Industrial Electric Power Cable

Industrial Electric Power Cable

 Cables are used for transmission of electrical power. They are mostly used for low voltage distribution in thickly populated area, in substations from transformers to main distribution panels and from main distribution panels to different distribution panels. Low voltage cables are also used in industries, workshops and maintenance shops / sheds. Medium voltage and high voltage transmission cables are also used for crossing the roads, railway lines and in densely populated areas in big cities.

Cables as compared to overhead lines have several advantages namely (i) the cable transmission and distribution are not subjected to supply interruptions caused by lightening or thunderstorms, birds, and other severe weather conditions, (ii) it reduces accidents caused by the breaking of the conductors, and (iii) its use does not spoil the beauty of place such as cities. But if a fault occurs because of any reason, it is not easily located.

The utilization of electricity in industry is mainly by cables as they present the most practical means of conveying electrical power to equipment, tools, and appliances of all types. Cable designs vary enormously to meet the diverse requirements but there are certain components which are common to all types of cables.

Electrical power cable can be considered to be just a conductor, overlying insulation, and frequently an exterior shield or jacket. It is used to convey electric power. It is the purpose of the cable to convey the electric current to the intended device or location. In order to accomplish this, a conductor is provided which is adequate to convey the electric current imposed. Equally important is the need to keep the current from flowing in unintended paths rather than the conductor provided. Electrical insulation (dielectric) is provided to largely isolate the conductor from other paths or surfaces through which the current can flow. Hence, it can be said that any conductor conveying electric signals or power is an insulated conductor. General awareness about the power cables is necessary for the electrical maintenance personnel to keep the power supply system in safe and reliable condition.

Power cables are used for transmission and distribution of electrical power in thickly populated areas, in sub-stations, in industries, and in workshops etc. These cables are extremely critical to the distribution and control of power, and in recent years, several innovations have been introduced in cable design. Improved dielectrics have resulted in better performance in addition to, in several cases, reduced costs. For the transmission, distribution, and utilization of electrical power, the choice normally lies between the use of overhead lines and underground cables.

Power cables are assembly of one or more individually insulated electrical conductors, normally held together with an overall sheath. The cables can be installed as permanent wiring within buildings, buried in the ground, and run overhead or exposed. Flexible power cables are used for portable devices, mobile tools, and machinery. Power cables are defined by voltage grade and nominal cross-sectional area.

Power cables play a very important role in the distribution system. There are several types of cables like ‘low tension’ (LT) cable, 11 kV (kilo volts) cable and 33 kV cable. They are designed and manufactured as per voltage, current to be carried, maximum operating temperature, and purpose of applications. For some applications such as mining, extra mechanical strength is given to the cable with double armouring. For wind power plants, flexible and UV (ultra-violet) light protected cables are needed. The underground cables have several advantages, e.g., less liable to damage because of storms.

All types of electric cable consist essentially of a low resistance conductor to carry the current except in special cases, such as heating cables, and insulation to isolate the conductors from each other and from their surroundings. In several types, such as single-core wiring cables, the two components form the finished cable, but normally as the voltage increases the construction becomes much more complex. Other main components can include screening to get a radial electrostatic field, a metal sheath to keep out moisture or to contain a pressurizing medium, armouring to get mechanical protection, corrosion protection for the metallic components, and a variety of additions extending, e.g., to internal and external pipes to remove the heat generated in the cable. Tab 1 gives important dates in cable developments.

Tab 1 Important dates in cable developments
1880sFirst gutta percha electric cable followed by rubber and vulcanized bitumen insulation
1890Ferranti 10 kV tubular cable and the introduction of paper insulation
1914Hochstadter development of screening which enabled distribution voltage to be increased to 33 kV
1926Emanueli provided the principle of pressurization with fluid-filled paper cables for voltages of 66 kV upwards
1930sPVC (poly-vinyl chloride) insulation first tried out in Germany
1943First 3-core 132 kV pressure cable in service
1949Introduction of the mass-impregnated non-draining cable in the United Kingdom to overcome the problems of drainage of oil-resin impregnant with cables installed on slopes
1950sFull commercial introduction of PVC and later thermoset insulation for wiring cables
PVC for power cables followed at the end of the decade
Successful development of aluminum sheaths, initially for pressure-assisted cables, and gradual adoption of aluminum conductors for power cables
First 275 kV FF (foundation fieldbus) cable (1954), operational use in 1959
1960sConsiderable distribution economies obtained by the use of combined neutral and earth cable
England / France + 100kV submarine DC (direct current) link inaugurated in 1961
First 400 kV FF cable, operational in 1969
1970sGradual extension of the use of thermoset insulation, mainly XLPE (cross linked poly-ethylene), as an alternative to paper insulation. Large commercial applications up to 15 kV but also experimental installations at higher voltages including transmission up to 132 kV
1980sIntroduction of optical fibre into overhead lines
Very widespread use of XLPE in the 11 kV – 33 kV range with significant quantities installed for transmission voltages of 66 kV – 240 kV
Discovery of high temperature superconducting materials
Development and growing use of cables designed to alleviate the effects when cables are involved in fires; properties include reduced flame propagation, low smoke emission, reduced emission of noxious fumes and corrosive gases and combinations of these characteristics
1990sWidespread use of optical fibres in overhead lines
Extension of polymerics to EHV (extra high voltage) and the commercialization of PPL (paper poly-propylene laminated)
Practical demonstrations of superconducting cables

The fundamental concern of power cable engineering is to transmit current (power) economically and efficiently. The choice of the conductor material, size, and design is to take into consideration such items as (i) ampacity (current carrying capacity), (ii) voltage stress at the conductor, (iii) voltage regulation, (iv) conductor losses, (v) bending radius and flexibility, (vi) overall economics, (vii) material considerations, and (viii) mechanical properties.

Main parts of cables – The main three parts of the cables are conductor, insulation, and protection.

Conductor – Conductor is a material which provides low resistance to the flow of electrical current. There are several low resistivity (or high conductivity) metals which can be used as conductors for power cables. Tab 2 shows examples of these as ranked by low resistivity at 20 deg C.

Tab 2 Resistivity of metals at 20 deg C
MetalOhm-sq-mm/[m x (10 to the power -8)]
Silver (Ag)1.629
Copper (Cu), annealed1.724
Copper, hard drawn1.777
Copper, tinned1.741-1.814
Aluminum (Al), soft, 61.2 % conductivity2.803
Aluminum, 1/2 hard to full hard2.828
Sodium (Na)4.3
Nickel (Ni)7.8

Considering these resistivity figures and cost of each of these materials, copper and aluminum become the logical choices. As such, they are the dominant metals used in the power cable industry today. The choice between copper and aluminum conductors is based on careful comparison of the properties of the two metals, since each has advantages which can outweigh the other under certain conditions.

Electrical grade high conductivity annealed copper or annealed aluminum conductors are used in cables. Normally all power cables have aluminum as the conductor material. Aluminum of high purity, (99.5 % pure electrical grade) which is highly anti-corrosive and highly conductive is used as conductor in cables. Annealing softens the aluminum, reduces tensile strength and increases the conductivity. The properties most important to the cable designer are shown below.

DC resistance – The conductivity of aluminum is around 61.2 % to 62 % that of copper. Hence, an aluminum conductor is required to have a cross-sectional area around 1.6 times that of a copper conductor to have the equivalent DC resistance. This difference in area is around equal to two AWG (American wire gauge) sizes.

Weight – One of the most important advantages of aluminum, other than economics, is its low density. A unit length of bare aluminum wire weighs only 48 % as much as the same length of copper wire having an equivalent DC resistance. However, some of this weight advantage is lost when the conductor is insulated, since more insulation volume is needed over the equivalent aluminum wire to cover the higher circumference.

Ampacity – The ampacity of aluminum against copper conductors can be compared by the use of several documents. It is obvious that more aluminum cross-sectional area is needed to carry the same current as a copper conductor as can be seen from Tab 2.

Voltage regulation – In AC (alternating current) circuits having small conductors (up to #2/0 AWG), and in all DC circuits, the effect of reactance is negligible. Equivalent voltage drops result with an aluminum conductor which has around 1.6 times the cross-sectional area of a copper conductor. In AC circuits having larger conductors, however, skin and proximity effects influence the resistance value (AC / DC ratio), and the effect of reactance becomes important. Under these conditions, the conversion factor drops slightly, reaching a value of around 1.4.

Short circuits – It is necessary to give consideration to possible short circuit conditions, since copper conductors have higher capabilities in short circuit operation. Caution is to be applied, when making this comparison of the thermal limits of materials in contact with the conductor (shields, insulation, coverings, and jackets etc.)

Other important factors – Additional care is to be taken when making connections with aluminum conductors. Not only does the metal tend to creep, but it also oxidizes rapidly. When aluminum is exposed to air, a thin, corrosion-resistant, high dielectric strength film quickly forms. When copper and aluminum conductors are connected together, special techniques are needed in order to make a satisfactory connection.

Aluminum is not used extensively in generating station, substation, or portable cables since the lower bending life of small strands of aluminum does not always meet the mechanical requirements of those cables. Space is frequently a consideration at such locations also. However, aluminum is the over-whelming choice for aerial conductors because of its high conductivity to weight ratio and for under-ground distribution for economy where space is not a consideration. The 8000 series alloys of aluminum (e.g., 8011, 8090, 8091 and 8093) have found good acceptance in larger size conductors used in large commercial, institutional and some industrial applications. Economics of the cost of the two metals (copper and aluminum) are, of course, to be considered, but always weighed after the cost of the overlying materials is added.

Voltage ratings of cables – The rating, or voltage class, of a cable is based on the phase-to-phase voltage of the system even though it is in a single or three phase circuit (Fig 1). For example, a 15 kV rated cable (or a higher value) is to be specified on a system which operates at 7.2 kV or 7.62 kV to ground on a grounded wye 12.5 kV or 13.2 kV system. This is based on the fact that the phase-to-phase voltage on a wye system is 1.732 (the square root of 3) times the phase-to-ground voltage. Another example is that a cable for operation at 14.4 kV to ground is to be rated at 25 kV or higher since 14.4 times 1.732 is 24.94 kV.

Fig 1 Voltage rating

The wye systems described above are normally protected by fuses or fast acting relays. This is normally known as the 100 % voltage level and has been previously known as a ‘grounded’ circuit. Additional insulation thickness is needed for systems which are not grounded, such as found in some delta systems, impedance or resistance grounded systems, or systems which have slow-acting isolation schemes.

Voltage designation – In the early days of electric power utilization, DC has been used, but little now remains except for special applications and for a few inter-connections in transmission networks. AC has several advantages and 3-phase alternating current is used almost exclusively throughout the world, so that suitable insulation and cable construction can be specified for the needed 3-phase AC service performance. The design voltages for cables are expressed in the form Uo / U (formerly Eo / E). Uo is the power frequency voltage between conductor and earth and U is the power frequency voltage between phase conductors for which the cable is designed, Uo and U both being r.m.s. values. (r.m.s. is measured under the root of the sum of squares of all instantaneous voltages divided by the number of instantaneous voltages. The r.m.s. value of an AC is equivalent to the power consumption by DC voltage).

Power cables in some of the standards are hence designated 600 V / 1 kV, 1.9 kV / 3.3 kV, 3.8 kV / 6.6 kV, 6.35 kV / 11 kV, 8.7 kV / 15 kV, 12.7 kV / 22 kV and 19 kV / 33 kV. For transmission voltages above this, it is normal to quote only the value of U and hence the higher standard voltages in the countries following these standards are 66 kV, 132 kV, 275 kV and 400 kV. The maximum voltage can be 10 % higher than the above values for voltages up to and including 275 kV and 5 % higher for 400 kV. Although the local distribution voltage in these countries is 240 V / 415 V, the cables are designed for 600 V /1 kV, largely since during manufacture and installation this grade of cable needs an insulation designed on mechanical rather than electrical parameters.

Standardization of system voltages has not been achieved world-wide although there is some move towards this. IEC (International Electrotechnical Commission) has published voltage designations which are approaching universal acceptance.

DC system voltages, by which is meant DC voltages with not more than 3 % ripple, are designated by the positive and negative value of the voltage above and below earth potential. The symbol Uo is used for the rated DC voltage between conductor and the earthed core screen.

Several references are found for cables being described as low voltage (LV), medium voltage (MV), high voltage (HV) and even extra high voltage (EHV) or ultra-high voltage (UHV). Apart from low voltage, which is defined internationally, these terms do not have normally accepted precise meanings and can be misleading. In some countries MV has in the past applied to 600 V / 1 kV cables (these now fall clearly within the LV designation), whereas others have taken it to mean 6 kV / 10 kV or 8.7 kV / 15 kV and misunderstanding can arise. For precision it is best to use the actual voltage rating of the cable.

Insulation – Insulation material means a material having good dielectric properties, which is used to separate or isolate the conducting electrical parts. Insulation to be used for cables is required to have several properties namely (i) it is to have a high specific resistance and dielectric strength; (ii) it is to be tough and flexible, (iii) it is not to be hygroscopic i.e. it does not absorb moisture from air or surroundings, (iv) it is to be capable of standing high temperatures without much deterioration, (v) it is to be non-inflammable and fire retardant, (vi) it does not be attacked by acids or alkalis, and (vi) it is to be capable of withstanding high rupturing voltages.

Electrical insulation materials are utilized to provide electrical isolation over the metallic conductors of underground cables. The insulating materials physically protect the conductor and provide a margin of safety. These materials are comprised of either synthetic or natural polymers. The polymeric insulation material selected for use can vary with the voltage class of the cable. Compatible polymeric shields are used between the insulation and the conductor, and over the insulation to grade the voltage stress, these are comprised of flexible polymers blended with conducting carbon black which imparts the semi-conducting characteristics.

The main types of insulation group of materials, which are used are (i) butyle rubber (BR), (ii) poly-ethylene (PE), (iii) polyvinyl chloride (PVC), (iv) fiberous material such as paper, and jute etc., (v) ethylene propylene rubber (EPR), (vi) cross linked poly-ethylene (XLPE), (vii) poly-chloroprene (PCP), and (viii) oil impregnated paper insulation.

Until the early 1990s, transmission class cables, defined as cables operating above 46 kV, had traditionally used oil-impregnated paper as the insulation. This paper insulation is applied as thin layers wound over the cable core, and is later impregnated with a dielectric fluid. As the application of synthetic polymers to cable technology matured, extruded poly-ethylene which has been subjected to cross-linking (XLPE) gradually displaced paper as the insulation material of choice for transmission voltages up to 230 kV, XLPE is the prime extruded material used for transmission cables. XLPE is considered to be the material of choice because of its ease of processing and handling, although paper / oil systems have a much longer history of usage and much more information on reliability exists. As a result of the higher stresses associated with transmission class cables (as compared to distribution cables) extreme care and cleanliness in handling materials and during manufacturing is needed. In the 1980s, a variation on paper as insulation was developed, the material being a laminate of poly-propylene paper (PPP) or poly-propylene laminated paper (PPLP), this material has been used for voltages above 230 kV.

For distribution voltage class cables (mostly 15 kV to 35 kV), the prime extruded material developed for use in the 1960s was conventional PE (high molecular weight poly-ethylene, or HMWPE). However, this has been replaced by XLPE as the material of choice during the late 1970s to early 1980s, as a result of unanticipated early failures in service because of the water-treeing problem. Installed PE-insulated cables are gradually being replaced (or rejuvenated in-situ for stranded construction.

Elastomeric ethylene-propylene co-polymer or ter-polymers have also been used (ethylene propylene rubber, EPR or ethylene propylene diene monomer, EPDM respectively) for medium voltage cable insulation. The term EPR has been used to generically describe both EPR and EPDM-insulated cables. EPR cables have been available since the 1960s, but their use had been consistently less common as compared to HMWPE or XLPE, because of higher costs and operating losses. EPR usage started to increase in the 1970s to 1980s partly because of easier processing as a result of modification of the EPR compound to facilitate easier extrusion (hence reducing the cost). In contrast to XLPE which is a semi-crystalline polymer, EPR is an elastomer (rubber) and hence needs the incorporation of inorganic mineral fillers in order to serve as a satisfactory functional insulation, this in turn leads to additional handling and processing requirements by materials suppliers.

Starting in the mid-1980s, XLPE has been gradually replaced by ‘tree resistant’ XLPE (TR-XLPE) as the material of choice for new distribution class cables. From the early 1980s and well into the late 1990s, a single grade of TR-XLPE has been used commercially in some countries. More recently several other grades of TR-XLPE have become available. Each insulation type has certain advantages and disadvantages. An overview is given in Tab 3.

Tab 3 Insulation type – key property information
PE (Low density poly-ethylene)Low dielectric losses, moisture sensitive under voltage stress
XLPE (Cross-linked polyethylene) Slightly higher dielectric losses than PE, ages better than PE, less moisture sensitive
EP (EPR/EPDM)Higher dielectric losses vs. XLPE or TRXLPE, more flexible, less moisture sensitive than XLPE or PE, needs inorganic filler
TR-XLPESimilar to XLPE, but slightly more lossy, because of additives; losses less than for EPR, ages ‘better’ than XLPE; less moisture sensitive
PILC (paper insulated lead covered)High reliability, possesses lead sheath
PVCMust contain a plasticizer for flexibility, higher dielectric losses, does not burn but yields toxic gases

Shielding of an electric power cable – Shielding layers which are provided for protection of the cable are (i) inner sheath, (ii) armouring, and (iii) outer sheath.

Inner sheath is provided over insulation for the protection from moisture and aggressive elements. For oil impregnated paper insulated cables, lead sheath or impregnated jute tapes with layers of bitumen compound are used. For polymeric material insulated cables, extruded PVC sheath or wrapping of plastic tapes are used.

Armouring is provided to avoid mechanical injury to the cable. Depending upon the application, the cable can be armoured or unarmoured. The armouring is applied over the core insulation or inner sheath for single core cables and over the inner sheath for the multi-core cables.

Armour is a metallic wrapping over the cable insulation. For single core cables, non-magnetic materials are used as armour, for example, flat aluminum wire. In multi-core cables, common armour is provided for all the laid-up cores and the armour material can be galvanized round steel wire or flat steel strip.

Single core and multi-core cables are provided with an extruded PVC outer sheath. The colour of the outer sheath is generally black. Fig 2 shows cross-sectional area of multi-core armoured cable.

Fig 2 Cross-sectional area of multi-core armoured cable

Shielding of an electric power cable is accomplished by surrounding the insulation of a single conductor or assembly of insulated conductors with a grounded, conducting medium. This confines the electric field to the inside of this shield. Two distinct types of shields are used namely metallic and a combination of non-metallic and metallic.

The purposes of the insulation shield are to (i) get symmetrical radial stress distribution within the insulation, (ii) eliminate tangential and longitudinal stresses on the surface of the insulation, (iii) exclude from the electric field those materials such as braids, tapes, and fillers which are not intended as insulation, and (iv) protect the cables from induced or direct over-voltages. Shields do this by making the surge impedance uniform along the length of the cable and by helping to weaken surge potentials.

The terms ‘sheaths’ and ‘jackets’ are frequently used as though they mean the same portion of a cable. Sheath is properly the term which applies to a metallic component over the insulation of a cable. An example is the lead sheath of a paper insulated, lead-covered cable.

Different metals can be used as the sheath of a cable such as lead, copper, aluminum, bronze, and steel etc. A sheath provides a barrier to moisture vapour or water ingress into the cable insulation. It is necessary to use such a sheath over paper insulation, but it also has a value over extruded materials because of water ingress.

The thickness of the metal sheath is covered by standards and specifications, but there are some constructions which are not covered. The thickness is dependent on the forces which can be anticipated during the installation and operation of the cable. Designs range from a standard tube to ones which are longitudinally corrugated. The bending radius of the finished cable is dependent on such configurations. For fully utilizing the metal chosen, one is to consider first cost, ampacity requirements, especially during fault conditions, and corrosion.

The term jacket is to be used for non-metallic coverings on the outer portions of a cable. They serve as electrical and mechanical protection for the underlying cable materials. There are several materials which can be used for cable jackets. The two broad categories are thermo-plastic and thermo-setting. For each application, the operating temperature and environment are important factors which are to be considered.

The armour consists of a single metal tape whose turns are shaped to inter-lock during the manufacturing process. Mechanical protection is hence provided along the entire cable length. Galvanized steel is the most common metal provided. Aluminum and bronze are used where magnetic effects or weight are to be considered. Other metals, such as stainless-steel or copper, are used for special applications.

Inter-locked-armour cables are frequently specified for use in cable trays and for aerial applications so that conduit and duct systems can be eliminated. The rounded surface of the armour withstands impact somewhat better than flat steel tapes. The inter-locked construction produces a relatively flexible cable which can be moved and repositioned to avoid obstacles during and after installation.

An overall jacket is frequently specified in industrial and power plants for corrosion protection and circuit identification. Neither flat-taped armour nor inter-locked armour is designed to withstand longitudinal stress, so long vertical runs are to be avoided.

Classification of electrical cables – The classification of electrical cables as per their application is given below.

Wiring cables – These cables are used for internal wiring of the buildings and other protected installations and have two components namely conductor and insulation. PVC as insulation material and annealed copper (solid or stranded) as conductor are normally used for wiring cables. Voltage grade of these cables are up to 1.1 kV.

Control cables – These are designed for control purposes or measuring circuits for carrying signals of DC up to 220 V and AC up to 440 V. These cables are available with armour and without armour. In these cables PVC, XLPE, EPR, and neoprene etc. are used as insulation. Control cables are available in 0.5 / 0.75 / 1 / 1.5 / 2.5 square millimeter (sq-mm) size copper conductor (solid / stranded) from 2 cores to 61 cores.

Power cables – Electrical power cables are used for distribution and transmission of electrical energy. These cables either single core or multi-core are particularly useful in power stations, sub-stations, house service connections, and street lighting etc. They can be installed indoors or outdoors, in air, in cable ducts, or under-ground.

Special application cables – Cables are also classified based on special applications such as (i) fire performance and heat resistant cables, (ii) pilot cables, (iii) instrumentation cables, (iv) submarine cables and ship board cables, (v) airport lighting cables, (vi) mining cables, (vii) cables for lifts and hoisting gears, (viii) welding cables, and (ix) cables for hazardous areas such as petro-chemical industries etc.

Classification of power cables – Electrical power cables are normally classified as per their designed (rated) voltages or the type of insulation used.

Classification as per designed voltage – Electrical power cables are normally classified as per their designed (rated) voltages. These are (i) low voltage cables – up to and including 1.1 kV, (ii) medium voltage cables – from 3.3 kV up to and including 33 kV, (iii) high voltage cables – above 33 kV and up to and including 132 kV, and (iv) Extra high voltage cables – above 132 kV and up to and including 700 kV. Tab 4 shows the normal voltage ratings of medium voltage electrical power cables.

Tab 4 Rated voltage of cables
Where Uo is the rated power frequency voltage between conductor and earth or metallic screen, U is the rated power frequency voltage between phase conductors, and Um is the maximum permissible continuous 3 phase system voltage.

Classification as per type of insulation used – Electrical power cables are normally classified as per the type of insulation used. These are (i) PILC (paper insulated lead sheath covered) cables, (ii) PVC (poly-vinyl chloride) cables, and (iii) XLPE (cross linked poly-ethylene) cables.

PILC cables – For several years, the superior insulation material for power cables from low voltage to high voltages was oil-impregnated paper. Oil-impregnated paper has very good electrical properties and a high degree of thermal overload capacity without excessive deterioration. However, PILC cables have the some dis-advantages namely (i) they are prone to moisture and damage, (ii) they have low current carrying capacities, (iii) they need low operating temperatures, (iv) they have heavier weight and they are difficult to handle during installation, and (v) there is migration of impregnating compound which do not permit laying cables vertically or on steep slopes. Because of these disadvantages, the use of PILC cables is limited.

PVC cables – PVC is a general-purpose thermo-plastic used for wires and cables insulation and is a suitable alternative to paper insulation. PVC is applied as continuous seam free extrusion as insulation and sheath. PVC cables has several properties and advantages namely (i) insulation resistance and breakdown strength are practically unaffected by moisture, (ii) there is no impregnating compound in these cables, hence these cables can be laid vertically and on steep slopes, (iii) these cables can withstand a high transient conductor temperature without any deformation of insulation, (iv) these cables are practically resistant to all chemicals encountered in practice, (v) these cables are flame retardant since PVC ignites with great difficulty and that too when directly exposed to a flame, (vi) these cables are easy to install and handle because of their lighter weight, (vii) small bending radii permit the termination of these cables in limited space, which eases the termination of PVC cables in switch boards and control panels etc., and (viii) PVC cables have a smooth outer surface resulting in a neat appearance when installed. PVC outer sheath is tough and abrasion proof.

The main disadvantage of PVC is that it becomes brittle because of high temperature variations. Normally there are two types of PVC, general purpose and fire retardant (FR-PVC). PVC insulation is suitable for voltages up to 11 kV.

XLPE cables – Poly-ethylene has a linear molecular structure. Molecules of poly-ethylene, not chemically bonded, are easily deformed at high temperatures. This linear structure is changed into cross-linked structure by special processes. This thermo-setting XLPE insulation material provide extra-ordinary electrical, thermal, and mechanical properties to the cables, like low dielectric loss, excellent dielectric strength, higher continuous current rating, high resistance to thermal ageing etc. The main advantages of XLPE cables over PVC cables are described below.

The first is excellent electrical and physical properties. High resistance to thermal deformation and the ageing property of XLPE cables provides higher continuous and short circuit current capacity ensuring higher degree of reliability over wide range of temperature variation as compared to PVC cables. Permissible maximum conductor temperature for XLPE cables for continuous duty is 90 deg C and for short circuit it is 250 deg C while that of PVC cables for continuous duty is 70 deg C and for short circuit it is 160 deg C. The second is higher current carrying capacity. Current carrying capacity of XLPE cables of the same size is around 20 % to 30 % higher than that of PVC because of the higher operating temperature. The third is resistant to heat. With cross-linked molecule’s structure, XLPE cables are excellently ozone resistant and provide outstanding stability and are resistant to heat. The fourth is that XLPE cables have lower dielectric loss, lower permittivity as compared to PVC cables. The fifth is that because of lower specific gravity, XLPE cables are comparatively lighter in weight than PVC cables, hence, ease in handling, laying, and installation. The cable needs less supporting because of lower weight. The sixth is that XLPE cable has higher mechanical properties and is more robust as compared to PVC cable because of thermo-setting process.

PVC insulated (heavy duty) electric power cables – These cables are normally used up to and including 11 kV installations. Insulation material used is poly-vinyl chloride (PVC) and conductors are made from electrical purity aluminum or copper. For giving flexibility to the conductors of cables are stranded. These cables are used where combination of ambient temperature and temperature rise because of load results in conductor temperature not exceeding 70 deg C under normal operation and 160 deg C under short circuit conditions.

Different cores in a cable are identified by the colours of PVC insulation. Accepted colour codes for PVC insulated cables are for (i) single core – red, yellow, blue, or black, (ii) tin core – red and black, (iii) three core – red, yellow, and blue, (iv) four core – red, yellow, blue, and black, and (v) five core – red, yellow, blue, black, and light grey. In 3.5 core cables, the three main cores are red, yellow, blue for phases and reduced core is black for neutral. Red, yellow, blue colours represent phase ‘R’, ‘Y’, ‘B’ phases and black colour represents neutral ‘N’ phase.

For cables of voltage grade up to and including 6.6 kV, method of core identification is (i) different colouring of the PVC insulation, (ii) coloured strips applied on the cores, or (iii) by numerals (1,2,3), either applying numbered strips or by printing on the cores. For cables of voltage grade of 6.35 kV / 11 kV, method of core identification is (i) coloured strips applied on the cores, or (ii) by numerals (1,2,3), either by applying numbered strips or by printing on the cores.

Constructional features – The construction features of the cables are described below.

Conductor – The conductor is made of electrical grade aluminum or copper. Normally, all power cables have aluminum as the conductor. The conductor is of stranded construction size 2.5 sq-mm and above.

Conductor screening – Cables rated for 6.35 kV / 11 kV are provided with conductor screening over the conductor by applying non-metallic semi-conducting tape or by extrusion of semi-conducting compound or a combination of the both.

Insulation – PVC compound is applied to the conductors by the extrusion process. It is so applied that it can be removed without damaging the conductor.

Insulation screening – Cables rated for 6.35 kV / 11 kV are provided with insulation screening. It consists of two parts, namely non-metallic (semi-conducting) and metallic.

Inner sheath (for multi-core cables) – The laid-up cores are surrounded by an inner sheath of any of the two types namely (i) extruded PVC compound (for armoured cables), and (ii) wrapping of PVC / plastic tapes for unarmoured cables. Inner sheath is also known as bedding in case of armoured cables.

Armouring – Depending upon the application, these cables can be armoured or unarmoured. For single core cables, flat aluminum wire armour is used, since aluminum being a non-magnetic material, does not induce stray current. For multi-core cables, galvanized round or flat steel wire armour or double steel tape armour is used. The armouring is applied over the core insulation or inner sheath in case of single core cables and over the inner sheath in case of multi=core cables.

Outer sheath – Outer sheaths are made of black poly-vinyl chloride compound, which protect the armour material from corrosion. This PVC compound is applied by extrusion method. Outer sheath is applied over the non-magnetic metallic tape covering the insulation or over the non-magnetic metallic part of insulation screening in case of unarmoured single core cables and over the armouring in case of armoured cables.

 Ratings and applications – PVC insulated power cables are normally designed and manufactured from rated voltage 650 V / 1.1 kV up to and including 6.35 kV / 11 kV and number of cores are 1, 2, 3, 3.5, 4, or 5 cores. Nominal area of aluminum conductor ranges from 1.5 sq-mm to 1,000 sq-mm for single core cables and from 2.5 sq-mm to 630 sq-mm for multi-core cables.

PVC unarmoured single core and multi-core cables are particularly useful in power stations, sub-stations, house service connections, street lighting, and building wiring etc. They can be installed indoors or outdoors, in air or in cable ducts.

PVC armoured single core and multi-core cables are useful in generating stations, sub-stations, distribution systems, street lighting, and industrial installations etc. On account of the armouring the cables can withstand rough installation, operation conditions and tensile stresses. They can be laid in water or buried direct in the ground even on steep slopes. Fig 3 shows the cross-sectional view of some of the PVC insulated cables.

Fig 3 Cross-section of PVC insulated cables

XLPE cables – Cross linked poly-ethylene (XLPE) insulated single core and multi-core cables are produced with colours of cores red, yellow, and blue to represent R, Y, B phases respectively and black colour to represent neutral N phase. It is measured under the root of the sum of squares of all instantaneous voltages divided by the number of instantaneous voltages. The RMS value of an AC is equivalent to the power consumption by DC voltage. Construction of XLPE insulated cables are similar to those of PVC cables. Hence, they have all the advantages of PVC cables in terms of cleanliness., ease of handling, and simple jointing and terminations.

The basic physical difference is that XLPE cables are more robust hence allow the thickness to be reduced which in turn allow a corresponding reduction in the overall size of the cables. These cables are suitable for use where combination of ambient temperature and temperature rise because of the load results in conductor temperature not exceeding 90 deg C under normal operating condition and 250 deg C under short circuit condition.

Low voltage XLPE cables – These cables are suitable for use on AC single phase or three phase (earthed or unearthed) systems for rated voltages up to and including 1.1 kV. These cables can be used on DC systems also for rated voltage up to and including 1.5 kV to earth. These cables are normally available in three configurations as described below.

The first configuration is low voltage XLPE insulated, unarmoured, PVC sheathed cables. These cables are designed for general purpose indoor power distribution application. Plain circular or sector shaped stranded annealed aluminum or copper conductors are used and insulation of core consists cross linked poly-ethylene. For multi-core cables, cores are laid up together and filled with non-hygroscopic material (plastic fillers) compatible with the insulation. Outer sheath consists of black colour PVC type ST2 (a compound for sheathing).

The second configuration is low voltage XLPE insulated, screened, PVC sheathed cables. Design of these cables are same as described in the first configuration except that aluminum mylar tape or annealed copper wire or tinned copper braid is used as screen material over XLPE insulation. Screening prevents external electro-magnetic influences to the cable.

The third configuration is the low voltage XLPE insulated armoured PVC sheathed cables. Constructional features of single core aluminum armoured cables and multi-core steel wire armoured cables are similar to PVC insulated cables. These cables are most suitable for under-ground power distribution application, where there is a risk of mechanical damages. Fig 4 shows cross-section of low voltage XLPE cables.

Fig 4 Cross-section of low voltage XLPE cables

Medium voltage XLPE cables – Cross linked poly-ethylene insulated and PVC sheathed medium voltage power cables are suitable for voltages from 3.3 kV and up to and including 33 kV. The categories of armoured screened or unscreened single core and three core XLPE insulated and PVC sheathed cables are available for electricity supply purposes are (i) earthed system (Uo / U) – 1.9 kV / 3.3 kV, 3.8 kV / 6.6 kV, 6.35 kV / 11 kV, 12.7 kV / 22 kV, and 19 kV / 33 kV, and (ii) unearthed system – 3.3 kV / 3.3 kV, 6.6 kV / 6.6 kV, and 11 kV / 11kV. In these cables, conductors are compacted stranded aluminum of smooth profile, free from sharp juts which can damage the insulation because of high local electric stresses.

XLPE insulation is processed using the triple layer dry curing extrusion method. These cables are supplied with extruded cross linked semi conducting screens to protect the main solid XLPE insulation. The conductor screen fills the interstices between wires and provides a smooth circular envelope around the conductor. This diminishes the concentration of flux lines around the individual wires and hence the electrical stress around the conductor.

Semi conductive insulation screen either strippable or bonded is applied over the core insulation. A layer of annealed uncoated copper tapes or copper wires is provided over the extruded insulation screen. This metallic screen provides an earthed envelope. This metallic shield provides protection from external fields, reduced stress concentration, and uniform radical field lines from conductor and does not cause induced current.

XLPE insulated medium voltage cables are normally available in several categories namely (i) armoured single core unscreened cable, (ii) armoured single core screened cable, (iii) armoured 3 core unscreened cable, and (iv) armoured 3 core screened cable. Fig 5 shows cross-section of medium voltage XLPE cables.

Fig 5 Cross-section of medium voltage XLPE cables

Selection, laying and installation of cables – Selecting the proper type and size of cable for the desired application is very important. Selecting the correct type and size of cable not only ensures the trouble-free performance but also optimizes the cost of material, installation, and the operation as well. While selecting the correct type and size of the cable, several factors as described below are to be kept in mind.

System voltage – Important factors to be considered are rated voltage, maximum operating voltage whether DC or AC, number of phases, and frequency. Tab 5 gives the permissible operating voltages.

Tab 5 Permissible operating voltage
Rated voltage of cableMaximum permissible continuous 3-phase system voltageMaximum permissible continuous 1-phase system voltageMaximum permissible DC voltage
UoUUmBoth cores insulatedOne core earthed 

Load conditions – Actual load conditions help in choosing correct cross section of conductors for the cable. The basic load conditions are given below.

Normal continuous load – It means that the given load current is going to flow continuously through cable. Standards are to be referred for current ratings for PVC cables which are based on the normal conditions of installation. If the actual conditions are not the same as the normal conditions, the values for the normal current ratings are to be multiplied by the relevant rating factors as given in the standards.

Intermittent load – If the cable is switched on and off periodically, so that the time between switching ‘off’ and then ‘on’ is not sufficient to cool the conductor to the ambient temperature during the rest period, then such load is called intermittent load. A proper cross-section of cable conductors for such load conditions can be decided in consultation with the cable manufacturers.

Short time load – Under these load conditions, the conductor is allowed to cool down to ambient temperature after the load period. Here again, the conductor cross-section can be decided in consultation with the cable manufacturers.

Cyclic load – If the load is cycle, the maximum permissible current can be increased by an amount depending on the shape of the load curve, type of cable, its heat capacity, and method of installation.

Earthing conditions – In 3 phase systems, it is necessary to know whether the neutral points are effectively earthed or earthed through resistance, inductance, or earthing transformer or if system is totally unearthed.

Permissible voltage drop – This factor also decides the minimum conductor size, particularly in long feeders so as to maintain voltage drop within statutory limits. Guidance about voltage drop in volts per kilometer per ampere, at the operating temperatures of the cables is available in standards. In case of very high voltage drop, it is necessary to choose a bigger conductor size.

In addition to above factors, the issues which are to be considered while selecting proper type of the cable are (i) soil conditions such as nature of soil, chemical action, and corrosions, (ii) installation conditions, (iii) economic considerations, and (iv) future expansions.

Selection of cable route – Prior to start excavation of cable trench, a preliminary survey of the cable route is done and a plan drawing is prepared and approval from all the concerned authorities is taken, if necessary. The points which are to be considered while selecting cable route are (i) selection of the shortest but the easiest route for reducing the overall cost, (ii) consideration is to be given for the access and transportation of cable drums by checking the road conditions, turns and width, (iii) paved roads are to be avoided as far as possible the footpaths are to be used, (iv) the route is to be as far as possible, away from parallel running gas, water pipelines and telephone / telecommunication cables, (v) suitable locations for cable joints and terminations are to be selected as needed, and consideration is to be given for future expansion or upgrading of the system.

Minimum permissible bending radii – The cable is not to be bent to a sharp radius. Minimum permissible bending radii for cables are available in the standards and is given in Tab 6.

Tab 6 Minimum permissible bending radii for cables
Voltage ratings (kV)PILC cablesPVC and XLPE cables
Single coreMulti coreSingle coreMulti core
Up to 1.120 D15 D15 D12 D
Above 1.1 to 1120 D15 D15 D15 D
Above 1125 D20 D15 D15 D
Note: D is outer diameter of cable.

Bending radius for the individual cores at the joints and terminations is to be above 12 times the diameter over the insulation.

Methods of cable laying and installation – The conventional methods for cable laying and installation are (i) laying direct in the ground, (ii) drawing in ducts, (iii) laying on racks in air, (iv) laying on racks inside a cable tunnel, and (v) laying along buildings or structures.

Laying direct in the ground – This method involves digging a trench in the ground and laying cables on a bedding of minimum 75 mm riddled soil or sand at the bottom of the trench, and covering it with additional riddled soil or sand of minimum 75 mm and protecting it by means of bricks, tiles, or slabs as shown in Fig 6a.

Fig 6 Laying and jointing of cables

Depth – The desired minimum depth of laying from ground surface to the top of the cable is to be (i) 0.9 m (metre) for the cables of 3.3 kV to 11 kV voltage rating , (ii) 1.05 m for the cables of 22 kV to 33 kV voltage rating, (iii) 0.75 m for the low voltage and control cables, (iv) 1 m for the cables at road crossings, and (v) and 1 m for the cables at railway level crossings (measured from bottom of sleepers to the top of pipe).

Clearances – The desired minimum clearances are (i) for power cable to power cable, clearance is not necessary, however larger the clearance, the better is the current carrying capacity, (ii) 0.2 m for the power cable to control cables, (iii) 0.3 m for the power cable to communication cable, and (iv) 0.3 m for the power cable to gas / water pipe-lines.

Cable laid crossing roads, railway track, and water pipe-lines – Hume pipe / galvanized iron pipe of suitable grade and size is to be used where cable cross roads, and railway tracks. Spare ducts for future extensions are also to be provided. The duct / pipe joints are to be covered by collars to prevent settlement in between pipes. The diameter of the cable conduit or pipe / duct is to be at least 1.5 times the outer diameter of cable. The ducts / pipes are to be mechanically strong to withstand forces because of the heavy traffic when they are laid across the road / railway tracks. · The cable entry and exit are to be through bell mouth or padding. The bending radii of steel or plastics ducts are not to be less than 1.5 m. Single core cables are not to be laid individually in steel ducts but instead, all three cables of the same system are to be laid in one duct. 

Cable over bridges – On bridges, the cables are normally supported on steel cable hooks or clamped on steel supports at regular intervals. It is advisable that cables laid on bridges are provided with sun shields to protect the cable from direct heating by the rays of the sun.

Trenching – The known methods of trenching are (i) manual excavation, (ii) excavation with mechanical force, (iii) thrust bore, and (iv) trench ploughing. Manual excavation method is normally in practice. Trenches are to be excavated as per the line and level shown on the cable route plan. If possible, the cable trench is to be of straight lines. All curves are to be smooth and suitable for laying the cable. The excavated trench sides and trench floor are to be trimmed to remove the sharp projections, if any, which can damage the cables. During excavation adequate measures are to be taken to protect all existing structures and existing services such as electrical cables, telecommunication cables, gas pipe-lines, and water pipe-lines etc.

Cable laying (by hand) – Before laying the cable, the cable is also to be examined for any exterior damage. The cable drum is mounted on a cable jack with a strong spindle. Drum is to be jacked high enough to fit in braking plank. Weak shaft is not to be used otherwise drum revolves unevenly.

The drum is never to be kept flat on its side on the ground and the cable taken away from the same. This invariably leads to kinking and bird caging. The pay in rollers, corner rollers and properly aligned smooth-running cable rollers are to be placed every 3 m to 4 m in the cable trench. At least three solid plates for guiding the cable around the bend is to be used for maintaining minimum bending radius. The drum is raised slowly equally from both the ends by using both the jacks. Now the cable is to be paid out from the top of the drum by rolling the drum in the direction of arrow marked on the drum. A cable grip can be provided at the end of the cable or persons can also directly grip the cable, positioning themselves near the cable rollers and pull after a sufficient length of around 50 m has been pulled.

The gangman is required to stand in a commanding position and to make evenly timed calls. This enables the persons positioned at each roller to pull the cable evenly, simultaneously and without jerks. The number of persons needed for pulling largely depends on the size and weight of the cable being laid. The persons at rollers are also to apply graphite grease in the course of pulling, as and when needed. When pulling round a bend, corner rollers are to be used so as to minimize abrasion. During the preliminary stages of laying the cable, consideration is to be given to proper location of the joint position so that when the cable is actually laid the joints are made in most suitable places.

There is to be sufficient overlap of cables to allow for the removal of cable ends which can have been damaged. This point is extremely important as otherwise it can result in a short piece of the cable having to be included. The joint is not to be near pipe end or at the bend.

Reinstatement – After laying the cable, it is to be checked again for ensuring that all the cable ends are undamaged and sealed. If trench is partially filled with water, cable ends are to be kept clear off water as far as possible. If cable has to be cut, both the cable ends are sealed immediately. Lead cap for paper cable and plastic cap for PVC cable / XLPE cable are to be used. As a temporary measure, end can be sealed by inserting them in an empty tin which is filled with hot bitumen-based compound.

Each cable length is to be aligned immediately after it is laid starting from one end. When aligning the cable, it is to be ensured that there is no external damage. If the joints are not to be made immediately after laying the cable, the cable ends are to be covered. The position of cable joint is to be marked with markers. The trench at the duct mouth at road or railway crossing is to be deepened to prevent the stone or the gravel from being drawn into duct and clogging it.

Before the trench is filled in, all joints and cable positions are to be carefully plotted. The requisite protective covering is then to be provided, the excavated soil replaced after removing large stones and well rammed in successive layers of not more than 0.3 m in depth. Where necessary, the trenches are to be watered to improve consolidation.

It is advisable to leave a crown of earth not less than 50 mm in the centre and tapering towards the sides of the trench to allow for settlement. After the subsidence has ceased, the trench can be permanently reinstated and the surface restored to its original conditions. Cable route markers are to be installed on either side of the cable trench at every 100 m interval on straight runs, and turning points. Joint markers are to be installed at all the four corners of the joint pit. 

Cable jointing – Cable joint is a device used to join two or more cables together for extension of lengths or to branch. These joints are made to perform at the same voltage class and ratings of the intended cables and are able to withstand the normal and emergency loading conditions. Selection of the proper cable accessories, proper jointing techniques, skill and workmanship is important. The quality of joint is to be such that it does not add any resistance to the circuit. All underground cable joints are to be mechanically and electrically sound and it is protected against moisture and mechanical damage. The joint is to be further resistant to corrosion and chemical effects.

Basic types of joints – The basic types of cable joints are described below.

Straight through joint is the type of joint which is used to connect two cables lengths together. This joint is further divided in two categories namely (i) simple straight through joints which are for jointing same type of cables such as PVC to PVC, and XLPE to XLPE etc. Transition straight through joints which are for jointing two different type cables such as XLPE to PILC.

Tee joint / branch joint which is normally used for jointing a service cable to the main distribution cable in distribution network. These joints are to be restricted to 1.1 kV grade cables. Tee joints on HT (high tension) cables up to and including 11 kV can be done only in exceptional cases. These joints are made either using cast resin kits or cast-iron boxes with or without sleeves for PILC cables and cast resin kits for PVC and XLPE cables.

Termination or sealing end is normally used to connect a cable to switch gear terminal in switch boards, distribution pillars, transformer box, motor terminal box, and to overhead lines.

Types of cable jointing accessories – Several types of jointing accessories are mainly used for jointing all types of low voltage and medium voltage power cables. Every jointing kit is provided with an instruction manual supplied by the manufacturer. Joints are to be made as per the guidelines given in the instruction manual. The cable jointing has (i) heat shrinkable jointing kit (preferred), (ii) cold shrinkable jointing kit, (iii tapex tape type jointing kit, (iv) push on type jointing kit, and (v) cast resin jointing kit.

Measurement of insulation resistance – Before jointing is commenced, it is desired that the insulation resistance of both sections of the cable to be jointed, be checked by insulation resistance testing instruments like megger. One example is given below for making straight joint for better understanding.

M-seal tapex type joint for 12 kV to 36 kV XLPE cables – Cable jointing is basically a technique of rebuilding the cable construction in the same formation as the original cables to be jointed. Jointing of XLPE power cables is based on several components namely (i) crimping type jointing ferrule, (ii) self-amalgamating insulating tapes, (iii) self-amalgamating semi-conducting tapes, (iv) non-linear stress grading pads, (v) earthing connector and clamps, and (vi) plastic mould and jointing compound etc.

The important steps in the cable jointing of medium voltage XLPE insulated screened armoured cable are (i) stripping of the jointing ends of both the cables to be done i.e., stripping of outer sheath, armour, inner sheath, insulation screen, core insulation and conductor screen, (ii) jointing of all the conductor cores are to be joint with the help of jointing ferrule and its crimping by suitable crimping tool, (iii) filling up the space between the ferrule and the core insulation and the crimped portion in ferrule with semi-conducting tape, so that it forms a smooth and round profile with 2 mm, (iv) measuring of a distance of 20 mm on both sides of the semi-conducting tape and applying stress grading pad of 30 mm width over the core covering 10 mm of the semi-conducting tape, (v) keeping of a gap of 5 mm from the semi-conducting layer of core, wrapping the self-amalgamating insulating tape so that the needed insulation thickness is built up, and ensuring a tapered profile of the tape towards the semi-conducting layer of the insulation, the self-amalgamating tape is to be stretched to 2/3rds of its original width while applying, and (vi) filling up the gap of 5 mm between self-amalgamating insulating tape and semi conducting layer of core by stress grading pad of 30 mm width, (vii) applying of semi-conducting tape one layer half over lapped around 10 mm on one side of metallic shielding to the other end in the same manner, (viii) wrapping of 2 layers of self-amalgamating insulating tape, each half overlapped to cover the semi-conducting tape, and stretching the tape 2/3rds of width while applying, (ix) wrapping one layer of copper wire mesh on the core to connect the copper tape from end to another over the tapes, and (x) after earthing, the mould is placed and filled it with cable jointing compound.

Cable end terminations – Termination kits are designed for terminating cable ends at an indoor type equipment with an indoor termination kit or on pole tops / outdoor transformer with an outdoor type termination. Both types of terminations are designed to operate at optimum level during normal loading and emergency condition of the cables.

The types of termination kits are mainly used for terminating all types of PVC / XLPE power cables up to medium voltage is (i) heat shrink termination kit, (ii) cold shrink termination kit, (iii) pre-moulded push on termination kit, (iv) cast resin termination kit, and (v) brass glands (for low voltage indoor terminations in dry and non-corrosive atmosphere) very terminating kit is provided with an instruction manual supplied by the manufacturer. Terminations are to be made as per the guidelines given in instruction manual. One example is given below for making end termination for better understanding.

M-seal push-on type pre moulded terminations for XLPE / EPR / PVC cables up to 36 kV – M-seal push on type termination kit comprises of intricately engineered and moulded EPDM (ethylene propylene diene monomer) rubber components and these are available up to 1,000 sq-mm for cables from 3.3 kV to 22 kV and up to 630 sq-mm for cables of 33 kV grade. This type of kit comprises of the following.

Stress cone which consists of highly track resistant insulating section is vulcanized to a semi-conducting section.

A semi conducting pad which is used to make the connection between screen and cone. The pad material has cold flow properties. When it is taped into position, the active pressure of the tape induces the cold flow property of the material so that it fills in all the cavities at the screen edge and in the folds of the material itself. This push on method suits all type of core screen including extruded or taped. This can be used on both type of conductors i.e. circular compacted or sector shaped.

The number of rain sheds to be provided is determined by the operating voltage and the location of the termination. The same termination can be used on 3.3 kV to 33 kV by only increasing or reducing the number of rain sheds. Rain sheds are normally provided for outdoor terminations. However, rain sheds can also be used on indoor terminations to increase creepage path in very highly polluted atmosphere or to match limited space availability. M-seal push on has an approximate creepage of 4 centimeters per kV.

A lug seal (for out-door terminations) is also provided to prevent any ingress of moisture. Important steps to be followed while carrying out M-seal push-on termination are given below.

The end of the cable is to be terminated i.e., stripping of outer sheath, armour, inner sheath, insulation screen, and core insulation etc. The stress cone is pushed on the prepared cable core, The stress cone and cable outer screen is connected with the semi-conducting cold flow material. The self-bonding insulating tape is wound over the semi-conducting cold flow material with active pressure. This active pressure ensures that voids are eliminated between the termination and the cable insulation.

Now if the termination is to be used for indoor use, the cable lug is to be provided and crimped using suitable crimping tool. This completed termination is ready for indoor use. For out-door termination, rain sheds and top cap are provided. Number of rain sheds vary with voltage rating of the cables. The cable lug is crimped by using suitable crimping tool and lug seal is provided. This completed crimped termination is ready for outdoor use.

Earthing and bonding of cables – The metal sheath, metal screen (if any), and armour of any cable are to be efficiently earthed at both ends. In case of single-core cables of larger sizes, the armour, lead sheath, and metal screen, if any, is bonded at times only at one point. Attention is drawn in this case to the presence of standing voltages along armour or lead sheath and to the considerable increase in such voltages when cables carry fault currents. These voltages are to be taken into account when considering safety and outer sheath insulation requirement.

All metal pipes or conduits in which the cables have been installed are to be efficiently bonded and earthed. Where cables not having metallic sheath are used, embedding additional earth electrodes and connecting the same with steel armour of cable becomes necessary. Earthing and bonding are to be done as per standards.

Testing of cables – Testing of cables is described below. 

Testing of cable installation – All the newly installed cables are to undergo the insulation resistance test before Jointing. After satisfactory results are achieved, cable jointing and termination work is to commence. It is to be noted here that insulation resistance test gives only approximate insulation resistance and the test is meant to reveal gross insulation faults. A fairly low insulation resistance reading compared to the values achieved at factory testing is not be a cause of worry since the insulation resistance varies greatly with parameters such as length and temperature. This is particularly more pronounced in the case of PVC cables. Table 7 gives the voltage rating of the insulation resistance tester for cables of different voltage grades.

Tab 7 Voltage rating of the insulation resistance tester for  voltage grades of cables
Voltage grade of cableVoltage testing of insulation resistance tester
1.1 kV500 V
3.3 kV1,000 V
11 kV1,000 V
22 kV2.5 kV
33 kV2.5 kV
Note: For long feeders, motorized insulation resistance tester is to used

 The test of completed installation can be measured and entered into record book for comparison purposes during service life of cable installation and during fault location.

Insulation resistance of completed installation is measured by a suitable bridge. In non-screened cables, the insulation resistance of each core is measured against all the other cores and armour / metal sheath connected to earth. With screened construction, the insulation resistance of each core is measured against all the other core and the metal screen connected to earth.

In case of conductor resistance (DC), the resistance of the conductor is measured by a suitable bridge. For this purpose, conductors at the other end are looped together with connecting bond of at least same effective electrical cross-section as conductor. The contact resistance is kept to a minimum by proper clamped or bolted connections. With properly installed and jointed cables, values hence measured and corrected to 20 deg C, are to be in general agreement with values given in test certificates.

The measured loop resistance is converted to ohms per km per conductor as ‘Rt = R / 2L’ (equation 1), Where ‘R’ is the measured loop resistance in ohms at temperature of ‘t’ deg C, ‘Rt’ is the measured resistance per conductor at ‘t’ deg C in ohms per kilometers, and ‘L’ is the length of cable (not the loop) in kilometers.


The ambient temperature at the time of measurement is to be recorded and the conductor resistance is to be corrected to 20 deg C by the formula ‘R20 = Rt / ((1+ a) x (t-20)’ (equation 2), where ‘R20’ is the conductor DC resistance at 20 deg C in ohms per kilometers, ‘t’ is the ambient temperature during measurement in deg C, and ‘a’ is the temperature coefficient of resistance (0.00393 ohms per deg C for aluminum).

For unscreened cables, capacitance is measured for one conductor against others and metal sheath / armour connected to earth. In case of screened cable, it is measured between conductor and screen. Capacitance bridge is used for this purpose. This measurement can be carried in case of cables above 11 kV. Alternatively, values given in test certificate are considered to be sufficient. Cables after jointing and terminating are subjected to DC high voltage test. Tab 8 gives the recommended values of test voltages.

Tab 8 Recommended values of test voltages
Rated voltage of cableTest voltage betweenDuration
Uo/UAny conductor and metallic sheath/screen/armourConductor to conductor (for unscreened cables)

The leakage current is also to be measured and recorded for future reference.

Normally DC test is to be preferred since the test equipment needed is compact, easily portable, and power requirements are low. The cable cores are to be discharged on completion of DC high voltage test and cable is to be kept earthed until it is put into service. DC test voltage for old cables is 1.5 times of the rated voltage or less depending on the age of cables, repair work, or nature of jointing work carried out etc. In any case, the test voltage is not to not be less than the rated voltage. Test voltage in these cases is to be determined by the engineer-in-charge of the work.

It is to be noted that frequent high voltage tests on cable installations are not to be carried out. This test is to be carried only when necessary. During the high voltage test, all other electrical equipment related to the cable installation, such as switches, instrument transformers, and bus bars etc. are to be earthed and adequate clearance is to be maintained from the other equipment and framework to prevent flashovers. In each test, the metallic sheath / screen / armour is to be connected to earth.

Cable installation plan – On completion of laying, terminating and jointing of the cables, a plan is to be prepared, which is to contain details of the installation such as (i) type of cables, cross-section area, and rated voltage, (ii) details of construction, cable number and drum number, (iii) year and month of laying, (iv) actual length between joint-to-joint or end, (v) location of cables and joints in relation to certain fixed reference points, e.g., buildings, hydrant, and boundary stones etc., (vi) name of the person who carried the jointing work, (vii) date of making joint, and (viii) results of original electrical measurements and testing on cable installation. All subsequent changes in the cable plan are also to be entered.

Maintenance – The maintenance of cable installation includes inspection, routine checking of current loading, as well as maintenance and care of all cables and end terminations.

Inspection – Whenever the cables or joints are accessible as in manholes, ducts, and distribution pillars etc., periodical inspection is to be made so that timely repairs can be made before the cables or joints actually cause interruption to service. The frequency of inspection is to be determined by individual from its own experience. Important heavily loaded lines need more frequent attention than less important lines.

Cables laid direct in the ground are not accessible for routine inspection, but such cables are frequently exposed when the ground is excavated by other utilities for installing or repairing their pipe-lines. Preventive maintenance in the form of regular inspection of all digging operations by other utilities or persons, carried out in areas where there are electric cables existing is of utmost importance. In an industrial plant, where the roads are congested with services of other utilities, the likelihood of damaging of electric cables is very high. Cable inspectors are required to patrol the various sections of the plant and where it is found that cables are exposed, these are to be examined thoroughly for any signs of damaged, such as deformation or dents in the cable or damage to earthenware troughs or ducts.

Checking of current loading – The life of paper insulated cables is considerably reduced through overloading. Hence, it is necessary to check the loads as frequently as possible for ensuring that the cables are not loaded beyond the safe current carrying capacities. The derating factors because of the grouping of several cables, higher ambient ground temperature, and higher thermal resistivity of soil are not to be neglected. In the case of high voltage (HV) feeder cables emanating from generating station, receiving station, or sub-station, panel mounted ammeters which are normally provided, are to be read daily. In the case of medium voltage distribution cables emanating from distribution pillars, the loads are conveniently checked by ‘clip-on’ type portable ammeters. Distributor loads are to be checked at intervals not exceeding three months.

Maintenance of cables – Repairs of cables normally involve replacement of a section of the defective cable by a length of new cable and insertion of two straight joints. All repairs and new joint in connection with repairs are to be made in the same manner as joints on new cables. In some cases where the insulation has not been damaged severely, or where moisture has not obtained ingress into the insulation, it can only be necessary to install a joint at the point of cable failure.

Maintenance of end terminations – Visual inspection of all the cable end termination is to be carried out regularly for any over-heating flashing mark, and insulation damage etc. Cable end terminations are to be checked for tightness with a suitable torque wrench / spanner periodically. The cable support clamps, glands are to be checked for proper position and intactness.

Storage and transportation of cables – No drums are to be stored one above the other. Drums are to be stored preferably on a plain ground without having any projected hard stones above the ground surface. The drums are to be stored preferably in the shed. Drums are to be kept in a such a way that bottom cable end does not get damaged. Both the ends of the cable are to be sealed with plastic caps.

The cable drums or coils are not to be dropped or thrown from railway wagons or trucks during the unloading operations. A ramp or crane is to be used for unloading cable drums. If neither of these is available, a temporary ramp with inclination of around 1:3 to 1:4 is to be constructed. The cable drum is then be rolled over the ramp by means of ropes and winches. Additionally, a sand bed at the foot of the ramp can be made to brake the rolling of cable drum.

The arrows painted on the flange of the drum indicate the direction in which the drum is to be rolled. The cable unwinds and becomes loose if the drum is rolled in the opposite direction. The site chosen for storage of cable drums is to be well-drained and preferably has a concrete surface / firm surface which does not cause the drums to sink and hence leads to flange rot and extreme difficulty in moving the drums.

All drums are to be stored in such a manner as to leave sufficient space between them for air circulation. It is desirable for the drums to stand on battens placed directly under the flanges. During storage, the drum is to be rolled to an angle of 90 deg once every three months. In no case the drums are to be stored on the flat, i.e., with flange horizontal.

Overhead covering is not necessary unless the storage is for a very long period. However, the cable is to be protected from direct rays of the sun by leaving the battens on or by providing some form of sun shielding. When for any reason, it is necessary to rewind a cable on to another drum, the barrel of the drum is to have a diameter not less than that of the original drum.

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