--- title: "Fuse and contactor ratings for induction motor" subject: "Switchgear and Protection" chapter: 3 key: "semester_6/switchgear_and_protection:motor-fuse-contactor-selection" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/motor-fuse-contactor-selection site: BCT Engineering --- # Fuse and contactor ratings for induction motor Switchgear and Protection · Chapter 3: Fuses - Asked in: 2071 Magh, 6 marks - Video: [How To Select Size of Contactor, Fuse and C.B For Motor in Urdu/Hindi](https://www.youtube.com/watch?v=kNEvCnID-rY) — kW Calc - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/motor-fuse-contactor-selection - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-3 ## Question An induction motor is required to protect through a fuse and contactor. Suggest a suitable guide line for selection of ratings of fuse and contactor. State any assumptions you make. ## Short answer For a 15kW, 415V motor with eta=0..9, pf=0.85: full-load current =27.28A. Select 63A gG HRC fuse (withstands 163.7A starting for 10s), 32A AC-3 contactor, and thermal overload relay set at 28A. Ensure fuse let-through I²t within contactor withstand for type-2 coordination. ## Answer **Assumptions:** 3-phase squirrel-cage induction motor, 15 kW, 415 V, 50 Hz, efficiency 90 %, power factor 0.85, direct-on-line (DOL) starting, starting current 6 × full-load for up to 10 s, prospective fault current at motor control centre 25 kA. The scheme is fuse (short-circuit protection) + contactor (switching) + thermal overload relay (overload protection). ```text 415 V ─[HRC fuse]─[Contactor]─[O/L relay]─ M short-ckt switching overload ``` #### Step 1: Full-load current ```text I_FL = P / (√3 · V · η · cosφ) = 15 000 / (√3 × 415 × 0.90 × 0.85) = 27.28 A Starting current = 6 × 27.28 = 163.7 A for ~10 s ``` #### Step 2: Fuse selection guidelines 1. **Type:** motor-rated HRC fuse (aM or gM) or time-delay gG fuse, because it must not melt on starting current. 2. **Rating:** for DOL start, about **2–2.5 × I_FL** for gG fuses → 54.6–68.2 A. Choose **63 A** gG HRC (or a 32 A/40 A aM fuse). 3. **Check starting:** the pre-arcing time of the chosen fuse at 163.7 A must be greater than the starting time (10 s) with margin, also allowing for repeated starts. 4. **Breaking capacity:** must exceed the prospective fault current (25 kA); LV HRC fuses with 80 kA are suitable. 5. **Back-up to contactor:** the fuse must clear all currents above the contactor's breaking capacity, and its cut-off current/I²t must be within the contactor's short-circuit withstand (type-2 coordination as per IEC 60947-4-1). 6. Overload protection is not expected from the fuse. #### Step 3: Contactor selection guidelines 1. **Utilisation category:** **AC-3** (starting squirrel-cage motors, switching off while running). 2. **Rated operational current Ie (AC-3)** ≥ I_FL at 415 V → choose a **32 A AC-3** contactor (≥ 27.28 A). 3. **Making/breaking capacity:** AC-3 contactor makes 10 × Ie (320 A) and breaks 8 × Ie (256 A), so it can make the 163.7 A starting current and break locked-rotor current. 4. **Coil voltage** suited to control supply (230 V AC or 110 V AC/24 V DC) and number of auxiliary contacts for control/interlocks. 5. Electrical and mechanical life suitable for number of operations per hour. #### Step 4: Overload relay Thermal (bimetal) relay with range including I_FL, set at **1.0–1.05 × I_FL ≈ 27.3–28.6 A**, class 10 trip (operates within 10 s at 7.2 × setting), with single-phasing protection. **Coordination:** overload relay curve protects for currents up to about 8–10 × I_FL; above the crossover point the fuse operates first, protecting the contactor. **Answer (for assumed motor):** I_FL = 27.28 A; fuse ≈ 63 A gG HRC (or 40 A aM); contactor 32 A AC-3; overload relay set ≈ 28 A. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the fuse + contactor + overload relay scheme, who provides overload protection? A. The HRC fuse B. The contactor C. The thermal overload relay D. The supply transformer Answer: C. The fuse gives short-circuit protection and the contactor switches; the thermal relay handles overload. (Key point: Role of each device) 2. For the assumed 15 kW, 415 V, eta 0.90, pf 0.85 motor, I_FL is: A. 27.28 A B. 24.6 A C. 36.4 A D. 163.7 A Answer: A. I_FL = 15000/(sqrt(3) x 415 x 0.9 x 0.85) = 27.28 A. (Key point: Full-load current) 3. Why must the fuse be rated about 2-2.5 x I_FL for DOL starting (63 A gG)? A. So it blows during every start B. So it does not melt on the 6 x I_FL starting current lasting about 10 s C. To provide overload protection D. To equal the contactor rating Answer: B. Its pre-arcing time at the starting current must exceed the starting time, avoiding nuisance blowing. (Key point: Fuse rating) 4. Which utilisation category and rating suits this motor's contactor? A. AC-1, 16 A B. AC-4, 20 A C. AC-3, 32 A D. AC-3, 16 A Answer: C. AC-3 covers squirrel-cage motor starting and running, with Ie of at least 27.28 A, so 32 A. 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