Saturday, June 11, 2016

Cables selection Methodology in electrical systems

1.       Cable Selection according to voltage level:
Cables are selected according to voltage level required to be carried by cable. Nominal voltage of the cable is usually expressed in Volts. Cables have nomenclature according to voltage level to be carried through the cables.

There are following types of cables according to voltage levels:-
Low tension cables:- Cables having voltage level upto 1000V
High tension cables:- Theses cables have voltage levels upto 11000V
Super tension Cables:- These cables having voltage level from 22KV to 33 KV
Extra High tension cables:- These cables having voltage level from 33KV to 66 KV
Extra Super voltage cables:- These cables having voltage levels beyond 132 KV
 (2) Cable selection according to Current carrying capacity of cables:
Cables are selected according to current carrying capacity of cables. This current carrying capacity is known as Ampacity. Cables are usually copper/ Aluminum cables , these cables are selected according to requirement of load connected at cable. Usually cables are selected 20% higher than the maximum current connected across cable.
Short circuit current values are also considered while selecting the cable.  Short circuit will leads to rise in temperature of cables. Cables are usually selected according to short circuit current carried by cables without much rise in temperature.
There is another factor which must be considered while selecting a power cable is voltage drop. Voltage drop depends upon resistance of power cable. Cable resistance depends upon length and diameter of cable. Cable size is selected according to voltage drop over the length of cable.

No of Cores in cable:
In electrical systems there are cables used single core to multicore cables. For Single phase power supply usually 2 core cable is used for three phase cables used are 3 core , 3.5 core or 4 core. Single core cables are used in electrical systems. 3.5 core is most widely used as 4 core cables uses neutral of same size of other cores but in 3.5 core cable neutral is half of size of other cores.
There are multicore cables such as 14 core/26 core/ 48 core etc.  these cables are used for control cables. These cables are both shielded and unshielded. Shielded cables are used for field instruments where interference from other signals to be avoided.

 Cable selection according to type of Insulation:
§  Cables are selected according to type of insulation used in cables e.g. PVC, XLPE , rubber. Usually XLPE and PVC insulated cables are used XLPE cables are cheaper than PVC cables. Cables outer sheath is designed as per standards : IS: IS:7098 / IEC:60502 / BS:6622/BS:7835.
 Cable selection according to Method of Installation:
Cables are both armored, Unarmored and flexible cables these cables are selected according to method of installation of cables.
Cables are to be selected according to method of laying of cables, there are following ways of laying the cables:-
1.     Underground
2.     Above ground in cable tray
3.     In air
If cable is to be laid underground than armored cable is used as it will provided more strength to cables.

If there is required to lay cables in cable tray than unarmored cables are used as these will leads to lower cost.
If we lay cables in air than these cable will carry higher current than the same cable laid underground.
Armoring of cable is done as per Codes: IS: 7098 / IS: 3975 / IEC:60502 / BS:6622/BS:7835.

Cable selection according to Environmental conditions:
§  Cables are usually selected according to industry and environment where these are used. E.g. XLPE cables are used where there are moisture conditions. A PVC jacket is a very stable material against a wide range of chemicals, while HDPE jacketed cables can serve better in wet locations.
       Elastomeric Cable is applied in trailing, coal cutter, wind mill, panel wiring, battery cable and such other areas.

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