--- title: "Selecting HRC fuse rating for motor" subject: "Switchgear and Protection" chapter: 3 key: "semester_6/switchgear_and_protection:hrc-fuse-motor-rating" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/hrc-fuse-motor-rating site: BCT Engineering --- # Selecting HRC fuse rating for motor Switchgear and Protection · Chapter 3: Fuses - Asked in: 2079 Chaitra, 5 marks - Video: [How to Select Fuse Rating | What is HRC Fuse | Hindi](https://www.youtube.com/watch?v=maWfYt0MHsQ) — Electrical With Abhishek Joshi - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/hrc-fuse-motor-rating - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-3 ## Question Discuss the method of selecting the rating of HRC fuse in motor stator. ## Short answer SelectHRCfuse by calculating motor full-load and starting current. Choose aM or time-delay gG fuse whose pre-arcing time at start current exceeds starting time, typically 2-3 times full-load for gG (e.g., for 27.28A motor, 63A gG). Verify breaking capacity and type-2 coordination with contactor, ensuring fuse cuts off before contactor damage. ## Answer An HRC fuse in a motor (stator) circuit provides **short-circuit protection** only; overload protection is given by a thermal overload relay. The fuse must not blow during starting, but must clear faults and protect the contactor. #### Method of selection 1. **Find full-load current:** ```text I_FL = P_out / (√3 · V_L · η · cosφ) ``` 2. **Find starting current and starting time:** DOL starting current I_st ≈ 5–7 × I_FL for 5–10 s (longer for high-inertia loads); star–delta ≈ 2–2.5 × I_FL; soft starter ≈ 3 × I_FL. 3. **Choose fuse type:** motor-circuit HRC fuse (**aM**, partial-range) or time-delay **gG** fuse. aM fuses withstand starting current and can be rated close to I_FL. 4. **Choose rating:** the smallest standard rating whose **pre-arcing time at I_st is greater than the starting time** (with margin for frequent starts and hot condition). As rough guides for gG fuses: - DOL start: fuse ≈ 2–3 × I_FL - Star–delta start: fuse ≈ 1.5–2 × I_FL 5. **Check breaking capacity** ≥ prospective fault current at the motor terminals. 6. **Coordinate with contactor and overload relay:** the fuse must interrupt all currents above the contactor's breaking capacity (≈ 8 × Ie for AC-3) and limit the let-through I²t to the contactor and cable withstand (type-2 coordination). 7. **Check cable protection:** fuse I²t below the cable's k²S². #### Example 15 kW, 415 V motor, η = 0.9, pf = 0.85, DOL: ```text I_FL = 15 000 / (√3 × 415 × 0.9 × 0.85) = 27.28 A I_st = 6 × 27.28 = 163.7 A for 10 s Fuse (gG) ≈ 2.5 × 27.28 = 68.2 A → 63 A standard, check its T–I curve: melting time at 163.7 A > 10 s ✓ ``` An aM fuse of about 32–40 A could also be used. #### Points to remember - Too small a fuse blows during starting (nuisance tripping); too large a fuse may not protect the contactor and cables. - Never rely on the fuse for overload; use the overload relay set at ≈ 1.0–1.05 × I_FL. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What protection does an HRC fuse in a motor circuit give? A. Overload only B. Short-circuit protection only C. Under-voltage protection D. Single phasing protection Answer: B. Overload is left to the thermal relay; the fuse is for short circuits. (Key point: Purpose of fuse) 2. The fuse rating chosen is the smallest standard rating whose: A. Current rating equals I_FL B. Breaking capacity equals starting current C. Pre-arcing time at starting current exceeds the starting time D. Voltage rating equals the motor power Answer: C. This prevents blowing during starting while keeping protection as tight as possible. (Key point: Rating criterion) 3. How does the gG fuse multiple of I_FL change with star-delta instead of DOL starting? A. It is higher, because starting current is higher B. It is lower (1.5-2 x I_FL), because starting current is lower C. It does not change D. It must be zero Answer: B. Star-delta starting current is about 2-2.5 x I_FL, so a smaller fuse suffices. (Key point: Starter type effect) 4. What is the consequence of choosing too large a fuse? A. It blows during starting B. It has lower breaking capacity C. It may not protect the contactor and cables D. The motor cannot start Answer: C. Too small causes nuisance blowing; too large leaves contactor and cable unprotected. 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