Type and classification of cables
Cables may be single core or multi-core cable according to the service it is to do. Multi-core cables may be three cores, four cores or four cores with reduced neutral.The underground cables may be classified into two ways according to:
(1)The type of insulation material used in their manufacture.
(2)The voltage for which they are manufactured
The next fig show the single core and multicore cables.
Single core cable
Multicore cable
To classified the cable according to operating voltage as follows:
(1) Low voltage for operating cable up to 1000 volt.
(2) High voltage for operating cable up to 11000 volt.
(3) Super voltage for operating cable up to 33000 volt.
(4) Extra high voltage for operating cable up to 66000 volt.
(5) Extra super voltage for operating cable up to 132000 volt.
The standard rated voltage of a cable is denoted by Uo/U (Um), where
Uo: is the rated voltage between conductor and neutral, i.e., volt between phase and neutral.
U : is the rated voltage between two conductors, i.e., volt between one phase and another one.
Um: is the maximum continuously permissible operating voltage of a cable at time or in any part of network.
The following table shows the rated voltage:
Table: the rated voltage
Uo/U (kV)
|
0.6/1
|
1.8/3
|
3.6/6
|
6/10
|
8.7/15
|
12/20
|
18/30
|
38/66
|
Um
|
1.2
|
3.6
|
7.2
|
12
|
17.5
|
24
|
36
|
72.5
|
Method of laying underground cables
(i) Cable trenches (direct laying)
This method of laying underground cables is simple and cheap and is much favoured in modern practice.
In this method, a trench of about 40cm wide is dug.(for medium voltage cables they are buried at a distance of 80 cm, for low voltage cables they are buried at a distance of 60 cm and for street lighting UGC are buried at a distance of 40 cm). The trench is covered with a layer of fine sand (of about 10 cm thickness) and the cable is laid over this sand bed. The sand prevents the entry of moisture from the ground and thus protects the cable from decay. After that it is covered with another layer of sand of about 10 cm thickness. The trench is then covered with bricks and other material in order to protect the cable from mechanical injury.
When more than one cable is to be laid in the same trench, a horizontal or vertical inter-axial spacing of at least 30 cm is provided in order to reduce the effect of mutual heating and also to ensure that a fault occurring on one cable does not damage the adjacent cable for the same purpose we put a brick between each two cables every reasonable distance.
From the advantages of direct cable laying:
-It is a simple and less costly method.
-It gives the best conditions for dissipating the heat generated in the cables.
-It is a clean and safe method as the cable is invisible and free from external disturbances.
The disadvantage of this method:
Is that the extension of load is possible only by a completely new excavation which may cost as much as the original work.
There is also another two type of laying but this type are preferable.
Direct laying method
Method of laying underground cable in special location
(i) Road crossing
For laying underground cables under the road, cast iron, galvanized-steel concrete pipes are employed for carrying them. Steel pipe is preferred as they can easily installed and having high resistance against mechanical damage. The pipe should be inclined to reduce the angle of bend as the cables enter and leave the road crossing.
Road crossing
(ii) Railway crossing
The laying underground cables is to be carried out in accordance with the rules and specification of the railway authorities and for that approval of railway authorities is essential the pipe should be laid at depth of 1m below formation level and should extend to minimum distance of 3m from the centre of the outer most track on either side .
(iii) Water main crossing
While crossing water supply main by a power cable, a minimum clearance of .4 m is to be provided between the two.
(iv) Communication line crossing
for laying power cable near communication line its desired to provide a minimum clearance of .6 m between the two horizontally and vertically, so that the disturbance to communication circuit are avoided.
(v) In tunnel
The cable normally cleated on walls or laid on earthed racks or brackets. The cables are run straight so as to avoid sag.
In tunnel
(vi) On bridge
In concrete bridge cover ducts in the form of pipe are provided but in the steel bridge steel pipe are laid on them and the cables are drawn in through them the pipe should be of a diameter large enough to accommodate required number of cables.
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