Supply Cables and Supports

4.1 INTRODUCTIONUnderground cables have several advantages compared to

overhead lines such as: ● high service reliability,● increased safety under a variety of environmental conditions, and ● better appearance. The major drawback is economical, because of their high insulation cost, especially at high voltage levels. Therefore, ● underground cable applications are restricted to populated areas.

CHAPTER

4

2

4.2 CLASSIFICATION OF CABLES

● Cables applied for distribution may be classified according to type of insulating material and their operating voltage. As for operating voltages cables can be divided into following groups:

● Low-voltage cables (up to 1000 V● High-voltage cables (up to 11 kV)● Super high-voltage cables (from 22 kV to 33 kV)● (Extra high-voltage cables (from 33 kV to 66 kV))● (Extra super high-voltage cables (above 132 kV))

4.3 LOW-VOLTAGE CABLES

CONSTRUCTIONA cable generally consists of three main components (Fig. 4.1); - conductor,

- insulation and - protective sheath

3

Stranded plainconductor Color-coded

PVC insulation

Sheath coloredPVC

Stranded tinnedcopper conductors

Inner plasticsheath

Galvanized steelwire

Color-codedcable insulation

Outer plasticsheath

Plastic filler

Figure 4.1 Low-voltage PVC cables:

(a) Two-core with PVC sheath

(b) four-core armoured with PVC sheath

4

(i) ConductorCopper or aluminum is the materials used as conductor in high and low voltage distribution.

To ensure flexibility and easy handling, conductors are stranded; a number of small size wires are twisted together spirally forming a core equivalent to a single wire of required size. The number of strands used are 1, 7, 19, 37 and 127.

(ii) InsulationIt is a layer of material surrounding the conductor. It should have the following properties:

- high dielectric strength- high resistivity- resistance to moisture and corrosion- high thermal conductivity- high tensile strength- high resistance to acids and alkalies,

5

• In earlier applications paper, vulcanized india rubber and similar insulating materials were exclusively used. However, recently PVC and XLPE materials are being extensively used in cable construction.

• PVC. Polyvinyl chloride is one of the commonest insulating materials used. It is a sythetic compound. It has high insulation resistance, good dielectric strength and is mechanically though over a wide range of temperatures. It is inert to oxgyen and almost all alkalies and acids. It has many application areas as power, installation control cables, in military and civil aircraft wiring, communication and signal wiring etc.

• It has one disadvantage that it softens at temperature above 80oC, therefore not to be used in ambient temperatures above 70oC for a prolonged period. The material will also hardened, and become brittle in temperatures below 0oC

• XLPE. Cross linked polyetylene compound is alternative to PVC, which has the advantage of being able to operate up to 90oC. Thus, it is possible to obtain cables of a smaller diameter for the same voltage rating. However, it has disadvantage of being chemically less inert compared with PVC.

6

(iii) Protective Inner SheathCables consisting of two or more cores are laid together and surrounded by a common covering or sheath (Fig. 4.1).

Sheathing is designed to provide a support to hold inner core conductors together and provide a degree of mechanical

protection. In most cable construction, PVC or fabric cotton is applied as sheathing material

(iv) Metallic Outer Sheath (Armour)The insulating material surrounding inner conductors should be protected mechanically under service conditions. The necessary protection is provided by means of metal 'armouring' ( Fig. 4.1 (b)).

Armouring may be provided either in the form of a single layer of galvanized iron wire or metal tape laid spirally over sheathing. In main and submain cables armouring is used as protective earthing (PE) conductor connected between the earth point of a distribution transformer and main earthing terminal of the installation (Fig. 2.2 (b)).

7

(v) PVC Protective Outer Sheath (Serving)In order to protect armouring from atmospheric conditions, a layer of

PVC or fibrous material is applied over the armour.

TYPES OF LOW-VOLTAGE PVC AND XLPE CABLES

Low-voltage distribution cables are almost made of PVC or XLPE. According to their construction, types:• single core non-armoured, • multi-core non-armoured and • multi-core or single core armoured. These cables are PVC or XLPE insulated cables, with PVC sheath and wire armour (SWA), and PVC sheathed overall (Fig.4.2). Types; • multicore armoured with PVC sheathed; CU/PVC/SWA/PVC or

CU/XLPE/SWA/PVC and • non-armoured with PVC sheathed; CU/PVC/ PVC or CU/XLPE

/PVC are armoured with PVC sheathed; CU/XLPE/ PVC or CU/XLPE /XLPE/PVC.

8

2 x 2.5 mm2

CU/PVC/PVC

4 x 2.5 mm2

CU/PVC/SWA/PVC

2 x 2.5 mm2

CU/PVC/PVC

16 x 1.5 mm2

CU/PVC/SWA/PVC

2 x 10 mm2

CU/PVC/SWA/PVC4 x 6 mm2

CU/PVC/SWA/PVC

Figure 4.2 Typical single-and multi-core PVC (or XLPE) insulated low-voltage cables.

9

● They are manufactured in all sizes up to 400 mm2 or to 1000 mm2. ● If in place of copper conductor or aluminum conductor is to be used, the current carrying capacity is reduced to 80% of a similar cable with copper conductor having the same csa, operated at the same ambient temperature and installed in the same manner.

Color Codes of Low-Voltage Cables ● Each separate single-core cable and each of single-core cable contained in a multi-core cable must be identified by either numerical or color coding. ● The same color coding must also be applied to the associated busbars. In Table 1.1 color coding identifications are given for non-flexible cables.

10

• Table 4.1. Color codes used to identify cores of non-flexible cables (514-06).

Function of core Color identification

Protective (including earthing) conductor

and circuits

Phase of a.c. single-phase circuits

Neutral of a.c. single- or three-phase

circuit

Phase R of thee-phase a.c. circuitPhase Y of thee-phase a.c. CircuitPhase B of thee-phase a.c. circuit

Green-and-yellow

Red

Black

Red

Yellow

Blue

4.4 HIGH-VOLTAGE CABLES

These cables are constructed as either one core or three-core cables applied at ● secondary transmission, and primary and secondary distribution systems (Fig. 4.3).

11

They are constructed as ● 3-core for voltages up to 66 kV in multi-core construction and ● single-core beyond 66 kV.

Construction- Conductors may be copper or aluminum solid or stranded.- Insulation material is often XLPE, but usage of polyethhylen andethylene propylene rubber (EPR) has recently ıncreased- On the conductor side there is a conductor screen and- On the insulation side semi-conducting screen. The functions of screens are to act as an interface between the conductor and the insulating material to provide even distribution of the electric field. - Filler material used in three-core cable is used to support the outer

shields. - Metallic sheath consists of copper wires or aluminum foil, or for one-

core cables as extruded lead sheath usually adopted to the insulation to prevent ingress of moisture into insulation. This sheath at the same time can serve as essential earth path around the cable

- The extruded metallic sheaths are protected by another protective oversheath which is made of PVC or polyethylene. Its flame resistant and flexible.

12

Lead or aluminumsheath

Aluminum orsteel sheath

Semi-conductinginsulation screen

Filler

XLPE insulation

Stranded copper conductor

Low smoke,non-halogen PVC orPE sheath

Stranded copperconductor

Conductor screen

XLPE insulation

Semi-conducting sheath

Low smoke,non-halogen PVC oePE sheath

Lead or aluminum sheath

(a)

(b)

Figure 4.3 High-voltage XLPE-insulated distribution cable constructions. (a) Three-core (<66 kV) cable. (b) Single-core (>66 kV) cable.

13

4.5 FLEXIBLE CABLES AND CORDS• The flexible cables or cords shall be sheathed with rubber or PVC

and armoured if necessary since they are exposed to risk of mechanical damage such as those used for portable tools

• Flexible cords are used for suspending light fittings and for connections to portable domestic appliances having low power consumption (Fig. 4.6). For example, for connection to supply from ceiling rose to pendant lamp fixtures or for connections to portable domestic appliance such as iron, kettles and so on, or for lift control traveling flexible wires are required.

• Flexible cables are made in 600/1000 V range, and include single-core, and circular twin and multi-core cables. The conductors are of tinned annealed copper. Various types of flexible cords and cables are available classified according to material used to insulate conductors. The insulating materials consist of vulcanized rubber, butyl rubber or a layer of silk or cotton braiding.

• They are classified according to their sizes. – Flexible cords vary in size from 0.5 mm2 to 4 mm2.– Flexible cables vary in sizes from 6-mm2 to 630 mm2.

• The following points should be cared for wiring flexible wires.- Under no circumstances they shall be used for fixed wiring.

14

- It is recommended that heath-resisting flexible cords be used between ceiling rose and lampholder in pendant fittings where tungsten-filament lamps are fitted.

- Exposed lengths of flexible cables or cords shall be as short as possible.

Cutton Braiding

CuttonPadding

Insulation

Conductor

Figure 4.6 Non-braided and braided flexible cord or cables.

15

Table 4.2. Color codes used to identify cores of flexible cables.

Number of cores Function of core Color identification

2 Phase

Neutral

Brown

Blue

3

4 or 5

Phase

Neutral

Protective

Phases

Neutral

Protective

Brown

Blue

Green-and yellow

All Brown or Black

Blue

Green-and yellow

16

4.6 FIRE RESISTANT CABLESA. MI CABLES• This type of cable consists of seamless metal tube containing tightly

packed magnesium oxide insulation accommodating copper single strand with one, two, three, four or seven conductors (Fig. 4.4(a)).

• The outer tube at the same time acts as an protective earth conductor (PE). MI cables may be used up to an ambient temperature of 150 oC.

Magnesium oxide insulation is hygroscopic, which it readily absorbsmoisture from the surrounding air, unless it is terminated and sealedwith special seal and gland (Fig. 4.4(b)). The insulating properties of the magnesium oxide is rapidly reduced with moisture. The ends of MI cables must therefore be sealed to prevent moisture penetration. Therefore, specially designed arrangements are needed at all joints and terminations. Fig. 4.4(b)shows the gland termination unit applied for coupling the terminalboxes to switch or socket outlet.• MI cable is extremely robust, fire, and water proof and corrosion

resistant. It has also indefinite life if installed properly.

17

Seamlesscopper shield

PVC shield

Copperconductors

MgOpowder

MgOpowder

Seamlesscopper shield

PVC shield

Sleve

Sealingdisc

Conductor

Conductors embedded inmagnesium powder

Glandbody

Compressionring

Glandnut Cable sheath

(also PE)

(a)

(b)

Figure 4.4 MI insulated copper sheathed cable and terminating components (seal and gland).

18

• These characteristics make it to be installed in hazardous and high temperature areas such as oil refineries, chemical factories, boiler houses and furnaces, petrol pump stations etc. The cable has color-coded PVC sheath; orange is for general electrical wiring, white for emergency lighting or red for fire alarm wiring.

• MI cables are often applied for three-phase installation and for sub-mains in larger buildings. They are used in industrial premises, where resistance to high temperature, corrosive chemical environment, and neat installation appearance are stringent requirements.

• The cable has color-coded PVC sheath; orange is for general electrical wiring, white for emergency lighting or red for fire alarm wiring.

19

B. CWZ CABLES (BS 6387)• With the advancement of building management systems

(BMS), types of cables to be installed gained importance to meet the stringent building safety standards for resistance to fire, smoke emission, acid gas emission, flame, and flame redardation in the event of a fire.

• Categorized CWZ cables and their accessories are produced as alternative to MI or MICC cables in many applications.

• The name of the cable associeted with C, W and Z in BS 6387 are IEC 60331 standards: - Resistance to fire alone (cat. C) - Resistance to water with fire (cat W)

- Resistance to fire with mechanical shock (cat. Z)

20

Stranded tinnedcopper conductors

Bedding (LSZH)

Color-codedcable insulation (XLPE)

Outer plasticsheath (LSZH)

Armour protection(SWA )

Fire barrier (MGT)

Figure 4.5 Fire performance CWZ cable.

21

Construction• CWZ cables are presently manufactured for several applications as

2 or 4 cores. The conductor size of 2 core CWZ cables vary in the range 1.5-400 mm2,and 4 core ones in the range 1.5-16 mm2 (Fig. 4.5).

- The cable conductor is plain annealed stranded copper - Insulated first with mica glass tape (MGT), then with XLPE

insulation. - The cables are embedded in Low Smoke Zero Hallogen (LSZH)

insulation. - The cable is surrounded metallic tape armour with copper earth-wire

maintaining longitudinal contact. - The finally cable is sheathed with LSZH insulation. The MGT is color coded as; 2 core with Brown and Blue; 3 core with Brown, Black and Grey 4 core with Brown, Grey, Blue and Black multicore all with Black.

22

4.7 VOLTAGE DROP ALONG FINAL CIRCUITSThe resistance of a cable causes voltage drop along its circuit • In residential distributions length of final circuits (E) and

distribution cable (D) needs voltage drop consideration. • In commercial and industrial installations lengths of the cables B

and C taking place at distribution levels is also subject to voltage drop consideration.

A B C D

Substation panelSub-main

distribution board

Distribution board

Fina

l circ

uits

Power levelDistribution level

Load levelSub-distribution level

Main distributionboard

E

Figure 4.6. Sections of a power distribution system

23

• The IEE Regulation (522-8) requires that the voltage drop at the terminals of any fixed current-using equipment must not allowed to drop so far to impair the safe operation of the equipment.

• The voltage drop between the origin of the installation (usually the supply terminals) and the fixed current-using equipment should not exceed 4 % of the nominal voltage of the supply.

• If the limit of 4% is to be applied the maximum permissible voltage drop

for 240 V is 9.6 V,

for 415 V is 16 V.

Voltage Drop EquationsFor dc and single-phase circuits, the mV/A/m values are linked to BS 6360 resistance values by mV6330BS invaluekmCatvauesmV/A/m pP /3012 TT o

24

Using the general approximate equations for single-phase and three-phase at 20 oC, the mV/A/m value for cables up to 35 mm2 at any cable conductor operating temperature T1

(oC) can be calculated using

mV/A/mtabulated230

230CatvauesmV/A/m

P

1a T

TT o

The operating temperature of a cable conductor T1 is the sum of ambient temperature Ta, and the temperature rise due to the current flowing through it. Assuming that conductor temperature rise is proportional to square of the current, hence,

at an ambient temperature of Ta, operating temperature of a cable conductor T1 is given by

CC

30 o22

a

2

Pa1z

b

I

ITTT

25

Voltage drop calculation for cables with sizes larger than 25 mm2 • For sizes larger than 25 mm2 the resistance and reactance of the

cable z=r+jx should be included after determining the magnitude of the line current phasor I =I /± . The cable voltage drop can simply be calculated using the following equation.

mVscosmV/A/m cLIinθxθr

4.8 MULTICORE CABLES IN PARALLEL

In large installations it is sometimes necessary to connect two or more similar multicore cable in parallel. Parallel connection facilitates; (i) easy installation of cables. Cables with smaller bending radius may easily be installed in confined spaces. For example, 300-mm2 cable is much larger than the bending radius of 120-mm2 cable; hence it is easier to install it along narrow cable routes. Further, weight of cables with smaller radius can easily be handled for storage, transportation and installation.

26

(ii) considerable increase in current carrying capabilities. For instance, a three-core armoured 95 mm2 cable carries 2.64 A per mm2 whereas a 400 mm2 three-core cable is rated to carry only 1.48 A per mm2 , almost half of the conductor of the larger cable performs no useful purpose.

• Regulation requires that for cables to be connected in parallel should be of

- the same type

- the same cross-sectional area and length

- the same dispositon (subject to the same type of installation methods) and also they should be

- arranged to carry substantially equal current.