The HV cable was meggered and had one phase reading lower than the other two phases, although the 5 kV Megger didn't break it down fully.
A low amperage high tension fuse was used to test the cable and avoid risking tripping the feeder.
The fault developed as a phase to screen fault which was later traced and thumped. Fault found in the crutch of the PILCpart of cable in the.300 kVA transformer.