Title of the invention

Fire and Heat resistant Power Cable with 150° C operating temperature
Field of the invention

The present invention relates to a power cable
that has excellent electrical, thermal, fire, chemical, weathering and mechanical properties even at elevated high temperatures continuously for a very long period.

Background of the invention

Modern power cables come in a variety of sizes, materials, and types, each particularly adapted to its uses. Large single insulated conductors are also sometimes called power cables in the industry. Cables consist of three major components: conductors, insulation, and protective jacket. The makeup of individual cables varies according to application. The construction and material are determined by three main factors:

• Working voltage, determining the thickness of the insulation;

• Current-carrying capacity, determining the cross-sectional size of the conductor(s) and insulating resistance at elevated temperature.

• Environmental conditions such as temperature, water, chemical or sunlight exposure, fire conditions and mechanical impact, determining the form and composition of the outer cable jacket.

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

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

Most of the power cables are made of Polyvinylchloride (PVC), Chemically cross- linked polyethylene/polyolefin (XLPE/XLPO) and by Electron beamed cross-linking Polyethylene/polyolefin (EB-XLPE/XLPO).

A good cable means it must be able to sustain extreme conditions such as high temperature resistance. But on the contrary, the cables available in the market do not have much temperature resistance and it has always failed to remain stable above 90°C. In cases when PVC insulated cables are exposed to flames during fire accidents, the cables form dense and dark fumes, which reduces the visibility to a larger extent and emit toxic gases, thereby making the extinguishing process, a herculean task.

Moreover in low voltage systems, when a linear load is connected either to a balanced or unbalanced three phase system amount of current flowing through the neutral core will be either zero or a very small amount when compared to that flowing through phase. Electrical harmonics are a common problem due to non-linear loads such as computers, battery chargers etc., which are extensively used in modern life. When such a load is connected to an unbalanced system amount of current flowing through the neutral will reach a value of 1.73 times the phase current in the worst conditions, thus increasing the temperature of the cable and heat emitted by conductors. Hence, the insulating material surrounding the conductors has to withstand this additional heat. Also, according to BS 7671:2001 the life of the cable decreases for every 8°C rise in temperature of cable. Voltage drop in the neutral conductor caused by the harmonics affects the voltage profile in all the three phases of the supply. This leads to further increase in the neutral conductor cross section. This suggest that neutral conductor should never have a cross section smaller than the phase conductors in armoured or metal screened cables where contribution of harmonics to eddy currents is considerable.

Non-linear loads which causes the current to vary disproportionately with the voltage during each half cycle has non-sinusoidal waveforms containing distortions whereby creating multiple frequencies within the source sine wave. When current is supplied to non-linear loads through normal cables, due to the presence of harmonics, the current rating factor increase which derates the rating of cable, hence heat increases on conductors and thus reduces the insulation life if the temperature increases over the specified limit.




From the above Figure l(a)-(d) it is clear that conductor size increases whenever harmonic content in the load current increases. For example, Maximum current carried by the PVC, XLPE/XLPO, and EB-XLPE/EB-XLPO insulated 2 and 4 sided irradiated electron beamed insulated 120 sq.mm copper conductor power cables are 240 Amps, 287 Amps, and 373 Amps respectively. However the current carried by the neutral conductor for 120 Sq.mm with 20% harmonic content are 279 Amps, 334 Amps, and 434 Amps for PVC, XLPE/XLPO, and EB-XLPE/EB-XLPO insulated 2 and 4 sided electron beamed irradiated power cables respectively. To carry this additional current created by the harmonics, the conductor size required are 185 Sq.mm, 240 Sq.mm, and 300 Sq.mm respectively for PVC, XLPE/XLPO and EB-XLPE/EB-XLPO insulated 2 and 4 sided electron beamed irradiated power cables. Therefore it is observed that as the harmonic content increases 20 %, 40 % and 50%, the required conductor size also increases by 50 %, 100% and 150% irrespective of the conductor being used. Hence to overcome all these problems and to enhance the performance of the power cables, new techniques in technology have become the need of the day.

Furthermore, the efficiency of the power cable drops down during summer time when there is an increase in ambient temperature and connected load to cable. The power cable has to meet these extreme conditions. In chemical cross linking, hygienic and efficient cross linking is not guaranteed in conditions like contaminated hot water. The electrical conducting properties of the cable might also be affected due to the presence of metallic contaminates in metallic production reels used to wind the cable during for cross linking process.

In addition to that, the electrical strain on the inner sheath which encompasses the insulated cores, in turn exerts a greater strain on the surface of the cores which leads to faster heating of the inner sheath and thereby reduces the efficiency of the inner sheath.

In view of all these disadvantages in the prior art, it is necessary to develop a power cable that can withstand high temperature and also some means to avert the reduction in efficiency caused due to the electrical strain on the inner sheath.

The present inventors have surprisingly found out a Electron Beam Cross Linked (EBXL) power cables with aluminum /copper conductor with primary insulation layer wrapped with glass mica, secondary insulation layer extruded with

Electron Beamed Cross Linked Poly Ethylene or Poly Olefin with enhanced properties.

Object of the invention

The first and foremost object of the present invention is to provide a power cable to withstand temperatures greater than 90 C.

It is another object of the present invention to make the cables withstand the high temperatures atleast for a time period of 20,000 hours.

It is another object of the present invention to provide cables having excellent electrical and mechanical properties at elevated temperatures.

It is another object of the present invention to provide a cable having both fire and heat resistant properties.

It is yet another object of the present invention to provide a conductor of any shape.

It is yet another object of the present invention to provide a primary insulation layer over a EBXL insulated core for 150° C operating temperature.

It is yet another object of the present invention to provide the primary insulation with materials selected from the group comprising glass mica tape, fiber glass tape, heat resistant sheathing, heat resistant tape for EBXL insulated core stated above to perform at elevated temperatures.

It is yet another object of the present invention to provide a heat resistant secondary insulation layer over any shape of conductor and with very high short circuit rating.

It is yet another object of the present invention to extrude the insulation layer with electron beamed cross linked polyethylene/ electron beamed cross linked poly olefin (EB-XLPE/EB-XLPO) for fire and heat resistant cable.

It is yet another object of the present invention to use glass mica tape to withstand high temperatures in two sided as well in four sided irradiated multi-core cross linked cables having sector shape or any other shape of the insulated cores.

It is yet another object of the present invention to encompass the insulated core with an inner sheath with glass mica/fibre glass tape to withstand very mechanical stress at elevated temperature from sharp edged armouring strips/wires.

It is yet another object of the present invention to envelop the sheath with glass mica and with Heat Resistant Flame Retardant low smoke and Zero-halogen low smoke with fire resistant properties.

It is yet another object of the present invention to have the sector shaped aluminium/Copper conductor wrapped with glass mica tape and secondary insulated core with 2 & 4 sided electron beam irradiation for multi-core cables.
Summary of the invention

According to one aspect of the present invention, there is provided a fire resistant power cable which has excellent electrical and mechanical properties which withstands high temperature as high as 150°C continuously for a period of atleast 20,000 hours.

According to another aspect of the present invention, there is provided a cable with aluminum or copper conductor encompassed by primary and secondary insulation layer. The insulation layers are enclosed by glass mica tape. The glass mica tape helps to withstand the high temperatures. The mica layer is surrounded by inner sheath. The inner sheath is selected from a group comprising heat resistant Polyvinylchloride, Zero-Halogen Low Smoke and Flame Retardant Low Smoke or glass mica tape / fiber glass tape.

Reference to Diagrams

The invention will now be described in greater detail with reference to the schematic drawings accompanying this complete specification, wherein,

Figure 1 shows an arrangement of four sided electron beam irradiation.

Figure 2 shows a comparison of current carrying capacity of 120 Sq.mm Polyvinylchloride (PVC) insulated Aluminium/Copper Conductor Cable

Figure 3 shows a comparison of current carrying capacity of 120 Sq.mm XLPE/XLPO insulated Aluminium/Copper Conductor Cable

Figure 4 shows a comparison of current carrying capacity of 120 Sq.mm electron beam irradiated cross linked polyethylene (EB-XLPE) or electron beam irradiated polyolefin (EB-XLPO) insulated Aluminium/Copper Conductor Cable

Figure 5 shows the arrangement of sector shaped conductor, primary/secondary insulation, glass mica tape, inner sheath, flat strip/round wire armouring and outerjacket of the cable

Figure 6 shows the arrangement of round shaped conductor, primary/secondary insulation, glass mica tape, inner sheath, flat strip/round wire armouring and outerjacket of the cable

Detailed description of the invention

The present invention relates to a power cable which has uses in distribution of power. More specifically, the present invention relates to an electric cable that has excellent electrical and mechanical properties even at elevated high temperatures.

The present inventors have surprisingly found electron beam cross linked (EBXL) power cable can withstand very high operating temperature continuously for a period of atleast 20,000 hours. The power cable comprised of aluminum or copper conductor with primary insulation layer, secondary insulation layer extruded with Electron Beamed Cross Linked Polyethylene or Polyolefin.

The insulation layer is wrapped with glass mica to withstand the high temperature and fire conditions in most complex shape of conductors.
The present invention revolutionizes the concept of providing enhanced thermal, electrical, mechanical properties in power cables for applications like wiring and others working at very high operating temperature. The present invention has an advantage over the conventional method of using XLPE, XLPO or similar temperature rated insulation in that we can operate at very high temperatures of 150°C for a time limit of atleast 20,000 hours irrespective of the shape and type of the conductor.

The present invention works well with all types of conductors whether round or sector shaped, single or multicore, armoured or unarmoured sheathed and jacketed.

In one embodiment of the present invention, the conductor is selected from a group comprising aluminium, bare copper and tinned copper. The conductor can be in any shape and specification, preferably either round or sector shaped. Fig.5 and Fig.6 show an arrangement of conductor, primary/secondary insulation, glass mica tape, inner sheath, flat strip/round wire armouring and outer jacket of sector shaped and round shaped cable respectively.

In another embodiment of the present invention, the conductor is surrounded by primary insulation layer. The primary insulation layer is selected from a group comprising glass mica tape, fiber glass tape, heat resistant sheathing and heat resistant type.

In yet another embodiment of the present invention, the conductor and primary insulation layer is enclosed by secondary insulation layer.

The secondary insulation layer is selected from a group comprising electron beam irradiated crosslinked polyethylene or electron beam irradiated polyolefin.

Fig 1 shows the arrangement of four sided electron beam irradiation. Cable is fed under the beam in such a way that upper and lower surfaces of the cable are reversed at each coil. Here the beam trajectories intersect at an angle of 90° in order to obtain four sided irradiation.

The XLPE and XLPO insulated four sided electron beamed irradiation cross linked power cables stand out as the best when compared with PVC insulated or conventionally cross linked cables/wires.1 The temperature rating, di-electric strength at elevated temperature and thermal and mechanical strengths of EB-XLPE/EB- XLPO cables are far better than conventional PVC and XLPE Cables. From the Fig. 2, Fig. 3 &Fig. 4, it is evident that for a 120 sq.mm conductor size the electrical current carried by EB-XLPE/EB-XLPO cable is more than conventionally used XLPE/XLPO and PVC cable for different ambient temperatures.

In some adverse conditions like fire accidents, the Polyvinylchloride (PVC) cables usually burn to produce smoke and fumes rich in halogen (toxic) content, thereby minimizing the visibility to large extent. Dense fumes bring the visibility factor down to a large scale. Hence the EB-XLPE / EB-XLPO power cables make use of ZHLS (Zero Halogen Low Smoke) insulations that prohibit the emission of halogen compounds in air thereby increasing the visibility which in turn reduce the risk while self-extinguishing.

In yet another embodiment of the present invention, the inner sheath surrounds the insulated cores. The inner sheath is made of material selected from a group comprising Heat resistant polyvinylchloride, flame retardant low smoke and zero halogen low smoke.

The inner sheath produces much heat and to overcome the heating, a coating of heat absorbent is Drovided between the conductor and the core. The heat absorbent comprises glass mica and fiber glass tape. The heat absorbent absorbs the heat from the conducting strands thus reducing the strain and work load of the inner sheath. Thus, irrespective of the type of insulation used in the conductor, the glass mica tape could increase the life of cable even under fire condition of a multicore two/four sided electron beam irradiated power cable.

The power cable was tested for different conditions as in fire resistant (IEC 60331, BS 6387; SS 299), low corrosivity and halogen free (IEC 60754; NFC 32-074; EN 50267; BS 6425), low smoke (IEC 61034;NFC 32-073;EN 50268;BS 7622), fire retardant (IEC 60332 Part 3 & 1; EN 50265 1 & 2.1) in accordance with different standards as mentioned within the brackets and the present inventors have observed and established that the power cable conformed to the requirements given in the above mentioned standards

The present invention of power cable is human and equipment friendly wherein it produces very low level of smoke and no harmful gases as in the case of conventional cables which was evident from fire retardant and fire resistant tests. When the power cable has also been tested to 150°C continuously, it surpassed insulation test requirements as against 90°C as specified in British/Indian/Interaational Electro technical Commission (IEC) standard for 90°C operation. When tested for short circuit rating the cable has passed 280°C as against 240°C as specified in British/Indian/ International Electro technical Commission (IEC) standard for the insulated cables. Uniform maintenance of circuit integrity is found when the cable is tested in free air or in a metallic conduit during fire.
Although the invention has been described in detail with reference to the drawings, it is to be construed that it is only to define the features of the invention and describe the same with respect to a particular exemplary unit. It is always to be construed that minor modifications are possible in the invention, without departing from the spirit and scope of the invention.

We Claim:

1. A power cable comprising a conductor, primary insulation material, secondary insulation material, heat absorbent material, inner sheath, wire / strip armoured, an outer jacket, wherein the said cable can be used for operations at high temperature such as 150° C irrespective of the shape and type of the conductor.

2. The cable as claimed in claim 1 wherein the said conductor is selected from a group comprising aluminium and copper having sector or any other shape.

3. The cable as claimed in claim 1 wherein the said primary insulation material is selected from a group comprising glass mica tape, fiber glass tape, heat resistant sheathing and heat resistant tape, Glass mica/fiber glass tape over sector or any other shaped cores.

4. The cable as claimed in claim 1 wherein the said secondary insulation material is selected from a group comprising electron beam irradiated polyethylene and electron beam irradiated polyolefin over sector or any other shaped cores.

5. The cable as claimed in claim 4 wherein the said insulation material is surrounded by glass mica which can withstand high temperature irrespective of the shape and type of the conductor.

6. The cable as claimed in claim 1 & claim 5 wherein the said cable can withstand operating temperature as high as 150°C.

7. The cable as claimed in claim 1 & claim 6 wherein the said cable can operate continuously for a very long time.

8. The cable as claimed in claim 7 wherein the said cable can operate continuously atleast for a period of 20,000 hours.

9. The cable as claimed in claim 1 wherein the said cable can be either single or multi core armoured or unarmoured cables.