Medium Voltage Cables 

Medium Voltage Cables to BS 6622

Single Core Cables to BS 6622

Application
The single core cables are designed for distribution of electrical power with nominal voltage Uo/U ranging from 3.8/6.6KV to 19/33KV and frequency 50Hz. They are suitable for installation mostly in power supply stations, indoors and in cable ducts, outdoors, underground and in water as well as for installation on cable trays for industries, switchboards and power stations.

Standards
BS 6622
BS 7835 (LSZH)

Construction

Conductor
Plain annealed copper or aluminium complying with IEC 60228/BS 6360. Copper conductors shall be stranded (class 2) and aluminium conductors shall be either solid or stranded (class 2).
Conductor Screen
Extruded layer of semi-conducting cross-linkable compound is applied over the conductor and shall cover the surface completely. The minimum thickness is 0.3mm and the maximum resistivity shall not exceed 500 Ohm-m at 90°C.
Insulation
Insulation is of cross-linked polyethylene compound XLPE (GP8) conforming to BS 7655-1.3 or EPR (GP7), conforming to BS 7655-1.2.

Table 1. Insulation Thickness

Nom. Cross
Section Area

Insulation Thickness at Nominal Voltage

3.8/6.6KV (Um=7.2KV)

6.35/11KV (Um=12KV)

8.7/15KV(Um=17.5KV)

12.7/22KV(Um=24KV)

19/33KV(Um=36KV)

mm²

mm

mm

mm

mm

mm

70 – 185

2.5

3.4

4.5

5.5

8.0

240

2.6

3.4

4.5

5.5

8.0

300

2.8

3.4

4.5

5.5

8.0

400

3.0

3.4

4.5

5.5

8.0

Above 500

3.2

3.4

4.5

5.5

8.0

Insulaton Screen
Extruded layer of semi-conducting cross-linkable compound is applied over the insulation. The extruded semi-conducting layer shall consist of bonded or cold strippable semi-conducting compound capable of removal for jointing or terminating. As an option, a semi- conducting tape may be applied over the extruded semi-conducting layer as a bedding for the metallic layer. The minimum thickness is 0.3 mm and the maximum resistivity is 500 Ohm-m at 90°C. The screen is tightly fitted to the insulation to exclude all air voids and can be easily hand stripped on site.
Metallic Layer
The metallic layer shall consist of either copper tapes or a concentric layer of copper wires or a combination of tapes and wires. The metallic layer provides an earth fault current path, capable of withstanding fault current to earth of 1000A for one second at maximum temperature 160°C. Copper wires are applied over the conducting water blocking layer with a minimum diameter of 0.5mm. And over the copper wires, copper tape with minimum thickness of 0.1mm can be applied helically with overlap. Total cross section of copper wire screen layer is shown in table 2.

Table 2. Minimum Total Cross Section of Copper Wire Screen & DC Resistance of the screen

Nominal Cross-Section of Cables

Minimum Cross-Section of Copper Wire Screen

DC Resistance of the Copper Wire Screen at 20°C

mm²

mm²

Ω

up to 120

16

1.06

150-300

25

0.72

400-630

35

0.51

Separation Sheath (for armoured cable)
The separation sheath comprises a layer of extruded PVC, PE or LSZH. The nominal thickness is calculated by 0.02Du + 0.6mm where Du is the fictitious diameter under the sheath in mm. The nominal separation sheath thickness shall not be less than 1.2mm.
Armour (for armoured cable)
The armour consists of round aluminium wire with diameter specified as in Table 3.

Table 3. Armour Wire Diameter

Fictitious Diameter Under the Armour

Armour Wire Diameter

mm

mm

> 

< 

 

 

25

1.6

25

35

2.0

35

60

2.5

60

-

3.15

Over Sheath
Overall sheath comprises a layer of extruded either PVC type 9 conforming to BS 7665-4.2 or MDPE type TS2 conforming to BS 7655-10.1. LSZH can be offered as an option. The oversheath is normally black in colour. When a DC voltage test is to be performed on the oversheath, a semi-conducting layer such as graphite coating shall be applied over the surface of the extruded oversheath. The nominal oversheath thickness is calculated by 0.035+D where D is the diameter immediately under the oversheath in mm. For cables with the oversheath not applied over the armour, the nominal oversheath thickness shall not be less than 1.4mm. And for cables with oversheath applied over the armour, the nominal oversheath thickness shall not be less than 1.8mm.

Physical Properties
Operating Temperature: up to 90°C
Temperature Range: -5°C ( PVC sheath ); -20°C ( PE sheath )
Short Circuit Temperature: 250°C (short circuit duration up to 5 seconds)
Bending Radius: 12 x OD

Table 4. Nominal /Operating /Test Voltages

Rated Voltage Uo/U

Operating Voltage (Um)

Testing Voltage (rms)

3.8/6.6KV

7.2KV

15KV

6.35/11KV

12KV

25.5KV

8.7/15KV

17.5KV

35KV

12.7/22KV

24KV

51KV

19/33KV

36KV

76KV

Single Core 3.8/6.6KV (Um=7.2KV) Dimensional Data

Nom.
Cross-
Section
Area

 

Unarmoured  Cables

Aluminium Wire Armoured Cables

Nom.
Insulation Thickness

Copper Wire Screen Area

Nom.
Sheath

Thickness

Approx. Overall Diameter

Approx. Weight

Copper Tape Screen Area

Nom. Bedding
Thickness

Nom. Armour Wire Diameter

Nom.
Sheath Thickness

Approx. Overall Diameter

Approx.
Weight

Cu

AL

Cu

 AL

mm2

mm

mm2

mm

mm

kg/km

mm2

mm

mm

mm

mm

kg / km

25

2.5

16

1.5

20.3

750

580

3.4

1.2

1.6

1.8

26.3

1150

1000

35

2.5

16

1.6

21.3

870

650

3.6

1.2

1.6

1.8

27.3

1250

1050

50

2.5

16

1.6

22.4

1090

770

3.9

1.2

1.6

1.8

28.4

1500

1190

70

2.5

16

1.7

24.2

1310

870

4.3

1.2

1.6

1.8

30.0

1740

1300

95

2.5

16

1.7

25.8

1560

960

4.7

1.2

1.6

1.9

31.8

2030

1430

120

2.5

16

1.8

27.4

1830

1070

5.0

1.2

1.6

1.9

33.2

2310

1550

150

2.5

25

1.8

29.1

2130

1190

5.3

1.2

1.6

2.0

35.1

2660

1720

185

2.5

25

1.9

30.8

2490

1320

5.8

1.2

2.0

2.0

37.4

3120

1950

240

2.6

25

1.9

33.5

3040

1520

6.3

1.2

2.0

2.1

40.3

3740

2200

300

2.8

25

2.0

36.1

3650

1770

7.0

1.2

2.0

2.2

42.9

4400

2520

400

3.0

35

2.2

40.6

4680

2170

7.7

1.2

2.0

2.3

47.2

5490

2960

500

3.2

35

2.3

43.6

5670

2570

8.5

1.3

2.5

2.5

51.6

6760

3660

630

3.2

35

2.4

48.8

6980

2990

9.5

1.4

2.5

2.6

57.0

8200

4160

Electrical Data

Nom.
Cross-
Section
Area

 

Unarmoured
Cables

Aluminium Wire Armoured Cables

 

D C
Resistance
CU / AL

A C
Resistance
CU / AL

Short Circuit Rating of
Conductor
CU / AL
1 sec

Capaci-
tance

Charging Current

Short
Circuit Rating of Copper
Wire
Screen
1 sec

Short Circuit Rating of Copper Tape Screen
1 sec

Reactance

Inductance

Impedance

Trefoil

Flat     spaced

Trefoil

Flat
Spaced

Trefoil

Flat Spaced

Cu

AL

Cu

AL

mm2

µΩ/m

µΩ/m

kA

pF/m

mA/m

kA

kA

µΩ/m

µΩ/m

nH/m

nH/m

µΩ/ m

µΩ/m

25

727/1200

927/1538

3.6/2.3

263

0.32

2.6

0.4

136

194

400

590

936

1544

952

1554

35

524/868

668/1113

5.0/3.2

291

0.35

2.6

0.5

127

185

390

580

679

1121

695

1131

50

387/641

494/822

6.8/4.4

321

0.39

2.6

0.5

121

179

380

570

511

834

527

844

70

268/443

343/568

9.8/6.3

371

0.45

2.6

0.6

115

173

370

550

364

583

386

597

95

193/320

248/410

13.3/8.5

417

0.50

2.6

0.6

110

168

350

540

272

427

300

446

120

153/253

196/325

17.2/11.0

459

0.55

2.6

0.7

107

165

340

520

225

345

257

367

150

124/206

159/265

21.2/13.5

494

0.59

4.3

0.7

103

161

330

510

193

287

229

313

185

99.1/164

128/211

26.6/17.0

543

0.65

4.3

0.8

100

158

320

500

165

237

206

267

240

75.4/125

98/161

34.9/22.3

583

0.70

4.3

0.9

97

155

310

490

140

191

185

226

300

60.1/100

80/130

43.8/28.0

602

0.72

4.3

1.0

95

153

300

490

126

163

174

203

400

47.0/77.8

64/102

57.3/36.6

627

0.75

5.8

1.1

92

150

290

480

113

141

164

184

500

36.6/60.5

51/81

72.3/46.2

654

0.79

5.8

1.2

90

149

290

470

105

124

158

171

630

28.3/46.9

42/64

91.2/58.3

726

0.87

5.8

1.3

87

145

280

460

97

110

151

160

Single Core 6.35/11KV (Um=12KV)
Single Core 8.7/15KV (Um=17.5kV)
Single Core 12.7/22KV (Um=24kV)
Single Core 19/33KV (Um=36kV)
Current Rating for 3.8/6.6KV (Um=7.2KV), 6.35/11KV (Um=12KV) & 8.7/15KV (Um=17.5KV)
Current Rating for 12.7/22KV (Um=24KV)
Current Rating for 19/33KV (Um=36KV)

Current Rating Conditions:

Ground Temperature:20°C
Ambient Temperature (air):
30°C
Depth of Soil: 0.8m
Thermal Resistance of Soil: 1.5K•m/W