Cable, wire construction. Applicable materials
Structural elements of power cables, wires and cords: conductive conductors, insulation, sheath and protective covers, screens.
- Differences between cable, wire and cord:
A cable is a single core or several insulated conductors that are covered with a metal or non-metallic sheath, in addition, there may be protective covers (steel or aluminum tapes or wire).
A wire is a single core or several conductors that are insulated, twisted and coated with a non-metallic sheath. The shell can be replaced with light protective covers (braid from yarn, fiberglass).
Cord - two or three flexible parallel conductors in isolation, which are enclosed in a non-metallic sheath, which can be replaced by light protective covers.
Hence make conclusions:
- cords are always flat conductors (most often used in electrical wiring), that's why they differ from wires;
- the cable may have reinforced protective covers, and the wire or cord may include light covers;
- only the cable can be enclosed in a metal sheath (lead, aluminum)
All conductors are operated for the purpose of transporting electrical energy with certain characteristics from point A to point B under specified operating conditions.
- Structural elements of cables, wires and cords and their functions.
Any conductor consists of all or several structural elements from the following list:
- conductive core;
- belt insulation;
- protective cover.
Purpose or function of elements：
- Conductive core - conducting electric current with the minimum possible losses in the form of heating (requirements - high conductivity, low cost, corrosion resistance, flexibility without breaks).
- Insulation - creating a barrier to electrical energy by providing the greatest resistance (requirements - playing the role of a dielectric in the widest temperature range possible, flexibility, adaptability).
- Screen - leveling of external electromagnetic interference (requirements - ease of manufacture, 100% coverage during bending).
- Belt insulation - additional insulation reducing the probability of breakdown.
- Sheath - performance of protective functions (counteraction to mechanical loads, atmospheric factors, ensuring tightness).
- Protective cover - functions similar to the shell, but in more severe operating conditions.
Let us make out in more detail each of the described structural elements of a power cable, wire or cord.
- Conductive core
All cores of cables, wires and cords for general industrial use are produced in accordance with the state standard regulatory document
The standard defines the electrical resistance to direct current of 1 kilometer of a conductive core, at the time of receiving the cable product at a temperature of +20 ⁰С. During operation, slight changes in resistance may be occurred.
To identify electrical resistance, you should know:
- flexibility class of the conductive core (indicates the manufacturer).
- material (copper or aluminum).
- cross-sectional area.
Flexibility class for conductive conductors for fixed cable products 1 or 2.
For conductors that mean mobility during operation, the flexibility classes of the conductive core are from 3 to 6. The higher the flexibility class of the conductive core, the more flexible the core is able to bending. Aluminum core can be maximum of 3 classes. Copper core can be tinned or non-tinned. Wire tinning is used to protect copper from corrosion in a tropical microclimate.
The advantages of copper core:
- good performance (it is possible to produce a monolithic and multi-wire core of almost any diameter);
- high electrical conductivity (second only to silver);
- Good mechanical performances.
- High price;
- Susceptibility to corrosion especially at high temperatures and humidity (protected by tinning or silvering).
Pluses of an aluminum conductors:
- low price;
- Upon contact with air, an oxide film forms which protects against corrosion.
- aluminum is a rather brittle metal, therefore it has low manufacturability and is used only for stationary laying;
- increased electrical resistance of the metal itself;
- oxide film leads to high transition resistance.
-Insulation of cables, wires:
Insulating covers can be made:
- from polyvinyl chloride plastic compound;
- from electrical insulating rubber;
- from silicone rubber;
- made of cross-linked polyethylene;
- from impregnated cable paper;
- from polyethylene;
- from polytetrafluoroethylene.
-Polyvinyl chloride plastic compound
The most common insulation material is polyvinyl chloride (abbreviation PVC) plastic compound.
Varying the composition of the polymer, we achieve its various technical characteristics.
The material has high electrical resistance at room temperature, but when the temperature rises to +70 ⁰С, it decreases by 3 orders of magnitude. In connection with this property, the operating temperature of the conductive core is +70 ⁰С, and this affects the rated current load.
Specialization: the most common insulation for general industrial use, used in cables for stationary laying.
- Low price;
- Availability of all components for the manufacture the polymer;
- Chemical resistance to many reagents;
- Low hygroscopicity (moisture absorption);
- provides tightness;
- does not spread flame.
- loss of electrical resistance when the temperature rises to +70 ° C and above;
- small permissible bending radius in comparison with rubber (used in conductors of fixed connection).
- Rubber insulation
Rubber insulation can only be used with a rubber hose sheath (in case if exists). Since rubber from natural сaoutchouc is quite expensive, almost all rubber used in the cable industry is artificial. Rubber is added:
- vulcanizing ingredients (elements that convert linear bonds in rubber into spatial bonds in insulation, for example, sulfur);
- vulcanization accelerators (reduce time consumption);
- extenders (reduce the price of the material without a significant reduction in technical characteristics);
- softeners (increase plastic properties)；
- age-resistors (is added for shells for resistance to solar radiation)
- Dyes (to give the desired color).
Rubber allows you to assign smaller bending radius of cable products, therefore, together with a multi-wire core, it is used in conductors for mobile connection （Flexible Cable, Cable - Flexible - Shielded - Shaft cables, rubber insulation based on natural and butadiene rubbers - polyethylene terephthalate film winding - polychloroprene based rubber hose wire).
Specialization: used in general industrial cables for mobile connection of consumers.
- low cost of artificial caoutchouc;
- good flexibility;
- high electrical insulation characteristics (6 times higher than the value for PVC compound);
- practically does not absorb water vapor from the air.
- decrease in electrical resistance when the temperature rises to +80 ° C;
- exposure to solar radiation (light oxidation), followed by characteristic cracking of the surface layer (in the absence of a shell);
- requires the introduction of special substances to obtain a certain chemical resistance;
- spreads burning.
Silicon insulation is resistant to low and high temperatures (estimated operating range from -60 to +180 ° C). Due to the siloxane bond of a large number of evident heat resistance, and resistance to oxidants.
The tensile strength of organosilicon rubbers decreases by a factor of 2 when the temperature is brought to +250 ° C; for organic rubbers, this value drops by a factor of 8. Alkalis and acids have a devastating effect. The electric strength does not change during aging at a temperature of +275 ° C for 135 days and decreases by only 10% with eight-month boiling in water.
Considering the described properties, it was found the main application in heat-resistant wires.
Specialization: used for wires that are operated at high temperatures.
- High electrical insulation properties in a wide temperature range;
- Good flexibility;
- improved durability in comparison with ordinary rubber.
Crosslinked polyethylene (abbreviation XLPE) is a modern material that, in addition to the bonds of molecules in a chain, has bonds between adjacent chains. It is used in high-voltage cables for fixed installation in the ground.
Production, classification, brands of cable products, their technical characteristics, as well as a video on the manufacture and photographs of samples are presented in this reference. On the left is a photo of the Conducting aluminum core - XLPE core insulation - Polyethylene sheath cable mark, which shows all the structural components: core, insulation, screen, belt insulation, sealed sheath.
Specialization: used in high voltage cables for use in the ground.
- high flexibility;
- allows a slightly greater heating of the core (without a serious decrease in electrical resistance) than is assigned for paper-insulated cables;
- low hygroscopicity, which does not require expensive metal shells.
- high price;
- manufacturing complexity;
- foreign equipment.
Impregnated-paper insulation (abbreviation IPI) is made of cable paper tapes, which are impregnated with a viscous compound or non-flowing compound (to lay cables with almost no vertical drop of up to 25 meters or with a slight vertical drop, respectively).
A restriction is imposed on matches of tapes that are superimposed on one another - no more than 3 matches and no more than 2 matches when the lower tape is directly adjacent to the core or screen.
The final insulation thickness varies depending on the cross-section of the core: for cables with voltage up to 1 kV within 1.2 - 2.4 mm, for 6 kV about 2 mm; 10 kV about 2.75 mm; 20 kV within 7 - 9 mm; 35 kV about 9 mm.
The main application is high-voltage cables for laying in the ground. A more modern development for the same purposes are high-voltage conductors with cross-linked polyethylene insulation.
Specialization: used in high voltage cables for laying in the ground.
- Low cost of production;
- Good dielectric properties;
- allows small bending radius of the conductor.
- can absorb moisture, which reduces dielectric characteristics (therefore, a sheath is necessary);
- the applied impregnation has good fluidity, especially when the temperature rises (cables for almost horizontal laying);
- high fire hazard;
- low mechanical strength (protective coatings are required).
There are two types of polyethylene that are used for cable products in compositions:
- low density polyethylene (LDPE);
- high density polyethylene (HDPE).
Produced at high and low pressure, respectively, in tubular reactors.
We list the differences in the properties of LDPE and HDPE, respectively:
- softening occurs at 105 ⁰С and 140 ⁰С;
- moisture permeability is 3 times worse for low density PE;
- yield strength up to 1130 and 245 MPa.
At +20 ° C, the polymer does not dissolve in any solvent. When the temperature rises to +70 ° C, it dissolves in carbon tetrachloride, toluene, chloroform and xylene. Most severe atmospheres do not any harm. Polyisobutylene combined with polyethylene, stearic acid and acetylene black (soot) are used to create screens (electrically conductive composition).
Specialization: general industrial cables and wires for fixed installation.
- high dielectric properties (300 times higher than the electrical resistance of PVC compound);
- resistance to chemical reagents;
- low hygroscopicity.
- relatively high cost;
- loss of resistance with increasing temperature;
- poor flexibility (applies only to fixed cabling cables).
Polytetrafluoroethylene (PTFE abbreviation) at +20 ⁰С consists of 90% of the crystalline phase, the rest is an amorphous state (molecules are in chaos). It has good mechanical characteristics under various temperature conditions from -90 to +250 ⁰С. When stretched, the crystals rearrange in the direction of the force.
Increasing temperature to +327 ° C leads to increasing in volume by 25% due to the transition of part of the molecules to the amorphous phase. Upon reaching +400 ° C decomposition begins with the formation of toxic gases. Perfectly resists most of the chemicals. At operating temperatures, it only reacts with molten potassium, sodium, and several fluoride compounds.
Specialization: operation in a wide temperature range in an environment with mechanical loads.
- high resistance to mechanical influences up to 250 ° C;
- low chemical activity.
- high price;
- contains toxic substances.
Electromagnetic shields are used to equalize the electric field in cables and wires. Mainly involved in high voltage cables and control cables.
- From metallized paper (with paper-impregnated insulation);
- From copper wires (with insulation of PVC and rubber);
- From electrically conductive rubber (with rubber insulation);
- galvanized steel wire (one-time reservation and screen).
The shield can be isolated (for each core) or general (protects the bundle of wires).
Specialization: protection of the signal passing through the core from electromagnetic interference.
- sufficient flexibility;
- little material is needed;
- may in turn include a mechanical protection function.
- expensive materials with high electrical conductivity are required.
Sheath of cables and wires
The main functions of the sheath:
- protection from solar radiation;
- protection against water and moisture;
- protection from aggressive substances;
- protection against mechanical damage.
Since paper-impregnated insulation has high moisture permeability, aluminum or lead sheath is used for high-voltage cables. The cable sheath with PVC or rubber insulation is made of PVC compound or hose rubber, respectively.
The main use of metal sheaths is high-voltage cables (for example, Aluminum conductor - Aluminum sheath - Armor of two steel tapes - There is a layer of plastic tapes in the pillow under the armor, Aluminum conductive core - Lead sheath - Armor from two steel tapes marks - aluminum and lead sheaths, respectively).
The use of lead is caused by its simplicity of processing - ductile metal with a low melting point, in addition, has good flexibility, resistance to aggressive environments, is easily soldered in the field when installing a cable line. However, under vibration loads and exposure to heat, cracks are occurred. Since lead has high creep, irreversible stretching of the shell can occur during severe slopes of the path, up to its rupture. Antimony additives (about 0.6%) increase vibration resistance, copper additive (up to 0.08%) reduces creep. Aluminum shells are 2 - 2.5 times stronger than lead shells, have a smaller mass, and resistant to vibrations. In some cases, replace the armoring. In addition, the shell can serve as a screen, having good electrical conductivity. A significant disadvantage is the low resistance to electrochemical and soil corrosion, as well as the high cost of the material.
Specialization: high-voltage cables with paper insulation for laying in the ground.
- High technology;
- Property management in the implementation of various additives;
- associated shielding of cores.
- need corrosion protection;
- Expensive metals;
- Lead is very plastic, so it easily loses its original shape.
PVC plastic sheaths.
The hose plastic compound for the sheath has a different composition compared to PVC insulating plastic compound. The polymer requires protection from light aging, high mechanical strength, sometimes should be resist to contact with oils and gasoline, be resist to severe frosts. If the lower limit of the operating temperature is in interruption, the PVC compound becomes stiff and crumbles upon impact (after restoration of the normal temperature, the properties return). During operation, due to the volatilization of the plasticizer, the characteristics of the polymer are reduced. Sheath thickness for cables up to 6 kV is set by the international standard.
Specialization: general industrial conductors for fixed connection, have become widespread.
- resistance to chemicals;
- sufficient mechanical strength;
- ensuring tightness.
- poor flexibility (compared to rubber);
- Serious mechanical loads are not permissible.
The main differences of hose rubber are its good susceptibility to tensile, twisting and shock loads. There are several types of rubber sheath (variety is determined by factors): for heavy, medium and light working conditions, which are resistant to oil, negative temperatures, which have the property of not spreading combustion.
The vulcanizing substance for artificial rubber is sulfur (2 - 3%), soot acts as a filler (carbon black, moreover, the smaller its particles, the higher mechanical parameters of rubber), thiuram are chosen as the vulcanization accelerator. A classic representative of a cable with rubber insulation and rubber sheath is a flexible cable CF for mobile use.
Specialization: in conductors for mobile connection of current collectors.
- good flexibility;
- affordable cost of artificial rubber;
- practical tightness.
- exposure to simultaneous exposure to solar radiation and oxygen, followed by surface destruction;
- poor resistance to chemicals.
The protective cover replaces the shell and may include: a cushion, armor and the outer cover. The cushion is used to protect the insulation from steel strips or wire, which is wound to book a conductor. Armoring is made of steel or aluminum tapes or wire (an element serving to protect the internal parts of the cable from shock and other mechanical influences). The outer cover must be airtight and resistant to atmospheric (or external) factors and in fact operating as a sheath.
Cushions consist of:
- from crepe paper (technical paper structure with high elongation to break);
- from a asphalt mastic or bitumen (adhesive);
- from plastic tapes (used instead of crepe paper tapes).
- steel tapes (lay two layers with overlap, tapes can be protected from corrosion by galvanizing, work on mechanical damage and do not perceive tensile loads);
- wire (superimposed in a continuous twist against the twisting the insulated cores in order to prevent unwinding; work well in tension and do not protect against shock and vibration).
- from cable yarn or fiberglass impregnated with bitumen (can be made non-combustible);
- made of plastic (polyethylene or polyvinyl chloride plastic compound, type of cover).
Protective covers are characteristic of high-voltage cables with paper-impregnated insulation, more about them can be found in the relevant international regulatory document.
Light protective covers are used for wires, they are used to protect insulation from insignificant mechanical influences. They can be folded from cotton yarn, linen thread, sewing thread, and fiberglass.
The braid can be treated with an antiseptic composition (inhibits the development of fungi, microorganisms, protects against termites) or weather-resistant composition. When used at high temperatures, varnish coatings are used to prevent the penetration of moisture and liquids, do not react with some chemically active substances.
Twisted fiberglass is used for conductors with organosilicon insulation (wire Silicone rubber insulation - flexible - fiberglass protective braid impregnated with enamel or heat-resistant varnish), twisted fiberglass has high mechanical strength (in comparison with yarn). To protect against the spread of harmful glass dust during fiber breakdown, such coatings are varnished.