A single core lead sheathed cable has a conductor diameter of $2$ cm ; the diameter of the cable being $6$ cm $.$ The cable is graded by using four insulating materials A$,$ B$,$ C and D have permittivities of $4,3\mathrm{.}5(5$ and $3$ respectively with the corresponding maximum safe working stress of $25k\frac{V}{c}m($r$.$m$.$s$.),40k\frac{V}{c}m.($r$.$m$.$s$.),30k\frac{V}{c}m.($r$.$m$.$s$.)$ and $35k\frac{V}{c}m($r$.$m$.$s$.)$ respectively.

Assess the arrangement of the insulating materials.

Determine the permissible voltage of the cable.