CLASSIFICATION OF CABLES

CLASSIFICATION OF CABLES

Cables are usually classified according to the voltage for which they are manufactured.

According to the voltage they can be classified as:-

1. L.T. [Low Tension cables] up to 1000V

2. H.T. [High Tension cables] up to 11KV

3. S.T. [Super Tension cables] from 22KV to 33KV

4. Extra High Tension cables from 33KV to 66KV

5. Oil filled under pressure & gas pressure cables for 66KV to 132KV.

Cable Conductors:-

The conductor of cables is usually stranded. I.e. it consists of a number of strands of wires of circular cross section so that it may become flexible and carry more current.

In the stranded conductor each wire lies on a helix the pitch of which is so adjusted that the cross section of the cable at right angles to its axis if practically circular.

To avoid the bending and deformation of the cable conductors under normal conditions the alternate layers have right and left spirals.

TYPES CABLES

There are many varieties of cables are available. Amongst them the common ones are

1. Paper Insulated cables

2. Rubber Insulated cables [Flexible cables]

a) Natural rubber

b) Butyl rubber

c) Ethylene propylene Rubber [EPR]

d) Silicon rubber

3. Plastic base insulated cables.

a) Polyvinyal chloride [PV ]

b) Polyethlene [PE]

c) Cross linked polyethlene [XLPE]

In Thermal Power Station paper insulated cables are normally NOT in use.

Rubber cables or Flexible cables mainly butye or EPR cables are used.

For low voltage & control application PVC cables are used.

For 6.6KV systems XLPE cables are used.

MAJOR TECHNICAL PARAMETERS OF MOST COMMONLY USED CABLES

PROPERTIESXLPEPVCEPR

Max. operating temperature 0C907090

Max. short circuit temperature 0C250160250

Tensile strength (Kg./Mm2)1.91.2 to 1.50.95

Elongation at break (%)300-500200-400300-800

Dielectric strength40-5020-3525-40

Heat deformation at 15CGOODPOOREXCELLENT

Application (in voltage)KV36KV1.1KV11KV

Cost1.2XX2X

Some times HR PVC is also used for insulation for 1.1KV Power & control cables

Maximum operating temperature of HR PVC is 850C.

FRLS CABLES.

Flame Retardant Low Smoke cables are FRLSPVC (or) FRLSXLPE

Conductor insulation is same as PVC & XLPE cables. But PVC material used for outer & inner sheath is modified by switable additives that they retard Flame and restrict smoke and acid gas.

Flame retardant is measured by Oxygen index and temperature index of the material.

OXYGEN INDEX:-

The part of oxygen needed in ambient atmosphere that will support combustion.

The material which ignite in air have O.I less than 21

Higher the O.I, better the flame resistance.

Temperature Index:- At what ambient temperature the material will

ignite in air. L.e. at O.I of 21.

Higher the Temperature Index better the flame retardancy.

Presently the oxygen Index for FRLs cable is 29/30

The temperature Index is around 250oC to 275oC

Low smoke is achieved by reducing HCL gas generation

FRLS cables are designed to generate HCL gas less than 15-20% by weight.

Less acid gas generation results in Low Smoke generation

FRLS cables normally generate smoke density less than 60%

Due to smoke density is 60% light transmitance is minimum 40%

PVC is inherent flame retardant enhanced by mixing certain additives special additives help in charformation reducing heat transfer & blocking additional fuel to flame.

On PVC Compound O.I, T.I, smoke density measurement tests are done

On finished material Flammability test is done

FIRE SURVIVAL CABLES

Elastomer or EPR insulated cables are manufactured to provide Fire survival properties even in case of fire for some hours.

In our country presently we have cables that can with stand fire for 3 hours duration at a temperature of 750oC with out effecting the normal performance of the cables.

The insulation, inner sheath and outer sheath of fire survival cables are elastomer (EPR) The outer sheath will have certain additives which will give low smoke, low hologen and flame retardant properties.

Fire survival cables are used in vital applications such as TRIPPING CIRCUITS, D.C CIRCUITS (DC System) & AC emergency circuits.

For all other areas FRLS cables are used.

COMPARASION BETWEEN FRLS AND Fire Survival Cables (FSC):-

Technical featuresFRLSFIRE SURVIVAL

InsulationPVC/HRPVC/XLPEEPR

Conductor operating temperature0C70/85/9090

Conductor short ckt temperatureoC160/160/250250

Inner sheathPVC/HRPVCEPR

Outer sheathWith or without FRLS Properties PVC/HRPVC with FRLS ProprtiesEPR wutg Fs properties

Oxygen Index29 3032

Temperature Index250-275oC300-350oC

Acid gas generattion by wt.Less than 20%Less than 2%

Smoke density60%20%

Transmittance40%80%

CABLES

Definitions:

a. Conductor: A manufactured product intended for transmitting electric energy is called a conductor, regardless of its construction (wire, cable, busbar, etc.)

b. Wire: is a bare of insulated conductor consisting of one or several small strands used for transmission & distribution of electric power.

c. Bare unprotected wire has no insulating or protecting sheathings.

d. Insulated wire has current conducting cores covered with an insulating sheathing (rubber, PVC, etc.)

e. Unprotected Insulated wire is a wire whose insulation is not protected with special sheathings from mechanical damage.

f. Protected Insulated Wire is a wire enclosed in a metal or any other mechanically strong sheathing.

g. Core is one or several twisted strands of wire used as a conductor of electric current.

h. Multicore or Multiconductor wire is a wire with several cores insulated from one another and enclosed in a common sheathing.

i. Cord is a wire consisting of two or more twisted insulated cores having considerable flexibility or a wire consisting of several such cores enclosed in a common flexible sheathing.

j. Cable is one or several single or multistrand insulated wires enclosed in a continuous protective sheath of metal, rubber or PVC.

POWER AND CONTROL CABLES

In power stations and substations cables are used for providing all the power lighting, protective, control and indication connections. The power and lighting circuits are installed with power cables while all the protective, control and indication connections are made with multicore control cables.

Power cables are manufactured with one, two, three and four cores of size 1.5 mm2. Control cables are of relatively small size 1.5mm2 to 10mm2 and a cable may consist of a large number of these cores.

CABLE CONSTRUCTION

Although the construction of a cable may differ in minor details to meet special service conditions, the basic make up consists of

a. A conductor to carry current

b. Insulation surrounding the conductor to withstand the voltage

c. A protecting sheath to guard against chemical corrosion and mechanical damage.

CONDUCTOR

Usually the conductor is composed of copper of purity of the order of 99, 95% as small quantities of impurity reduce considerably the electric conductance of copper (e.g.) 1% nickel with small amount of phosphorous and sulfur will reduce the conductivity by 35%. Aluminum is now used as an alternative to copper for economic reasons.

Annealed Copper:

The wires are annealed in electricity heated, thermostatically controlled furnaces in an oxygen free atmosphere to restore the electrical conductivity and the physical properties lost in the wire drawing operations.

Insulation:

At present, insulation materials for most cables are limited to

a. Natural or synthetic rubber (becoming obsolete)

b. Oil impregnated paper (For 11KV & above power cables)

c. PVC (For control cables, LT power cables, HT power cables upto 6 KV)

d. XLPE (Cross-linked polyethylene) (New comer)

Sheathing and Overall protection:

The insulated cores are laid up (spiraled to form the cable and then covered with a common layer of insulation. To protect the sheath from mechanical injury and attack by gases, acids, the cables are armoured.

CONSTRUCTIONAL FEATURES OF PVC INSULATED HEAVY DUTY CABLES AS PER IS : 1554 (PART I) 1964

1. Conductor: Annealed copper or Electrical conductor (EC) grade AC. Generally all power cables have AC conductor control cables have Al or Cu conductor.

2. Insulation is a suitably compounded PVC applied to the conductor by the extrusion process.

3. Core identification : Colour scheme

a. Single core

: Red, Yellow, Blue or Black

b. 2 core

: Red and Black

c. 3 core

: Red, Yellow and Blue

d. 3 core

: Red, Yellow, Blue and Black ( Core)

e. 4 core

: Red, Yellow, Blue and Black

f. 5 core

: Red, Yellow, Blue, Black and Light grey

g. More than 5 core: To adjacent cores (Counting and direction

core) in each layer are coloured Blue and

Yellow and remaining are Light Grey.

4. Laying-up: All multicore cables are laid up as per colour scheme indicated above with thermoplastic fillers in interstices wherever applicable to make the cable circular.

5. Inner sheath: The laid-up cores are surrounded by and thinner sheath of any of the following:

a. Extruded unvulcanised rubber

b. Extruded PVC

c. Wrapping of PVC tapes.

6. Armouring : The armouring is applied over the inner sheath. When the dia of inner sheath is not more than 13mm, the armour consists of galvanized steel wire. In other cases, it can be either galvonised round steel wire or steel strip. For single core cable used in Ac system, the armour should be non-magnetic material (H grade Al); if used in D.C. system the armour can be of steel.

7. Outer sheath: is a heavy duty PVC compound by extrusion method, applied over armour in case of around cable and over inner sheath in case of unarmound cables.

Manufacturing Range:

a. Single core cables 1.5mm2 to 1000mm2.

b. Multicore cable 1.5mm2 to 500mm2.

c. Control cable (1.5mm2 to 2.5mm2)2 to 61 cores.

CONSTRUCTIONAL FEATURES OF PILC (PAPER INSULATED) CABLES

a. Conductor : Cu or Al

b. Insulation: Paper tapes of 5 mils. Thick made from wood pulp or manila or both, are lapped one by one on to the conductor until the required thickness of insulation is obtained. The paper insulated conductor is dried at high temperature and vacuum to remove air and moisture and then impregnated with refined mineral oil to improve electrical strength and stability of insulation.

c. Sheathing and overall protection: The paper insulated cores are twisted together to from one cable, the interstices are filled with paper, called fillers, to make a compact cylindrical structure. Paper tapes are laped over this for required thickness of insulation. Paper being of a frigroscopic nature is protected from the ingress of moisture by extruding a continuous sheath of lead (or Aluminum). To protect the lead against damage when laid in ground impregnated biturmous jute strands is applied over lead sheath and a metal armounding of two steel tapes is applied. To preserve the armour, bituminous impregnated jute is served.

Selection of cable:

Depends on location, environmental conditions and method of laying.

a. Unarmounded PVC cables should not be installed in premises with temperature higher than 65oC.

b. PVC cables are to be protected against direct sun rays.

c. PVC cables are not suitable for laying under water because sheath does not provide sufficient water eightness.

d. Jute served PIVC cables not to installed within premises, tunnels due to risk of fire.

e. For laying in ground jute served PILC cable or PVC sheathed armoured cable.

f. Non-armoured PVC cables may be laid in cable tunnels of power stations.

Advantage of PVC insulated cables:

a. No plumb joints and terminations (end joints) simple taped terminations for indoor use and conventional compound filled boxed for outdoor use can be employed.

b. High resistance to corrosion

c. Unsusceptible to breakdown due to moistures entry.

d. Easier and cheaper installation due to lighter weight.

e. Less liable to damage during installation due to light weight.

f. Good fire retarding properties.

g. Easy to keep clean in textile and other industries where cleanliness is important.

Cable Laying:

Cable drums are usually of wood and vary in size with the diameter and length of cable to be accommodated. Arrow mark is indicated on the side of the drum indicate the direction in which it should be rolled. If the direction is not followed, cables will tend to accumulate towards the inner turns, possibly resulting indamage. Spindle of adequate strength should be used to mount the cable drums on the jacks. Heavy cables are handled over special rollers for easy laying. The minimum bending radius (recommended by the manufacture) depending on the size of the cable should be adhered since severe or sharp bends may cause displacement and fracture of insulating papers in PILC cables or damage the sheathing. Unnecessary turning and twistings are to be avoided. The cables are laid generally in underground (Burried cables) and in ducts.

Burried Cables:

Cables laid directly in the ground should be an armoured cable for mechanical protection. In PVC cables armour is protected by an outersheath of heavy duty PVC and in PILC cables, the armour is protected from dust by a covering of bitumous impregnated jute covering. The depth of trench will be about 3 ft. depending on surface conditions. The cable will be covered with sand for about 3 to 6 and then covered with bricks. In the initial stages of backfilling the trench, care must be taken to see that there are no large stones in the soil surrounding the cables. The soil should be riddled so that the cable has good surrounding bed of soft material, when road crossings are involved cables should be laid through pipes.

Cable in ducts (tunnels):

When cables are installed in tunnels, the support racks should not placed more than 700mm apart to prevent the cable from sagging. The cables pass through wall should run through pipes. All cables are to be tagged (identification mark tags) at intervals of 10 to 15M. To provide access for inspection, the tunnels are built with manholes. The cables should properly arranged at the joints of cross-over and take-off so that they are unable to come in contact in contact with each other.

Cable Joints:

Cable joints over required in cables for the purpose of

a. Connecting together two lengths to make a continuous cables (i.e.) straight through joint.b. Connecting a subsidiary cable to a main cable to form a branch from the main (i.e.) branch joint or tee joint.c. Connecting a cable to switchgear, Transformer etc. (i.e.) end joint.

The main requirement of a joint is that it should not introduce any weakness into the cable system. It must be so constructed that

a. The resistance of a jointed conductor is not greater than that of are unjointed conductor.

b. The insulation of the cable is maintained and at the joint is not less effective that that of the cable cores.

c. The joint is properly enclosed to prevent deterioration due to ingress of moisture and to avoid mechanical damage.

d. The joint should withstand the mechanical stresses imposed by a short circuit and the thermal effect of fault currents.

Joints:

Joints in conductors are commonly made by soldering or crimping. For straight this joints, the ends of two cables are sweated into suitable split ferrules. For tee joints, the branch conductor may be wrapped round the main conductor and sweated or tee fittings may be used. Crimping may be used in which a sleeve is fitted over the cable cores and then compressed to make an apparently solid section. The essential feature of a soldered joint is that each of the jointed surfaces is wetted by a solder film and the two films are continuous with the solder filling the space between them. Any larger space may be detrimental to the joint electrically as solders have only to about 10% of electrically conductivity of copper.

Solders an frixtures of 70% tin and 40% lead, whereas plumbing lead sticks contain 40% tin and 60% lead; lower tin content in plumbing metal gives a plastic range (i.e.) metal can be easily manipulated. For copper joints, Frysol soldering flux and soldering lead sticks are used. For Aluminum joints ALCAP soldering sticks and Eyre No.7 Aluminum flux are used.

Plumbing:

Lead sheath of the cable and jointing sleeve (or) gland of an end jointing box may be jointed by plumbing, when a man of metal is formed outside the jointed surfaces to provide rigidity.

Joints Insulation:

Materials used for joint insulation include bitumous impregnated cotton tape (HT tape), Empire tape and porcelain spacers.

Filling compound:

The compound used for filling the joint box is usually hard-setting bituminous compound of grade to suit the type and voltage of the joint. Cold settings compound consisting of a polyester resin and a hardening agent which sets to a solid, solid when mixed are also used now (M-seal epoxy cable jointing compounds).

Joint Sleeves and Boxes:

Joints in lead sheathed cables are enclosed in lead sleeve and plumbed. Outer protection boxes are fitting on to the cable joints and may be of cast iron, the spacer between the joints and the box being filled with bitumous compound.

Testing:

1. Insulation Testing: Any new length of cable should be tested for insulation valve before jointing is commenced and also after the completion of joint. After the test, the cable should be discharged.

2. D.C. High voltage test: This test is conducted to the cable to reveal any concentrated local faults that can not be deducted by megger. A Kenotron testing set is used for this purpose. The high d.c. voltage is applied to each conductor with the other conductors and sheath connected to earth for atleast 5mm. D.C. voltage is adjusted to be 5 to 6 times the voltage of the cables upto 10KV cables. A microammeter connected to the HV line gives the leakage which should not exceed 500 microamps for cable upto 10KV.

Cable Line Faults:

The method selected for locating a fault in a cable line depends on the nature of the fault that has occurred. Among the faults that may arise in a cable are brake-down in cable insulation resulting in fault of one phase to earth, 2 or all 3 phase to earth, or in shorting of 2 to 3 phase to each other with or without earth.

The first step in detecting the fault in the cable is to test it with a megger after the cable is disconnected at both ends. The IR value of each conductors to earth and to every other conductor is to be checked. This permits the nature of fault. The next step is to locate the selection within which the fault has occurred. The end, joints and straight this joints if any, are to be inspected for signs of bleeding of compound, smoke, etc.

With the aid of universal type cable bridge, the fault may be located by the loop and capacitance methods in which the ratio of the distance of the fault from the ends of the cable may be determined. If a break exists, the capacitance method is used to measure the capacitance of the broken conductor which is proportional to the total length of the conductor.

Murray Loop Test:

This is employed for locating a fault where low insulation exists but conductors are intact. In fig one core of a cable has a fault to earth and a sound core is available.

Higher fault location accuracy may be attained by employing the induction or acoustic methods. In the induction method, an andio currents of 10 to 20 amps are fed into the faulted conductor from a suitable generator and the fault is located by using a search coil and earphones.

In the acoustic method, a kenotron rectifier unit serves to feed pulses of high voltage do into the faulted conductor and cause discharges of current at the fault so that they can be detected by using a sensitive crystal pick up and amplifier.

When a joint is found to be faulty, the entire joint is discorded by cutting at least 150mm from each end of the jointed cable. Testing for moisture is to be done as follows:

Samples of paper are taken from cable belting and from the layers nearest to the conductor and immersed in a treated (105o - 120o) tape impregnating oil or transformer oil. If moisture presents, it will cause bubbling or frothing.

Current Rating of cables:

The current carried by copper or Aluminum conductor in a cable generator heat by virtue or I2R losses raises its temperature until equilibrium is reached with outside air or ground, when the heat generated is equal to heat dissipated to outside air or ground. The parameters which govern load capability of power cables can be divided into 1) internal and 2) External.

1. Internal

a. Conductivity of conductor material

b. Maximum permissible temperature of conductor material

c. Resistivity of Insulation

2. Externala. Thermal Resistivity of air or soil.

b. Ambient temperature

Conductivity

Cu

AlFor equal ampacity and Area Ratio

1

1.39

Temperature rise weight ratio

1

0.42

For equal diameter Resistance Ratio

1

1.65

Current carrying capacity

1

0.78

Insulation

Insulant

Max. conductor operating temperaturea. PILC

70 - 80o C

b. PVC

60 - 70o C

c. XLPE (Cross-linked Polyethylene)

90o C

The continuous load current for PVC insulated heavy duty cables are given in the table on the following assumptions.

1. Maximum conductor temperature

80o C

2. Ground temperature

30o C

3. Air temperature

40o C

4. Depth of Laying

75o C

5. Thunnal Resistivity of Soil

150o C cm/w

6. Thunnal Resistivity of PVC

650o C cm/w

The current ratings given in various tables are based on the above assumption. In actual practice, then conditions may be different. Therefore, to determine the current rating, the tabulated ratings should be multipled with appropriate rating factor or factors.

Rating Factors:

1. Underground cables

a. Depth of laying

b. Variation in ground temperature

c. Variation in Thermal Resistivity of soil

d. Group rating factor.

2. Cables in air

a. Variation in ambient temperature

b. Group Rating Factor

Supervision of cables, joints etc.

To ensure reliable operation of cables, it is necessary to patrol and inspect them regularly check their loads, keep watch over their conditions of insulation.

The aims of patrolling are to see that

a. The condition of the route remains satisfactory.

b. The warning notices showing the presence of underground cables and marker posts indicating the joint boxes are in place.

c. The protection pipes where the cable rise from the ground up a wall are in place.

d. The sealing boxes and st. this joint boxes show no signs of bleeding of compound.

e. The cable anchoring and protective devices are undisturbed.

f. The armour shows no signs of rust, dents or other damages and protective coating is satisfactory.

g. The cable temperature is normal.

h. Tunnel lighting and ventilation systems are in proper order. Air temperature within a tunnel should not exceed outdoor temperature by more than 10%.

i. Cables have been correctly segregated with adequate clearance between each other tagged and corrected marked for identification.

j. No cable should be under tension or sags excessively.

k. To prevent fire within tunnel, it is important to keep the tunnel cleared of all debris.

l. Secl. of all openings in the cable trunches to prevent ingress of ashes, water and oil.