Underground Cable

Come on, Mildred. Make a phone call or two to a couple of cable suppliers!

In my opinion the insulation thickness , conductor shield and insulation screen -and all other parts -except armour and serving - are the same for the same rated voltage.

In USA U.L. standards state the cable building rules .See:

UL Standard 1072 Standard for Medium-Voltage Power Cables

For IEC world see: IEC 60502-2 Cables for rated voltage from 6 kV up to 30 kV.

Yes and no depending on the application.

Try looking at cable manufacturer's websites for more details.

In India it is NO . Special considerations can be paid by the owner if one needs to. But yes, As all around the world the cable capacity gets de-rated.

Insulation thickness of ue rated cables higher than earthed system cables due to sqrt^3 times higher voltage between phases during earth fault in unearthed system

You are right, tigerdon. The grounding fault duration state the insulation thickness.

From IEC 60502-2 ch. 4.1 Rated voltages:

"The rated voltage of the cable for a given application shall be suitable for the operating conditions in the system in which the cable is used. To facilitate the selection of the cable, systems are divided into three categories:"

Category A: ground fault is disconnected from the system within 1 min.

Category B: ground fault is disconnected from the system within 1 h. [up 8 hours could be tolerated if the total duration in one year should not exceed 125 hours.]

Category C: [more than category A or B].

Table 1 presents the recommended rated voltage-to-ground [or neutral] Uo. For category A and B it will be a half of maximum permitted phase-to-phase. For C category more is required.

Category A corresponds to 100% and Category B to 133% insulation level according to UL 1072.