--- title: "Cable fault location techniques and Murray loop" subject: "Power System Analysis I" chapter: 2 key: "semester_4/power_system_analysis_i:cable-fault-techniques-murray" source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/cable-fault-techniques-murray site: BCT Engineering --- # Cable fault location techniques and Murray loop Power System Analysis I · Chapter 2: Overhead and Underground Transmission - Asked in: 2079 Chaitra, 6 marks - Video: [Underground Cable Faults Explained | Murray & Varley Loop Test with Numerical Insight](https://www.youtube.com/watch?v=DQ9kt5wLWQ0) — Electrology - 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-techniques-murray - Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-2 ## Question What are the techniques of locating faults in underground cable? Explain Murray loop test. ## Short answer Techniques include Murray loop, Varley loop, capacitance comparison, TDR, Blavier test, and thumper. The Murray loop test for short-circuit faults connects faulty and sound cores at the far end, balances a bridge, and locates the fault at x = (Q/(P+Q))*2L from the test end. ## Answer Underground cable faults are first found to exist with an insulation (megger) test. They are then located by measuring from the cable ends, and finally pinpointed on the route. #### Techniques of locating faults | Method | Used for | |---|---| | Murray loop test | Earth and short-circuit faults (resistance bridge) | | Varley loop test | Earth and short-circuit faults (bridge with an extra variable resistor) | | Murray capacitance loop / capacitance comparison | Open-circuit (broken conductor) faults | | Pulse echo (TDR) | Any fault, from the travel time of a reflected pulse | | Blavier test | Earth fault when no sound core is available | | Thumper (surge generator) and acoustic detection | Pinpointing on the route | #### Murray loop test (short-circuit or earth fault) This is a Wheatstone bridge method. It needs one sound core running alongside the faulty core. ```text +--------------(G)--------------+ | | B ---/\/\------+------/\/\----- A | P | Q | | battery | | | | | earth | sound core faulty core | x <- fault to earth | | +====== loop at far end ========+ ``` 1. At the far end, connect the faulty core to the sound core with a low-resistance link, so the two cores form a loop. 2. Connect ratio arm $P$ (a variable resistor) to the test end B of the sound core and ratio arm $Q$ to the test end A of the faulty core, and connect the galvanometer across A and B. Connect the battery between the junction of $P$ and $Q$ and earth (for an earth fault). For a short circuit between two cores, connect the battery to the other shorted core instead of earth. 3. Adjust $P$ and $Q$ until the galvanometer reads zero. Let $R_x$ be the resistance of the faulty core from the test end to the fault, and $R_L$ the resistance of the whole loop. The arm next to $P$ is the rest of the loop, $R_L - R_x$, and the arm next to $Q$ is $R_x$. At balance: $$ \frac{P}{Q} = \frac{R_L - R_x}{R_x} \;\Rightarrow\; R_x = \frac{Q}{P+Q}\,R_L $$ If both cores have the same cross-section and the cable length is $L$, resistance is proportional to length: $$ x = \frac{Q}{P+Q}\times 2L $$ Here $x$ is the distance from the test end to the fault. The fault resistance carries only battery current, so it does not affect the balance. #### Example Take a 2 km cable (loop length 4 km) that balances at $P = 3\ \Omega$ and $Q = 1\ \Omega$: $$ x = \frac{1}{3+1}\times 4 = 1\ \text{km} $$ So the fault is 1 km from the test end. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which test is used for open-circuit (broken conductor) faults? A. Murray resistance loop B. Murray capacitance loop (capacitance comparison) C. Blavier test D. Megger test Answer: B. An open circuit is found by comparing capacitances since there is no conductive loop. (Key point: Murray vs capacitance test) 2. When is the Blavier test used? A. Earth fault when no sound core is available B. Open-circuit faults only C. Pinpointing on the route D. Faults between two sound cores Answer: A. Blavier test needs only the faulty core, unlike Murray which needs a sound core. (Key point: Blavier test) 3. What does the Murray loop test need besides the faulty core? A. An earthed sheath of different material B. A pulse generator C. One sound core of the same size running alongside D. Access to the cable midpoint Answer: C. The sound core and faulty core are linked at the far end to form the loop. (Key point: Murray loop condition) 4. A 2 km cable (loop 4 km) balances at P = 3 ohm, Q = 1 ohm. Where is the fault from the test end? A. 3 km B. 1.5 km C. 0.5 km D. 1 km Answer: D. x = Q/(P+Q) x 4 = 1/4 x 4 = 1 km. (Key point: Fault distance example) ## Related in Power System Analysis I - [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× - [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× - [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 - [Locating faults in underground cables](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Acable-fault-location-method) — Power System Analysis I, chapter 2, asked 3× - [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