Diff between Earthed & Unearthed cable

In 3phase earthed system, phase to earth voltage is 1.732 times less than phase to phase 

voltage. Therefore voltage stress on cable to armor is 1.732 times less than voltage stress

between conductor to conductor. Whereas in unearthed system, (if system  neutral is not

grounded) phase to ground voltage can be equal to phase to phase voltage. In such case the

insulation level of conductor to armor should be equal to insulation level of conductor to 

conductor.

Also can be detailed as:

For cables to be used in solidly earthed systems, the phase-to-armour insulation has to be rated for U/root(3) only which is the phase-to-ground voltage when the system-neutral is  solidly earthed with no intentional resistance in the neutral grounding circuit. But in the case of system-neutral being resistance-earthed, then the phase-to-ground voltage of the two healthy phases rise up when an earth fault occurs on the third phase. When the system-neutral is high-resistance-earthed or left unearthed, the phase-to-ground voltage of healthy phases come close to or attain phase-to-phase values depending on the degree of effectiveness of system-neutral earthing. Therefore the phase-to-armour insulation of cables used in ungrounded systems could be rated for the full phase-to-phase voltage U instead of for U/root(3). The cables to be used in solidly earthed systems can have the phase-to-armour insulation rated for U/root(3). The U/Uo rating of the cable indicates the voltage rating of the core-to-core insulation and the core-to-armour insulation. For example for a 6.6kV ungrounded system, 6.6kV/6.6kV (UE) class cable has to be used while 6.6kV/3.8kV (E) class cables are adequate for solidly earthed systems. The UE-class cable is
naturally costlier than the earthed class of cable.