--- title: "MCB principle, MCB/MCCB/RCCB, HRC cut-off" subject: "Industrial Power Distribution and Illumination" chapter: 3 key: "semester_6/industrial_power_distribution_and_illumination:mcb-mccb-rccb-hrc-cutoff" source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/mcb-mccb-rccb-hrc-cutoff site: BCT Engineering --- # MCB principle, MCB/MCCB/RCCB, HRC cut-off Industrial Power Distribution and Illumination · Chapter 3: Power Carrying Devices - Asked in: 2078 Chaitra, 4 marks - Video: [Difference between MCB, MCCB, ELCB, RCCB, RCBO, RCD and MPCB || How does this device work?](https://www.youtube.com/watch?v=iqVm_gNa67M) — Electrical Technician - 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/mcb-mccb-rccb-hrc-cutoff - Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-3 ## Question Explain the operating principle of MCB. What are the differences between MCB, MCCB and RCCB? What are the cut-off current characteristics of HRC fuse. ## Short answer MCB uses bimetal for overload, solenoid for short circuit, arc chute quenching. MCB (≤125A, 6-10kA) for final circuits; MCCB (≤2500A, adjustable) for feeders; RCCB detects earth leakage via CT for shock protection. HRC fuse cut-off: melts before peak, limiting let-through current and energy, enabling high breaking capacity (80-120 kA). ## Answer #### Operating principle of MCB A miniature circuit breaker trips automatically on overload and short circuit: - **Thermal trip:** load current heats a **bimetal strip**. On a sustained overload it bends and releases the trip latch after a time inversely related to the current. - **Magnetic trip:** a **solenoid coil** carries the current. A short-circuit current (several times rated, e.g. 5–10 × In for C curve) pulls the plunger instantly and trips the latch. - The contacts open and the arc is driven into an **arc chute**, where it is split and extinguished. The MCB can be reset by its lever. #### Differences between MCB, MCCB and RCCB | Point | MCB | MCCB | RCCB | |---|---|---|---| | Full name | Miniature CB | Moulded case CB | Residual current CB | | Protects against | Overload, short circuit | Overload, short circuit (some earth fault) | Earth leakage, shock | | Current rating | Up to about 100–125 A | About 16 A to 2500 A | 16–100 A | | Breaking capacity | 6–10 kA (up to 25 kA) | 16–200 kA | Not for short circuit | | Trip setting | Fixed | Often adjustable | Fixed sensitivity (30, 100, 300 mA) | | Principle | Bimetal + solenoid | Thermal-magnetic or electronic | Core-balance CT senses I(L) − I(N) | | Use | Final circuits | Main feeders, motors | With MCB for shock protection | #### Cut-off current characteristic of HRC fuse When a short circuit occurs, the prospective current would rise to a very high peak. An HRC fuse melts during the first rising part of the current wave, **before the first peak**, so the current is cut off at a much lower value. ```text i prospective current ^ .-. | / \ | / . \ | / /\ \ cut-off | / / \ \ ← current (actual) | / / \ \ |/_/______\____\______> t |←pre-arcing→|←arcing→| total operating time ``` - **Pre-arcing time:** from fault start until the element melts. - **Arcing time:** from melting until the arc is quenched by the quartz sand. - **Cut-off current:** the peak current actually reached, much less than the prospective peak. - The cut-off characteristic is a graph of cut-off current against prospective current for each fuse rating. It shows that the HRC fuse is **current limiting**: the let-through energy (I²t) and electromagnetic forces are very small, which protects cables and switchgear. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does an MCB trip on a sustained overload? A. A bimetal strip heated by the load current bends and releases the latch after an inverse-time delay B. A solenoid plunger pulls instantly C. A core-balance CT senses the current difference D. A quartz-sand element melts Answer: A. The thermal element is a bimetal strip; the more the overload, the faster it bends. (Key point: MCB thermal trip) 2. What causes the instantaneous trip of an MCB on a short circuit? A. The bimetal strip melts B. The solenoid coil carrying the current pulls the plunger when current is several times rated (e.g. 5-10 In for C curve) C. The arc chute plates overheat D. The earth leakage exceeds 30 mA Answer: B. The magnetic trip uses a solenoid and plunger for fast operation at high currents. (Key point: MCB magnetic trip) 3. Which statement about the RCCB is correct? A. It protects mainly against overload using a bimetal strip B. It has a breaking capacity of 200 kA for short circuits C. It senses the imbalance I(L) - I(N) with a core-balance CT and protects against earth leakage and shock D. It has adjustable trip settings for motor feeders Answer: C. An RCCB detects residual current with a core-balance CT; it is not a short-circuit device. (Key point: RCCB principle) 4. What does the HRC fuse do to the prospective short-circuit current? A. It lets the full prospective peak flow, then opens at current zero B. It only operates after several cycles C. It raises the current to clear the fault faster D. It melts before the first peak, so the actual peak (cut-off current) is much lower than the prospective peak Answer: D. The HRC fuse is current limiting: it interrupts in the rising part of the first wave. (Key point: HRC fuse cut-off) ## Related in Industrial Power Distribution and Illumination - [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 - [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× - [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 - [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× - [Selecting MCCB and MCB for MDB and SDB](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Amccb-mcb-selection-mdb-sdb) — Industrial Power Distribution and Illumination, chapter 5 - [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 - [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 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 - [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 - [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