HIGH VOLTAGE CABLE SELECTION

http://en.wikipedia.org/wiki/Earthing_system

In a simple system where generators and transformers are having capacities of few MVA the fault current is less, the star point or neutral point is solidly grounded. This is called earthed system.
Depending up on the application of use, the cables are classified as "earthed" and "unearthed" .In a three phase system with star point is earthed directly it is an earthed system. In the event of the earth fault, the voltage in the faulty phase will become zero. The voltage between the healthy phases and the ground will be the same. In a large and complex power generation and distribution Systems where large generators (to the tune of 500 MVA) capacity are in commercial use and also several mega power stations are connected to grid, the fault level will be increased tremendously. In case of an earth fault, a heavy current flows in to the fault and this may damage the costly generators and power transformers. To reduce the fault current, the star point is connected to the earth through a resistance or a reactance In this case if an earth fault occurs on one phase, the voltage of the faulty phase with respect to the earth will be potential difference across the limiting resistance or reactance. Due to this the voltages of other two healthy phases with respect to the earth rises by 1.7 times (Vector sum ). Due to this reason the insulation of these phases have to be designed for these increased voltage.

You being a new comer to this forum I would like to alert you that use of capital letters in a thread is considered by members as shouting and indecent and not liked. So avoid using capital letters in your threads in future.

Hi! I am also from Salem in Tamilnadu.

Well! Let us understand what is the effect of an earthed system and an unearthed system.

In an unearthed system, during a single phase to earth fault, the voltage of the healthy phases with respect to earth will be the line voltage. And, in a (solidly) earthed system, during a single phase to earth fault, the voltage of the healthy phases with respect to earth will be only the phase voltage.

Now, let us take a 6.6kV (UE)/11kV(E) cable. This means that the insultaion of the cable is designed for 6.6kV. In a 6.6kV unearthed system, during a single phase to earth fault, the voltage of the healthy phase conductors of this cable with respect to the earthed armour will be 6.6kV and as the insulation is deisgned for this voltage, it would withstand the same.

Now, the same cable with the same insulation can also be used in a 11kV (Solidly) Earthed system; In a 11kV (solidly) earthed system, during a single phase to earth fault, the voltage of the healthy phase conductors of this cable with respect to the earthed armour will only be the phase voltage (about 6.35kV); as the cable insultaion is designed for 6.6kV, the cable would withstand this stress too.

So, the above cable can either be used on a 6.6 kV unearthed system or on a 11kV (solidly) earthed system.

Hope you understand!