Time domain reflectometry (TDR) sends a low-voltage pulse down the cable and records the echoes that return wherever the impedance changes: splices, branches, the open end or a fault. From the round-trip time and the propagation velocity of the cable (v/2) it calculates the distance. It is used to measure length, locate splices and find opens and low-resistance faults; high-resistance faults barely reflect the pulse and require high-voltage methods.
What TDR is
The time domain reflectometer (TDR) is the basic instrument of cable fault location. It injects a low-voltage pulse at one end, between a conductor and the shield or between two conductors, and records everything that comes back. Each change of impedance along the cable returns part of the pulse: a splice, a branch, a change of cross-section, the end of the cable or a fault.
The result is a trace (reflectogram): a horizontal axis in distance and a vertical one with the amplitude of the echoes. Reading it well is the difference between digging in the right place or opening more trench than needed. The other cable tests are in the guide to cable diagnostic tests.
How the distance is calculated
The pulse travels to the impedance change and back. The instrument measures the total time t and multiplies it by half the propagation velocity, the value reflectometers call v/2:
distance = v/2 × t
Propagation velocity depends on the insulation (its permittivity), not on the conductor; the layers of each cable type are described in power cables. In general physics, v = c / √εr, where c is the speed of light and εr the relative permittivity of the insulation. That is why two cables of the same cross-section with different insulations have different v/2, and a circuit with different cable sections does not have a single velocity.
How to get a reliable v/2: from the cable manufacturer, or by calibrating on a section of known length of the same cable, adjusting v/2 until the trace shows that length. We do not publish values by cable type because they depend on the specific product.
How to read the trace
| What appears | Shape of the reflection | What it means |
|---|---|---|
| Open end or cut conductor | Large positive reflection (reflection coefficient close to 1) | If it appears before the expected length, there is an open |
| Short circuit or low-resistance fault | Negative reflection (impedance drops) | Solid fault between conductor and shield or between phases |
| Splice | A small horizontal “S” shape | Impedance change at the joint between sections |
| Change of cable type | Step in the trace | Sections with different impedance and velocity |
| High-resistance fault | Almost nothing | The fault impedance is very high compared with the cable’s |
The polarity comes from transmission-line physics: the reflection coefficient is positive when the impedance rises (open) and negative when it falls (short). BAUR describes the open end as a positive reflection with a coefficient close to 1 and the splice as a horizontal “S”.
- Does the trace show a reflection different from the healthy phase?
- Yes: Is the reflection negative or positive before the end?
- Negative: Low-resistance fault: mark the distance and move to route tracing and pinpointing
- Positive: Conductor open: confirm by measuring from the other end
- No: High-resistance or intermittent fault: move to arc reflection, impulse current or decay
- Yes: Is the reflection negative or positive before the end?
Why measure from both ends
In a circuit with different cable sections the velocity changes, and the distance calculated with a single v/2 has error. BAUR notes that measuring from both ends gives two different positions and the fault lies between them. The sum of the two distances should be close to the total cable length; if not, the v/2 or the reference length is wrong. This check costs little and avoids digging in the wrong place.
Limits of the method
- High-resistance faults: they are the majority in medium-voltage cables damaged by aging or moisture. The pulse passes through them almost without reflecting. The solution is to make the fault break down with a high-voltage impulse and measure during the arc, with methods such as arc reflection, impulse current or decay.
- Dead zone at the start: the pulse has a width and the first meters are hidden by the pulse itself. Some instruments have specific modes for that zone.
- Attenuation in long cables: the pulse loses amplitude and broadens with distance; in very long cables the echo from the end arrives weak.
- Sheath faults: the TDR is no use, because ground is not a defined return path. The bridge method and the step voltage method are used.
- Corroded shields or open neutrals: they distort the trace and can mimic reflections.
The reference trace
A trace taken when the cable is healthy (on receipt or at commissioning) is the best tool for the next fault: it shows the real length, the position of each splice and the normal shape of the cable. Comparing the current trace with that reference, or with a healthy phase, makes the fault stand out. That is why we recommend keeping the trace of each circuit in the handover protocol of the collector system or the industrial network.
What can falsify the reading
- v/2 taken from a different cable.
- Test leads and connection cables that are not subtracted.
- Noise from neighboring energized cables.
- Circuits with branches: each branch generates its own echo and the trace becomes harder to read.
- Connection to the wrong phase: always compare with the other phases.
What not to do
- Dig using the TDR distance without tracing the route: the electrical length is not the position on the ground.
- Conclude that “the cable is fine” because the TDR shows nothing.
- Apply high voltage to the cable without isolating and grounding the ends according to the safety rules.
Cable testing with Tiza Group
If you have a faulted cable or a new network and want its reference fingerprint, we help you plan and carry out the tests on your cable network with test equipment from recognized manufacturers (OMICRON, Doble, Baur, among others). Field work is done in Colombia, or elsewhere on request. Learn about the cable fault location service or write to us.
Related guides
- Insulation resistance test and polarization index
- Power cable diagnostic tests
- Power cables
- Medium-voltage splices and terminations
- Fault location in underground cables: methods
- High-resistance faults in cables: ARM, ICE and decay
- Bridge methods for cable faults: Murray and Glaser
References
- IEEE Std 1234-2019, Guide for Fault-Locating Techniques on Shielded Power Cable Systems.
- BAUR GmbH, Time domain reflectometry (TDR), technical page on the fault location process.
- BAUR GmbH, Cable fault location: process steps and methods.
Frequently asked questions
What v/2 value should I use on the reflectometer?
The one that matches the actual insulation of the cable, taken from the cable manufacturer or, better, calibrated by measuring a section of known length of the same cable. A wrongly set v/2 shifts all distances by the same proportion.
Why does the TDR show the full length of the cable but not the fault?
Because the fault is high resistance: its impedance is much higher than the characteristic impedance of the cable and it produces almost no reflection. In that case, methods that make the fault break down at high voltage are used, such as arc reflection (SIM/MIM), impulse current or decay.
How do I tell a splice from a fault on the trace?
By comparing with a reference trace of the same cable or with the healthy phase: splices appear on all three phases at the same distance and with a small shape; the fault appears only on the damaged phase.