- Asked 3/20
- 2072 Asoj · 8 marks
MCB, MCCB and HRC fuse comparison
Short answer
MCB trips via thermal bimetal for overload and magnetic solenoid for short circuit. MCB rated ≤125A, breaking capacity 6-25kA, fixed; MCCB rated up to 2500A, breaking 16-200kA, adjustable. HRC fuse melts, very fast, breaking 80-200kA, must replace; MCCB is reusable, adjustable, can switch. Rupturing capacity: MCB 6-25kA, MCCB 16-200kA, HRC 80-200kA.
Explain the operating principle of MCB. What are the difference between MCB and MCCB? Compare HRC fuse with MCCB. What is rupturing capacity of MCB, MCCB and HRC fuse?
Operating principle of MCB
An MCB has a thermal and a magnetic trip acting on a common latch:
- Thermal (overload): load current heats a bimetal strip; on sustained overload it bends and trips the latch after an inverse-time delay.
- Magnetic (short circuit): a solenoid in series pulls its plunger instantly when current exceeds a set multiple of rated current (B: 3–5 × In, C: 5–10 × In, D: 10–20 × In), tripping the latch and helping to open the contacts.
- The arc is drawn into an arc chute of metal plates, split and cooled until it goes out. The MCB is then reset by its lever.
MCB vs MCCB
| Point | MCB | MCCB |
|---|---|---|
| Current rating | Up to about 100–125 A | About 16 A to 2500 A |
| Breaking capacity | 6–10 kA (max about 25 kA) | 16–200 kA |
| Trip setting | Fixed | Adjustable (thermal and magnetic) |
| Trip unit | Thermal-magnetic | Thermal-magnetic or electronic |
| Poles | 1, 2, 3, 4 (DIN rail) | 3 or 4, bolted |
| Extras | Few | Shunt trip, under-voltage, aux contacts, motor operator |
| Use | Final circuits in buildings | Feeders, motors, main incomers |
HRC fuse vs MCCB
| Point | HRC fuse | MCCB |
|---|---|---|
| Principle | Melting of silver element in quartz sand | Thermal-magnetic/electronic trip with contacts |
| After fault | Must be replaced | Reset and reused |
| Speed | Very fast; current limiting | Slower (unless current-limiting type) |
| Breaking capacity | 80–120 kA (up to 200 kA) | 16–200 kA |
| Adjustability | None | Adjustable settings |
| Switching duty | Cannot switch load | Can switch load on/off |
| Single phasing | One fuse may blow, motor single-phases | All poles open together |
| Cost | Low | High |
| Remote operation | Not possible | Possible (shunt trip) |
Rupturing (breaking) capacity
The rupturing capacity is the maximum prospective short-circuit current a device can interrupt safely at rated voltage. Typical values:
- MCB: 6 kA or 10 kA (IEC 60898), up to about 25 kA.
- MCCB: 16 kA to about 100 kA, some up to 200 kA (IEC 60947-2).
- HRC fuse: 80 kA to 120 kA, some up to 200 kA.
The device's breaking capacity must be higher than the fault level at the point where it is installed.
✓ Answer checked by Claude Opus 5.5 (high effort)
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