TDR Cable Fault Testing: How to Read and Interpret Waveforms

Underground power cables are the quiet workhorses of any electricity network. They run for kilometers under roads, pavements and fields, so when one of them fails, you cannot simply "look" for the problem. The cable is out of sight, and every minute of downtime costs money. The first question on site is always the same: where exactly is the fault?

The fastest way to answer that question is a technique called Time-Domain Reflectometry (TDR), also known in the industry as the "traveling wave method." This guide explains, in plain language that beginners can follow, how TDR works, how to read its waveforms correctly, and why an all-in-one instrument like the XHGG502A+ Cable Fault Pre-locator makes the whole job faster, easier and more reliable.

1. What Is TDR and How Does It Work?

Think of TDR like shouting into a long tunnel. You shout at the entrance, and if the tunnel is clear, the sound simply travels away. But if there is a wall, a door, or a hole somewhere inside, part of your shout bounces back as an echo. The time it takes for the echo to return tells you roughly how far away the obstruction is.

A cable fault locator works the same way, but with an electrical pulse instead of sound. It fires a short pulse into the cable. Wherever the cable’s impedance changes — at a fault, a joint, or the open end of the cable — a small part of the pulse is reflected back to the instrument. By measuring the time delay between the outgoing pulse and the returning echo, the instrument calculates the distance to that point. The basic formula is simple:

The division by two is needed because the pulse travels to the fault and then all the way back — it covers the distance twice.

Two useful facts make waveform reading easy to learn. First, the polarity of the echo tells you what kind of point the pulse has hit. A reflection from an open circuit (or the cable terminal) comes back with the same polarity as the outgoing pulse, while a reflection from a short circuit or a low-impedance fault comes back inverted. Second, TDR gives you a rough, or "pre-located," distance. After TDR narrows the search down to a short section of cable, you still do precise pinpointing on the spot with an acoustic-magnetic device to find the exact point before digging.

2. Know Your Cable Faults First

Before touching the instrument, it helps to understand the four main types of cable fault, because each one needs a different test method:

• Open-circuit fault: the insulation is fine but the conductor cannot pass the voltage — for example a broken core or a poor connection. Pure open-circuit faults are rare; most appear together with a low- or high-resistance fault.
• Low-resistance (leakage) fault: the insulation is damaged and the fault can be measured directly with the low-voltage pulse method. If the resistance is close to 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 (usually above a few hundred ohms) that the low-voltage pulse method cannot see directly. It needs the high-voltage flashover method.
• Flashover high-resistance fault: the insulation is damaged but no fixed resistance channel is formed. When the pre-test voltage reaches a certain level, the cable suddenly breaks down and flashes over. It also needs the high-voltage flashover method.

In practice, you first judge the nature of the fault — for example by measuring insulation resistance with a megohmmeter — and then choose the matching test mode. This is exactly the workflow a full-featured locator like the XHGG502A+ is designed to support.

3. How to Read the Waveforms

3.1 The polarity rule

The most important habit to build is checking polarity. If the returning echo has the same polarity as the outgoing pulse, it came from an open circuit or the cable terminal. If the echo is inverted (opposite polarity), it came from a short circuit or a low-impedance fault. This single rule lets you identify the fault nature at a glance.

3.2 Open-circuit fault waveform

On the screen you see the outgoing pulse at time t1, then a reflected pulse at time t3 that has the same polarity as t1 (a positive echo). Between them, a small same-polarity echo at t2 is the reflection from an intermediate joint. At t4 you see a secondary reflection from the fault, smaller than the primary one because the pulse loses energy on its way. The fault distance is measured from t1 to t3. On the XHGG502A+, you simply move the cursor to these two points and the instrument shows the calculated distance automatically.

3.3 Low-impedance (short-circuit) waveform

Here the story is almost the same, except that the echo at t3 has the opposite polarity to the outgoing pulse. There may also be a reflection from the cable terminal further along. Once you see an inverted echo, you know the fault is a low-resistance or short-circuit type and you can read the distance the same way.

3.4 Beware of joint reflections

Intermediate joints and connectors also reflect the pulse, and their echo is usually of the same polarity (though this depends on the connector structure). A beginner can easily mistake a joint reflection for a real fault. The trick is to save a reference waveform of the healthy, full-length cable. Comparing the fault waveform against the reference makes the real fault echo stand out clearly.

3.5 High-voltage flashover and near-end waveforms

When testing high-resistance faults with the high-voltage flashover method, the waveform begins with a spark-gap discharge at t0, followed by the primary reflected wave from the fault at t1 and the secondary reflection at t2. Measure the distance from the test end to the fault point as usual.

One special case is a fault very close to the test end. The waveform then shows a series of reflected waves that look like a decaying cosine oscillation. Instead of measuring one single echo, you take several periods, measure the distance across them, and divide by the number of rising and falling edges to get the average. This is a small detail, but it prevents big errors in near-end faults — the area inside the instrument’s test blind zone (for the XHGG502A+, about 10 m).

4. Three Test Methods on the XHGG502A+

The XHGG502A+ integrates three TDR test methods plus a built-in megohmmeter, so one instrument covers virtually every power cable fault you are likely to meet.

4.1 Low-voltage pulse method

This method sends a low-voltage pulse (300 V ± 15%) into the cable and is ideal for open-circuit, low-resistance and short-circuit faults. It is the fastest and simplest method, and it is also the one you use to measure cable length and to calibrate wave velocity.

4.2 High-voltage flashover method

For leakage and flashover high-resistance faults, the low-voltage pulse cannot "see" the fault directly. The XHGG502A+ therefore uses the high-voltage flashover method, which comes in two forms. Direct flashover uses a DC high-voltage supply to force a flashover discharge at the fault; impulse flashover (also called the universal method) uses a high-voltage pulse signal source. In both cases the reflected signal is picked up by a dedicated sampler and fed into the tester for analysis.

4.3 Multiple pulse method (optional pulse coupler)

The most challenging faults are high-resistance faults that need high voltage. Right at the moment of the impulse, the cable can be swamped by a large cosine oscillation that hides the useful echo. The multiple pulse method solves this elegantly: after the high-voltage unit breaks down the fault point, a trigger device immediately fires several low-voltage test pulses. While the arc is still conducting, these pulses see a short-circuit-like echo and form eight fault-reflection pulses; after the arc goes out, eight more pulses are sent to record the open-circuit reflection. The result is eight sets of clean, high/low-voltage comparison waveforms. Among them you will almost always find a set that is easy to read, and the XHGG502A+ displays these 1–8 groups for automatic or manual distance measurement.

5. Step-by-Step Field Workflow

Here is a simple routine that works for most jobs. It follows the workflow recommended in the XHGG502A+ manual.

1. Safety first. Make sure the cable is fully de-energized, connect the ground wire correctly, and confirm the battery is charged. Note: when using high-voltage flashover or multiple pulse modes, never select the low-voltage pulse mode in the software.
2. Judge the fault nature first. Use the built-in megohmmeter function of the XHGG502A+ (0.1 MΩ–40 MΩ) to measure insulation resistance and decide which fault type you are dealing with.
3. Set the instrument. Choose the dielectric (wave) velocity — for example cross-linked polyethylene (XLPE) cable uses about 172 m/µs — and pick a test range. If the cable length is unknown, simply enable the automatic range function.
4. Sample and adjust. Start sampling, then use the amplitude and baseline controls until the echo is clear and sits neatly within the display.
5. Hold and measure. Press "Hold" to freeze the waveform, then either use the cursor to mark the leading edges of the outgoing pulse and the fault echo, or press automatic measurement to read the distance instantly.
6. Save and export. Store the waveform with a name for later comparison, and export files to a PC via USB for archiving and printing.

A practical tip from the manual: on long or unknown cables, first use the low-voltage pulse method to measure the full length of every phase and calibrate the wave velocity. Then run the actual fault test. This one habit removes the most common source of distance error.

6. Common Mistakes and Quick Tips

• Mistaking a joint reflection for the fault — a joint echo can look like a fault. Compare with a reference waveform of the healthy cable before you trust it.
• Using the wrong wave velocity — a wrong wave velocity produces a wrong distance even if the waveform looks perfect. Always calibrate the velocity against a known cable length first.
• Reading the wrong peak — in high-voltage flashover mode, do not confuse the spark-gap discharge or the large cosine oscillation with the fault echo. Rely on the stable reflections at t1 and t2.
• Near-end faults — for faults close to the test end, use the multi-period average method instead of a single echo, and remember the blind zone.
• Noisy waveforms — poor grounding or loose wiring produces a noisy, messy waveform. Check the ground wire and signal connections before suspecting the instrument.

7. Why the XHGG502A+ Is the All-in-One Choice

For maintenance teams who want one instrument that covers the whole job — not a drawer full of separate boxes — the XHGG502A+ is designed around two ideas: comprehensive and convenient.

7.1 Comprehensive

• one instrument handles open-circuit, short-circuit, low-resistance, leakage high-resistance and flashover faults — from a quick low-voltage pulse check to a full high-voltage multiple pulse test with an optional coupler.
• a built-in megohmmeter for fast fault-nature pre-judgment, so you rarely need a second tool on site.
• measurement performance you can rely on: sampling frequency from 1 MHz to 400 MHz, a minimum resolution of 0.07 m, a measurement error of no more than ±(0.5% * L + 1 m), a maximum test range of 150 km, and a test blind zone of ≤ 10 m.
• complies with industry standards GB/T 18268.1, DL/T 849.1-2019 and JJF 1042-2020.

7.2 Convenient

• a bright 10.1-inch industrial touch screen (280-nit, 1024*600) that stays readable even in direct sunlight, with both touch and encoder-knob operation.
• automatic range setting and automatic waveform analysis, plus the multiple pulse mode showing 8 groups of comparison waveforms — far easier for beginners to interpret high-resistance faults.
• large on-board storage (over 8,000 low-voltage/flashover records and over 250 multiple pulse records), Chinese-friendly naming, and USB export to PC management software for reports and printing.
• built-in large-capacity lithium battery that charges in about 2 hours and runs for more than 5 hours, a rugged casing rated to IP54, and an operating range of −20 °C to +65 °C — made for real field conditions.

Conclusion

Reading TDR waveforms is not magic. Once you understand the polarity rule, learn to measure from the leading edges, and learn to separate joint echoes and oscillations from the real fault, you can locate cable faults quickly and confidently. Choosing the right instrument makes that learning curve far shorter.

The XHGG502A+ Cable Fault Pre-locator brings low-voltage pulse, high-voltage flashover and multiple pulse testing together with a built-in megohmmeter in a single rugged, easy-to-use package. Its comprehensive modes, automatic analysis and clear waveform display help your team find faults faster — with less training and fewer mistakes. For any team that works on underground power cables, it is the all-in-one, convenient choice.