How to Locate Intermittent Cable Faults: Proven Methods, Equipment and a Step-by-Step Field Guide

Introduction

Intermittent cable faults are among the most frustrating and time-consuming challenges in power cable maintenance. Unlike permanent faults that present a stable, measurable problem, intermittent faults come and go—triggering an outage one day, disappearing for weeks, and then returning without warning. By the time a crew arrives on site, the fault has often vanished, leaving no trace of its location.

Locating an intermittent fault requires a different approach than locating a permanent one. Standard Time Domain Reflectometry (TDR) often fails because the fault resistance is too high or the fault only occurs under specific thermal, electrical, or mechanical conditions. This article explains what causes intermittent cable faults, why they are so difficult to find, and the proven methods and equipment that field engineers use to pinpoint them quickly and accurately.

1. What Are Intermittent Cable Faults?

An intermittent cable fault is a defect that does not produce a stable, continuous short circuit. Instead, the fault appears and disappears depending on operating conditions such as load current, temperature, voltage level, soil moisture, or mechanical movement. Common causes include:

• Water treeing in XLPE insulation — Moisture penetrates the insulation over time, creating tree-like channels that conduct only when the cable is energized under load or at elevated temperature.
• Hairline cracks in insulation — Tiny cracks that open and close with thermal expansion and contraction as cable load changes throughout the day.
• Loose or corroded connections — Joints or splices with high contact resistance that fail under high load current but appear normal at low load.
• Mechanical damage that is not fully penetrated — A cable that has been nicked or abraded but still has enough insulation to hold off voltage under normal conditions, breaking down only when voltage surges or temperature rises.
• Moisture in joints or terminations — Water ingress that creates a conductive path only when the joint is wet or when the cable warms up.
• Semi-conductive layer damage — Knife marks or incomplete removal of the semicon screen at a joint create local field stress that breaks down intermittently.

1.1 Why Intermittent Faults Are So Hard to Locate

The core difficulty with intermittent faults is that the fault resistance is not stable. When you arrive on site and connect your TDR, the fault may have sealed itself—insulation resistance reads normal, and the TDR shows no reflection. By the time you leave, the fault returns and trips the circuit again. This cycle can repeat for weeks or months, causing repeated outages and costly emergency call-outs.

Standard TDR works by sending a low-voltage pulse and measuring reflections from impedance changes. It performs best on low-resistance faults (short circuits and open circuits). For intermittent faults, the resistance is typically very high when the fault is not active, and TDR cannot detect it at all. Even when the fault does break down, the reflection may be weak and masked by noise.

2. Proven Methods for Locating Intermittent 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 most widely used technique for locating high-resistance and intermittent cable faults. The principle is straightforward:

A high-voltage surge generator (thumper) applies a high-energy voltage pulse to the faulty cable. This pulse forces the intermittent fault to break down—creating a temporary arc that lowers the fault resistance to nearly zero for the duration of the pulse. While the arc is active, a TDR pulse is sent down the cable and reflects off the low-resistance arc, producing a clear, easily measurable reflection.

The time between the outgoing TDR pulse and the reflection from the arc is measured, and using the cable’s propagation velocity, the distance to the fault is calculated. ARM works because it actively forces the fault to present a low-impedance reflection, regardless of its normal resting resistance.

Key advantages of ARM:

• Works on high-resistance faults that TDR cannot detect
• Does not require the fault to be stable—each surge pulse temporarily creates the reflection
• Can locate faults from 1 meter to several kilometers away
• Provides a visual waveform display that experienced operators can interpret

2.2 Pulse Current Method

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

The pulse current method is particularly useful for very high resistance faults and long cables because it uses the high-energy surge pulse as both the fault activator and the measurement signal. It is less affected by cable attenuation than a low-voltage TDR pulse.

2.3 Burn-Down / Fault Conditioning

When an intermittent fault has an extremely high resistance (often above 500 kΩ), it may not break down even at the maximum surge generator voltage. In these cases, a burn-down (or fault conditioning) procedure can help:

A controlled DC or AC voltage is applied to the cable at a level just below the breakdown threshold. The energy at the fault gradually carbonizes the insulation and lowers the fault resistance over several minutes, until the fault becomes a low-resistance short that can be located by TDR or ARM. Burn-down should be used carefully—it creates permanent damage at the fault point, which is acceptable because the fault will need to be repaired anyway, but it should not be used on healthy cable sections.

2.4 VLF / High-Voltage Stress to Activate the Fault

Some intermittent faults only appear under sustained overvoltage or thermal stress. In these cases, applying a VLF (Very Low Frequency) AC voltage or a DC hipot voltage at elevated levels can force the fault to break down consistently. Once the fault is breaking down under sustained voltage, ARM or pulse current measurements can be performed to locate it.

This approach is especially useful for water-tree-damaged cables, where the fault resistance varies with voltage and temperature. Applying 1.5–2.0 times rated voltage at 0.1 Hz for several minutes often triggers stable breakdown that allows accurate pre-location.

2.5 Acoustic-Magnetic Pinpointing

After pre-location with ARM or pulse current methods narrows the fault to a specific section, the fault must be pinpointed on the ground surface. The acoustic-magnetic method is the standard approach: the surge generator pulse causes the fault to discharge (arc), producing both an acoustic sound (from thermal expansion at the fault point) 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 a few tens of centimeters.

2.6 Monitoring and Event Logging

For intermittent faults that are truly difficult to reproduce—those that occur only under specific load or weather conditions—permanent or temporary monitoring systems can be deployed. These systems continuously monitor the cable for partial discharge activity, leakage current, or voltage transients, and log the time and severity of each fault event. Over days or weeks, the data reveals patterns (e.g., the fault always occurs when ambient temperature drops below 5°C, or when load exceeds 80%). This information helps engineers choose the right test conditions and timing to force the fault to appear during controlled testing.

3. Step-by-Step: Locating an Intermittent Fault

Step 1: Confirm the Fault is Intermittent — First, verify that the fault is truly intermittent. Measure insulation resistance at both ends. If readings are normal but the cable still trips under load, the fault is likely intermittent. Review outage logs to identify patterns (time of day, load level, weather).

Step 2: Perform a TDR Scan — Start with a low-voltage TDR pulse. If the fault is currently active (low resistance), TDR may locate it directly. If no reflection is visible, proceed to ARM.

Step 3: Set Up the Surge Generator — Connect the high-voltage surge generator (thumper) to the faulty phase. Start at a lower voltage (e.g., 8–10 kV) and gradually increase until the fault breaks down consistently. Look for a sudden voltage drop or current spike on the thumper meters.

Step 4: Perform ARM / Pulse Current Pre-Location — With the fault breaking down, use the ARM or pulse current function on the fault locator. The waveform will show the outgoing pulse and the reflection from the arc. Measure the distance and calculate the fault position along the cable.

Step 5: If No Breakdown — Try Burn-Down or VLF Stress — If the fault does not break down at available surge voltage, attempt burn-down with sustained DC voltage, or apply VLF overvoltage stress for several minutes to force consistent breakdown.

Step 6: Pinpoint with Acoustic-Magnetic Method — Using the pre-location distance as a guide, walk the cable route with the acoustic-magnetic receiver. Use the magnetic signal for wide-area scanning, then use acoustic listening to mark the exact fault location on the ground.

Step 7: Excavate and Verify — Dig at the marked location. Inspect the cable for visible damage. After repair, re-test the cable and perform a final TDR to confirm the fault is cleared.

4. Best Practices

• Start low, go high — Begin surge voltage at the lowest level that causes breakdown. Excessive voltage can create additional damage at other weak points in the cable.
• Record waveforms — Save every ARM/TDR waveform. Comparing waveforms from multiple pulses helps confirm the fault location and distinguish it from noise.
• Use the right propagation velocity — Input the correct cable velocity factor (from manufacturer data or TDR calibration on a known cable length). An incorrect velocity produces inaccurate distance calculations.
• Test all phases — Intermittent faults may not affect every phase. Test each phase-to-ground and phase-to-phase to identify the exact faulted circuit.
• Be patient — Intermittent faults may require multiple surge pulses, elevated voltage, or sustained stress before they break down consistently. Rushing the process often means returning to the same fault later.
• Prioritize safety — High-voltage surges are dangerous. Follow lockout/tagout, use proper PPE, maintain a safe distance from the cable during pulsing, and always discharge the cable after testing.

5. XZH TEST Solutions for Intermittent Fault Location

XZH TEST offers a complete cable fault location system designed to handle intermittent and high-resistance faults:

• Integrated Cable Fault Locators — Combine TDR pre-location, ARM (arc reflection), and pulse current methods in a single portable instrument. The receiver automatically switches between methods based on fault type.
• High-Voltage Surge Generators (Thumpers) — Available in portable and trolley-mounted configurations up to 35 kV, with adjustable energy output and automatic cycling. Enough energy to break down stubborn intermittent faults in medium and high-voltage cables.

• Acoustic-Magnetic Pinpointing Receivers — Detect both acoustic and magnetic signals from fault discharges, with built-in correlation for noise rejection. Pinpoint accuracy to within ±10–30 cm.
• VLF Test Systems — Apply sustained overvoltage stress to activate water-tree and voltage-dependent intermittent faults, with optional Tan Delta and PD measurement for insulation condition assessment.
• Complete System Packages — All-in-one kits combining TDR, surge generator, acoustic-magnetic pinpointing, and cable route tracing in a single portable setup for efficient field operations.

Conclusion

Intermittent cable faults are challenging but not impossible to locate. The key is to actively force the fault to present a measurable signal—using high-voltage surges for ARM or pulse current pre-location, burn-down or VLF stress for stubborn high-resistance faults, and acoustic-magnetic pinpointing for final ground location. With the right equipment, a systematic procedure, and patience, even the most elusive intermittent fault can be found and repaired—often in a single site visit rather than weeks of repeated outages.

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, surge generators, acoustic-magnetic pinpointers, TDR instruments, VLF test systems, and cable route tracers—all engineered for field durability, measurement accuracy, and operator safety.