Time Domain Reflectometry on Shielded Power Cable: Locating Faults and Defects

What it is

Time domain reflectometry, usually shortened to TDR, is a cable test that works like radar on a conductor. A reflectometer launches a fast, low-energy pulse into one end of the cable and displays the echoes that return, plotted against time and converted to distance. A uniform cable returns little, while any place where the characteristic impedance changes sends a reflection back: a splice, a termination, a tap, a section of damaged insulation, a crushed or wet region, a short circuit, or an open end. On medium- and high-voltage shielded power cable the test is used to find the electrical length, to confirm the positions of joints and terminations against the route drawings, and to estimate the distance to a fault after the cable has been shown to be bad by other means. It is applied to a cable that is out of service, de-energized and grounded, and it does not stress the insulation. It is a companion to insulation, jacket and shield continuity tests, which say whether the cable is sound, while the reflectometer says where along its length a problem is.

Why it is done

A fault on a buried or ducted cable is useless information until it can be located. Digging a long route to find a failure is costly and disruptive, and a cable that is slowly degrading may fail again after a repair that missed the real problem. A reflectometer turns a vague diagnosis into a distance, so the work crew can start the final search in the right stretch of route, and it shows how many joints and terminations a cable actually has, which may differ from the drawings after years of repairs. The test is also valuable after installation and after pulling: a cable damaged by a pulling eye, a sharp bend, a crush or water entering a termination may still pass a simple resistance check but will show an anomaly on the trace. A trace taken when the cable is new serves as a baseline for later faults, and it helps to separate a cable fault from a problem in a termination or an accessory. Because the test is low-energy, it can be repeated freely without further damaging a weakened cable.

The principle

A pulse traveling along a cable meets the cable's characteristic impedance, which depends on the geometry and materials of the conductor, the insulation and the shield. As long as the impedance is uniform, the pulse proceeds without reflection. Where the impedance changes, part of the pulse is reflected, with a polarity and size that depend on the change. An open circuit reflects the pulse with the same polarity, and a short circuit reflects it inverted. A region of lower impedance, such as water-soaked insulation, shows as a dip, and a region of higher impedance as a rise. The instrument measures the time between the launch and the echo, and because the speed at which the pulse travels in the cable is known as a fraction of the speed of light, the time becomes a distance along the cable. That fraction is called the velocity of propagation, and it depends on the insulation material, so an incorrect setting gives a wrong distance. Cable losses weaken the echo from far away, so longer cables need stronger or wider pulses, which in turn make nearby events harder to resolve.

How it is performed

The cable is de-energized, isolated from the system, tested for absence of voltage and grounded under the owner's safety procedures, with all connected equipment disconnected so that only the cable is measured. The reflectometer is connected to the conductor and the shield at one end, using the proper leads, after confirming that the cable has been discharged. The cable type is selected and the velocity of propagation is set from the manufacturer's data, or calibrated on a known length of the same cable. The pulse width and the range are chosen to suit the length of the cable and the fault being sought, then the trace is acquired and examined. The measured length is read from the end reflection and compared with the route length and the records. Reflections along the length are identified and matched with the known joints, terminations and taps, and any other anomaly is noted and its distance recorded. For a cable that has failed, the test can be run from both ends to cross-check the distance of the fault, and the other conductors can be compared as a reference. The trace is saved, and the instrument identity and settings, the ambient conditions and the cable details are noted.

What is recorded

The record identifies the cable and the end tested, the instrument with its calibration status and the settings used, and keeps the saved trace together with the reflections found, both expected and unexpected. When a fault is being located, the estimated position and how it was confirmed are included. Conditions at the time and the connection arrangement are noted. These details matter because the next trace should be taken in the same way, so that a later reader comparing the two sees a change in the cable and not a change in the settings or the hookup.

How results are judged

A reflectometer trace is read as a picture, and this page gives no numerical limits. The shape of the judgment is a comparison. The measured length is compared with the recorded route and the cable records. The reflections are compared with the expected pattern of joints, terminations and taps, and any reflection that cannot be explained is treated as suspect, whether it is a dip, a rise, an open or a short. A trace is compared with the baseline taken when the cable was new, with the other phases of the same circuit, or with a similar cable, and a change is more significant than any single feature. The distance to a fault is cross-checked from both ends and with other methods before excavation. Results are judged together with the insulation, jacket and shield tests, since the reflectometer locates problems but does not by itself rate the insulation. The medium- and high-voltage shielded cable test items are described in ANSI/NETA ATS-2025; refer to your licensed copy for the requirements, and to the cable manufacturer for the propagation data.

Variations

At acceptance a trace of every new cable becomes the baseline, and anomalies from pulling or handling can be caught before energization. In maintenance and after a fault the purpose is to locate and confirm a problem, and the reflectometer gives an approximate distance that is often followed by pinpointing methods, such as high-voltage thumping and tracing along the route, to find the exact spot before excavation. Extruded and laminated insulation differ in their propagation velocity and in the way water and damage show on the trace. Cables with many joints, such as old networks with repairs, produce busy traces that need the records to interpret, while a single long run in a duct is simpler. Concentric neutral and tape-shielded designs behave differently when the shield is part of the measurement path. Very long cables lose pulse energy and may need to be tested from both ends. The same technique is used on communication cable and, with optical pulses, on fiber, where it is known as optical time-domain reflectometry.