Gautam:
These are the two type of cable.
large cable have shielding of metal over cable to get good 
mechnical strenth. some time this shielding has been earth.
when ever short ckt accurs between line and earth. it will 
use to locet the fault by fault locater...

no above is wrong
earth cable and unearth cable get differantiated with size 
of insulation( thickness) , unearth cable it is more and if 
l-g fault takes place on unearth cable voltage rise will be 
sqrt 3 times phase voltage  on healthy phase

In an earthed cable, the three phase of a cable are earthed 
to a ground.Each of the phase of system is grounded to 
earth.
Examples are 1.9/3.3 kv, 3.8/6.6 kv system
whereas, in an unearth system, the phases are not grounded 
to earth .As a result of which there are chances of getting 
shock by personnel who are operating it. Examples are:
6.6/6.6 kv, 3.3/3.3 kv system.

Unearthed cable have more insulation strength as compared 
to earthed cable.When fault occure phase to ground volage 
is root 3 time the normal phase to ground voltage. so If we 
used earthed cable It may be chances of insulation puncture.
SO unearthed cable are used.Such type of cable are used in 
6.6 KV system where resistance type earthing are used.

what nirav said is right but the thing is he didn mention 
on which phases the voltage will be root 3 times the phase 
to ground voltage. actually it is on the other two phases 
other than the faulted phase that the voltage will shoot to 
1.73 times the normal phase to ground.
regards.