[tialaramex]

However "a fast device that measures return latency on the shorted circuit" is presumably very cheap today.

20+ years ago, rewiring buildings for 1000baseT the Cisco switches (3650s maybe? 3750s? I know they had IPv6 multicast acceleration in the switch fabric because that's why we had them) would do this for any circuit on command, OK, 18 metres from here there's a fault, pace, pace, pace, I reckon it'll be up on this cable tray... yup, some fool tried to "repair" a broken Cat5 cable, we'll just rip it out, meanwhile patch to a different circuit.

[jmole]

Time Domain Reflectometry - https://community.cisco.com/t5/networking-knowledge-base/how...

[sandos]

I believe I even had a BIOS in a PC that did this in some way....

Actually, I think it wasn't detecting faults but rather the length of cable connected.