--- title: "Fuse vs MCB; HRC fuse time terms" subject: "Switchgear and Protection" chapter: 3 key: "semester_6/switchgear_and_protection:fuse-vs-mcb-hrc-terms" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/fuse-vs-mcb-hrc-terms site: BCT Engineering --- # Fuse vs MCB; HRC fuse time terms Switchgear and Protection · Chapter 3: Fuses - Asked in: 2070 Bhadra, 2+2+2+2 marks - Video: [What is Cut Off Characteristics Of Fuse.](https://www.youtube.com/watch?v=fXrXmJ_oQ3A) — IB TECHNICAL CLASSES - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/fuse-vs-mcb-hrc-terms - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-3 ## Question 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. ## 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. ## Answer #### 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. ```text i prospective │ ╱‾╲ │ cut-off ●╲ actual current │ ╱│ ╲ │ ╱ │ ╲___ └──┴──┴──────┴────► t pre-arc arcing ◄──────►◄────► total operating time ``` Total operating (clearing) time = pre-arcing time + arcing time. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which is a correct difference? A. A fuse element melts and must be replaced, while an MCB uses bimetal and magnetic trips and can be reset B. An MCB has higher breaking capacity than an HRC fuse C. A fuse can be used as a switch D. MCB element melts Answer: A. Fuse is one-time; MCB is reusable with thermal and magnetic trips. (Key point: Fuse vs MCB) 2. What is cut-off current? A. Rated current B. Maximum instantaneous current reached before the element melts when prospective current is large C. Minimum fusing current D. Arc voltage Answer: B. HRC fuse melts before the first peak, limiting the let-through. (Key point: Cut-off) 3. What is pre-arcing time? A. Time from arc start to extinction B. Total of both C. Time from fault start until the element melts and arc is initiated D. Time to reset Answer: C. It is the melting time. (Key point: Pre-arcing time) 4. How is total clearing time related? A. Pre-arcing - arcing time B. Arcing time only C. Cut-off / rated D. Pre-arcing + arcing time Answer: D. Total = pre-arcing time + arcing time; arcing time ends when current is zero. (Key point: Total time) ## Related in Switchgear and Protection - [Fuse terms with cut-off characteristic sketch](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Afuse-terms-cutoff-sketch) — Switchgear and Protection, chapter 3 - [HRC fuse construction; fuse vs MCB](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Ahrc-fuse-construction-vs-mcb) — Switchgear and Protection, chapter 3 - [Prospective current, cut-off current, time delay fuse](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aprospective-cutoff-time-delay-fuse) — Switchgear and Protection, chapter 3 - [Cut-off and time-current characteristics of fuse](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Afuse-cutoff-time-current) — Switchgear and Protection, chapter 3 - [HRC fuse I²t and fuses in series](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Ahrc-fuse-i2t-series) — Switchgear and Protection, chapter 3 - [HRC fuse construction and advantages](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Ahrc-fuse-construction-advantages) — Switchgear and Protection, chapter 3 ## Related in other subjects - [HRC fuse quartz sand, cut-off, vs Kit-Kat](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Ahrc-quartz-sand-kitkat) — Industrial Power Distribution and Illumination, chapter 3 - [MCB principle, MCB/MCCB/RCCB, HRC cut-off](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Amcb-mccb-rccb-hrc-cutoff) — Industrial Power Distribution and Illumination, chapter 3 - [Cut-off current characteristics of HRC fuse](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Ahrc-cutoff-characteristics) — Industrial Power Distribution and Illumination, chapter 3 - [Fuse vs MCB and HRC rupturing capacity](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Afuse-mcb-hrc-rupturing) — Industrial Power Distribution and Illumination, chapter 3, asked 3×