If you want to understand how TDR works, watch this classic tutorial from the mid-50s in the AT&T archive. The first 5 minutes, showing the reflection of an open and short circuit is enough.
Many are surprised when they learned that it's possible to detect the location of a break in the mains wiring or an Ethernet cable, just by doing an electrical test from one end of the cable (some Ethernet cards even have builtin TDR support!). There's no closed loop, why is it possible to test anything electrically? Because at high frequency, the circuit becomes an electromagnetic waveguide.
Basically like a radar, you send a fast electrical or optical pulse to the electrical cable or the optical fiber, the EM wave travels along the cable until it hits the break and reflects back. You record the time and listen for the echo. All discontinuities (such as damages) on the line will reflect and distort the original pulse. Use the speed of light to pinpoint the location of the fault.
* AT&T Archives: Similiarities of Wave Behavior
https://www.youtube.com/watch?v=DovunOxlY1k
Many are surprised when they learned that it's possible to detect the location of a break in the mains wiring or an Ethernet cable, just by doing an electrical test from one end of the cable (some Ethernet cards even have builtin TDR support!). There's no closed loop, why is it possible to test anything electrically? Because at high frequency, the circuit becomes an electromagnetic waveguide.
Basically like a radar, you send a fast electrical or optical pulse to the electrical cable or the optical fiber, the EM wave travels along the cable until it hits the break and reflects back. You record the time and listen for the echo. All discontinuities (such as damages) on the line will reflect and distort the original pulse. Use the speed of light to pinpoint the location of the fault.