- 2070 Bhadra · 2+2+2+2 marks
Fuse vs MCB; HRC fuse time terms
Short answer
Fuse melts irreversibly; MCB uses thermal (bimetal) and magnetic trips, is resettable and usable as a switch. HRC fuse cut-off current is the maximum instantaneous current reached before element melts, limiting peak fault current. Pre-arcing time is the interval from fault inception to element melting. Arcing time is the duration from arc initiation to final extinction; total clearing time is their sum.
What is the main difference between Fuse and MCB? Define the following terms for HRC fuse: (i) cut off (ii) pre-arcing time (iii) arcing time.
Difference between fuse and MCB
| Point | Fuse | MCB (miniature circuit breaker) |
|---|---|---|
| Operating principle | Fuse element melts by I²R heating | Thermal (bimetal) trip for overload + magnetic trip for short circuit |
| Reuse | One-time; element must be replaced | Reset and reused after tripping |
| Speed on short circuit | Very fast; HRC fuse cuts off before first peak | Fast (magnetic, ~5–10 ms), but slower than HRC |
| Breaking capacity | Very high (HRC up to 80–120 kA) | Limited (typically 6–10 kA) |
| Switching | Cannot be used as a switch | Can be used as an ON/OFF switch |
| Indication | Not always visible | Handle position shows tripped state |
| Cost | Low initial cost | Higher initial, but no replacement cost |
| Single phasing | Possible when one fuse blows | Multi-pole MCB trips all poles |
HRC fuse terms
(i) Cut-off (cut-off current): the maximum instantaneous value of current actually reached before the fuse element melts, when the prospective fault current is large. An HRC fuse melts before the first peak of the fault current, so the cut-off current is much lower than the prospective peak; this limits thermal and electromagnetic stress on the circuit.
(ii) Pre-arcing time (melting time): the time from the start of the fault current until the fuse element melts and an arc is initiated. It depends on the magnitude of current; for heavy faults it is a few milliseconds.
(iii) Arcing time: the time from the instant the arc is initiated (element melts) until the arc is finally extinguished and the current becomes zero. Silica sand absorbs the arc energy and quenches the arc.
i prospective
│ ╱‾╲
│ cut-off ●╲ actual current
│ ╱│ ╲
│ ╱ │ ╲___
└──┴──┴──────┴────► t
pre-arc arcing
◄──────►◄────►
total operating time
Total operating (clearing) time = pre-arcing time + arcing time.
✓ Answer checked by Claude Opus 5.5 (high effort)
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