Fuse vs MCB and HRC rupturing capacity

Difference between fuse and MCB

Operating principle of MCB

An MCB (miniature circuit breaker) has two trip elements acting on one latch mechanism:

 Line ─┬─[contacts]──[bimetal]──[coil]── Load
       │      ▲          │         │
       │      └── latch ─┴─────────┘
       └ arc chute (splits the arc)

1. Thermal trip (overload): the load current passes through a bimetal strip. On a sustained overload the strip heats, bends and releases the latch. The time delay is inversely related to the current, so small overloads take longer to trip.
2. Magnetic trip (short circuit): a solenoid coil carries the current. A large short-circuit current (for example 3–5 × In for B curve, 5–10 × In for C curve, 10–20 × In for D curve) pulls the plunger instantly, which trips the latch and also pushes the moving contact open.
3. Arc quenching: the arc formed between opening contacts is driven into an arc chute of steel plates, where it is split, cooled and extinguished.

Rupturing capacity of HRC fuse

The rupturing (breaking) capacity of a fuse is the largest prospective fault current (r.m.s.) that it can safely interrupt at rated voltage without damage or danger. It is expressed in kA (or MVA).

• HRC (high rupturing capacity) fuses have a silver element inside a ceramic body filled with pure quartz sand. When the element melts, the sand absorbs the arc energy and forms a high-resistance glass-like substance (fulgurite), which quenches the arc quickly.
• Because the fuse melts before the fault current reaches its peak, it limits the cut-off current, so the actual current is much lower than the prospective current.
• Typical breaking capacity of LV HRC fuses is 80 kA to 120 kA (some up to 200 kA), far higher than MCBs (6–10 kA) and comparable to large MCCBs.
• This makes HRC fuses suitable near transformers and in main panels where fault levels are very high, and as back-up protection for MCBs.