How to Locate High-Resistance Cable Faults: Practical Field Methods for Underground Cables

For field technicians, this means the cable may still carry load at low voltage, the insulation resistance meter may read borderline-normal, and the TDR may show no clear reflection. Yet the cable trips under fault conditions, or fails intermittently under load. Locating these faults requires specialized techniques that actively force the fault to present a measurable signature. This article explains why high-resistance faults are so difficult and walks through the proven methods used by professional cable fault locators.

1. What Is a High-Resistance Cable Fault?

A high-resistance fault is a dielectric breakdown in which the insulation is damaged but has not collapsed into a direct metal-to-metal short. Instead, the fault point conducts current through a resistive path — often through carbonized insulation, water trees, or moisture-contaminated defects — rather than through a solid metallic path. Typical fault resistance values range from several hundred kiloohms to several megaohms.

Common causes of high-resistance faults include:

• Water treeing in XLPE insulation — moisture migrates through the insulation over years, creating tree-like micro-channels that conduct weakly; these faults often measure in the hundreds of kiloohms.
• Carbonized tracking from prior overvoltage events — a previous surge that partially burned a conductive track through the insulation leaves a carbonized path with moderate-to-high resistance.
• Moisture in joints or terminations — water contamination in a joint creates a resistive leakage path rather than a direct short.
• Partial mechanical damage — a cable nicked or abraded during installation may have damaged insulation but retained enough dielectric strength to avoid a dead short.
• Semi-conductive layer defects — knife marks or incomplete removal of the semicon screen at a termination create field concentrations that break down only under high voltage.

1.1 Why Standard TDR Cannot Detect Them

TDR works by sending a low-voltage pulse and measuring reflections from impedance mismatches. A solid short circuit (near-zero resistance) produces a strong, easily recognized reflection. But a high-resistance fault presents an impedance close to the cable's characteristic impedance — typically 20–50 Ω — so the reflection is tiny, often buried in noise, or absent entirely. In practice, TDR reliably detects faults below about 100 Ω; above that, the reflection amplitude drops sharply and becomes difficult to distinguish from normal cable features (joints, splices, branches).

This is why a technician can connect a TDR to a faulted cable, see a perfectly clean trace, and still know the cable is defective. The fault resistance is simply too high to produce a measurable reflection with a low-voltage pulse.

2. Proven Methods for Locating High-Resistance Faults

2.1 Arc Reflection Method (ARM) / Surge Wave Method

The Arc Reflection Method — also called ARM, surge wave method, or impulse current method — is the industry standard for locating high-resistance faults. Instead of using a low-voltage TDR pulse, apply a high-voltage surge (8–35 kV) that forces the fault to break down. During the breakdown the fault resistance momentarily drops to nearly zero as an arc forms; while the arc is active, a TDR pulse measures a strong reflection from the fault point, exactly as if it were a solid short.

The high-voltage surge is generated by a thumper (surge generator), which charges a capacitor and discharges it into the cable. The energy of the surge — typically 500–2450 joules — must be sufficient to bridge the fault resistance and sustain an arc for several microseconds. Once the arc forms, the TDR reflection is clear and the distance to the fault is calculated from the pulse round-trip time and the cable propagation velocity.

Key advantages of ARM:

• Works on faults up to several megaohms — far beyond TDR's practical range.
• Requires only a few seconds per pulse; fault location can be obtained in minutes.
• Provides a visual waveform that experienced operators can interpret and verify.
• Does not require a good return conductor (unlike bridge methods).

2.2 Pulse Current Method

The pulse current method (also called the surge current method) is closely related to ARM. Instead of sending a separate low-voltage TDR pulse, it measures the current wave of the high-voltage surge itself as it travels down the cable, reflects from the fault arc, and returns. A current coupler (clamp-on CT) is placed around the test lead or cable at the near end, and the receiver displays the outgoing surge and its reflection.

This method is particularly effective for very high-resistance faults and long cables, because it uses the high-energy surge as both the fault activator and the measurement signal. The high-voltage pulse has much greater energy and travels further with less attenuation than a low-voltage TDR pulse, making it ideal for long or attenuated cables.

2.3 Burn-Down / Fault Conditioning

When the fault resistance is so high that even a 35 kV surge cannot break it down, a burn-down (fault conditioning) procedure is used. Burn-down applies a sustained DC voltage at a level just below the breakdown threshold, delivering continuous energy into the fault point. Over minutes the energy carbonizes the insulation and creates a conductive path, gradually lowering the fault resistance from megaohms to a few hundred ohms or less.

Once the fault resistance drops, standard TDR or ARM can be used for pre-location. Burn-down should be used cautiously: it permanently damages the fault point (acceptable, since repair is needed anyway), but it must be controlled to avoid creating secondary faults elsewhere in the cable. Dedicated burn-down sets include current limiting and automatic cutoff to prevent overcurrent damage.

2.4 Bridge Methods (Murray / Varley Loop)

Bridge methods are among the oldest fault location techniques, but they remain useful for high-resistance faults when a parallel good conductor is available. The Murray loop and Varley loop methods use a Wheatstone bridge configuration to measure the resistance ratio between the faulty conductor and a healthy conductor in the same cable. By comparing the resistance of the full cable length to the resistance from the test end to the fault, the distance to the fault is calculated.

Bridge methods are particularly effective for high-resistance ground faults (phase-to-screen or phase-to-earth) because they measure DC resistance directly and do not depend on pulse reflections. However, they require a good conductor of the same gauge and length to form the loop, and they cannot locate open-circuit faults or phase-to-phase faults without a suitable return path. Accuracy is typically ±1–2% of cable length.

2.5 VLF / High-Voltage Stress Activation

Some high-resistance faults — especially water-tree faults — only break down under sustained AC overvoltage. In these cases, applying a VLF (Very Low Frequency) AC voltage at 0.1 Hz, typically at 1.5–2.0 times rated voltage, for several minutes forces the fault to break down repeatedly and consistently. Once the fault is actively arcing under sustained voltage, ARM or pulse current pre-location can be performed.

VLF stress is also useful for diagnosing whether a high-resistance fault is voltage-dependent: if the fault appears at 1.5 U0 but disappears at 1.0 U0, the condition is typically water treeing or void discharge rather than a permanent carbonized track.

2.6 Acoustic-Magnetic Pinpointing

After pre-location narrows the fault to a specific section, the exact ground-surface position must be found. The acoustic-magnetic method is universal for this final step: the thumper surge causes the fault to arc, producing both an acoustic sound (from rapid thermal expansion at the fault) and a magnetic signal (from the surge current). A handheld receiver with a ground microphone and magnetic sensor is walked along the cable route. The operator first uses the magnetic signal to find the general area, then switches to acoustic listening to pinpoint the exact fault location to within 10–30 cm.

3. Step-by-Step Procedure

4. Best Practices

• Start with the lowest effective surge voltage — excessive voltage can damage healthy cable sections; increase voltage only until consistent breakdown occurs.
• Confirm the propagation velocity — input the correct VF (velocity factor) from the cable manufacturer; an incorrect VF produces distance errors of 5–15%.
• Save and compare multiple waveforms — a genuine fault reflection is consistent across multiple pulses while noise appears randomly.
• Test all conductors — high-resistance faults may affect one phase-to-ground, phase-to-phase, or the sheath; test each combination.
• Use adequate surge energy — energy, not just voltage, matters for high-resistance faults.
• Prioritize safety — follow lockout/tagout, use insulated PPE, maintain a safe distance during pulsing, and always discharge and ground the cable after testing.

5. XZH TEST Solutions for High-Resistance Fault Location

XZH TEST offers a complete cable fault location system engineered specifically for high-resistance and intermittent faults:

• Integrated Fault Locators — combine TDR, ARM (arc reflection) and pulse current methods in one portable receiver with touchscreen display; automatic mode selects the best method based on fault type.
• High-Voltage Surge Generators — portable and trolley-mounted thumpers up to 35 kV with adjustable energy (up to 2450 J), sufficient to break down even very high-resistance faults in medium and high-voltage cables.
• Acoustic-Magnetic Pinpointing Receivers — detect both acoustic and magnetic signals from fault arcs, with built-in correlation for field noise rejection and pinpoint accuracy to ±10–30 cm.
• Burn-Down Capability — selected surge generator models include controlled burn-down output for lowering extremely high fault resistance before ARM pre-location.
• Complete System Packages — all-in-one kits combining TDR/ARM locator, surge generator, acoustic-magnetic pinpointing and cable route tracing in a single portable setup.

Conclusion

High-resistance cable faults are the norm in real-world underground cable systems, not the exception. Standard TDR fails on these faults because the fault resistance is too high to produce a measurable reflection. The solution is to actively force the fault to present a low-impedance signature — using high-voltage surges for ARM or pulse current pre-location, burn-down or VLF stress for the most stubborn faults, and acoustic-magnetic pinpointing for final ground location. With the right equipment, a systematic procedure, and attention to voltage, energy and propagation velocity, even megaohm-level faults can be located accurately and efficiently.

About XZH TEST

XZH TEST specializes in cable fault detection and electrical testing equipment, providing practical solutions for power utilities, electrical contractors, testing companies and field engineers. Product range includes cable fault locators, high-voltage surge generators, acoustic-magnetic pinpointers, TDR instruments, VLF test systems and cable route tracers — all engineered for field durability, measurement accuracy and operator safety.