In daylight, we look for the smoke. At night, we look for the flame. When we get close, we sniff for the ozone. But seriously, we have a gizmo with a 600V contactor and a bank of lights. We place it between our DC bus and the load side of an open bridge feeder or traction power breaker. It flashes the lights through the fault. Then we clamp on in various manholes and see the current pulses before the fault, and not after. For signal or control cables I'll give it a shot with a metallic TDR. Guessing the velocity of propagation is always fun but not really that important.
Joe
