Locate a high-resistance underground cable fault by confirming the fault class first, then using bridge or insulation-based measurements where a TDR trace shows nothing, conditioning the fault with controlled surge impulses so it breaks down repeatedly, and finally pinpointing with acoustic and electromagnetic methods before excavation. Work in defined steps and respect the cable’s safe test limits at every stage.
Confirm the Fault Is High Resistance and Not a Setup Error
Before selecting methods, confirm that a high-resistance fault exists and that the measurement setup is valid. Isolate the cable at both ends according to the approved procedure, verify the phase and sheath connections, and measure insulation resistance with an insulation tester at the appropriate voltage. A clean, repeatable low insulation reading on the affected phase, with healthy readings on the other phases, confirms the fault class.
Check the setup at the same time: a poor temporary connection, a wet termination or an unisolated parallel path can produce a misleading resistance value. Record the insulation readings, the test voltage and the temperature before moving to the location stage.
The isolation and grounding sequence must be completed and verified at both ends before any measurement, and the work area controlled so no one can re-energise the circuit during the test. The cable’s route, length and construction data are gathered in the same step, because the location methods depend on them and the record must be complete.
Why a Standard TDR Trace May Show Little Evidence
A standard TDR sends a low-voltage pulse and looks for a reflection. A high-resistance fault reflects very little of that pulse, so the trace often shows a flat line where a fault should appear. This is expected behaviour: the absence of an echo is not evidence that the cable is healthy, and it does not mean the fault does not exist.
Record the flat trace anyway, because it becomes the baseline for comparison after fault conditioning. Some TDR units have trace comparison features that highlight the change once the fault is made to break down, which turns the earlier “empty” trace into useful evidence.
When taking the trace, set the velocity factor for the cable construction, choose a pulse width matched to the length, and adjust the gain so the healthy end reflections are visible. Save the trace with its settings, because a trace without settings cannot be reproduced and the comparison after conditioning will not be meaningful.
If the cable has parallel sections or branches, note that a single TDR trace can include reflections from junctions as well as the fault, and the operator should compare the trace with the known route geometry. A branch reflection that is mistaken for a fault is one of the classic errors in TDR interpretation, so the route drawing belongs next to the instrument during the measurement.
Use Insulation and Bridge Measurements to Refine the Fault Class
Insulation and resistance measurements refine the fault class: whether the fault is to earth, between conductors, or both, and roughly how high the resistance is. Bridge methods then convert the resistance ratio into a distance estimate using the known cable length and conductor parameters. A bridge can produce a stable distance value where a TDR shows nothing, which makes it the natural prelocation tool for high-resistance faults.
Document the measurement conditions: the bridge variant, the fault configuration assumed, the cable length used in the calculation and the conductor data. The accuracy of the distance estimate depends on these inputs, and the estimate is still a prelocation value that must be refined by pinpointing.
Choose the bridge variant to match the fault: a loop or Murray-type arrangement for a fault to earth on one conductor, and the appropriate configuration for a fault between conductors. The operator must know which configuration is being applied, because a distance calculated with the wrong configuration is confidently wrong.
Fault Conditioning and Arc-Reflection Options
Fault conditioning applies controlled high-voltage impulses to break the high-resistance fault down into an intermittent arc. Once the fault flashes over, arc-reflection systems re-measure the TDR echo while the arc is present, producing a reflection that a standard TDR cannot see. The impulse voltage and energy are matched to the cable rating and increased in controlled steps, and the operation is stopped as soon as the fault becomes locatable.
Conditioning stresses the cable, so it must follow the site’s safety and isolation rules and the manufacturer’s guidance for the cable type. Do not keep raising the voltage indefinitely: if the fault does not respond within the defined limits, stop and review the fault class and the test setup rather than continuing to stress the cable.
Each impulse cycle should be timed and observed: the charging voltage, the breakdown, the arc duration and the receiver response. The arc-reflection trace captured during the flashover is the evidence that the fault is locatable, and it should be saved with the conditioning record so the prelocation value can be traced back to the settings that produced it.
Move from Prelocation to Pinpointing
Prelocation gives a distance from the test end; pinpointing finds the position on the ground. During each surge discharge, an acoustic receiver detects the sound at the surface and an electromagnetic receiver detects the discharge pulse. The operator moves along the route and marks the point where both signals peak, which is the position to excavate.
Where the acoustic signal is weak, because of depth, soil conditions or background noise, the electromagnetic signal provides the primary evidence, and the two receivers together confirm the location. Mark the route, record the measurements and keep the prelocation distance in the record so the crew can verify the result against the expected position.
Walk the route in a consistent pattern, stopping at regular intervals and at every surface anomaly, and compare the receiver response at each stop. The peak point should be confirmed by approaching it from both directions, because a single maximum on one pass can be an artefact of the walking pattern rather than the true location.
Stop Conditions and Post-Repair Verification
Define the stop conditions before starting: the maximum test voltage and energy for the cable, the number of impulses allowed at each level, and the conditions that trigger a review of the fault class or the setup. If the fault does not break down within the limits, or if the cable shows signs of additional stress, stop and escalate to the responsible engineer.
| Step | Evidence | Stop condition | Next action |
|---|---|---|---|
| Fault class confirmation | Insulation and resistance readings | Readings not repeatable or setup suspect | Correct setup and repeat |
| Prelocation | TDR trace or bridge distance | No usable reflection; distance inconsistent | Switch to bridge or conditioning |
| Conditioning | Arc reflection appears | Voltage or energy limit reached | Stop and review fault class |
| Pinpointing | Acoustic and electromagnetic peak | No clear peak after full route pass | Recheck prelocation and route data |
| Post-repair verification | Insulation, TDR and service tests | Results below the required level | Reopen and repair |
After repair, verify the cable with insulation testing and, where applicable, a TDR length or profile check, so the repair is proven before the cable is re-energised. Keep the full record: fault class, prelocation, conditioning settings, pinpointing evidence and verification results.
Review the whole record against the excavation and repair findings: did the pinpointed position match the visible damage, and did the prelocation distance agree with the repaired location? This reconciliation closes the loop on the method’s accuracy and gives the network team the confidence to apply the same sequence to the next high-resistance fault.
If the reconciliation shows a consistent error, review the input data: the cable length, the velocity factor or the conductor parameters used in the prelocation. Correcting these inputs for future jobs improves the accuracy of the whole fleet’s fault-location practice, which is the practical payoff of keeping complete records.
Frequently Asked Questions
Why can’t a TDR find a high-resistance fault?
Because a high-resistance fault reflects too little of the TDR pulse energy to produce a measurable echo. This is expected, and the correct response is to confirm the fault class with insulation and bridge measurements, then use fault conditioning and arc reflection to make it locatable.
What is arc reflection?
Arc reflection is a technique that measures the TDR echo while a surge impulse is keeping the fault flashed over. During the arc, the fault behaves like a low-resistance point, producing a reflection that a standard TDR cannot see without the conditioning.
How is the exact location pinpointed?
The exact location is found by moving an acoustic and electromagnetic receiver along the route during the repeated surge discharges and marking where both signals peak. The two signals together confirm the position before excavation.
The pinpointing pass should cover the full prelocation window and be repeated from both directions so the peak is confirmed, not assumed from a single pass.
For the fault-type-to-method framework, see the types of cable faults guide. To qualify a fault-location system that handles high-resistance faults, review cable fault testing solutions and request a technical proposal with your cable data and fault records.