Mshaw,
The device you mentioned sounds like a TDR. I guess I should have mentioned that TDR stands for Time Domain Reflectometer. They work by sending pulses down conductors or fiber for Optical TDRs. When you get to an impedance change, some energy couples forward, while some reflects back. It works similar to SONAR or RADAR where they gate on a sensitve receiver after the pulse is sent and time the round trip. They calculate the distance to the fault by using the time, speed of light, and the velocity of propagation percentage correction to the speed of light. The guess work I mentioned is because those characteristics vary alot with power conductors and paired cables when compared to coax, and especially fiber. I usually just check a known length of the closest cable I can find and adjust the VP to jive with my sample.
Joe
