Selecting HRC fuse rating for motor

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:

 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:

 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.