohibitive cost.

Cabling Elements

CORE
PAIR
TRIAD
QUAD
Insulated Conductor is known asCore Two cores twisted to form a Pair Three cores twisted to form a Triad Four cores twisted to form a Quad

Individual Shielding Of Elements

In case where individual shielding is required, the above elements are wrapped first with Plain Polyester, then the drain wire is laid parallel, over it Aluminum Polyester tape is applied with Aluminum side touching the drain wire. Above this a plain polyester tape is applied for shield isolation from other shields


Shielded Pair
Shielded Triad
    
Shielded Quad

Cabling Elements


Laying Up Of Elements

The required number of cabling elements (Shielded or Unshielded) or cores are stranded (Laid up) concentrically in layers to form the laid up cables .Cables up to eight elements will have one layer. For cable having more that eight elements there will be two or more layers and they are laid in opposite direction.

Overall Shielding (SCREENING)

As discussed earlier shielding or screening is provided over the laid up cable elements in order to protect the signal inside from out side interferences. There are two method of shielding as under:

Alu-Polyester Shielding

Here the shielding is provided by wrapping of Aluminum foil bonded with Polyester film .Polyester film is also known as Mylarwhich is a trade name and hence some times it is calledAlu-mylar shielding.It is observ:d: that in case of Electrostatic Interference the coverage is more important thanconductance of the shield and hence thin Aluminium foil ( B.S. 5308 specifies a minimum thickness of 0.008 mm i.e. 8 Miaron thickness for Aluminium foil) is used for shielding for economic reasons. This foil has poor tensile strength and it breaks while wrapping. To provide mechanical strength Polyester film is laminated to Aluminum foil. The Polyester film apart from providing strength also acts as additional insulation for shield isolation.

We generally use.N.uminum Polyester tape having 0.015 mm (15 Micron)Aluminum foil laminated to 0.012 mm (12 Micron) Polyester film for shielding. Some of the clients insist on 50 to 75 Micron Aluminum foil which according to us does not offer any additional advantage but adds to the cost. Moreover thicker tape does not fit tightly on small cables and it may cut the insulation when the cable is bent due to its sharp edges.

A tinned copper drain wire is provided in continuous contact with the Aluminum side of the tape all along the length of the cable. As the name suggests it drains off electrostatic interference. It helps to connect the Alu-polyester tape to earth at the termination. More over it takes care of any breakage in the tape in providing continuous earth around the circuit. The most common size of the drain wire is 0.50 sq. mm (20 AWG). Multi-wire construction is preferred as it does not protrude above the tape and chances of its breakage are less.

Generally a plain polyester tape of thickness 0.025 mm (25 Micron) is provided over the laid up cable elements, over which the drain wire runs and above that Alu-Polyester tape is wrapped. The pain polyester tape keeps the drain wire always in contact with theAluminum foil.

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

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