Choose between TDR, bridge and surge-generation methods by the fault class, the cable type and the stage of the job: time-domain reflectometry estimates distance for low-resistance and open faults, bridge methods measure resistive faults precisely, and surge generators break down high-resistance faults so they can be located and pinpointed. A complete fault-location job normally uses them in sequence rather than as alternatives.
The Methods Operate at Different Stages of the Job
Cable fault location is a staged process: prepare the cable, classify the fault, prelocate the distance, then pinpoint the exact position for excavation. The three method families belong to different stages. TDR and bridge methods are prelocation tools that estimate or measure distance from the cable end. Surge generation is both a conditioning tool and the excitation for pinpointing, because it makes a high-resistance fault break down repeatedly so a locator can find it.
The stage determines the requirement. A distance estimate with a stated accuracy band may be enough to dispatch a crew, while pinpointing accuracy of less than a metre is needed before digging. Choosing a method without defining the stage usually produces the wrong tool for the job.
Preparation comes before every stage: the cable must be isolated, verified de-energised and grounded according to the approved procedure, with both ends identified and access confirmed. The fault class and the cable data, including length, type and test history, are recorded at the start because every method’s accuracy depends on them. Safety preparation is not a separate checklist item; it is the first stage of the location job itself.
Where TDR Provides a Useful Distance Estimate
A time-domain reflectometer (TDR) sends a pulse down the cable and measures the time until the reflection returns, converting the time to distance using the cable’s velocity of propagation. It works well for low-resistance faults, open circuits and cable length measurement, and it gives an immediate distance estimate that can guide the rest of the operation.
TDR has limits: high-resistance faults reflect little energy, so the trace may show no usable echo, and the distance calculation depends on the velocity factor of the specific cable construction. The result is an estimate that should be refined by pinpointing, not a dig-ready coordinate on its own. For high-resistance faults, the operator should not conclude “no fault found” from a flat TDR trace.
Use the TDR’s controls deliberately: the velocity factor must match the cable construction, the pulse width should suit the cable length, and the gain should be adjusted so weak reflections are visible without saturating the trace. Compare the trace with the healthy length or with a loop measurement where available, and save the trace for later comparison after conditioning.
When Bridge Methods Help with Resistive Faults
Bridge methods apply a DC measurement between the cable conductor and the fault path to calculate distance from resistance ratios. They are well suited to resistive faults, including high-resistance faults that a TDR cannot see, and they can produce a stable distance value from a measured resistance ratio rather than from a weak reflection.
Bridge accuracy depends on knowing the cable length, the conductor cross-section and the fault path resistance, and on the cable being isolated from other circuits. Different bridge variants handle different fault configurations, so the method must match the fault class: a short between conductors is measured differently from a short to earth. Where the resistance is too high for a clean bridge balance, fault conditioning comes next.
The inputs to the bridge calculation are the same inputs that limit its accuracy: the exact cable length, the conductor resistance per unit length and the fault resistance. Errors in these inputs move the calculated distance, so the record should state the source of each input. A bridge result is a prelocation value with its own accuracy band, to be confirmed by pinpointing before any excavation.
Why Surge Generation Is Used for Breakdown Faults
Surge generators, often called thumpers, apply high-voltage impulses that repeatedly break down a high-resistance fault, converting it into an intermittent arc. Once the fault flashes over, other methods can respond: an arc-reflection system re-measures the TDR echo during the arc, and the acoustic or electromagnetic pulse of each breakdown is used to pinpoint the location from the surface.
Surge generation stresses the cable, so it must be applied with voltage and energy limits matched to the cable rating and the fault behaviour, and repeated only in controlled steps. The goal is to make the fault locatable, not to damage the cable further or to excite a healthy section beyond its rating.
The way the surge system operates also matters: single-shot impulse mode is used for acoustic and electromagnetic pinpointing, while arc-reflection mode triggers a TDR measurement during the arc. The repeat rate affects both the safety of the operation and the ability of the receiver to lock onto the signal, so the system’s duty and control features belong in the specification as much as its voltage and energy ratings.
Acoustic and Electromagnetic Pinpointing After Prelocation
Once the distance is prelocated, pinpointing finds the exact position. Acoustic pinpointing listens for the sound of the discharge at the surface, with the operator moving along the cable route until the loudest point is found. Electromagnetic pinpointing detects the electromagnetic pulse of each discharge and can work where the acoustic signal is weak or the route is hard to access.
The two techniques complement each other: acoustic pinpointing is direct but depends on surface conditions and background noise, while electromagnetic pinpointing is less affected by sound but requires a clear signal path. A combined receiver that shows both signals helps the operator confirm the location before excavation.
Route mapping improves every pinpointing pass. Mark the cable route from the drawings and the route tracer before the discharge starts, walk the full prelocation window, and note the surface conditions along the way. The operator then interprets the receiver readings against the known route rather than guessing, which reduces the risk of excavating the wrong point on a long or unbounded route.
A Combined Method Sequence for Difficult Faults
Difficult faults are solved by sequence, not by one instrument. A typical flow is: isolate and verify safety, classify the fault with insulation and resistance measurements, run a TDR to capture the trace, use a bridge or a prelocator for resistive faults, condition the fault with controlled surge if needed, then pinpoint acoustically and electromagnetically before excavation.
| Fault class | Prelocation method | Conditioning | Pinpointing |
|---|---|---|---|
| Open circuit | TDR | Not required | Electromagnetic tracing |
| Low-resistance short | TDR or bridge | Usually not required | Acoustic and electromagnetic |
| High-resistance fault | Bridge or arc-reflection TDR | Controlled surge breakdown | Acoustic and electromagnetic during discharge |
| Intermittent fault | TDR with repeated traces | Often required | Monitor and pinpoint during breakdown |
The sequence and the choice of method follow the fault class and the cable system’s rating and construction, and every step must respect the cable’s safe test limits. The table shows the normal pattern; the responsible engineer confirms the exact sequence for the cable being tested.
Document each stage in the job record: the fault class evidence, the prelocation method and result, the conditioning settings, the pinpointing signals and the excavation point. A complete record supports the repair decision, becomes the baseline for post-repair verification and feeds the network’s fault statistics, which are the data that future method selection should be based on.
Frequently Asked Questions
Why doesn’t the TDR show a high-resistance fault?
Because a high-resistance fault reflects little of the TDR pulse energy, so the echo is too small to see or measure. That is expected behaviour, not a sign the cable is healthy; the fault class should be confirmed with insulation and resistance measurements, and a bridge or surge-based method used instead.
When is a bridge method better than a TDR?
Bridge methods measure resistance ratios, so they work on resistive faults including high-resistance ones that produce no TDR echo. They require the cable to be isolated and the conductor and fault path parameters to be known, and they are typically used when the fault is resistive and a stable distance value is needed.
Why is surge generation needed at all?
Because many real faults are high-resistance until they flash over. Controlled surge impulses break the fault down repeatedly, making it locatable by arc reflection and by acoustic and electromagnetic pinpointing. The energy and voltage are matched to the cable rating to avoid overstressing healthy insulation.
Without conditioning, a high-resistance fault may remain invisible to the prelocation instruments, and the crew is left excavating on a guess rather than on measured evidence.
For the fault-type-to-method framework behind this sequence, see the types of cable faults guide. When you need to build a fault-location toolkit for your cable network, review cable fault testing solutions and request a technical proposal with your cable types and fault history.
The right toolkit is the one that covers your fault classes and route conditions in sequence, with training and procedure to match; a single instrument, however capable, rarely finishes the job alone.