Cable Troubleshooting Strategies in Different Application Scenarios
The cable laying method and application environment significantly affect the difficulty of troubleshooting and the choice of methods.
Direct-Buried Cable Fault Diagnosis: Challenges and Solutions
Challenge: The cable is buried in the ground and is not visible; Soil moisture and composition variations affect the electric field and sound wave propagation. Adjacent pipelines (water pipes, gas pipes, other cables) can generate interference signals; Accurate cable path information is difficult to obtain.
Recommended Procedures:
Preliminary Judgment: Megohmmeter and multimeter are used to judge the fault type (short circuit, open circuit, ground fault, etc.).
Route Confirmation: Use a cable route tracer to accurately track and mark the cable direction to avoid deviations in subsequent positioning.
Pre-location: Select the appropriate method based on the fault type.
Low-impedance short circuit/open circuit: TDR is preferred.
High-impedance ground fault: The Secondary Impulse Method (SIM/MIM) is preferred. If the device does not support it, you can try the High Voltage Bridge method (which requires burning the fault point first) or the acoustomagnetic method after a high voltage impulse.
Fault Point Location (Pin-pointing): Accurate positioning using the acoustomagnetic synchronous timing method within the area indicated by the pre-location results. A pulsed high voltage is applied to the cable, and the loudest sound is located by listening to the discharge sound on the ground. For ground faults that do not produce a clear discharge sound, the step voltage method can be tried.
Verification: After the suspected fault point is determined, a small area can be excavated, or local acoustomagnetic and step voltage method verification can be conducted again.
Addressing the Challenges: Reduce route errors through high-quality route tracers; Choose an acoustomagnetic receiver with strong anti-interference capability; Adjust the high-pressure impact energy according to soil conditions; A combination of methods corroborates the results with each other.
Insulated Aerial Cable (ABC) Troubleshooting: Quick Location Tips
Challenge: Fault points are often visible, but they are widely distributed and involve working at high altitudes, which can be dangerous to operate.
Typical Faults: Insulation layer aging and cracking, branch scratches, lightning strikes, bird and animal damage, joint process issues.
Test Process:
Visual Inspection: Carefully inspect the line, using a telescope, to look for obvious carbonization traces, burn marks, cracks, foreign body overlap, and other obvious traces of the insulation layer. Bucket trucks or drones increase efficiency and safety.
Thermal Imaging: Thermal cameras are used to detect abnormal temperature rises in the cable body, especially at joints and terminals, when the cable is operating under load. Temperature rise is an important sign of early failure or overload.
Basic Electrical Measurement: After a power outage, use a megohmmeter and multimeter to test insulation resistance and continuity to determine the fault type.
Fault Location: While visual inspection may reveal the fault point, TDR or acoustomagnetic (if high voltage impulse can be applied) can also be used to locate the fault point if it is not obvious (e.g., internal breakdown).
Skills: Use route maps and geographical indications to assist in positioning; Pay attention to the influence of weather factors on infrared thermography and visual inspection.
Cable Fault Diagnosis in Tunnels/Cable Trenches: Environmental Impact and Detection Methods
Challenge: The environment is enclosed, and there may be risks such as harmful gases, oxygen deficiency, high temperature, and high humidity; The space is narrow, and equipment is inconvenient to carry and operate; There are many cables, and it is difficult to identify the target cable; Ambient noise may interfere with acoustic detection.
Recommended Procedures:
Safety Assessment: Gas detection and ventilation should be carried out before entry to ensure safety.
Target Identification: Confirm the faulty cables using cable identification tags and system drawings.
Visual Inspection: Carefully inspect along the cable path, especially at joints and supports, for signs of insulation damage, ablation, deformation, etc.
Infrared Thermal Imaging: Conducted during loading, to detect abnormal hot spots.
Pre-location: TDR (for low resistance/open circuit) or Dual Pulse Method (for high resistance).
Fault Point Location: Acoustomagnetic synchronous positioning in tunnels/trenches is generally easier than direct burial because the discharge sound propagation is more direct. Use a contact acoustic sensor (placed on the cable surface) or an air-coupled sensor in combination with a magnetic field sensor.
Partial Discharge (PD) Detection: Tunnels/trenches are a favorable environment for partial discharge detection, and the background noise is relatively stable. Online or offline PD inspections can be performed using TEV sensors (on metal brackets or trays), HFCT sensors (on grounding wires), or ultrasonic sensors (on the cable body surface or accessories) to detect early insulation defects.
Submarine Cable Fault Diagnosis: Special Requirements and Technology
Challenge: The environment is extreme, requiring professional waterproof and pressure-resistant equipment; High positioning accuracy is required because the repair cost is extremely high; Repair work is complicated.
Typical Faults: Anchor hooks, fishing net scratches, ship anchor damage, earthquake and tsunami, internal water tree/electrical tree breakdown.
Recommended Procedures:
Pre-location: Primarily relies on high-precision submarine-specific TDR equipment, which usually requires the use of buoys or GPS-assisted surface position measurement. The high voltage bridge method can also be used, if possible.
Precise Location and Detection: Extremely difficult. Detailed search may be required in conjunction with sonars, underwater robots equipped with acoustomagnetic sensors, or flux sensors that detect changes in the magnetic field caused by leakage currents.
Fault Repair: Professional submarine cable laying and repair vessels are often required, and repair is conducted using wet or dry joint technology, which is costly.
Special Equipment: Submarine TDR probe, underwater acoustomagnetic synchronous receiver, ROV (Remotely Operated Vehicle).
Communication Cable (Fiber/Copper) Troubleshooting: OTDR and Other Tools
Communication cable fault diagnosis is different from power cables, especially fiber optic cables.
Fiber Optic Cable Fault:
Typical Faults: Broken fibers, dirty/damaged connectors, excessive splice loss, excessive bending radius (macrobend/microbend).
Basic Tool: Optical Time Domain Reflectometer (OTDR).
Principle: Similar to TDR, the OTDR transmits light pulses into the fiber and analyzes Rayleigh scattering and Fresnel reflection signals along the fiber path. By analyzing the shape and position of the reflection/scattering curve, it is possible to determine the length, attenuation, splice loss, connector loss, and the location of the fiber break point.
Applications: Accurately measure the loss distribution of fiber links, locate breaks, high-loss points, connector, or splice issues.
Other Tools:
Light Source and Power Meter: Used to measure the overall loss of the optical link and determine if there is a problem.
Visual Fault Locator (VFL): Shines a visible red light to detect fiber breaks, bends, or connector problems over short distances (the fiber jacket must be optically non-dense).
Fiber Microscope: Inspects connector end faces for cleanliness, scratches, or damage.
Copper Cable Fault:
Typical Faults: Open circuit, short circuit, wrong wiring, open circuit, crosstalk, excessive return loss.
Basic Tools: Cable Certifier/Tester or TDR (for open circuits, short circuits).
Applications: Measure pair length, wiring scheme (to determine short circuits, opens, mis-wires, crossed pairs), Near-End Crosstalk (NEXT), Far-End Crosstalk (FEXT), return loss, insertion loss, and other parameters to evaluate copper performance and locate faults. The TDR function is often used to pinpoint open or short circuit points.
In-depth Analysis of Typical Cable Fault Cases
Combining theory and practice is the key to mastering the technology. Here are some typical cable fault diagnosis cases in different scenarios.
Case 1: Single-Phase Ground Fault of a High-Voltage Power Cable in a Chemical Plant
Background: In the area of a large chemical plant, a single-phase ground fault alarm occurred on the outgoing feeder of a 35kV XLPE insulated power cable in operation, causing a power outage in the affected area.
Fault Phenomenon: The system’s ground protection device operated, and the circuit breaker tripped. The operator tried to reclose, but the relay operated again.
Diagnostic Steps and Procedures:
Preliminary Judgment
After the power outage, use a 2500V megohmmeter to test the insulation resistance of the faulty cable. The insulation resistance of phases A and B is normal (> 2000 MΩ), and the insulation resistance between phase C and ground decreases significantly, to only 5 MΩ. It is preliminarily judged to be a ground fault on phase C, and the resistance at the fault point is medium-to-high resistance.
Pre-location
Since it is a high-impedance fault, using conventional TDR directly may not be effective. The operating team decided to use Ultra-Low Frequency AC Hipot (VLF) testing with Dielectric Loss (Tan Delta) and Partial Discharge (PD) detection for pre-location and to assess the cable condition at the same time. Connect the VLF tester between phase C and ground, and apply 0.1 Hz, 2U0 (approximately 40kV) AC voltage. During the test, it was found that the tanδ value of phase C rapidly increased with increasing voltage, and a continuous large-amplitude partial discharge signal was detected. By analyzing the signal propagation characteristics (such as time difference positioning), the fault point is estimated to be located about 1.2 km away from the substation.
Precise Positioning (Quadratic Impulse Method)
In order to pre-locate more accurately for subsequent pinpointing, the O&M team used a cable fault tester with a quadratic impulse function. Connect the high voltage impulse generator (set to 15kV) to phase C and ground, and set the cable tester to secondary impulse mode. After applying a high voltage impulse, a flashover occurs at the fault point, and the cable tester captures a clear arc reflection waveform. The waveform was analyzed, and the fault distance was calculated to be 1.22 km. The results of the two pre-locations were fundamentally consistent.
Fault Point Detection (Acoustomagnetic Method)
According to the pre-location result of 1.22 km, O&M personnel carried the acoustomagnetic synchronous receiver and listened to the sound on the ground in the area around 1.2 km along the direction indicated by the radiometer (route tracer). The cable route tracer confirmed the precise cable direction on the ground beforehand. The operator carefully listened to the ground while applying a 15kV high voltage impulse, and finally heard the loudest discharge sound at a distance of 1225 meters from the test end. Combined with the synchronous judgment of the magnetic field signal, the precise location of the fault point was determined.
Excavation and Verification
A small excavation area was made at the location determined by the acoustomagnetic method, and it was found that the cable had a joint with blackened traces on the outer insulation. Dissection of the joint revealed that the internal filling (e.g., silicone grease) had failed, and moisture intrusion had led to moisture deterioration of the insulation, forming electrical trees, which eventually broke down and discharged at high voltage. The fault point was exactly the same as the diagnostic result.
Solution: Replace the faulty joint and check other joints from the same batch, performing preventive replacement or hidden danger treatment.
Case 2: Rapid Repair of Communication Cable Fiber Fault in a Data Center
Background: A large data center expanded its capacity and laid a new batch of multimode fiber optic cables. During the commissioning process, it was found that a fiber optic link connecting the two buildings could not communicate normally, and the optical signal loss was huge.
Fault Phenomenon: Through optical power meter testing, it was found that the optical link loss was much higher than expected, close to infinity, and the fiber optic was suspected to be broken.
Diagnostic Steps and Procedures:
Preliminary Judgment
End-to-end tests were performed using a light source and optical power meter, and it was confirmed that the link was not open circuit and the loss was extremely high. Suspected broken or severely bent fiber.
Fault Location (OTDR)
Connect the OTDR to one end in the equipment room and select the appropriate optical wavelength (e.g., 850nm or 1300nm, corresponding to multimode fiber). After the OTDR emitted a light pulse, a large Fresnel reflection peak was clearly displayed on the waveform graph, followed by no scattered or reflected signal. This indicates that the fiber was completely broken at that point. The OTDR automatically calculated that the break point was located 356 meters from the test end.
On-site Search and Verification
According to the distance of 356 meters, O&M personnel combined with the pipeline manhole and bridge wiring drawings to conduct a search. In a pipe manhole approximately 350 meters from the optical fiber outlet of the equipment room, it was found that the optical fiber might have been crushed or bent during the pipe threading process, causing the optical fiber to break. Visual inspection also confirmed the break.
Solution
Fiber optic splicing repair in a pipe manhole. Use a fiber cleaver to cut the broken ends, clean the fiber, and use a fusion splicer to precisely align and weld the ends. After splicing is completed, the link is re-tested with an OTDR to confirm that the splice loss is qualified (usually < 0.1 dB) and the signal at the end of the link is normal. The link restored communication.
Lesson Learned
Fiber break point location is one of the most classic applications of OTDR, which is fast and accurate. For communication cables, in addition to break points, OTDR can effectively diagnose faults such as high-loss splices, connector issues, and macrobends.
Case 3: Comprehensive Diagnosis of High-Resistance Faults in Medium Voltage Cables in Industrial Parks
Background: A 10kV ring main unit (RMU) outgoing cable (XLPE insulation) in an industrial park frequently experiences instantaneous single-phase ground faults, causing the RMU to trip, but most reclosures are successful. The fault phenomenon is intermittent.
Fault Phenomenon: The system’s protection device operates instantaneously, and the record shows it is a single-phase ground fault, but the fault does not continue, and reclosing is successful. Megohmmeter test insulation resistance is within the normal range, but breakdown occurs when performing the VLF withstand voltage test.
Diagnostic Steps and Procedures:
Preliminary Judgment
Instantaneous, intermittent failure and normal megohmmeter test, high suspicion is a high-impedance fault or flashover fault, which may be related to voltage level and environmental changes. Megohmmeters are unable to detect such faults.
Insulation Assessment (VLF + Tan Delta + PD)
A 0.1 Hz, 1.5 U0 voltage boosting test is performed on the cable using VLF withstand voltage test equipment (lower than the standard withstand voltage value to avoid burning the fault point). In the process of boosting the voltage, it is found that the dielectric loss tanδ value increases significantly and non-linearly with increasing voltage, and a continuous partial discharge signal appears when a certain voltage is reached. Analyze the PD signal characteristics to determine whether the fault may exist in the cable body or at a joint. The location function indicates that the fault is roughly at a certain distance in the cable area.
Precise Positioning (Quadratic Impulse Method + Acoustomagnetic Method)
In order to pre-locate and precisely locate, it is necessary to “excite” the fault point to make it stable during high-voltage discharge or breakdown. Connect the cable to the cable fault test van (containing the high voltage impulse generator and the secondary impulse main unit). First, try to pre-locate using the quadratic impulse method, setting the voltage to be close to the peak operating voltage (e.g., 15kV). After several impulses (thumps), a distance estimation (e.g., 750 meters) is obtained. Then, acoustomagnetic pinpointing is conducted on the cable path around 750 meters. A pulsed high voltage was applied, the ground sound was carefully listened to, the magnetic field signal was observed, and finally, the loudest discharge sound was heard at a distance of 755 meters from the test end.
Excavation and Verification
Excavation at this point revealed that the cable was located in an underground trench with a prefabricated joint at this location. Inspect the appearance of the joint and find that the sealing tape was slightly damaged, and moisture intrusion was suspected. After dissecting the joint, small electric discharge traces were found at the interface between the insulation stress cone and the cable body insulation layer, which proved that the defect here was the cause of the intermittent high-resistance flashover fault.
Solution
Replace the faulty connector (joint). Since the connector is prefabricated and has a long service life, other joints on the same cable section are tested for preventive testing (e.g., ultrasonic or TEV partial discharge testing) to assess their condition.
Lesson Learned
For intermittent high-impedance faults, basic megohmmeter tests are often ineffective and need to be combined with high voltage testing (VLF) and advanced diagnostic techniques (quadratic impulse method, acoustomagnetic method) to effectively diagnose and locate. Patience and meticulous on-site investigation are critical.
Building an Effective Cable Fault Prevention and Maintenance System
“Prevention is better than a cure”. Effective preventive maintenance can significantly reduce cable failure rates, extend cable life, reduce power outages, and lower O&M costs.
Periodic Preventive Testing and Inspection Programs
Establishing and strictly implementing a cable inspection program is the basis for preventing failures:
Annual/Term Items:
Insulation Resistance Test: Measure regularly to observe its changing trend. The continuous decrease in insulation resistance value is an important signal of insulation aging.
Partial Discharge (PD) Monitoring: Especially for critical lines and aging cables. Early insulation defects can be detected offline (e.g., in combination with VLF withstand voltage) or through online monitoring.
Tan Delta Test: Usually performed in conjunction with VLF withstand voltage, it evaluates the overall degree of moisture or general aging of the cable.
DC Withstand Voltage Leakage Current Test: While VLF is more recommended for XLPE cables, there are still applications for DC testing for oil-paper cables, etc., focusing on the change of leakage current over time.
Quarterly/Inspection Items:
Connector/Termination Temperature Inspection: Use a thermal camera or infrared thermometer to regularly check the surface temperature of cable joints and terminal heads. Abnormally high temperatures may indicate poor connection, excessive contact resistance, or internal defects.
Operating Environment Inspection: Check whether the cable trench, tunnel, manhole cover, support, fire blocking, etc., are in good condition, and whether there are issues such as standing water, miscellaneous items, corrosive gases, and animal infestation.
Appearance Inspection: Inspect and check whether the cable body, sheath, armor layer, and anti-corrosion layer have damage, deformation, bulging, and other abnormal phenomena.
Introducing Smart Online Monitoring Technology
With the development of technology, smart online monitoring systems can provide more continuous and comprehensive information on the operating status of cables, achieving the transformation from periodic maintenance to condition monitoring and predictive maintenance.
Distributed Temperature Sensing (DTS): The temperature distribution of the entire cable line is monitored in real time using optical fiber laid next to the cable. This is an effective means to prevent thermal aging and overload faults by being able to detect cable overloads, poor heat dissipation, or the influence of external heat sources in time.
Online Partial Discharge (PD) Monitoring System: HFCT, TEV, or ultrasonic sensors are installed at cable terminals and critical joints to monitor PD signals 24/7. Through data collection, analysis, and trend assessment, early insulation defects can be found in time.
Conditional Online Monitoring Platform: Integrate DTS, online PD, current, voltage, temperature, humidity, and other sensor data, through big data analysis and artificial intelligence algorithms, comprehensively evaluate and predictively diagnose the health status of cables, and find hidden dangers in advance.
Optimizing Design, Construction, and Operation Management
Design Stage: Reasonable selection of cable type and cross-section, consideration of laying environment, load characteristics, and short-circuit capacity; Optimize routing to avoid corrosive areas and areas prone to external damage; Standardize the design of cable tunnels and channels to ensure good ventilation and heat dissipation.
Construction Stage: Strictly implement installation process regulations, control cable pulling tension and bending radius; Ensure the quality of cable heads and joints, use qualified materials, and ensure good sealing; Specification of backfill material and depth (for direct-buried cables); Do a good job of sealing the tube well and tunnel entrance to prevent animals and moisture from entering; Strict handover tests (e.g., VLF withstand voltage + tanδ test + PD test) are performed on newly laid cables.
Operation Management: Avoid long-term overload operation of cables; Strengthen trustee management of construction to prevent external force damage; Clean water and debris in the cable channel in time; Operational data is monitored and analyzed.
Improving Personnel Skills and Emergency Response Capabilities
Professional Training: Regularly train cable O&M personnel on fault diagnosis technology and safety operating procedures to ensure they are proficient in using advanced testing equipment and fault analysis capabilities.
Emergency Plan: Formulate a detailed emergency plan for cable failures, clarify the responsible person, disposal process, and material preparation for each link, and shorten the fault response time.
Tools: Equipped with comprehensive and reliable fault diagnosis equipment and safety protection equipment.
Conclusion: Towards a Smart and Predictive Future of Cable Operation and Maintenance
Cable faults are a significant challenge affecting the reliability of power, communication, and industrial systems. Mastering systematic fault identification and diagnosis technology is the key to reducing losses and ensuring safe operation. This guide sorts out common cable fault types and causes, introduces common and advanced detection technologies and equipment in detail, and provides practical troubleshooting strategies for different scenarios, supplemented with typical cases to help you understand.
Looking forward, with the deep integration of technologies such as the Internet of Things, big data, and artificial intelligence, cable operation and maintenance are accelerating development towards intelligence and prediction. The smart diagnostic system based on online monitoring data can achieve continuous evaluation and early warning of cable status, so as to change from passive emergency repair to active maintenance, maximize the value of cable assets, and build a more reliable and resilient power transmission and information network.
We recommend that relevant industries continue to invest in advanced detection technologies and smart monitoring systems, strengthen personnel training, and continuously optimize operation and maintenance strategies to cope with the increasingly complex operating environment and growing reliability requirements
