Conductors

UNDERGROUND LINE CONDUCTORS

In underground lines, conductors are really close to the floor so they are insulated and protected with different layers:

Conductor layers

Usually stranded copper (Cu) or aluminium (Al). Copper is densier and heavier, but more conductive than aluminium. Electrically equivalent aluminium conductors have a cross-sectional area approximately 1.6 times larger than copper, but are half the weight (which may save on material cost).

Annealing – is the process of gradually heating and cooling the conductor material to make it more malleable and less brittle.

Coating – surface coating (eg. tin, nickel, silver, lead alloy) of copper conductors is common to prevent the insulation from attacking or adhering to the copper conductor and prevents deterioration of copper at high temperatures. Tin coatings were used in the past to protect against corrosion from rubber insulation, which contained traces of the sulfur used in the vulcanising process.

Conductor Screen

A semi-conducting tape to maintain a uniform electric field and minimise electrostatic stresses (for MV/HV power cables).

Insulation

Commonly thermoplastic (eg. PVC) or thermosetting (eg. EPR, XLPE) type materials. Mineral insulation is sometimes used, but the construction of MI cables are entirely different to normal plastic / rubber insulated cables. Typically a thermosetting(eg. EPR, XLPE) or paper/lead insulation for cables under 22kV. Paper-based insulation in combination with oil or gas-filled cables are generally used for higher voltages.

Plastics are one of the more commonly used types of insulating materials for electrical conductors. It has good insulating, flexibility, and moisture-resistant qualities. Although there are many types of plastic insulating materials, thermoplastic is one of the most common. With the use of thermoplastic, the conductor temperature can be higher than with some other types of insulating materials without damage to the insulating quality of the material. Plastic insulation is normally used for low- or medium-range voltage.

The designators used with thermoplastics are much like those used with rubber insulators. The following letters are used when dealing with NEC type designators for thermoplastics:

T - Thermoplastic

H - Heat-resistant

W - Moisture-resistant

A - Asbestos

N - Outer nylon jacket

M - Oil-resistant

Paper has little insulation value alone. However, when impregnated with a high grade of mineral oil, it serves as a satisfactory insulation for extremely high-voltage cables. The oil has a high dielectric strength, and tends to prevent breakdown of the paper insulation. The paper must be thoroughly saturated with the oil. The thin paper tape is wrapped in many layers around the conductors, and then soaked with oil.

Enamel: the wire used on the coils of meters, relays, small transformers, motor windings, and so forth, is called magnet wire. This wire is insulated with an enamel coating. The enamel is a synthetic compound of cellulose acetate (wood pulp and magnesium). In the manufacturing process, the bare wire is passed through a solution of hot enamel and then cooled. This process is repeated until the wire acquires from 6 to 10 coatings. Thickness for thickness, enamel has higher dielectric strength than rubber. It is not practical for large wires because of the expense and because the insulation is readily fractured when large wires are bent.

Mineral-insulated (MI) cable was developed to meet the needs of a noncombustible, high heat- resistant, and water-resistant cable. MI cable has from one to seven electrical conductors. These conductors are insulated in a highly compressed mineral, normally magnesium oxide, and sealed in a liquidtight, gastight metallic tube, normally made of seamless copper.

Silk and Cotton: in certain types of circuits (for example, communications circuits), a large number of conductors are needed, perhaps as many as several hundred. Because the insulation in this type of cable is not subjected to high voltage, the use of thin layers of silk and cotton is satisfactory.

Silk and cotton insulation keeps the size of the cable small enough to be handled easily. The silk and cotton threads are wrapped around the individual conductors in reverse directions. The covering is then impregnated with a special wax compound.

Insulation Screen

A semi-conducting material that has a similar function as the conductor screen (ie. control of the electric field for MV/HV power cables).

Conductor Sheath

A conductive sheath / shield, typically of copper tape or sometimes lead alloy, is used as a shield to keep electromagnetic radiation in, and also provide a path for fault and leakage currents (sheaths are earthed at one cable end). Lead sheaths are heavier and potentially more difficult to terminate than copper tape, but generally provide better earth fault capacity.

Filler

The interstices of the insulated conductor bundle is sometimes filled, usually with a soft polymer material.

Bedding / Inner Sheath

Typically a thermoplastic (eg. PVC) or thermosetting (eg. CSP) compound, the inner sheath is there to keep the bundle together and to provide a bedding for the cable armour.

Individual Screen (Instrument Cables)

An individual screen is occasionally applied over each insulated conductor bundle for shielding against noise / radiation and interference from other conductor bundles. Screens are usually a metallic (copper, aluminium) or semi-metallic (PETP/Al) tape or braid. Typically used in instrument cables, but not in power cables.

Drain Wire (Instrument Cables)

Each screen has an associated drain wire, which assists in the termination of the screen. Typically used in instrument cables, but not in power cables.

Overall Screen (Instrument Cables)

An overall screen is applied over all the insulated conductor bundles for shielding against noise / radiation, interference from other cables and surge / lightning protection. Screens are usually a metallic (copper, aluminium) or semi-metallic (PETP/Al) tape or braid. Typically used in instrument cables, but not in power cables.

Armour

For mechanical protection of the conductor bundle. Steel wire armour or braid is typically used. Tinning or galvanising is used for rust prevention. Phosphor bronze or tinned copper braid is also used when steel armour is not allowed.

• SWA - Steel wire armour, used in multi-core cables (magnetic),
• AWA - Aluminium wire armour, used in single-core cables (non-magnetic).

When an electric current passes through a cable it produces a magnetic field (the higher the voltage the bigger the field). The magnetic field will induce an electric current in steel armour (eddy currents), which can cause overheating in AC systems. The non-magnetic aluminium armour prevents this from happening.

Outer Sheath

Applied over the armour for overall mechanical, weather, chemical and electrical protection. Typically a thermoplastic (eg. PVC) or thermosetting(eg. CSP) compound, and often the same material as the bedding. Outer sheath is normally colour coded to differentiate between LV, HV and instrumentation cables. Manufacturer’s markings and length markings are also printed on the outer sheath.

Termite Protection

For underground cables, a nylon jacket can be applied for termite protection, although sometimes a phosphor bronze tape is used.

Conductor Protection (Appendix)

Wires and cables are generally subject to abuse. The type and amount of abuse depends on how and where they are installed and the manner in which they are used. Cables buried directly in the ground must resist moisture, chemical action, and abrasion. Wires installed in buildings must be protected against mechanical injury and overloading. Wires strung on crossarms on poles must be kept far enough apart so that the wires do not touch. Snow, ice, and strong winds make it necessary to use conductors having high tensile strength and substantial frame structures.

Generally, except for overhead transmission lines, wires or cables are protected by some form of covering. The covering may be some type of insulator like rubber or plastic. Over this, additional layers of fibrous braid or tape may be used and then covered with a finish or saturated with a protective coating. If the wire or cable is installed where it is likely to receive rough treatment, a metallic coat should be added.

The materials used to make up the conductor protection for a wire or cable are grouped into one of two categories: non-metallic or metallic.

Non-Metallic

The category of non-metallic protective coverings is divided into three areas. These areas are:

(1) according to the material used as the covering,

(2) according to the saturant in which the covering was impregnated, and

(3) according to the external finish on the wire or cable.

These three areas reflect three different methods of protecting the wire or cable. These methods allow some wire or cable to be classified under more than one category. Most of the time, however, the wire or cable will be classified based upon the material used as the covering regardless of whether or not a saturant or finish is applied.

Many types of non-metallic materials are used to protect wires and cables. Fibrous braid is by far the most common and will be discussed first.

Fibrous Braid

Fibrous braid is used extensively as a protective covering for cables. This braid is woven over the insulation to form a continuous covering without joints. The braid is generally saturated with asphalt, paint, or varnish to give added protection against moisture, flame, weathering, oil, or acid. Additionally, the outside braid is often given a finish of stearin pitch and mica flakes, paint, wax, lacquer, or varnish depending on the environment where the cable is to be used.

Woven Covers

Woven covers, commonly called loom, are used when exceptional abrasion-resistant qualities are required. These covers are composed of thick, heavy, long-fibered cotton yarns woven around the cable in a circular loom, much like that used on a fire hose. They are not braids, although braid covering are also woven; they are designated differently.

Rubber and Synthetic Coverings

Rubber and synthetic coverings are not standardized. Different manufactures have their own special compounds designated by individual trade names. These compounds are different from the rubber compounds used to insulate cable. These compounds have been perfected not for insulation qualities but for resistance to abrasion, moisture, oil, gasoline, acids, earth solutions, and alkalies. None of these coverings will provide protection against all types of exposure. Each covering has its own particular limitations and qualifications.

Jute and Asphalt Coverings

Jute and asphalt coverings are commonly used as a cushion between cable insulation and metallic armour. Frequently, they are also used as a corrosive-resistant covering over a lead sheath or metallic armour. Jute and asphalt coverings consist of asphalt-impregnated jute yarn heli-wrapped around the cable or of alternate layers of asphalt-impregnated jute yarn. These coverings serve as a weatherproofing.

Unspun Felted Cotton

Unspun felted cotton is commonly used only in special classes of service. It is made as a solid felted covering for a cable.

Metallic

Metallic protection is of two types: sheath or armour. As with all wires and cables, the type of protection needed will depend on the environment where the wire or cable will be used.

Metallic Sheath

Cables or wires that are continually subjected to water must be protected by a watertight cover. This watertight cover is either a continuous metal jacket or a rubber sheath molded around the cable.

Lead-sheathed cable is one of three types currently being used: alloy lead, pure lead, and reinforced lead. An alloy-lead sheath is much like a pure lead sheath but is manufactured with 2-percent tin. This alloy is more resistant to gouging and abrasion during and after installation. Reinforced lead sheath is used mainly for oil-filled cables where high internal pressures can be expected. Reinforced lead sheath consists of a double lead sheath. A thin tape of hard-drawn copper, bronze, or other elastic metal (preferably nonmagnetic) is wrapped around the inner sheath. This tape gives considerable additional strength and elasticity to the sheath, but must be protected from corrosion. For this reason, a second lead sheath is applied over the tape.

Metallic Armour

Metallic armour provides a tough protective covering for wires and cables. The type, thickness, and kind of metal used to make the armour depend on three factors:

(1) the use of the conductors,

(2) the environment where the conductors are to be used, and

(3) the amount of rough treatment that is expected.

1. Wire-braid armour

Wire-braid armour, also known as basket-weave armour, is used when light and flexible protection is needed. Wire braid is constructed much like fibrous braid. The metal is woven directly over the cable as the outer covering. The metal used in this braid is galvanized steel, bronze, copper, or aluminum. Wire-braid armour is mainly for shipboard use.

2. Steel tape

A second type of metallic armour is steel tape. Steel tape covering is wrapped around the cable and then covered with a serving of jute. There are two types of steel tape armour. The first is called interlocking armour. Interlocking armour is applied by wrapping the tape around the cable so that each turn is overlapped by the next and is locked in place. The second type is flat- band armour. Flat-band armour consists of two layers of steel tape. The first layer is wrapped around the cable but is not overlapped. The second layer is then wrapped around the cable covering the area that was not covered by the first layer.

3. Wire armour

Wire armour is a layer of wound metal wire wrapped around the cable. Wire armour is usually made of galvanized steel and can be used over a lead sheath (see view C of the figure above). It can be used with the sheath as a buried cable where moisture is a concern, or without the sheath when used in buildings.

4. Coaxial cable

Coaxial cable is defined as two concentric wires, cylindrical in shape, separated by a dielectric of some type. One wire is the center conductor and the other is the outer conductor. These conductors are covered by a protective jacket. The protective jacket is then covered by an outer protective armour.

Coaxial cables are used as transmission lines and are constructed to provide protection against outside signal interference.

General conductor layers:

Conductors with specific layers:

Designation of conductors for cables from 1 to 30 kV

Designation of conductors according to UNE 21024 standards for cables insulated in impregnated paper and nominal voltages of 1’8/3 kV and 26/45 kV, UNE 21123 for insulated cables from 1 to 30 kV.

1. Material de aislamiento.

B: Goma butílica.
D: Etileno-propileno.
E: Polietileno.
G: Goma.
P: Papel impregnado.
R: Polietileno reticulado.
S: Silicona.
V: PVC (cloruro de polivinilo).

2. Pantalla y envoltura metálica.

H: Pantalla.
AL: Tubo continuo de aluminio.
AW: Tubo corrugado de aluminio.
C: Tubo liso de cobre.
CW: Tubo corrugado de cobre.
P: Tubo continuo de plomo.
3P: Caso de cables tripolares con envolvente de plomo individual.
3(PV): en el caso de que cada cable de plomo esté protegido con una capa de PVC.

3. Tipo de recubrimientos.

B: Goma butílica.
D: Etileno-propileno.
E: Polietileno.
G: Goma.
P: Papel impregnado.
R: Polietileno reticulado.
S: Silicona.
V: PVC (cloruro de polivinilo).
J: Material textil.

4.Tipo de armadura.

F: Fleje de hierro.
EI: Flejes de acero reforzado.
FA: Flejes de aluminio.
M: Alambres de hierro.
MA: Alambres de aluminio reforzado.
MV: Alambres de acero recubiertos de PVC.
Q: Pletinas de hierro.
QA: Pletinas de aluminio.

5.Materiales de la cubierta.

E: Polietileno.
G: Goma.
J: Material textil.
N: Policloropreno.
V: Cloruro de polivinilo.
K: Caucho de silicona.

6.Tensión nominal.

Viene expresada en kilovoltios y se designará con los valores U0/U, que indican la tensión nominal entre conductores y tierra el primer valor y tensión entre conductores el segundo valor, por ejemplo 1,8/3 Kv.

7.Número de conductores y sección nominal.

La primera cifra indicará el número de conductores seguido de un signo x, una segunda cifrá indicará la sección nominal del conductor en mm², y, si existe conductor de sección reducida, se indica igualmente en mm² despúes del signo /, por ejemplo, 3 x 50/35.

8. Forma de los conductores y/o tratamientos recibidos por los mismos.

E: Conductor estañado.
F: Conductor flexible.
K: Conductor compacto.
S: Conductores sectoriales.

9.Material del conductor.

Al: Aluminio.
AC: Acero.
ALM: Aleación de aluminio (Almelec).

Connections betwen two conductors:
The splice is the accessory that connects two sections of an underground line. In the connection of insulated cables the continuity of all the elements that make up the cable has to be ensured: the driver, the screen, the insulation, the cover, etc. The connection of the conductor is made by sleeves or metal elements which contain both conductors, constituting the body of the joint.
The rest of the elements are joined by accessories that are mostly retractable in tubular form which are concentric to the sleeve.