Cable failures can arise from various causes, among which mechanical damage due to external forces, moisture absorption, and aging-leading to reduced insulation performance-are common. Factors such as atmospheric overvoltage, switching overvoltage, and long-term overload operation can also cause cable breakdown. Cable faults can generally be categorized into ground faults, short circuits, and broken conductors. Specific fault types include single- or two-phase grounding, inter-phase short circuits, three-phase complete short circuits, and single- or multi-phase conductor breakage.
Cable breakdowns are generally classified as high-resistance breakdown or low-resistance breakdown. When measured with an ohmmeter, insulation resistance below 10 kΩ indicates low-resistance breakdown, while values above 10 kΩ indicate high-resistance breakdown. Locating the breakdown point is typically a two-step process: first, a rough measurement to determine the general area of the fault, followed by precise localization of the exact point.
I. Rough Measurement Methods for Cable Fault Location
Wheatstone Bridge Method
Still widely used today, the Wheatstone DC bridge method is applied for locating single-phase low-resistance ground faults. Based on the principle that a uniform cable's resistance is proportional to its length, the method incorporates the loop resistance on both sides of the fault into a DC bridge. When the bridge is balanced, the resistances of the ratio arm and the adjustable measuring arm are measured. Using these values and the total cable length, the distance from the measuring end to the fault point can be calculated.
Standing Wave Method
Based on microwave transmission line theory, this method utilizes the standing wave resonance phenomenon in transmission lines to test faulty cables. It is suitable for low-resistance and open-circuit faults.
Pulse Reflection Method
This technique leverages the phenomenon of wave reflection when the characteristic impedance of a transmission line changes. A voltage is applied to the cable core-sufficient to cause discharge without breakdown. The discharge pulse propagates and reflects along the cable, and a digital oscilloscope is used to capture the positions of three or more pulses. The fault location is then calculated based on the pulse position ratios. This method is suitable for high-resistance breakdowns.
Flashover Method
The DC high-voltage flashover test method is used for faults that break down under flashover conditions, i.e., faults with extremely high resistance that break down when the voltage from a high-voltage signal generator reaches a certain level. This method is effective for locating high-resistance faults that are challenging for other techniques.
II. Precise Localization Methods for Cable Fault Points
Acoustic Method
Sensitive and reliable, the acoustic method is commonly used for all fault types except those with very low ground resistance (below 50 Ω). After a rough estimation of the fault distance, an impulse high voltage is applied between the faulty cable core and the copper sheath. This causes intermittent discharge at the fault point, generating electromagnetic radiation and audible vibrations. Using a locator (acoustic wave receiver) with a piezoelectric crystal probe, the discharge sound waves are detected along the known cable path within the estimated range. When the headphone sound is loudest or the signal peaks, the probe's ground position corresponds exactly to the fault point. Since break and flashover faults often occur at intermediate joints, these should be prioritized during acoustic testing.
Audio Frequency Current Induction Method
This method is mainly used for metallic low-resistance faults (ground resistance below 10 Ω) and for locating open-circuit and broken conductor faults. An audio frequency signal generator injects current into the cable, which then emits electromagnetic waves. A probe (inductive coil) is moved along the cable path above ground to detect changes in the electromagnetic field. The amplified signal is fed into headphones or an indicator instrument. The fault location is identified where the audio signal is strongest or the meter reading is highest.
