Under ground (UG) cables - construction - materials - types - joints – testing

Under ground (UG) cables - construction - materials - types - joints – testing

Under Ground (UG) cables:-

“A cable so prepared that it can withstand pressure and can be installed below the ground level and normally two or more conductors are placed in an UG cable with separate insulation on each conductor”. Or  A typical underground cable will consist of a conductor/s covered by a number of insulating and protective layers necessary for its satisfactory operation.

Advantages

•  Less chance to damage through storms or lightning.
•  Low maintenance cost.
•  Less chances of fault.
•  Voltage regulation in UG cables system is much  better, because they have less inductive losses.

Disadvantages

However, their major draw back / disadvantages are

•  Initial cost of UG cable system is heavy.
•  The cost of joints are more.
•  Introduce insulation problems at high voltages compared with O.H lines.

General construction of UG cables:-

Cores or conductors: A cable may have one or more than one core (conductor) and they are made of tinned copper or aluminum conductor.

Insulation:- Each core or conductor is provided with a suitable thickness of insulation. The commonly used materials for insulation are impregnated paper, varnished cambric or rubber mineral compound.

Metallic sheath: In order to protect the cable from moisture, gases or other damaging liquids (acids or alkalies) in the soil and atmosphere, a metallic sheath of lead or aluminum is provided over the insulation. The metallic sheath is usually a lead or lead alloy.

Paper Belt: Layer of imprignated paper tape is wound round the grouped insulated cores.

Bedding: Over the metallic sheath is applied a layer of bedding which consists of a fibrous material like jute or hessian tape and it protect against corrosion.

Armouring: Over the bedding, armouring is provided which consists of one or two layers of galvanized steel wire or steel tape. Its purpose is to protect the cable from mechanical injury.

Serving: In order to protect armouring from atmospheric conditions, a layer of fibrous material (like jute) similar to bedding is provided over the armouring.

Classification of cables:

Method of classification is generally preferred as

1. (i) Low-tension (L.T) cables – upto 1100 V
2. (ii) High-tension (H.T) cables – upto 11,000 V
3. (iii) Super-tension (S.T cables – from 22 KV to 33 KV
4. (iv)  Extra high-tension (E.H.T) cables – from 33 to 66 KV
5. (v) Extra super voltage cables – beyond 132 KV

    A cable may have one or more than one core depending upon the type of service for which it is intended. It may be (i) single-core (ii) two-core (iii) three-core (iv) four-core etc.

     Single-core low tension cable has ordinary construction because the stresses developed in the cable for low voltages (upto 6600 V). Single-core cables are not usually armoured in order to avoid excessive sheath losses.

                                        

Cables for 3-Phase Service:

 For the purpose, either three-core cables or three single core cables may be used. There are three types

1. Belted cables – upto 11 KV

2. Screened cables – from 22 KV to 66 KV

3. Pressure cables – beyond 66 KV

Belted cables:

                                  

    The cores are insulated from each other by layers of impregnated paper. Another layer of impregnated paper tape called paper belt is wound round the grouped insulated cores. The belted type construction is suitable only for low and medium voltages.

Screened cable: Two principal types of screened cables are H-type cable and S.L. type cables.

H-type cables.: This type of cable was first designed by H. Horchstadter and hence the name. Each core is insulated by layers of impregnated paper .The insulation on each core is covered with a metallic screen which usually consists of a perforated aluminum foil.

                               

S.L. type cables: the constructional details of 3-core S.L (separate lead) type cable. It is basically H-type cable but the screen round each core insulation is covered by its own lead sheath. There is no overall lead sheath but only armouring and serving are provided.

   

Pressure cables: For voltages beyond 66 KV, solid type cables are unreliable because there is a danger of breakdown of insulation due to the presence of voids. Two types of pressure cables viz oil filled cables and gas pressure cables are commonly used.

Oil filled cables: In such type of cables, channels of ducts are provided in the cable for oil circulation.

Oil-filled cables are of three types viz.

(i) Single-core conductor channel

(ii) Single-core sheath channel and

(iii) Three-core filler-space channels.

                    

Gas pressure cables: Pressurized gas (usually dry nitrogen) is circulated around cables in an air-tight steel pipe. Such cables are cable of carrying higher values of load current and can operate at higher values of voltage.

The specification of underground cables:

1   Reference to the Indian Standard; for example Ref. IS 694-1977.

2   Manufacturer’s name, brand name or trademark.

3  Type of cable and voltage grade.

4   Number of cores.

5  Nominal cross-sectional area of conductor.

6  Cable code.

7  Colour of cores (in case of single core cables)

8  Length of cable on the reel, drum or coil

9  Number of lengths on the reel, drum or coil (if more than one).

10  Direction of rotation of drum (by means of arrow).

11 Approximate gross weight.

12  Country of manufacturing.

13  Year of manufacture

Methods of laying of UG cables

The following are the methods of laying underground cables

1.  Laying direct in ground
2.  Laying in ducts
3.  Laying on racks in air.
4.  Laying on racks inside a cable tunnel.
5.  Laying along buildings or structures

Laying direct in ground: This method involves digging a trench in the ground This method involves digging a trench in the ground and laying cable(s)on a bedding of minimum75mm riddled soil or sand at the bottom of the trench, and covering it with additional riddled soil or sand of minimum 75 mm and protecting it by means of tiles, bricks or slabs.

Depth: The desired minimum depth from ground surface to the top of cable is as follows:

a) High voltage cables, 3.3 KV to 11 KV rating : 0.9 m.

b) High voltage cables, 22 KV, 33 KV rating : 1.05 m.

c) Low and medium voltage and control cables : 0.75 m.

d) Cables at road crossings : 1.00 m.

e) Cables at railway level crossings (measured from bottom

of sleepers to the top of pipe) : 1.00 m.

Width: The width of a trench for laying a single cable should be minimum 35 cm. When more than one cable is laid spacing between two cables is 20 cm.

                     

Laying in ducts: When drawing the cables through ducts, lack of space in the drawing pits usually restricts the distance from the cable drum to the duct mouth.

Laying cables on racks in air: Inside buildings, industrial plants, generating stations, substations and tunnels, cables are generally installed on racks fixed to the walls or supported from the ceiling.

Laying cables along buildings or structures: Cables can be routed inside the building along with structural elements or with trenches under floor ducts or tunnels.

Cable jointing methods: This process consists of the following steps.

a) Exact measurement of the cable for insulation removal.

b) Removal of insulation.

c) Replacing of the original insulation with high grade tapes and sleeves.

d) Dressing the cable ends and conductor joints through sleeves/split sleeves.

e) Providing separators between cables.

f) Fixing a cast iron or any other protective shell around the joint and filling the joint boxes with molten bitumen compound.

g) Plumbing metallic sleeves or brass glands to the lead sheath of the cable to prevent moisture from entering the joint in case of cast iron joint boxes or tape insulation in case of cast resin kit joint boxes.

Straight through joints:

This type proves useful when a need is felt for extending cable pieces in any application. They are suited for outdoor, indoor, underground and submerged cable jointing.

              

Tee joint: Tee joints are suited for all voltage levels, construction and insulation materials of cables. A one core cable can be safely connected to a three core cable through this joint.

                           

Types of cable faults and testing procedure: The common faults which are likely to occur in cables are:

1. Ground fault. The insulation of the cable may breakdown causing a flow of current from the core of the cable to the lead sheath or to the earth. This is called “Ground Fault”.

2. Short circuit fault. If the insulation between two conductors is faulty, a current flows between them. This is called a “short circuit fault”.