--- title: "Locating faults in underground cables" subject: "Power System Analysis I" chapter: 2 key: "semester_4/power_system_analysis_i:cable-fault-location-method" source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/cable-fault-location-method site: BCT Engineering --- # Locating faults in underground cables Power System Analysis I · Chapter 2: Overhead and Underground Transmission - Asked in: 2077 Chaitra, 5 marks; 2071 Bhadra, 5 marks; 2069 Poush, 6 marks (asked 3 times) - Video: [How to Locate Cable Faults with the Murray Loop Test](https://www.youtube.com/watch?v=IbeTNogACYQ) — The Learning Curve - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/cable-fault-location-method - Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-2 ## Question How do we detect underground fault in a power cable? Explain with necessary diagram. ## Short answer Fault detection starts with insulation testing to identify the faulty core and type. Pre-location uses the Murray loop test or time-domain reflectometry (TDR) to measure fault distance. Exact pinpointing is done with an acoustic thumper or audio-frequency tracer along the cable route. ## Answer Faults in underground cables (insulation breakdown to sheath, short between cores, or open conductor) cannot be seen, so they are detected and located electrically in three steps. #### 1. Identify the faulty core and type of fault - Use an **insulation tester (megger)** between each core and earth/sheath and between cores. Very low reading = earth or short-circuit fault. - Use a continuity test (cores shorted at far end) to find open-circuit faults. #### 2. Locate the fault (pre-location) – loop tests **Murray loop test** (for earth and short-circuit faults): the faulty core and a sound core are joined at the far end to form a loop, which with ratio arms $P$ and $Q$ forms a Wheatstone bridge. The battery is connected from the junction of $P$ and $Q$ to earth, and the galvanometer across the two cable ends at the test end. ```text A o-----[Battery]------+ Earth (sheath) | : +---+---+ : fault resistance P Q : | | : D o--(G)--o B : | | : | +== faulty core =X========+ | <------ x -----> | +========= sound core ============+ Test end Far end (cores joined) ``` At balance: $$ \frac{P}{Q} = \frac{R}{R_x},\qquad x = \frac{Q}{P+Q}\times2l $$ where $x$ = distance of fault from test end and $l$ = cable length (cores of equal area). **Varley loop test:** Similar loop, but with fixed ratio arms $P$, $Q$ and a variable resistor $S$ in series with the faulty-core arm; the loop resistance $(R_1+R_x)$ is first measured by a separate balance. Balance gives $$ R_x = \frac{(R_1+R_x)Q - SP}{P+Q} $$ and $x = R_x/r$. It is useful when the loop resistance is not known. **Pulse echo (time domain reflectometry, TDR):** A short pulse is sent into the cable; it reflects at the fault. Distance $= v t/2$, where $v$ is the pulse velocity in the cable and $t$ the echo time. Fast and suitable for open and short faults. #### 3. Pinpoint the exact spot Use an acoustic thumper (surge generator, listen for the discharge sound) or tracer/audio-frequency methods along the route before excavating. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Fault type is first identified by A. Murray loop B. TDR C. Megger and continuity tests D. Excavation Answer: C. Insulation test then continuity. (Key point: First step) 2. In pulse echo, distance is A. vt/2 B. vt C. 2vt D. v/t Answer: A. The pulse travels there and back. (Key point: Pulse echo) 3. Exact spot is found using A. Megger B. Murray loop C. Continuity D. Acoustic thumper or tracer Answer: D. Surge generator discharge is heard. (Key point: Pinpoint) 4. Varley loop is useful when A. Cable is open B. Core absent C. Loop resistance is not known D. Fault is overhead Answer: C. A separate balance measures loop resistance. (Key point: Varley) ## Related in Power System Analysis I - [Locating phase-to-phase cable fault](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aphase-to-phase-cable-fault) — Power System Analysis I, chapter 2 - [Murray loop test for cable faults](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Amurray-loop-test) — Power System Analysis I, chapter 2, asked 5× - [Varley loop test for cable faults](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Avarley-loop-test) — Power System Analysis I, chapter 2 - [Cable fault location techniques and Murray loop](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Acable-fault-techniques-murray) — Power System Analysis I, chapter 2 - [Underground cables vs overhead lines](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Acables-vs-overhead-lines) — Power System Analysis I, chapter 2 - [Murray loop test for open and short circuit faults](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Amurray-loop-open-short-circuit) — Power System Analysis I, chapter 2, asked 5× ## Related in other subjects - [Underground cable laying systems](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aunderground-cable-laying) — Industrial Power Distribution and Illumination, chapter 3 - [Fault tolerance and types of faults](https://www.bctengineer.com/question.md?key=semester_7%2Fdistributed_system%3Afault-types) — Distributed System, chapter 9 - [OFC block diagram vs cable](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aofc-block-diagram-vs-cable) — Propagation and Antenna, chapter 6 - [Power vs lighting system and cable sizing factors](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Apower-vs-lighting-cable-factors) — Industrial Power Distribution and Illumination, chapter 5