Fuse and contactor ratings for induction motor

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).

 415 V ─[HRC fuse]─[Contactor]─[O/L relay]─ M
        short-ckt   switching   overload

Step 1: Full-load current

 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.