Megger cable first, 500 Vdc

Ph to Neutral.
Ph to Earth
Neutr to Earth
and from each individual core to ground, to establish if it's external damage

Have both ends disconnected,

Is it a screened cable?

or 2 or 3 core with no screen.

then fault locaion is easy.

POPIE for an earth fault or Thumper for internal cable fault.

The product of rotation, excitation and flux produces electricty.