Rubber Cables

INTRODUCTION:

A cable is a flexible metal or glass wire or group of wires. All cables used in electronics are insulated with materials like plastic or rubber. These cables are widely used in various applications. They are used for trailing and flexible supply leads in either single or multi core versions. They will remain flexible even at sub zero temperatures.  These cables, though expensive have its advantages. Rubber is more durable, with high levels of water and abrasion resistance and hence these factors make cables made of rubber an attractive choice for outdoor applications.

A high voltage cable must  have three components in it

a)A Metallic conductor generally made of copper or aluminium. some times the conductor might be coated with Tin, gold, silver for better durability.
b)An insulation that prevents direct contact unsafe proximity between conductor and other objects
c)A seath or a jacket which is the protective covering from outside influence like weather, water and any other mechanical damages.

Cable technology:

In cable technology both insulation and seath or jacket releated part required good and adequate knowledge in rubber technology for rubberized insulated electric cables.

In selectimg  rubber isolation compound, the selection of polymer and other rubber ingredients are very important because incorrect selection of polymer or elastomer can damage the cable sustem and additionally, contaminaton, moisture and impurities can change the character of insulation and subsequently can cause a lot of energy loss in the system.


                                                           


For cable seath or jacket applications, there are a number of polymers used from NR to PVC,NBR,CR,CSM etc. but on selecting the right polymer, it is most important to know the electrical properties of those polymers.

For example under water or under sea cable applications.EPM,EPDM are most important as seath and during extrusion,flaw less extrudate is also equally important.

A cable is most often might have two or more wires running side by side and bonded, twisted or braided together to form a single assembly but can also refer to a heavy strong rope.

In mechanics, cables,otherwise known as wire ropes are used for lifting, hauling,and towing or conveying force through tension.An optical cable contains one or more optocal fibres ina protective jacket that supports the fibres.

History:

Early telegraph system used the first forms of electrical cabling, transmitting tiny amount of power, Gutta percha insulation used on the first submarine cables,was however unsuitable for building wiring use since it deteriorated rapidly when exposed to air.

The first power distribution system developed by thomas edison in 1882  used copper rods, wrapped in jute and placed in rigid pipes filled with a bituminous compound.

Althought vulcanized rubber had been patented by charles goodyear in 1844,it was not applied to cable insulation until in 1880 s when is was used for lighting circuits.Rubber insulated cable was used for 11,000 volts circuits in 1897 installed for the Niagara falls power project.


The use of electric cable first became an identifiable occupation in the latter half of the 19 th century after commercialization of the electric telegraph,the telephone and electric power distribution.Ropes made of multiple strands of natural fibers such as hemp, sisal,manila and cotton have been used for millennia for hoisting and hauling.

Invention of improved steel making techniques made high quality steel available at lower cost, and therfore wire ropes become common in mining and other industrial applications.By the middle of 19 th century, manufacture of large submarines telegraph cables was done using machines similar to those used for manufacure of mechanical cables.

Designing of power cables:

Designing of electric cable is important with respect to both metallic wire as well as the insulation of the wire and the number of such insulated wire, compiling the cable.Modern power cables come  in a variety of sizes,materials and types each particualrly adapted to its uses. The concept of cable is due to the fact that it contains a number of insulated wire to perform a specific power function.
Large  single insualted conductors are also sometimes called power cables in the industry.

The construction and material are determined by three main factors:

1.Working voltage,determining the thickness of the insulation.
2. Current carrying capacity,determining the cross sectional size of the conductor.
3. Environmental conditions such as temperature, water, chemical or sunlight exposure and mechanical impact determining the form and composition of the outer cable jacket.

Solid wire also called solid-core or single strand wire, consists of one piece of metal wire.Stranded wire is composed of smaller gauge wire bundled or wrapped together to form a larger conductor.Stranded wire is more flexible than solid wire of the same total cross sectional area.Solid wire is cheaper to manufacture than stranded wire and is used where flexibility is major requirement in the application.


At high frequencies, current tends to run along the surfaces of the conductor.This is known as the skin effect.Cables can be securely fastened and organized, such as by using  cable trays, cable ties or cable lacing etc.Continuous flex or flexible cables used in moving applications within cable carriers can be secured using strian relief devices or cable ties.

Any current carrying counductor, including a cable, radiates an electromagnetic field.Likewise, any conductor or cable will pick up energy from existing electromagnetic field around it.These effects are often undesirable in the first case amounting to unwanted transmission of energy which may adversely effect nearby equipment or other parts of the same piece of equipment and in the second case unwanted pickup of noise which may mask the desired signal being carried by the cables or if the cable is carrying power supply or control voltages,pollute them to such an extent as to cause equipment malfunction.

The first solution to these problems is to keep cable lengths in building short,since pickup and transmission are essential proportional to the length of the cable.The second solution is to route cables away from trouble.Beyond this, there are particular cable designs that minimize electromagentic pickup and transmission.

Three of the principal design techniques are shielding, twisted pair geometry and coaxial geometry.

Shielding cables makes use of the electrical principle of the faraday cage.The cable is encased for its entire length in foil or wire mesh .All wires running inside this shielding layer will be to a large extent decoupled from external electric fields,particularly if the shield is connected to a point of constant voltage such as earth.

A twisted pair cable has two wires of a cable twisted around each other.This can be determined by putting one end of a pair of wires in a hand drill and turning while maintaining moderate tension on the line.Where the  interfering signal has a wavelength that is long compared to the pitch of the twisted pair,alternate lengths of wires develop opposing voltages,tending to cancel the effect of the interference.

     Twisted pair cable

Two or more wires may be wrapped concentrically separated by insulation to form coaxial cable.Coaxial design helps to further reduce low frequency magnetic transmission and pickup.In this design the foil or mesh shield has a circular cross section and the inner conductor is exactly at its centre.This causes the voltage induced by a magentic filed between the shield and the core conductor to consists of two nearly equal magnitude which cancel each other.

Cables for direct burial or for exposed installations may also include metal armor in the form of wires spiraled around the cable or a corrugated tape wrapped around it.The armor may be made of steel  or aluminum and although connected to earth ground is not intended to carry current during normal operation.

 Power cables use stranded copper or aluminum conductors, although small power cables may use solid conductors.The cable may include uninsulated conductors used for the circuit neutral of for earth connection.

Some power cables for outdoor overhead use may have no overall sheath. It can only be a overhead solid wire.Other cables may have a plastic or metal sheath enclosing all the conductors. The material for the sheath will be selected for resistance to water, oil, sunlight, chemical vapours.

In general plastic coated metallic conductors are mostly used for household purpose and they are used in low voltage application and generally termed as electric wire. Major plastic polymers used are PE ,PVC. In low voltage application. Electrical parameter are not very critical and are usually limited to volume resistivity and dielectric strength. 

The word cable is mostly based for heavy duty purpose. Elastomer compounds are mostly designed for higher voltages cables.The normal  criterion of performance of an elastomeric high voltage cable insulant is the apparent dielectric strength against time characteristics measured on cable samples.

Electrical cable types based on construction and cable properties can be sorted into the following types.

• Coaxial cable

• Mineral insulated copper clad cable

• Twinax cable

• Flexible cables

• Non metallic sheathed cable

• Metallic sheathed cable

• Multi core cable( consist of more than one wire and is covered by cable jacket).

• Shielded cable

• Single cable

• Twisted pair.

There are also special cable types.In the special purpose applications electrical voltage, environmental and mechanical performance requirements can limit the choice of polymer.For special purpose category there are several requirements like long time ageing resistance covering the maximum operating temperature, dielectric strength, volume resistivity, dielectric loss, corona resistance.

Rubberized cable material should be non polar in nature such  that the electric discharge is minimum and it must be with high volume resistivity such aNR,IIR.etc.

Usually NR,EPDM,EPM, cross linked PE, VMQ,SI etc polymers are used widely. Elastomeric material is chosen for cable world wide because of its better high voltage insulation property, weathering property, good flexibility property and its mechanical property for cut and wear resistance.

cables should be made such a way that it provides trouble free services, superior performance and durability standards.It should have good mechanical properties,ozone and weather resistance property it might have good flame resistance capability(CR, CIIR,CSM with aluminium hydroxide, antimony trioxide) or low smoke and low toxity property(EPDM,EPM with aluminium hydroxide, Antimony trioxide  formulated compound), high temperature use(Si) .

Generally cables are categorized of two types

a) General purpose type

b) Special purpose type.

For general purpose type NR is the usual choice however it can also be blended with SBR,BR and IR elastomers for processing and handling economics. In general purpose rubber sheath can be made for low voltage and also for high voltage heavy duty purpose.

Special purpose cable might have flame resistance property,low smoke generating property, oil resistance property, heat resistant property,weather resistance property and so on.with high voltage cables,corono discharge is a common phenomenon.The normal criterion of performance of elastomer for high voltage cable is the apparent dielectric strength against time. measured on a cable sample.

Degradation of polymer should be minimum for high voltage cables. Corono discharge is the result of the ionization of a gaseous atmosphere and the acceleration of the ions in the electric filed at the surface of the insulant or in voids associate with filler agglomerates.If the gas contains oxygen then once of the products of ionization is ozone,which chemically attacks polymers.

For elastomeric insulation  with high voltage electric field, heat is generated owing to the energy loss in the dielectric.The energy loss is a function of volume resistivity, dielectric. The energy loss is a function of volume resistivity, dielectric constant and dispersion factor.Which one of these parameters is dominant depends on whether the electric field is the source of DC or AC and the frequency of alternating field.

NR has been used successfully over a number of years as high voltage cable insulant but NR also prone to degrade by corona discharge and needs protection on both inner and outer surfaces by a thin  layer of corona resistance compound based on CR halogenated polymers, having  high dielectric constants modify favourably near to the conductor where the stress highest and where discharge attack is most damaging.

With high voltage cables elastomers should not contains any ionic ingredients because even very little ionic material can absorb 2-3% water with out serious effect on electrical properties.However with higher absorption of moisture the cable can be affected with electrical properties such as volume resistivity and dielectric strength.Higher purity of elastomer and fillers are essential for cable insulation material because impurities  can cause problem on the electrical properties of insulation.

Manufacturing Of Rubberized Cables:

A copper cable consists of two  or more copper wires running side by side and bonded.Twisted or braided together to form a  single asssembly. We also know that electrical cables may be made more flexible by stranding the wires.

Manufacturing of rubberized cables can be done either in four stages.

a) drawing of metallic wire

b) Mixing of insulated compound and seath compound.

c) Rubberized coating on metallic wire by extrusion process 

d) Vulcanization of cable.

Alternatively it can also be done in three stages from b to d with the metallic wire in the shop floor.

High quality and perfromance of cables are basically due to use of standard elastomers like CR,EPDM,CSM,EPM,SBR,NR,etc.Several properties of seath compounds are described depending on its uses.But most important part is compound should be processed easily an with out any difficulties in processing in mixing mill or in extrusion.

Larger volume of rubber compound could be mixed in intermix or banbury. However use of two roll mill and kneader are also popular.Internal mixing is preferred because of faster productivity with good dispersion.Insulation of electrical failure due to any unwanted agglomerates and contamination.Both cold feed extruder and hot feed extruders are in use.
Both insulation compound could be extruded in different cross head extrusion and the unvulcanized cables could be coiled in trays or in reels either for batch curing and continuous curing.

Extruder in the industry where the metallic conductor is axially passed through the extruder and the insulatin compound is coated first over which the seath compound coated and passed directly in vulcanizing tube for  continuous vulcanization.

EXTRUSION TECHNOLOGY FOR CABLES:

Extrusion is a process used to create objects of a cross sectional profile.A material is pushed through a die of desired cross section.The advantage of this material over other is its ability to creat very complex cross section. Extrusion may be continuous or semi continuous .The extrusion of electric cables can done hot feed extruder  or by cold feed extruder.Cold extruder with length to diameter ratio of 12:1 and 15;1 have used widely.

Extrusion defects:

Surface cracking;

When the surface of an extrusion splits.this is often caused by the extrusion temperature, friction or speed being too high.It can also happen at lower temperatures if the extruded product temporarily sticks to the die.

PIPE:

A flow pattern that draws the surface oxides and impurities to the center of the product.Such a pattern is often caused by high friction or cooling  of the outer regions of the billet.

INTERNAL CRACKING:

When the center of the extrusion develops cracks or voids.These cracks are attributed to a state of hydro static tensile stress at the center line in the deformation zone in the die.

SURFACE LINES:

When there are lines visible on the surface of the extruded profile.This depends heavily on the quality of the die production and how well the die is maintained as some residues of the material extruded can stick to the die surfaces and produce the embossed lines.

VULCANIZATION:

Cable vulcanization is done by two methods
1.Batch process
2. Continuous process

In batch process or discontinuous process the uncured cable is coiled in trays or reeled in drums.During coiling or winding, talc is sprayed which acts as a anti tack agent.A number of trays  or drums could be put in  a autoclave for cable curing.

In the autoclave a steam pressure is maintained equivalent to 150-160 degree celsius  with a cure cycle between 15- 90 minutes depending on the cable gauges.When a rubber seath is applied over a thermoplastic insulated core, the curing temperature is generally kept below the softening point of the insulant

To provide support during handling and restraint during cure spiral over lapped cotton tape may also be applied as a rubber covering operation after extrusion,Steam during curing causes the tapes to absorb moisture to shrink and thereby consolidates the covering.

The consolidation is also reinforced by the expansion forces within the rubber in the later pat of vulcanization.Other low stretch tapes such as woven glass, fibre etc have been used for the purpose.

For higher consolidation of seath or for better heat transfer.A metal seath could be applied.In many industries high voltage cable vulcanization is only done by lead coverage to have high degree of material consolidation and also it helps to avoid deformation under processing.

After application of cover compound the cable is passed through lead extrude.The drum is then taken to autoclave for vulcanization.After vulcanization the drum is cooled and lead is removed by slitting by a lead stripping machine installed along the length of the cable.

Continuous vulcanization

In continuous vulcanization system a high pressure steam tube is attached to the die face of the extruder head.The cable material, the steam tube length  and the temperature of the steam   (kept between  170- 200 degree Celsius the speed of extruder are so adjusted that at the end of steam tube the cable is cured.At the remote end of the steam tube,steam pressure is balance by water pressure in a cooling tube maintained against a controlled leak at the cable exit gland.

Conventional continuous vulcanization of cable began with different placement of extruder, heating section,cleaning section and winding section like

a. Horizontal continuous vulcanization(CCV)

b. Catenary vulcanization(VCC)

c. Vertical Continuous Vulcanization(VCC)

system but out of all these CCV process is much popular in the industry because it gives higher throughput speeds for a given cables size.The CCV process with respect to continuous extrusion curing and winding the vulcanized cable.

Curing medium however varies between fluidised salt bed and steam.

These days three different medium of vulcanizing systems are used and they are.

1. Salt melts vulcanizing systems without air pressure

2. Salt melts air pressure vulcanizing systems

3. Steam/Nitrogen vulcanizing systems.

In a conventional continuous vulcanization system liquid bath is very popular.It consist a tank containing salts at a temperature of around 230 degree Celsius.
Salt bath is a good choice for a CV method because it requires relatively short length curing units, it as good heat exhange properties and can be cured at the necessary high temperatures.Also salt does not cause surface oxidation and it is easy to clean off with water.

The entire salt bath operation involves at four main steps

1.The rubber is fed through a cold feed vented extruder.

2.The extrudate is conveyed through the salt bath and cured.

3.Cured cable is rinsed and cooled.

4. Cables are then cut and processed according specification.

This process is also known as LCM process or liquid curing method where a heated molten mixture of  53% potassium nitrate 40% sodium nitrate and 7% sodium nitrate.Since this process is on line to the extrusion line the resident time and the mixture temperature is important to establish the correct cure time of cable.

Cable coming out of the bath should be cured correctly with satisfactory hardness results on line.In this system vulcanization could be done with or without air pressure.The disadvantage of the salt bath LCM is that it has been associated with the formation of nitrosamines which are suspected human carcinogens.These chemicals are formed when a nitrogen and an oxygen from a nitrosating compound bind to the amino group nitrogen of the amine compound.

In salt bath melts vulcanizing systems without air pressure or atmosphere vulcanizing furnace will essential contain a fluidized bed of salt  bath tank with larger capacity to accommodate multiple passes of the cable through which followed by washing unit.

In salt melts air pressure vulcanizing systems will essentially contain a fluidized bed of salt bath enclosed in a telescopic section that is actually a tube sliding into another tube which has larger diameter.

Sealing devices are located between the fixed tube sliding one as well as between the sliding tube and extrusion head.A proper device connects the extrusion and the vulcanizing unit through the telescopic section.

Steam nitrogen vulcanizing system also contain telescopic section like salt melt vulcanization system.Telescopic section is actually a tube,sliding into  another tube that has larger diameter.Sliding of the tube can be operated either by pneumatic or by hydraulic cylinders.Here the heating is done by steam under nitrogen gas pressure.table sealing devices are located between the fixed tube and sliding one as well as between the sliding tube and the extrusion head. A proper device connects the extrusion head to vulcanizing unit through the telescopic section.

Lead Presses

Large diameter cables can also be vulcanized in ‘short’ lengths using a lead cover. This method can be a problem for coloured cables coated with rubbers with sulphur content, which can form a lead sulfide stain on the surface of the rubber in contact with the lead sheath. It is necessary to use very large drums to support the cable during vulcanisation in an autoclave, and it may also be necessary to rotate the drum to offset any eccentricity of the cable cover which may occur, due to gravity causing compound movement, owing to the reduction of compound viscosity with temperature rise.

It is used for some electrical cables.

Hot Air/Infrared Tunnels:

Hot air tunnels are often used for vulcanisation of cable covered with rubbers such as silicone.These systems may also incorporate infrared radiation as a means of boosting heat transfer to the product.

Application:

Cable design changes with respect to different application of cables.
In nuclear industry applications the cable may have special requirement ionizing resistance.
Cable materials may also be specified not to large amounts of smoke if buried.Cables inter underground use or direct burial in earth will have plastic or metal, most often lead sheaths or may  special direct buried construction.When cables in where exposed to mechanical impact damage, they protected with flexible steel tape or wire armor which also be covered by a water resistance jacket.in house application insulation made with polymers.
major application of cables include

• Ship wiring
• mining
• railway signaling
• railway track feed
• furnace cables
• Locomotives
• Open pit mining
• Medium Voltage 
• X ray tube supply
• Nuclear Power cable
• Automobiles
• Audiovisuals cable
• Bowden cable for bicycle
• Communication cable
• Computer cable
• Sensing cable.