1. Introduction: The Importance of Cable Fault Diagnosis
In modern society, cables serve as core carriers in power, telecommunications, and industrial fields, with their reliability directly impacting system safety and stable operation. However, cable faults are inevitable due to environmental factors, mechanical stress, insulation aging, and other influences. Outages or communication interruptions caused by these faults result in significant economic losses annually. Therefore, mastering systematic and efficient cable fault identification and diagnosis techniques is critically important.
The Cable System Expert Team compiles this guide based on standards from the International Electrotechnical Commission (IEC) and the Institute of Electrical and Electronics Engineers (IEEE), combined with extensive field experience. It aims to provide a full-process technical framework, from fault pre-assessment to precise repair, assisting technical personnel in quickly locating fault types and positions, effectively shortening repair times, minimizing losses, and comprehensively enhancing cable system reliability.
2. Cable Fault Classification, Characteristics, and Underlying Causes
To diagnose cable faults effectively, it is essential to first understand the types of faults and their underlying causes. Different fault types exhibit different electrical characteristics and require different detection strategies.
2.1 Common Fault Types and Their Electrical Characteristics
Cable faults are typically classified based on the resistance characteristics and connection state at the fault point:
Short Circuit Fault:
Characteristic: Abnormal connection occurs between phases, or between a phase and ground (or neutral). The fault point resistance is typically very low, close to zero (known as a Low Resistance Short Circuit).
Electrical Characteristic: Insulation resistance is close to zero, and loop resistance is abnormally low.
Manifestation: May lead to tripping, fuse blowing, or equipment damage.
Open Circuit Fault:
Characteristic: The cable conductor is interrupted, preventing current flow. This can be a complete or partial break in one, two, or three phases.
Electrical Characteristic: Conductor resistance is abnormally high, or even infinite; insulation resistance may be normal or damaged.
Manifestation: The Equipment fails to receive power, or the communication signal is interrupted.
Ground Fault:
Characteristic: The cable conductor (or the insulation layer after breakdown) connects to the earth. This is one of the most common types of cable faults. Based on the contact resistance at the fault point to the ground, it can be classified as a Low Resistance Ground Fault or a High Resistance Ground Fault.
Electrical Characteristic: Insulation resistance drops significantly, potentially from hundreds of MΩ or even infinity down to tens or a few MΩ, or even below 1kΩ (low resistance) or above 1kΩ (high resistance), sometimes reaching hundreds of MΩ (high resistance).
Manifestation: Ground fault protection device operates, system ground current increases abnormally, and may cause a voltage shift.
High Resistance Fault:
Characteristic: The fault point resistance is high, possibly ranging from several kΩ to several MΩ. This usually results from insulation degradation, carbonization, or partial breakdown, but has not yet formed a complete low-resistance path. High-resistance faults are often an early stage of many low-resistance and breakdown faults.
Electrical Characteristic: Insulation resistance drops, but still has a certain value. Under high voltage, the fault point may experience flashover or discharge, leading to unstable resistance values.
Manifestation: May cause local heating, increased dielectric loss, partial discharge, etc. Early on, there might be no obvious external signs, but it is easily revealed during withstand tests.
Flashover Fault:
Characteristic: Under high voltage, discharge occurs on the surface or within the insulator, forming a transient or intermittent conduction. Insulation performance may temporarily recover after the voltage is removed.
Electrical Characteristic: Fault point resistance drops sharply with increasing voltage and increases when the voltage is lowered or removed.
Manifestation: The system may experience an instantaneous ground fault or short circuit, causing protection actions, but reclosing may be successful. Diagnosis is challenging.
Intermittent Fault:
Characteristic: Fault symptoms appear and disappear intermittently, possibly related to factors such as temperature, humidity, voltage level, or mechanical vibration. For example, a tiny crack may expand with temperature rise, causing contact, and separate when the temperature drops.
Electrical Characteristic: The resistance and connection state of the fault point are unstable and change with external conditions.
Manifestation: System protection devices operate intermittently, making fault capture difficult and posing a significant challenge for diagnosis.
2.2 Analysis of Internal and External Factors Leading to Cable Faults
Cable faults are not random; their causes are complex and diverse, usually resulting from the long-term or transient action of multiple factors:
Mechanical Damage:
External Causes: Accidental damage by excavators, pipe jacking equipment, etc., during construction; damage from road construction or third-party activities; tensile or compressive stress from foundation settlement or soil movement; animal (e.g., rats, termites) gnawing on the sheath.
Internal Causes: Excessive bending or pulling tension during installation; poor installation quality or external force impact on cable accessories (e.g., joints, terminations).
Chemical Corrosion:
Corrosive substances in the soil, such as acids, alkalis, and salt,s erode the cable sheath and armor layers; industrial waste liquids, oil stains, etc., penetrate the cable structure; electrolytic corrosion (especially in stray current areas).
Thermal Aging:
Long-term overload operation or high ambient temperature during laying causes accelerated aging, hardening, embrittlement, or even carbonization of cable insulation and sheath materials, leading to loss of insulation performance. Poor heat dissipation (e.g., densely packed cables, insufficient ventilation) exacerbates thermal aging.
Moisture Ingress and Humidity:
Damage to the cable sheath, poor sealing of joints, or moisture ingress into terminations allows water to enter the cable interior. Under the action of the electric field, moisture forms Water Trees, microscopic deterioration channels in the insulation material, which significantly reduce dielectric strength and eventually lead to breakdown (Electrical Trees).
Electrical Stress:
Overvoltage: Overvoltage impulses caused by lightning strikes, switching operations, resonance, etc., may exceed the cable insulation’s withstand capability, leading to insulation breakdown.
Electric Field Concentration: Design or installation defects in cable accessories (joints, terminations) lead to uneven electric field distribution, creating excessively high electric field strength in local areas, accelerating insulation degradation, and partial discharge.
Partial Discharge (PD): When tiny voids, impurities, moisture, or other defects exist within, on the surface, or at interfaces of the insulation material, partial discharge may occur under operating voltage, releasing energy, gradually eroding the insulation material, forming discharge channels, and ultimately leading to insulation breakdown.
Design and Manufacturing Defects:
Impurities, voids, or foreign matter in the insulation material during cable body manufacturing; improper extrusion process leading to uneven insulation thickness or microcracks; rough surface or protrusions on metal shields or semi-conductive layers.
Quality issues with materials for cable accessories (joints, terminations) or unreasonable structural design.
Installation and Construction Defects:
Improper cable laying (too small bending radius, excessive pulling tension, proximity to heat or corrosive sources); non-standard cable termination fabrication processes (inaccurate stripping dimensions, improper semi-conductive layer treatment, poor sealing, incorrect stress cone installation); use of unqualified backfill material.
Understanding these fault types and causes is fundamental to effective fault diagnosis and the formulation of preventive strategies.
3. Cable Fault Diagnosis Core Techniques and Equipment
Cable fault diagnosis is a step-by-step process, typically including fault assessment, pre-location, precise fault location, and pinpointing the fault location on the ground. Different tools and techniques are needed for each stage.
3.1 Basic Testing and Preliminary Assessment
After confirming a potential cable fault, the initial step is to perform basic electrical parameter measurements to make a preliminary assessment of the fault nature.
Megohmmeter (Insulation Resistance Tester):
Purpose: Measures the insulation resistance between cable conductors and between conductors and the shield (or ground). This is the most common and basic method for assessing cable insulation condition.
Operation: Apply a DC test voltage (typically 500V, 1000V, 2500V, 5000V, selected according to the cable voltage rating), and record the insulation resistance value after a specified time (e.g., 1 minute or 10 minutes).
Assessment: Insulation resistance significantly lower than normal values or specification requirements (e.g., recommended standards: low voltage cables ≥ 100 MΩ/km, 10kV cables ≥ 1000 MΩ/km) indicates potential insulation degradation or a ground fault. If the resistance value is close to zero, it indicates a low resistance ground fault or short circuit.
Multimeter:
Purpose: Measures conductor DC resistance, checks continuity (open circuit), and measures inter-phase or phase-to-ground resistance (suitable for low voltage or situations with low fault point resistance).
Operation: Use the resistance range to measure the resistance across the conductor ends to determine if it’s an open circuit; measure inter-phase or phase-to-ground resistance to determine if it’s a short circuit or low resistance ground fault.
Assessment: Infinite conductor resistance indicates an open circuit; inter-phase or phase-to-ground resistance close to zero indicates a short circuit or low resistance ground fault.
Cable Route Tracer:
Purpose: Used to determine the precise route of cables in invisible laying scenarios like underground direct burial. Particularly important in the fault pinpointing stage.
Principle: A signal of a specific frequency is applied to the cable, and a receiver detects the induced electromagnetic field to track the cable path.
Models: Common models include RD8000, vLocPro, etc.
3.2 Precise Fault Location Techniques
Basic tests can only determine the fault type, not the exact location. Precise fault location techniques aim to measure the distance between the test end and the fault point.
3.2.1 Time Domain Reflectometry (TDR)
Principle: A fast-rising voltage pulse is injected into the cable and propagates along it. When the pulse encounters an impedance mismatch (such as a fault point, joint, termination, or open end), part or all of the pulse is reflected back. By measuring the time interval between the transmitted and reflected pulses, and knowing the propagation speed of the signal in the cable (velocity of propagation, Vp), the fault distance can be calculated: Distance = (Time Difference / 2) * Vp.
Applicable Scenarios: Excellent for locating open circuits and low-resistance short circuits. Reflected signals are clear and easy to interpret.
Limitations: For high resistance faults (especially very high resistance), the pulse energy may be attenuated or absorbed at the fault point, resulting in weak or distorted reflected signals, reducing location accuracy or even making location impossible.
Accuracy: Generally high, can reach ±0.5% or even higher (depending on equipment performance, accuracy of known Vp, and operator experience). VP needs to be calibrated by testing a known length of a healthy cable section.
3.2.2 High Voltage Bridge Method (Murray Loop, Bridge Method)
Principle: Utilizes the principle of the classical Wheatstone bridge. A healthy cable segment or a healthy phase from the faulty cable is used to construct a bridge circuit. When the bridge is balanced, the fault point distance is calculated based on the resistance ratio of the cable conductors. The commonly used Murray Loop bridge is suitable for single-phase ground faults or phase-to-phase short circuits.
Advantage: Especially suitable for high resistance ground faults (even up to several MΩ), which is a weakness for TDR. The principle is based on DC resistance measurement, unaffected by reflected signal attenuation.
Operation Points: Requires at least one healthy conductor as a return path; requires precise measurement of total cable length and conductor resistance; requires the use of a High Voltage Generator (such as DC withstand test equipment) to “condition” or “burn” the insulation near the high resistance fault point to lower the fault point resistance, facilitating bridge measurement or subsequent acoustic-magnetic location. The burning voltage is often high, such as 8kV, 15kV, or even higher, and operation must be extremely cautious and adhere to safety regulations.
3.2.3 Impulse Current Method (ICE) and Secondary Impulse Method (SIM/MIM)
Principle: These methods are improvements on TDR for locating high-resistance faults. They apply a high-voltage pulse to the faulty cable, causing breakdown or flashover at the high-resistance fault point, generating a current pulse. Sensors then capture the current pulse waveform propagating along the cable, and analysis similar to TDR is used to locate the fault by analyzing the reflected wave.
ICE: Directly analyzes the reflected current pulse generated at the fault point.
SIM/MIM (also known as Arc Reflection Method): Utilizes the arc formed during fault point breakdown to create a low-impedance “short circuit” for the TDR pulse at the fault point, generating a clear reflected waveform. This overcomes the issue of weak TDR reflections in high-resistance faults and is currently a very effective method for dealing with them.
Applicable Scenarios: Precise pre-location of high-resistance ground faults and flashover faults.
Equipment: Usually integrated into professional cable fault locators, requiring coordination with a surge high-voltage generator (high-voltage equipment in a cable fault test van).
3.2.4 Fault Point Pinpointing
Pre-location techniques provide the fault distance, but the actual fault point may be within a small area. Fault point pinpointing uses external methods based on the pre-location result to accurately determine the fault location on the ground.
Acoustic-Magnetic Method:
Principle: A high-voltage surge (using a surge high-voltage generator) is applied to the faulty cable. When the fault point breaks down and discharges, it produces sound (pressure wave) and electromagnetic signals. An operator uses an Acoustic-Magnetic Synchronized Receiver to listen to the sound through headphones and receive the electromagnetic signal via an induction coil. Due to the significant difference in propagation speeds between sound and electromagnetic waves, the equipment can determine if the sound and electromagnetic signal originate from the same location and if the sound lags the electromagnetic signal (electromagnetic wave speed is close to the speed of light, sound wave speed is much slower), thus indicating the direction and location of the fault point. The sound signal is strongest directly above the fault point.
Applicable Scenarios: Various types of breakdown discharge faults (ground, short circuit, flashover), particularly effective for underground direct-buried cables.
Operation Points: Ambient background noise can affect listening; the surge energy needs to be adjusted to cause continuous discharge at the fault point without damaging healthy parts of the cable; the operator requires experience to distinguish fault discharge sounds from other noises.
Step Voltage Method:
Principle: A DC or low-frequency AC voltage is applied to a ground-faulted cable, causing current to leak into the earth at the fault point. This creates a voltage gradient field around the fault point. Two probes are inserted into the ground and connected to a high-sensitivity voltmeter, and moved along the cable path. Directly above the fault point, the voltage difference will reverse polarity.
Applicable Scenarios: Low or medium resistance ground faults, particularly useful for fault points that do not produce a clear discharge sound.
Operation Points: Significantly affected by soil moisture and uniformity; requires sufficient test voltage and current; probe insertion depth and spacing affect accuracy.
Minimum Current / Maximum Magnetic Field Method:
Principle: An audio frequency or specific frequency current signal is applied to the faulty cable. If the fault is a short circuit or low resistance ground fault, the current forms a loop at the fault point; if it’s an open circuit, the current stops at the break point. A current clamp or magnetic field sensor is used to detect current or magnetic field strength along the cable path. After a short circuit or low resistance ground fault point, the current will significantly decrease or disappear (minimum current), or the magnetic field will change. Before an open circuit point, the current is normal, and after the point, the current is zero.
Applicable Scenarios: Low resistance short circuits, ground faults, or open circuit faults. Also often used in conjunction with a route tracer to confirm the path.
3.3 Insulation State Assessment and Early Warning Techniques
These techniques are primarily used to assess the overall health of the cable insulation and detect potential defects. They fall under the category of preventive maintenance or the diagnosis of high resistance/early-stage faults.
Partial Discharge (PD) Detection:
Principle: Defects in the insulation material (such as voids, impurities) cause partial discharge under the influence of the electric field, generating electrical pulses, electromagnetic waves, acoustic waves, light, and chemical byproducts. PD detectors capture these signals to assess the extent of insulation degradation and the type of defect.
Technical Parameters: Sensitivity is typically measured in picocoulombs (pC), capable of detecting very weak discharge signals (e.g., 1 pC).
Methods:
Electrical Method: Detects current pulses generated by discharge (e.g., through High Frequency Current Transformer HFCT sensors on ground leads, or by measuring capacitively coupled signals). Applicable for online or offline testing.
Acoustic Method: Detects ultrasonic waves generated by discharge (e.g., through contact or air-coupled sensors). Suitable for testing cable accessories.
Ultra-High Frequency (UHF) Method: Detects UHF electromagnetic waves (300 MHz – 3 GHz) generated by discharge. Offers strong interference immunity, commonly used for GIS, transformers, etc., and can also be used for cable terminations.
Transient Earth Voltage (TEV) Method: Detects transient voltages to ground coupled onto the metal enclosures of switchgear, etc., from internal PD.
Purpose: Detects early insulation defects in cables and their accessories (e.g., voids in joints, moisture ingress into terminations, water trees/electrical trees in the cable body). It is a key technology for predictive maintenance.
Dielectric Loss (Tan Delta, tgδ) Test:
Principle: Measures the tangent of the dielectric loss angle of the cable insulation material under AC voltage. Dielectric loss represents the insulation material’s ability to convert electrical energy into heat. Healthy insulation materials have low losses, a low tanδ value, and the value changes little with increasing voltage. Moisture ingress, aging, or the presence of water trees and other defects in the insulation will cause the tanδ value to increase and increase rapidly with rising voltage.
Purpose: Assesses the overall level of moisture ingress or widespread aging in the cable insulation. Often performed in conjunction with AC or VLF withstand testing.
Withstand Test:
Purpose: Verifies the cable’s ability to withstand a certain level of overvoltage without insulation breakdown. It effectively exposes defects that only manifest under high voltage.
Methods:
DC Withstand: A traditional method, but DC voltage can accumulate space charge in XLPE and other extruded insulations, potentially damaging healthy cables. It is gradually being replaced by VLF.
AC Withstand: More closely simulates actual cable operating conditions, but test equipment is large and requires high energy.
Very Low Frequency (VLF) AC Withstand (0.1 Hz): Widely used today for withstand testing of XLPE and other extruded insulation cables. Equipment is portable, requires low energy, and does not cause space charge accumulation. Often combined with tanδ and PD measurements.
In the next article, we will explain cable troubleshooting in different scenarios with specific cases. Follow ZMS CABLE FR to learn more about cables.
