How to Use a Cable Fault Pre-locator

1. Introduction

1.1 Why Cable Fault Location Matters

Underground power cables are the hidden arteries that keep cities, factories, and homes running. But when a buried cable fails, the damage is not always easy to find. A single open, short, or high-resistance fault can knock out power to an entire area, and every extra hour of downtime means lost production, frustrated customers, and rising repair costs.

The biggest challenge is not repairing the cable — it is finding the fault. Digging up kilometers of cable hoping to strike lucky is slow, expensive, and disruptive. This is why technicians rely on a cable fault pre-locator: a smart instrument that measures the approximate distance to the fault from the test end, so your crew can walk straight to the right spot and pinpoint precisely.

1.2 What This Guide Covers

This guide is written for everyone — including technicians who are testing a cable fault for the very first time. We explain every step in plain language, avoid unnecessary jargon, and show you exactly how to operate the instrument, read the waveforms, and stay safe. By the end, you will be able to pre-locate the four most common cable fault types with confidence.

Throughout this guide, we recommend the XHGG502A+ Cable Fault Pre-locator, a professional all-in-one instrument designed for fast, reliable pre-location of cable faults in the field.

1.3 Introducing the XHGG502A+ Cable Fault Pre-locator

The XHGG502A+ is a specialized instrument for measuring and analyzing the condition of power cables and locating cable faults. It combines modern electronic and computer technologies to perform signal filtering, acquisition, data processing, graphic display, and graphic analysis. With it you can measure cable wave velocity, cable length, and cable fault distance.

The instrument is suitable for a wide range of power cables, high-frequency coaxial cables, street-light cables, and buried wires of different cross-sections and materials. It detects low-resistance, short-circuit, open-circuit, and broken-wire faults, as well as high-resistance leakage and high-resistance flashover faults.

Its technical parameters comply with recognized standards, including GB/T 18268.1 (electromagnetic compatibility requirements for test equipment used in industrial environments), DL/T 849.1-2019 (general technical conditions for cable fault flash testers, Part 1), and JJF1042-2020 (calibration specification for cable fault testers).

2. Cable Fault Basics Anyone Can Understand

Before you switch on the instrument, it helps to understand what you are actually looking for. Don't worry — you do not need to be an electrical engineer. Here is the essential background in simple terms.

2.1 What Kinds of Cables Can Be Tested

Power cables are usually classified by their voltage rating. This matters because it tells you how much care is needed and which method to use:

• Low-voltage cables — rated at 6 kV and below;
• Medium- and high-voltage cables — rated between 6 kV and 35 kV (inclusive);
• High-voltage cables — rated at 66 kV and above.

Cables are also classified by their insulation medium, for example oil-impregnated paper insulation, cross-linked polyethylene (XLPE) insulation, and rubber or plastic insulation. The XHGG502A+ works across these common types, and the propagation speed (wave velocity) of each insulation type can be set in the instrument.

2.2 The Four Common Cable Fault Types

Almost every cable fault you meet in the field falls into one of these four categories:

• Open-circuit fault — the cable insulation is fine, but the conductor cannot transmit normally (for example, a broken core wire or a very high-resistance point). Pure open-circuit faults are rare; they usually appear together with low- or high-resistance faults.
• Low-resistance fault — the insulation is damaged and the fault can be measured directly with the low-voltage pulse method. The resistance is generally below a few hundred ohms. When the resistance is zero, it is called a short-circuit fault — a special case of a low-resistance fault.
• Leakage high-resistance fault — the insulation is damaged and forms a fixed resistance channel, but the fault cannot be measured directly with the low-voltage pulse method. The resistance is usually above a few hundred ohms.
• Flashover high-resistance fault — the insulation is damaged, but no fixed resistance channel forms. Instead, when the test voltage reaches a certain value, the leakage current suddenly jumps and a flashover (breakdown) occurs at the fault point.

2.3 How to Tell What Kind of Fault You Have

There are generally three ways to determine the nature of a cable fault: (1) judge it with an insulation resistance meter (megohmmeter) and a multimeter; (2) judge it from the results of cable pre-testing; and (3) judge it with a cable fault tester such as the XHGG502A+.

Low-resistance and open-circuit faults can usually be determined directly from the waveform tested with a multimeter or with the low-voltage pulse method. High-resistance faults need the high-voltage flashover method or the multiple-pulse method, which are explained later in this guide.

2.4 The Standard Fault-Testing Workflow

Professional cable fault location generally follows five steps. A pre-locator handles the heart of this workflow:

1. Analyze the nature of the fault and understand the voltage rating and insulation characteristics of the faulty cable.
2. Use the low-voltage pulse function to measure the length of all phases of the faulty cable and calibrate the cable's wave propagation speed.
3. Select an appropriate testing method and perform a preliminary (rough) fault location with the cable fault tester.
4. Perform precise fault location — determining the route and depth of the buried cable and accurately locating the fault point (with a path tracer and pinpoint locator).
5. Analyze the errors in the test results (measurement error, transmission speed error, interpretation error, and instrument error).

3. How the Pre-locator Works (Simple Explanation)

3.1 The Traveling-Wave Idea — Like Radar Under the Ground

The XHGG502A+ works on the traveling-wave (reflection) principle. Think of it like radar: the instrument sends a short electrical pulse down the cable. When that pulse meets a place where the cable's characteristic impedance changes — such as a fault point or the cable end — part of the wave bounces back to the test end. This returning wave is called a reflected wave, and the time it takes to come back tells you how far away the fault is.

Two simple rules let you read the result: at a short-circuit point, the reflected wave has the opposite polarity of the transmitted pulse; at an open-circuit point (including the cable terminal), the reflected wave has the same polarity. By measuring the round-trip time and knowing the wave velocity, the instrument calculates the distance to the fault.

3.2 The Three Testing Methods of the XHGG502A+

Different faults need different testing methods. The XHGG502A+ gives you three powerful tools in one instrument:

• Low-Voltage Pulse Method (TDR) — sends a low-voltage pulse to detect open-circuit, low-resistance, and short-circuit faults. This is the fastest, safest first step, and it is also used to measure cable length and calibrate wave velocity.
• High-Voltage Flashover Method — uses a high-voltage source to trigger a flashover discharge at high-resistance fault points. It includes direct flashover (for flashover-type high-resistance faults) and impulse flashover / the universal method (for high-resistance faults with high leakage).
• Multiple-Pulse Method (ARC, optional) — works together with the optional pulse coupler. It suppresses the large cosine oscillation interference that appears at the moment of impulse high voltage, and displays eight sets of high/low voltage comparison waveforms. This makes high-resistance flashover faults much easier to analyze.

  

4. Why Choose the XHGG502A+

The XHGG502A+ is designed with the field technician in mind. Here are the features that make it easy to learn, quick to use, and dependable on site.

4.1 Key Features at a Glance

Table 1. Key features of the XHGG502A+

4.2 Core Technical Parameters

Table 2. Core technical parameters

4.3 What's in the Box

The standard packing list includes the cable fault tester, charger, single Q-line, double Q-line, sampling box (sampler), USB communication cable, and — as optional accessories — the coupler main unit, coupler high-voltage cable A and B, and a transparent grounding wire. The optional pulse coupler is needed only when you plan to use the multiple-pulse method for complex high-resistance flashover faults.

5. Before You Start: Safety & Preparation

Safety comes first, always. Cable fault testing involves high voltage, and a few simple rules protect you and your equipment.

5.1 Critical Safety Rules

• De-energize first. Make sure the cable under test is completely de-energized before you begin. Never test a live cable.
• Ground correctly. Connect the test leads correctly — especially the ground wire and the high-voltage wire.
• Don't mix methods. When using the high-voltage flashover or multiple-pulse methods, never select the low-voltage pulse method in the instrument software.
• Handle with care. Avoid subjecting the instrument to strong impacts, and do not let non-professionals disassemble it.
• Look after the battery. If the battery is low, charge the instrument before use. If the instrument is idle for a long period, charge and maintain it every two months to prevent battery self-discharge and damage.

For high-voltage and megohmmeter tests, always use a discharge rod to fully discharge the high-voltage energy-storage capacitor and the tested cable after testing, and before touching any wiring.

5.2 Pre-Test Checklist

• Read the instrument manual and understand the operating procedures and wiring methods.
• Check that the battery is sufficiently charged.
• Inspect the test leads, sampler, and coupler (if used).
• Confirm the cable's rated voltage and insulation condition, and make a preliminary judgment of the fault type.
• If you will use the multiple-pulse method, confirm the optional pulse coupler is available.

6. Step-by-Step Operation Guide

6.1 Power On and Understand the Screen

Turn on the instrument with the power switch. After the startup screen, the main operating interface appears. It is divided into five areas: a working status indicator, a waveform display area, an instrument parameter display area, a function-setting area, and an instrument operation area.

When the status indicator shows Holding State, the instrument is ready. Press the Sampling button and it switches to Sampling State. In the function-setting area you set the wave velocity (dielectric velocity), the operating mode, the test range, and system options such as automatic ranging, screen brightness, and the operation feedback buzzer.

6.2 Operation 1: Low-Voltage Pulse Method

Best for: open-circuit, low-resistance, and short-circuit faults, plus cable length measurement and wave-velocity calibration.

1. In the function-setting area, select the dielectric velocity (for example, Cross-linked polyethylene: 172 m/µs), choose the operating mode Low-Voltage Pulse, and select an appropriate test range. If the cable length is unknown, enable Automatic Range in the system settings.
2. Connect the single Q-line from the instrument to the cable under test according to the low-voltage pulse wiring diagram.
3. Click Sampling. The instrument starts low-voltage pulse sampling automatically. Use the Baseline and Amplitude controls so the echo fits neatly within the display and is easy to analyze.
4. Press Hold to stop sampling. Use the waveform controls to move and compress the waveform for the best view, then use the cursor or Automatic Measurement to read the fault distance.
5. Click File Processing to save the waveform for later viewing and comparison.

6.3 Operation 2: High-Voltage Flashover Method

Best for: high-resistance leakage and flashover faults. Caution: it is strictly prohibited to connect the sampler for high-voltage sampling while the software is still in low-voltage pulse mode.

1. Select the dielectric velocity, choose the operating mode High-Voltage Flashover, and set the test range (or enable Automatic Range).
2. Connect the high-voltage wiring (high-voltage supply, capacitor, sphere gap, and tester) according to the flashover wiring diagram, but do not connect it to the main unit yet.
3. Adjust the impulse discharge voltage of the high-voltage source so that it triggers a breakdown discharge at the fault point. You can confirm breakdown when the voltage indicator of the high-voltage equipment instantly drops to zero and the current indicator instantly increases.
4. Click Sampling and wait for the sampler to provide feedback.
5. Connect the sampler to the tester main unit with the dedicated dual Q-line, and slowly move the sampler parallel to the sampling ground wire — usually within about 10 cm the sampler captures the reflected signal.
6. Adjust the amplitude knob so the sampled waveform is fully displayed. Press Hold to stop sampling and analyze the waveform. If it does not look standard, click Sampling again to resample.
7. After testing, remove the sampler, turn off the power, and disconnect the tester. Then use a sound-magnetic synchronous pinpointing device above the cable within the rough range for precise location, and use a discharge rod to fully discharge the high-voltage capacitor and the cable.

6.4 Operation 3: Multiple-Pulse Method (Optional)

Best for: high-resistance flashover faults where the flashover waveform is disturbed by cosine oscillation. It requires the optional pulse coupler.

1. Select the dielectric velocity, choose the operating mode Multiple Pulse, and set the test range (or enable Automatic Range).
2. Connect the wiring according to the multiple-pulse diagram. Note: when connecting the coupler wires, only plug and unplug them — never rotate or twist them.
3. Adjust the high-voltage impulse so it triggers a breakdown discharge at the fault point (voltage indicator drops to zero, current indicator rises).
4. Click Sampling. Connect the dual-Q cable to the main unit and acquire the waveform, then adjust the amplitude knob so the waveform is fully displayed.
5. The display defaults to waveform group 4. If you want a different group, click the 1–8 keys on the left side of the screen to view the corresponding multiple-pulse group waveform. Overlap the upper and lower waveforms and measure the distance.
6. Use Automatic Measurement or the cursor for manual analysis, then save the waveform through File Processing.

6.5 Operation 4: Built-in Megohmmeter

The built-in megohmmeter measures cable insulation resistance. Ensure the cable is completely discharged before testing, and note that the megohmmeter outputs high voltage at both ends of the single Q-wire during testing — take precautions against electric shock, and discharge the cable with a discharge rod afterward.

1. On the main interface, select the Megohmmeter mode.
2. Click Start to run the resistance test and read the value.
3. After the test, click End to stop, then Exit to return to the main interface. Discharge the tested cable with a discharge rod.

6.6 Saving and Exporting Waveforms

To review a saved record, click File Processing, choose the file name, and click Open — the waveform appears on screen.

To export waveforms to a computer for archiving or printing: install the waveform file-management software, copy the required folder to the root of the C drive, connect the instrument to the computer with the USB cable, install the driver, then restart the instrument and open the cable fault browser software. Scan the test type, select the serial port assigned by the computer (for example COM*), and click Open. Back in the instrument's File Processing screen, select the waveform and click Export to display it on the computer, where you can save or print it.

7. Reading the Waveforms

Reading a waveform is easier than it looks. Here is what the common patterns mean.

7.1 Open-Circuit Fault Waveform

The transmitted pulse appears at time t1. A reflection from an intermediate connector (if any) appears at t2. At t3, the reflected pulse from the open-circuit fault appears with the same polarity as the transmitted pulse. A secondary reflection at t4 has a smaller amplitude because of attenuation. Move the cursor to the fault reflection, and the instrument displays the calculated distance.

7.2 Low-Resistance (Short-Circuit) Fault Waveform

As before, the transmitted pulse is at t1 and any connector reflection at t2. At t3, the reflected waveform from the low-impedance fault point appears with opposite polarity (opposite to the transmitted pulse). The distance to the fault is read from this reflection.

7.3 Full-Length and Intermediate Connector Waveforms

When there is no fault, you see the reflection from the whole cable length (the terminal), which has the same polarity like an open circuit, plus a secondary reflection with lower amplitude. This pattern is useful for measuring the full cable length and locating intermediate connectors, and it is also used to calibrate the wave velocity.

7.4 Impulse Flashover Waveforms

In impulse flashover testing, t0 is the spark-gap discharge, t1 is the primary reflection from the fault point, and t2 is the secondary reflection. For faults very near the test end, the waveform may appear as a decaying cosine oscillation; in that case, take several reflected waves and calculate their average distance for a more reliable result.

7.5 Tips for Accurate Reading

Keep the transmitted pulse and the fault echo clearly visible before measuring. Align the cursor with the leading-edge inflection points of the transmitted pulse and the fault echo pulse. When a waveform is unclear, resample rather than guess — and use Automatic Measurement to let the instrument compute the distance for you.

8. Common Problems and Troubleshooting

Table 3. Common problems and solutions

9. Real-World Usage Tips and Application Scenarios

9.1 Where It Is Most Useful

The XHGG502A+ is ideal for utility and industrial teams working on urban distribution cables, buried industrial cables, street-light cables, and other underground power lines. It is equally useful for technicians checking open, short, and high-resistance faults before excavation.

9.2 Field Best Practices

• Calibrate the wave velocity using a healthy cable phase of known length before testing the faulty phase.
• Treat the pre-location result as a rough distance — always confirm with precise pinpointing equipment above the cable.
• Compare several waveform groups (especially in multiple-pulse mode) to reduce interpretation error.
• Choose the optional pulse coupler for complex high-resistance flashover faults to make analysis easier.
• Follow the five-step workflow in Section 2.4 so nothing is skipped and results are consistent.

10. Conclusion

Cable fault pre-location does not have to be a guessing game. With a clear understanding of cable fault types, a simple traveling-wave principle, and the right instrument, you can quickly find the rough distance to a fault and send your crew straight to the right location.

The XHGG502A+ makes this easy even for beginners: three testing methods in one portable unit, an automatic range and waveform analysis, a bright touchscreen readable in sunlight, a built-in megohmmeter, a built-in battery for field use, and a robust IP54-protected design. Whether you are dealing with open, low-resistance, or high-resistance flashover faults, the XHGG502A+ gives you fast, reliable pre-location.

For a complete cable fault location solution, combine the pre-locator with a path tracer and a pinpoint locator: the pre-locator gives you the distance, and the pinpoint locator confirms the exact spot. Always follow the safety rules in this guide and use a discharge rod before touching any high-voltage circuit.

The XHGG502A+ is covered by a one-year free warranty for product quality issues, with lifetime repair and technical support. If you notice any abnormal condition or malfunction, contact us and we will arrange the most convenient and effective solution for you.