--- title: "Fuse vs MCB and HRC rupturing capacity" subject: "Industrial Power Distribution and Illumination" chapter: 3 key: "semester_6/industrial_power_distribution_and_illumination:fuse-mcb-hrc-rupturing" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/fuse-mcb-hrc-rupturing site: BCT Engineering --- # Fuse vs MCB and HRC rupturing capacity Industrial Power Distribution and Illumination · Chapter 3: Power Carrying Devices - Asked in: 2075 Bhadra, 8 marks; 2074 Bhadra, 4 marks (asked 2 times) - Video: [HRC FUSE (HIGH RUPTURING CAPACITY FUSE) AND ITS CUTOFF CHARACTERISTICS FULL EXPLAINATION](https://www.youtube.com/watch?v=KefixMzqcz4) — Asw Education - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/fuse-mcb-hrc-rupturing - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-3 ## Question What are the difference between Fuse and MCB? Explain the operating principle of MCB. Discuss about rupturing capacity of HRC fuse. ## Short answer Fuse melts and needs replacement; MCB trips on overload/short circuit via bimetal and solenoid, and resets. MCB's thermal trip bends bimetal, magnetic trip pulls plunger. HRC fuse rupturing capacity (80-120 kA) is its safe interruption limit, using quartz sand to quench arc and limit cut-off current. ## Answer #### Difference between fuse and MCB | Point | Fuse | MCB | |---|---|---| | Principle | Fuse wire melts by I²R heat | Thermal bimetal + magnetic trip | | After operation | Must be replaced/rewired | Simply reset by the lever | | Speed on short circuit | Very fast (current limiting) | Fast (magnetic trip, 2–10 ms) | | Overload protection | Poor, not exact | Accurate, inverse-time | | Switching | Cannot be used as switch | Also works as an ON/OFF switch | | Indication | Not clear which fuse blew | Lever shows tripped state | | Tampering | Can be wrongly rewired | Cannot be tampered easily | | Cost | Low | Higher | #### Operating principle of MCB An MCB (miniature circuit breaker) has two trip elements acting on one latch mechanism: ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is a key practical difference between a fuse and an MCB after operation? A. Both must be replaced B. Fuse is reset by a lever; MCB must be rewired C. MCB is simply reset by its lever; the fuse must be replaced or rewired D. Neither can be reused Answer: C. An MCB is reset by the lever, while a blown fuse must be replaced or rewired. (Key point: Fuse vs MCB reset) 2. How does an MCB respond to a sustained overload versus a short circuit? A. Magnetic coil for overload, bimetal for short circuit B. Bimetal heats and bends (thermal, inverse time) for overload; solenoid trips instantly for short circuit C. Both use a bimetal only D. Both rely on a fuse wire melting Answer: B. Thermal bimetal gives inverse-time overload trip; the magnetic solenoid acts instantly on short circuit. (Key point: MCB trip elements) 3. What is the rupturing capacity of a fuse? A. Its rated continuous current B. The largest prospective fault current (rms) it can safely interrupt at rated voltage C. Its melting temperature D. The voltage at which it blows Answer: B. It is the maximum fault current the fuse can interrupt without damage or danger, given in kA or MVA. (Key point: Rupturing capacity) 4. Why is an HRC fuse able to limit the fault current? A. It blows after the current peaks B. It opens the line slowly C. It has no element D. The element melts before the fault current peaks and sand absorbs the arc energy, giving current cut-off Answer: D. Quick melting limits the cut-off current, and quartz sand quenches the arc. (Key point: Quartz sand and cut-off) ## Close variants set in other exams - [MCB, MCCB and HRC fuse comparison](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Amcb-mccb-hrc-comparison) — Industrial Power Distribution and Illumination, chapter 3, asked 3× ## Related in Industrial Power Distribution and Illumination - [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 - [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 - [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 - [Types of fuse, phase wire, HRC use](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Afuse-types-hrc-use) — Industrial Power Distribution and Illumination, chapter 3 ## Related in other subjects - [Fuse vs MCB; HRC fuse time terms](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Afuse-vs-mcb-hrc-terms) — 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 - [HRC fuse operating characteristics and applications](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Ahrc-fuse-operating-characteristics) — Switchgear and Protection, chapter 3 - [Operating principle of MCB](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_and_electronics_engineering%3Amcb-operating-principle) — Basic Electrical and Electronics Engineering, chapter 4