Short Answer: Where the Search Begins
The search for damage to a power cable begins with determining the nature of the fault: conductor break, short circuit, low-resistance or high-resistance insulation damage. Primary diagnostics are performed by measuring insulation resistance with a megohmmeter and checking conductor integrity. Based on the results, a localisation method is chosen: for breaks and short circuits, pulse reflectometry is used; for high-resistance damage, burning methods followed by acoustic or inductive location; for damage with a flashing breakdown, the oscillating discharge method.
The accuracy of the search depends on the correct choice of method and consideration of installation conditions. For cables in the ground, the acoustic method gives an error within 0.5–1 m; for cables in trays and pipes, the inductive method with a search coil is used. Increased voltage testing and partial discharge measurement relate to diagnosing the condition of insulation, not to finding an already existing fault. The distinction between these tasks is important: diagnostics reveal developing defects, while the search localises an existing fault.
What Is Checked and Why
Physical meaning of fault location. Cable damage disrupts the distribution of the electric field and the conductivity of the circuit. A conductor break interrupts the current path, a short circuit creates a low-resistance path between conductors or to earth, and insulation damage creates a leakage current. Each of these disruptions affects the propagation of the probing signal differently, which is used for localisation.
Why localisation is carried out. Precise determination of the fault location allows:
- reducing the volume of excavation work;
- reducing line downtime;
- choosing the correct repair method — section replacement, joint installation, re-termination;
- preventing damage to adjacent utilities when opening the route.
Search methods are divided into relative (remote) and absolute (topographic). Relative methods determine the distance to the fault from the beginning of the line: pulse reflectometry, oscillating discharge method, loop method. Absolute methods indicate the point on the route: acoustic, inductive, contact. In practice, relative and absolute methods are applied sequentially: first the distance is determined, then the location is refined on site. Design features of the cable, for example single-core or multi-core design, affect the choice of method and the accuracy of localisation.
When the Check Is Carried Out
After installation. Before commissioning a cable line, tests provided for by the PUE RK are carried out: checking the integrity and phasing of conductors, measuring insulation resistance, and for cables above 1 kV — testing with increased rectified voltage. These tests reveal installation defects, but do not replace fault location if the line fails the tests. If deviations are detected, they proceed to localisation methods.
Operation. Damage may occur as a result of mechanical impact during earthworks, insulation ageing, overload, or moisture in joints. When protection operates or voltage disappears on the line, primary diagnostics are performed: measuring insulation resistance and checking conductor integrity. If damage is confirmed, they proceed to locate the fault.
Troubleshooting. This is a separate task, distinct from incoming inspection and acceptance tests. For the search, specialised equipment is used: reflectometers, high-voltage pulse generators, acoustic receivers, cable locators. The general design and characteristics of a power cable, including design features affecting signal propagation, are described in a separate material: power cable: design, types, characteristics.
How It Is Carried Out
Primary Diagnostics
Before choosing a localisation method, the nature of the damage is determined:
- Disconnect the cable from the network, check for absence of voltage, install earthing.
- Measure insulation resistance with a megohmmeter. For power cables up to 1 kV, the norm is not less than 0.5 MΩ. If the resistance is lower, record the value for method selection.
- Check conductor integrity: measure the DC resistance of the conductor. A break gives infinitely high resistance, a short circuit — close to zero.
- Determine the transition resistance at the fault location. The choice of method depends on this: low-resistance (up to several tens of ohms) or high-resistance.
Primary diagnostics do not require specialised equipment other than a megohmmeter and a DC bridge. More about the insulation resistance measurement procedure — in the material measuring cable insulation resistance with a megohmmeter.
Pulse Reflectometry
The method is based on probing the line with a short pulse and measuring the return time of the reflected signal. The instrument — a pulse reflectometer — generates a probing pulse that propagates along the cable at a speed depending on the insulation type (specified in the passport of the instrument or cable). Upon reaching an inhomogeneity — the fault location, a joint, a branch — the pulse is partially reflected. The distance is calculated from the reflection delay time: L = v · t / 2, where v is the propagation speed, t is the delay time.
The method allows determining breaks, short circuits and low-resistance connections with a leakage resistance of up to several tens of kΩ (depending on line length and attenuation). For high-resistance damage, reflectometry is ineffective without preliminary burning. Reflectometers operate on low-voltage direct current, are safe for the operator and do not change the characteristics of the damage. The result is presented as a reflectogram — a graph of the reflected signal amplitude versus distance.
Oscillating Discharge Method
Used for damage with flashing breakdown, typical of joints. The essence of the method: high voltage from a generator is applied to the cable, charging the cable capacitance until breakdown. At the moment of breakdown, an electric discharge occurs at the fault location, and free oscillations are excited in the line. By measuring the period of these oscillations, the distance to the fault location is determined. The method requires caution: high voltage is dangerous for personnel.
Acoustic Method
Based on listening to sound oscillations arising from a spark discharge at the fault location. Periodic high-voltage pulses from a generator are applied to the damaged conductor. A spark discharge occurs at the fault location, accompanied by sound. The sound is picked up from the surface using a stethoscope or piezoelectric sensor. The acoustic method is used to determine fault locations with flashing breakdown, single-phase and multiphase stable short circuits, and breaks with earthing at the break point. Efficiency depends on the depth of the cable and the level of extraneous noise.
Inductive Method
Based on picking up the magnetic field above the cable. An audio-frequency current (800–1000 Hz) from a generator is passed through the damaged conductors. An alternating magnetic field is formed around the cable. Moving along the route with a receiving coil and amplifier, the signal is listened to. Beyond the fault location, the sound volume decreases or disappears. The inductive method allows determining two-phase and three-phase stable short circuits with a transition resistance of not more than 20–25 Ω. The method is not applicable at great cable depths (more than 1.5–2.0 m).
Loop Method
Used when one or two conductors are damaged and at least one undamaged conductor is available. The damaged conductor is short-circuited with the intact conductor, forming a loop. A DC measuring bridge is connected to the conductor ends. By measuring the bridge arm resistances, the distance to the fault location is calculated. The method is used in cases where the pulse method cannot be applied due to high transition resistance or lack of equipment. In modern practice, it is used less often due to its labour intensity.
Increased Voltage Test
According to the PUE RK, power cables above 1 kV are tested with increased rectified voltage. The values of the test voltage and the duration of application are standardised in Table 97 of Appendix 1 to the Rules. During the test, attention is paid to the nature of the leakage current change. The cable is considered to have passed the test if there was no breakdown, no sliding discharges and no surges in leakage current. The increased voltage test is a check of the dielectric strength of the insulation, not a search for damage. But if a breakdown occurs during the test, this indicates the presence of a defect, which is then localised by other methods.
Partial Discharge Measurement
Diagnostics of the insulation condition of cables with cross-linked polyethylene (XLPE) insulation includes partial discharge (PD) measurement according to ST RK IEC 60270-2013. Partial discharges are local electric discharges that bridge only part of the insulation between conductors. They do not lead to immediate breakdown, but gradually destroy the insulation. PD measurement allows identifying developing defects at an early stage, before breakdown occurs. This is a diagnostic method, not a search for an already existing fault.
How to Evaluate the Result
Interpretation of results depends on the method. The main criteria are summarised below.
| Method | Normal result | Deviation | Probable cause |
| Insulation resistance measurement | Not less than 0.5 MΩ for cables up to 1 kV | Below 0.5 MΩ | Insulation damage, moisture, contamination |
| Conductor integrity check | Resistance corresponds to the norm for cross-section and length | Infinitely high or close to zero | Break or short circuit |
| Pulse reflectometry | The reflectogram shows reflections from joints and line ends typical of a healthy cable | Appearance of an additional reflection at a distance not corresponding to known inhomogeneities | Break, short circuit, low-resistance damage |
| Acoustic method | Maximum sound volume at the fault location | Weak signal or its absence | Great depth, high noise level, poor contact at the discharge point |
| Inductive method | Sharp decrease in signal volume beyond the fault location | Signal does not disappear or disappears at an unexpected section | Transition resistance above 20–25 Ω, great depth, interference |
| Increased voltage test | No breakdown, no sliding discharges, leakage current stable | Breakdown, sliding discharges, increase in leakage current | Insulation defect, local weakening |
Sources: PUE RK, paragraph 29, clause 311; reflectometer operating manuals; methodological guidelines for determining the location of damage to power cables.
When evaluating the reflectometry result, it is important to take into account that reflections arise not only from damage but also from joints, branches, and changes in cross-section. Therefore, before measurement, it is necessary to have passport data of the line indicating known inhomogeneities. Comparing reflectograms taken from both ends of the line increases reliability. For cables with PVC or cross-linked polyethylene insulation, the signal propagation speed differs, which affects the distance calculation.
What Can Distort the Measurement
| Factor | How it affects | What to do |
| Signal propagation speed | Depends on the insulation type. An incorrect value gives an error in distance. | Verify from the cable passport or calibrate using a known length. |
| Signal attenuation | On long lines, the reflection from a remote fault weakens and may be indistinguishable. | Measure from both ends, use amplifiers. |
| Known inhomogeneities | Joints and branches give reflections that can be confused with damage. | Have a line passport indicating all inhomogeneities. |
| Transition resistance | High-resistance damage does not give a clear reflection during reflectometry. | Apply burning or the oscillating discharge method. |
| Cable depth | For the inductive method, at a depth of more than 1.5–2.0 m the signal weakens. | Apply the acoustic method or combine methods. |
| Extraneous noise | Street noise, operation of machinery interfere with the acoustic method. | Conduct the search at a quiet time, use filters. |
| Condition of end terminations | Contamination or moisture of terminations distorts the insulation resistance measurement. | Inspect and clean the terminations before measurement. |
What to Do in Case of Deviation
Sequence of diagnostic steps for an unsuccessful search:
- Clarify the nature of the damage. Re-measure insulation resistance and conductor integrity. It is possible that the damage is high-resistance, and the reflectometry method is not suitable.
- Apply burning. For high-resistance damage, perform insulation burning to reduce the transition resistance. After burning, apply reflectometry or the acoustic method.
- Measure from both ends. If reflectometry from one end does not give a clear result, repeat the measurement from the other end. Comparing reflectograms helps to isolate the reflection from the damage.
- Combine methods. If the relative method gave a distance but the absolute method does not confirm the point, apply another absolute method: inductive instead of acoustic or vice versa.
- Check the route. Make sure the cable is laid along a known route. If there is no route, use a cable locator to determine the cable position before localising the damage. Installation features are described in the material power cable installation.
Typical Mistakes
Applying reflectometry for high-resistance damage without burning. If the transition resistance at the fault location is tens and hundreds of kΩ, the reflection will be too weak to detect. The method will not give a result, and time will be lost. Before reflectometry, the transition resistance must be assessed.
Ignoring line passport data. On the reflectogram, reflections from joints and branches can be mistaken for damage. Without a passport indicating known inhomogeneities, the risk of false localisation is high.
Using an incorrect signal propagation speed. The speed depends on the insulation type and cable design. If a value for PVC insulation is entered into the reflectometer, and the cable has XLPE insulation, the distance will be determined with an error.
Searching with the acoustic method under high noise level. Street noise, operation of machinery, and traffic drown out the discharge sound. Searching under such conditions is ineffective. A time with minimal extraneous noise should be chosen.
Attempting to find damage without disconnecting the cable. All fault location methods, except passive tracing, require disconnecting the line and removing voltage. Working under voltage with test equipment is dangerous and prohibited.
FAQ
Which method to choose for locating a conductor break?
For a conductor break, pulse reflectometry is the most effective. It allows quickly determining the distance to the break point with an accuracy depending on line length and attenuation. To refine the location on the route after reflectometry, the inductive or acoustic method is used if there is earthing at the break point.
What to do if the reflectometer does not see the damage?
The probable cause is high-resistance damage. It is necessary to perform insulation burning to reduce the transition resistance, then repeat the measurement. If burning is impossible, apply the oscillating discharge method or the loop method (if an intact conductor is available). It is also worth measuring from the other end of the line.
Can damage to a cable under voltage be found?
No. All fault location methods, except passive tracing, require disconnecting the cable and removing voltage. Working with test equipment on a cable under voltage is dangerous and prohibited by safety rules.
What is the accuracy of the acoustic method?
The accuracy of the acoustic method depends on the depth of the cable and the noise level. For cables in the ground at a depth of up to 1.5 m, the error is within 0.5–1 m. At greater depth or high noise levels, the accuracy decreases. To refine the location, the inductive method or a combination of methods is used.
What is the oscillating discharge method and when is it applied?
The oscillating discharge method is used for damage with flashing breakdown, which often occurs in cable joints. High voltage is applied to the cable until breakdown; at the moment of breakdown, free oscillations arise. The distance to the fault location is determined from the oscillation period. The method requires high-voltage equipment and compliance with safety measures.
Sources
- Electrical Installation Code of the Republic of Kazakhstan 2015, as amended by the order of the Minister of Energy of the Republic of Kazakhstan dated 31.10.2022 No. 340 — paragraph 29 “Power cable lines”, clause 311 — in force — IPS “Adilet”, adilet.zan.kz — composition of cable line tests, insulation resistance norms, test voltages.
- Electrical Installation Code of the Republic of Kazakhstan 2015, as amended by the order of the Minister of Energy of the Republic of Kazakhstan dated 31.10.2022 No. 340 — Table 97 of Appendix 1 — in force — IPS “Adilet”, adilet.zan.kz — values of the rectified test voltage for cables above 1 kV.
- GOST 2990-2013 “Cables, wires and cords. Voltage test methods” — in force — official text on meganorm.ru — methods of testing with alternating, direct and pulse voltage.
- ST RK IEC 60270-2013 “High-voltage test techniques. Partial discharge measurements” — in force — official text — diagnostics of XLPE cable insulation, partial discharge measurement.
- IEC 60270:2000 “High-voltage test techniques — Partial discharge measurements” — in force — official IEC website, webstore.iec.ch — international standard to which ST RK IEC 60270-2013 is identical.
- Methodological guidelines for determining the location of damage to power cables with voltage up to 10 kV (RD 34.20.516-90) — in force — official text on files.stroyinf.ru — fault location methods, including the loop method, acoustic and inductive.
- Operating manual for the REIS-205 reflectometer — in force — official text on rkpo.ru — pulse reflectometry method, determination of break, short circuit, low-resistance connections.
