functions and applications of fuse, MCB, and MCCB

functions and applications of fuse, MCB, and MCCB

1. What They Are — Overcurrent Protection Devices

All three protect against overcurrent (overload and short circuit) by interrupting circuit when $I$ exceeds rating, but different technology, rating, reusability.

2. Comparison Table (exam must-draw)

Feature	Fuse (wire)	MCB (Miniature Circuit Breaker)	MCCB (Moulded Case CB)
Principle	Heats and melts ($I²R$) wire element, one-time, needs replacement	Thermal (bimetal) for overload (slow) + magnetic (solenoid) for short circuit (fast, <10ms) — resettable (switch)	Same as MCB but larger, adjustable, with thermal + magnetic + electronic trip
Breaking capacity	Low–medium (kA, e.g., 1kA for glass, 100kA for HRC fuse)	Medium (e.g., 6–10kA)	High (10–100kA) for industrial
Current rating	mA to 100A (HRC up to 600A)	0.5–63A (common home 6,10,16,32A)	16–1600A (industrial)
Operation time	Fast for short (ms, depends on $I²t$), slow for overload (inverse)	Inverse time (overload) + instant magnetic at ~5–10× rated for short	Adjustable trip settings (long-time, short-time, instantaneous)
Reusability	One-time (replace)	Resettable (flip switch)	Resettable, adjustable
Cost	Cheapest	Moderate	Expensive
Functions	Overload + short circuit (HRC also current limiting — cuts before peak)	Overload + short circuit + isolation (can switch off)	Same as MCB + adjustable, higher breaking, for motors/industrial

3. Selection

Application	Choice	Why
Home lighting (6A)	MCB 6A, B-curve (3–5× In magnetic)	Resettable, handles inrush of bulbs
Home socket (16A)	MCB 16A, C-curve (5–10× for motor start)
Semiconductor protection	Fast-acting HRC fuse (e.g., aR)	Faster than MCB for sensitive device
Industrial motor 100A	MCCB 100A with adjustable 80–100A, magnetic at 10×	High breaking, adjustable, handles motor inrush
Distribution board incomer 200A	MCCB 200A

4. Worked Checks

Need to protect 5A load: fuse rating slightly above (e.g., 6A) to avoid nuisance, but not too high (16A would allow 3× overload before blowing).

Why HRC fuse current limiting? Silver element in silica sand quenches arc quickly, cuts $I²t$ let-through.

MCB B vs C: B trips magnetic at 3–5× In (sensitive, lighting), C at 5–10× (motors).

Need to coordinate? Fuse/MCB time-current curves must be selective (downstream blows before upstream).

5. Exam Points

• Compare fuse vs MCB vs MCCB (principle, breaking capacity, rating, reusability) — 7 marks, very frequent (the most common protection comparison question).
• State fuse melts $I²R$, MCB thermal+magnetic, MCCB adjustable — 5 marks, very frequent.
• Give current ranges (MCB ≤63A, MCCB 16–1600A) — 3-5 marks, frequent.
• Show selection for home vs industrial — 3-5 marks, frequent.