--- title: "Restricted earth fault for generator" subject: "Switchgear and Protection" chapter: 7 key: "semester_6/switchgear_and_protection:ref-generator-solidly-grounded" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/ref-generator-solidly-grounded site: BCT Engineering --- # Restricted earth fault for generator Switchgear and Protection · Chapter 7: Protective Relays - Asked in: 2070 Magh, 4 marks - Video: [SGP415 Balanced or Restricted Earth Fault Protection of Alternators](https://www.youtube.com/watch?v=IbUK5IEA_a0) — Lectures in Electrical Engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/ref-generator-solidly-grounded - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-7 ## Question Draw the connection diagram of restricted earth fault protection scheme for a generator with solidly grounded neutral. ## Short answer Connect three line CT secondaries in parallel to form residual current, and one matching CT in the neutral-to-earth lead. Connect both to a high-impedance earth fault relay with stabilizing resistor and metrosil; internal faults trip the generator, protecting about 90-95% of the winding. ## Answer **Restricted earth fault (REF) protection** is a unit protection that responds only to earth faults inside the generator stator winding (the zone between the neutral CT and the line CTs). It is not affected by external earth faults. #### Connection - Three line CTs (one per phase at the generator terminals) have their secondaries connected in **parallel (residual connection)**; their sum is the residual current 3I₀ on the line side. - One CT of the same ratio is placed in the **neutral-to-earth connection** of the solidly grounded star point. - The residual circuit of the line CTs and the neutral CT secondary are connected so that their currents circulate; an **earth fault relay** (a high-impedance relay with stabilising resistor R_st and a non-linear resistor/metrosil) is connected across them. ```text Generator stator Line CTs ┌──── R winding ──────────────(CT)──── R star ──┼──── Y winding ──────────────(CT)──── Y point └──── B winding ──────────────(CT)──── B │ │ │ │ (CT) neutral CT └─┼─┘ secondaries │ │ in parallel ═╧═ earth (solid) │ │ neutral CT sec ──────────┬──────────────┘ │ [R_st] │ [ 64 ] REF relay │ common return ───────────┴── other ends of all CT secondaries ``` #### Operation - **Normal or external earth fault:** the residual current of the line CTs equals the current in the neutral CT; the two secondary currents circulate and **no current flows in the relay**. - **Internal earth fault:** fault current flows through the neutral CT but not through the line CTs (or flows in the opposite sense if other sources feed it); the unbalance current passes through the relay, which trips the generator breaker and field breaker. - Since the neutral is solidly grounded, the earth fault current is large and almost the whole winding (about 90–95%) is protected. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does restricted earth fault protection respond to? A. Earth faults inside the stator zone between the neutral CT and line CTs only B. All earth faults including external ones C. Overloads D. Phase-to-phase faults only Answer: A. REF is a unit scheme limited to its zone and unaffected by external earth faults. (Key point: REF zone) 2. How are the CTs connected in the REF scheme? A. Line CTs in series with the neutral CT B. Line CT secondaries paralleled (residual) and connected to circulate against the neutral CT, with the relay across them C. Only the neutral CT is used D. Line CTs are star-delta connected to the field Answer: B. The residual of the three line CTs balances the neutral CT current; the relay is connected across the circulating circuit. (Key point: Connection) 3. What happens during an external earth fault? A. The relay trips immediately B. The residual line current equals the neutral CT current, they circulate, and no current flows in the relay C. The neutral CT is bypassed D. Only the field breaker trips Answer: B. For external faults both currents match and circulate; only an internal fault leaves an unbalance in the relay. (Key point: Operation) ## Related in Switchgear and Protection - [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 - [Why and how the neutral is grounded](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aneutral-grounding-methods) — Switchgear and Protection, chapter 5, asked 4× - [Converting IDMT scheme to differential protection](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aidmt-to-differential-transformer) — Switchgear and Protection, chapter 7 - [Differential protection: 11 kV, 100 MVA generator](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Agenerator-differential-11kv-6-ohm) — Switchgear and Protection, chapter 7 - [Unit protection scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aunit-protection-scheme) — Switchgear and Protection, chapter 7 ## Related in other subjects - [Restricted earth fault protection](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Arestricted-earth-fault) — Power Plant Design, chapter 4 - [Advantages of solidly grounded generators](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Asolidly-grounded-advantages) — Power Plant Design, chapter 4 - [Fault currents of isolated-neutral 380 V generator](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Agenerator-faults-380v-isolated-neutral) — Power System Analysis II, chapter 5 - [Neutral reactance and L-G fault severity](https://www.bctengineer.com/question.md?key=semester_5%2Fpower_system_analysis_ii%3Agenerator-neutral-reactance-lg-severity) — Power System Analysis II, chapter 5, asked 2×