You need to have both ends of the cable clear from any bus, or termination or lightning arrestor.
A 5 kV megger test is acceptable and does not damage an HT cable.
By testing each phase at the time to ground and testing phase to phase a good overall idea of the condition of the cable can be found.
In your case with a ground fault you probably have a < 1 M.ohm reading on one of the phases.
If all phases give equal readings say 500M.ohms or more the cable may be ok and the OCB or lightning arrestors may need to be checked.
Prior to commisioning after repair a VLF test is the best way and does not damage new or aged cables.
We, in NZ, VLF test aged 11 kV cables at ± 17 kV. so a 15 kV cable requires about ± 23 kV for a VLF test. check what your POCO requires.