Every cable specification out there does not recommend DC hipotting service aged (>5 years) cables. DC hipot is considered a destructive test and will most likely damage the cable. Ghost307 is right about disconnecting the cable from the bus, the cable should be disconnected with corona supression before any overpotential test.
You can locate your problem without a shutdown with an online Partial Discharge test, or if shutting down is not a major issue a VLF or Tan Delta test would be more appropriate.
When is the last time you tested your relays? What type of relays are they? What indications are you getting? What type of cable?
