--- title: "Restricted earth fault protection" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:restricted-earth-fault" source: https://www.bctengineer.com/q/semester_8/power_plant_design/restricted-earth-fault site: BCT Engineering --- # Restricted earth fault protection Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2081 Chaitra, 6 marks - Video: [Restricted Earth Fault Protection | Restricted Earth Fault Relay | Hindi](https://www.youtube.com/watch?v=88S6p48gPNE) — Electrical With Abhishek Joshi - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/restricted-earth-fault - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question What is restricted earth fault protection? Illustrate its use with suitable diagram. ## Short answer Restricted earth fault (REF) is a sensitive unit protection for earth faults within a star winding. It compares residual current of phase CTs with neutral CT current; for internal faults, difference flows to relay. High-impedance relay stabilises against CT saturation. Used on HV star winding of GSU transformers and small resistance-earthed generators. ## Answer **Restricted earth fault (REF) protection** is a sensitive **unit protection** that detects earth faults inside a defined zone only, usually a star-connected transformer or generator winding. It compares the sum of the three phase currents with the neutral current. #### Principle - CTs are placed in each phase and in the neutral-to-earth connection, all of the same ratio. - The three phase CTs are connected in parallel (residual connection) and then in parallel with the neutral CT, with a relay across them. - **External earth fault** (outside the zone): the residual current from the phase CTs equals the neutral CT current, so they circulate between CTs and no current flows through the relay. - **Internal earth fault** (inside the winding): the neutral current has no matching residual current, so the difference flows through the relay and it trips. ```text A ---[CT]-----+---- to system B ---[CT]-----+ C ---[CT]-----+ Star | | | | winding \ | / | residual \|/ | N ---+--[CT_N]---+---[ R ]--+ | relay | earth (high-impedance, with stabilising resistor and Metrosil) ``` #### Features - Usually a **high-impedance relay** with a stabilising resistor so CT saturation on heavy external faults does not cause false trips. A non-linear resistor (Metrosil) limits voltage during internal faults. - Very sensitive: it covers faults close to the neutral end, where the ordinary transformer differential (87T) is not sensitive enough. On a resistance-earthed winding a fault near the neutral gives very little phase current. - Fast and stable. #### Use - **HV star winding of the generator step-up transformer** (solidly earthed side). - Star winding of station/auxiliary transformers. - Small generators with resistance earthing. For a delta winding, REF is applied using an earthing transformer or the three phase CTs alone (balanced earth fault protection). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is restricted earth fault protection? A. Protection against phase-to-phase faults of the whole system B. A sensitive unit protection for earth faults inside a defined zone, such as a star winding C. Overcurrent backup for external faults D. Protection against overfluxing Answer: B. REF is a unit scheme that responds only to earth faults inside the zone between the phase CTs and the neutral CT. (Key point: REF definition) 2. For an external earth fault, why does no current flow through the REF relay? A. The relay is disconnected B. The residual of the phase CTs equals the neutral CT current and they circulate between CTs C. The neutral CT is short circuited D. The stabilising resistor blocks it Answer: B. With equal CT ratios the two currents balance and circulate, so the relay branch carries almost nothing. (Key point: External fault behaviour) 3. What happens for an internal earth fault in the winding? A. Neutral current has no matching residual current, so the difference flows through the relay and it trips B. Currents still cancel and the relay stays idle C. Only the phase CTs carry current and the relay blocks D. The Metrosil opens the circuit without tripping Answer: A. The unbalance between neutral and residual currents flows through the relay and operates it. (Key point: Internal fault behaviour) 4. Why is a stabilising resistor used with a high-impedance REF relay, and what does the Metrosil do? A. Resistor speeds tripping; Metrosil increases sensitivity B. Resistor prevents false trips from CT saturation on heavy external faults; Metrosil limits voltage during internal faults C. Resistor provides inrush restraint; Metrosil provides 2nd harmonic block D. Both are only for alarm purposes Answer: B. The stabilising resistor keeps the relay stable if a CT saturates on an external fault, and the non-linear Metrosil limits the high voltage on internal faults. (Key point: Stabilising resistor and Metrosil) ## Related in Power Plant Design - [Unit generator-transformer schemes](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Aunit-generator-transformer-schemes) — Power Plant Design, chapter 4, asked 3× ## Related in other subjects - [Restricted earth fault for generator](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aref-generator-solidly-grounded) — Switchgear and Protection, chapter 7 - [Combined phase and earth fault protection of generator](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Acombined-phase-earth-fault-generator) — Switchgear and Protection, chapter 7 - [Earth fault protection: 20 MVA alternator](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aearth-fault-20mva-5-ohm) — Switchgear and Protection, chapter 7 - [Earth fault neutralizer reactance](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aearth-fault-neutralizer-reactance) — Switchgear and Protection, chapter 5