--- title: "Fuse terms with cut-off characteristic sketch" subject: "Switchgear and Protection" chapter: 3 key: "semester_6/switchgear_and_protection:fuse-terms-cutoff-sketch" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/fuse-terms-cutoff-sketch site: BCT Engineering --- # Fuse terms with cut-off characteristic sketch Switchgear and Protection · Chapter 3: Fuses - Asked in: 2073 Bhadra, 6 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-terms-cutoff-sketch - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-3 ## Question Explain the following terms showing the sketch of the cut off characteristics of fuse: i) Minimum fusing current ii) Rated current iii) Prospective current iv) Cutoff current v) Fusing factor. ## Short answer Minimum fusing current is the lowest current that melts the element; rated current is the continuous current carried without overheating. Prospective current is rms fault current that would flow if the fuse were replaced by a link; cut-off current is maximum instantaneous current actually reached. Fusing factor equals minimum fusing current divided by rated current, greater than one. ## Answer #### Sketch of cut-off characteristic of a fuse ```text i prospective current │ ╱‾‾‾╲ │ ╱ ╲ │ cut-off ╲ │ ●──╲ ╲ │ ╱ ╲ ╲ │ ╱ ╲___ ╲___ └┴────┴────┴─────────────► t 0 melting arc (pre-arc) extinction Without fuse, current follows the prospective curve; with fuse it is cut off at a lower peak. ``` #### (i) Minimum fusing current The minimum value of current that will cause the fuse element to melt (and thus operate) under specified conditions. Below this value the fuse carries current indefinitely. It depends on element material, length, cross-section, enclosure and ambient temperature. #### (ii) Rated current (current rating) The current that the fuse element can carry **continuously without overheating or melting**. It is always less than the minimum fusing current. *Example:* a 32 A fuse can carry 32 A for an indefinite time. #### (iii) Prospective current The r.m.s. value of the fault current (first loop, symmetrical) that **would flow** in the circuit if the fuse were replaced by a link of negligible impedance. It depends only on the source and circuit impedance, not on the fuse. #### (iv) Cut-off current The **maximum instantaneous current** actually reached before the fuse element melts, when the prospective current is large enough. Because an HRC fuse melts before the first peak of the fault current, the cut-off current is much smaller than the prospective peak, limiting electromagnetic and thermal stress. #### (v) Fusing factor ```text Fusing factor = Minimum fusing current / Current rating ``` - It is always greater than 1. - A value close to 1 gives closer protection, but too close causes nuisance blowing. - Typical values: about 1.1–1.5 for HRC cartridge fuses, about 1.8–2 for semi-enclosed rewirable fuses. - *Example:* a 100 A fuse with minimum fusing current 140 A has a fusing factor of 1.4. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How is fusing factor defined and what is its value? A. Minimum fusing current / current rating, always > 1 B. Rated / minimum fusing, always < 1 C. Prospective / cut-off D. Rated x 2 Answer: A. E.g. 140 A / 100 A = 1.4. (Key point: Fusing factor) 2. What is true of the rated current? A. It melts the element B. It is the continuous carrying current, less than minimum fusing current C. Equals prospective current D. Greater than fusing current Answer: B. A 32 A fuse can carry 32 A indefinitely. (Key point: Rated current) 3. What is prospective current? A. Current after cut-off B. Rated current C. R.m.s. fault current that would flow if the fuse were replaced by a negligible-impedance link D. Peak cut-off Answer: C. It depends on source and circuit impedance, not on the fuse. (Key point: Prospective current) 4. Typical fusing factors? A. HRC 3; rewirable 1 B. Both 1 C. HRC 2; rewirable 1.1 D. HRC about 1.1-1.5; semi-enclosed rewirable about 1.8-2 Answer: D. Rewirable fuses need a higher factor; HRC gives closer protection. 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