Electric power can be transmitted or distributed by Power cables. Power cable may be used as overhead cable or underground cable. The underground cables have some advantages such as less damage through storms or lightning, less chances of faults, low maintenance cost, low voltage drop and better appearance. cables are designed and manufactured as per voltage, current rating , maximum operating temperature and purpose of applications. We can give extra mechanical strength to cable with double armouring, so cable became hard.
Construction of Power Cable :-
During the construction of cable There are various parts are to consider is more important.The power cable mainly consists of,
Conductor sizes are identified with conductor resistance. A cable may have one or more than one core (conductor) depending upon the type of service. For instance, the 3-conductor cable shown in Figs used for 3-phase service. The conductors are made of tinned copper or aluminium and are usually stranded in order to provide flexibility to the cable. There are different types of core as, Single core, Two core , Three core, Four core.
Each core or conductor is provided with a suitable thickness of insulation. the thickness of layer depending upon the voltage to be withstand by the cable and also temperature. The commonly used materials for insulation are impregnated paper, varnished cambric or rubber mineral compound. so insulation is mainly divided in different categories according to temperature rating, class Y, class A, class E, class B ,class F class H, class C and temperature rating is 90°,110°,130°,150°,180° and above 180°c Reaspectively.
Mostly it is used in Multi core cables. In order to protect the cable from moisture, gases or other damaging liquids like acids, in the soil and atmosphere, a metallic sheath of lead or aluminum is provided over the insulation as shown in Fig. used for multi core hence, It works as binder for insulated conductors together in multi-core power cables.
Over the metallic sheath is applied a layer of bedding which consists of a fibrous material like annealed tinned copper wire, nylon braid, cotton braides and hessian tape. in addition The purpose of bedding is to protect the metallic sheath against corrosion and from mechanical injury due to armouring.
we can give extra mechanical strength to cable with double 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. it is used for providing earthing shield to the current carrying conductors as well as it is also used for earthing purpose of the cable for safety. Armouring may be not done in the case of some cables.
In order to protect armouring from atmospheric conditions, mechanical, weather, chemical and electrical protection. layer of fibrous material similar to bedding is provided over the armouring. its provide more mechanical protection compare to electrical protection. so this is known as serving.
Classification of Cable :-
Cables are classified according to there power rating as following,
|1.||Low tension cable||Up to 1000KV|
|2.||High tension cable||Up to 11KV|
|3.||Super tension cable||From 22KV to 33KV|
|4.||Extra high tension cable||From 33KV to 66KV|
|5.||Extra super voltage cable||Above 66KV|
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