--- title: "Purpose of earthing, earthed vs unearthed system" subject: "Industrial Power Distribution and Illumination" chapter: 2 key: "semester_6/industrial_power_distribution_and_illumination:earthing-purpose-earthed-unearthed" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/earthing-purpose-earthed-unearthed site: BCT Engineering --- # Purpose of earthing, earthed vs unearthed system Industrial Power Distribution and Illumination · Chapter 2: Earthing - Asked in: 2074 Bhadra, 4 marks - Video: [Earthing Systems Explained: TT, TN-S, TN-C, TN-C-S & IT (Complete Guide)](https://www.youtube.com/watch?v=PYCH4yRwbAk) — Gaurav J - TheElectricalGuy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/earthing-purpose-earthed-unearthed - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-2 ## Question What is the major purpose of earthing? Describe the consequences of earth and unearthed system. ## Short answer The major purpose is to protect people by keeping enclosures near earth potential and to ensure fast fault clearance. In an earthed system, a fault causes a large current that trips protection, while an unearthed system allows supply to continue but causes healthy phases to rise to line voltage and may produce transient overvoltages. ## Answer #### Major purpose of earthing Earthing connects the system neutral and the exposed metal parts of equipment to earth through a low-resistance path. Its main purposes are: - **Safety of persons:** keeps metal bodies near earth potential so a fault does not cause a dangerous shock. - **Quick fault clearance:** a large earth-fault current makes fuses, MCBs and relays trip quickly. - **Stable system voltage:** the earthed neutral fixes phase-to-earth voltage and limits overvoltages. - **Safe discharge of lightning and static charges.** #### Consequences of earthed and unearthed systems | Point | Earthed system | Unearthed system | |---|---|---| | Fault current on first earth fault | Large; protection trips | Very small (capacitive); nothing trips | | Shock risk | Low; faulty part isolated fast | Healthy phases rise to line voltage; shock risk from second fault | | Voltage of healthy phases | Stays near phase voltage | Rises to √3 × phase voltage | | Overvoltages | Limited | Arcing ground gives transient overvoltages up to 5–6 times normal | | Fault location | Easy (relay shows faulty feeder) | Difficult | | Continuity of supply | Feeder trips on first fault | Supply can continue after one fault | | Insulation needed | Phase voltage level | Line voltage level (costlier) | **Summary:** an earthed system is safer and is used for almost all LV industrial and building installations. An unearthed system gives continuity during a single fault, but causes overvoltage, insulation stress, arcing grounds and hidden faults, so it is used only in special cases (some mines and process plants with insulation monitoring). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does earthing help fuses and MCBs clear faults quickly? A. It reduces the supply voltage B. It gives a large earth-fault current that makes protection trip C. It removes the load D. It blocks leakage current Answer: B. A low-resistance earth path allows a large fault current, so the protective device operates fast. (Key point: Quick fault clearance) 2. In an unearthed system with a first earth fault, what happens to the healthy phases? A. They fall to zero B. They stay at phase voltage C. They rise to sqrt(3) times the phase voltage, i.e. line voltage D. They rise to 5 times phase voltage permanently Answer: C. With one phase at earth potential, the other phases rise to line voltage relative to earth. (Key point: Healthy phase voltage) 3. Which problem is specific to the unearthed system? A. Fault current is large B. Arcing ground causing transient overvoltages up to 5-6 times normal C. Easy fault location D. Supply is lost on the first fault Answer: B. Arcing grounds in an unearthed system create severe transient overvoltages and the fault is hard to locate. 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