--- title: "Cut-off and time-current characteristics of fuse" subject: "Switchgear and Protection" chapter: 3 key: "semester_6/switchgear_and_protection:fuse-cutoff-time-current" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/fuse-cutoff-time-current site: BCT Engineering --- # Cut-off and time-current characteristics of fuse Switchgear and Protection · Chapter 3: Fuses - Asked in: 2078 Chaitra, 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-cutoff-time-current - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-3 ## Question Explain the cut-off characteristics and the time-current characteristics of a fuse. ## Short answer Time-current characteristic is inverse: time decreases as current increases, with pre-arcing and total clearing curves; below minimum fusing current no operation. Cut-off characteristic shows maximum instantaneous current versus prospective rms current; for large faults the fuse melts before first peak, limiting peak current and let-through energy. ## Answer #### Time–current characteristic of a fuse The time–current (T–I) characteristic shows the time taken by a fuse to operate for different values of current. It is plotted on log–log scales. ```text t (s, log) 10⁴│╲ │ ╲ 10²│ ╲ pre-arcing curve │ ╲ ╲ total clearing curve 1 │ ╲ ╲ │ ╲__╲__ 0.01│ ‾‾‾──____ └──┬──────────────────► I (log) I_min fusing ``` - It is an **inverse** characteristic: small overcurrent → long time; large current → very short time. - Below the **minimum fusing current** the fuse does not operate at all. - Two curves are usually given: **pre-arcing (melting) time** and **total operating time** (pre-arcing + arcing). The gap between them is the arcing time; it is important only for short times. - The characteristic depends on element material, cross-section, shape (notches), the filler and the ambient temperature. - It is used for coordination: the T–I curves of fuses, relays and breakers in series must not cross. #### Cut-off characteristic of a fuse When the prospective fault current is large, an HRC fuse melts before the current reaches its first peak. The maximum instantaneous current actually reached is the **cut-off current**. ```text i │ ╱‾‾╲ prospective current │ ╱ ╲ │ ●╲╱ cut-off │ ╱ ╲ │ ╱ ╲___ actual (limited) └┴──┴───┴──────► t t_p t_a ``` The **cut-off characteristic** is a graph of cut-off current (peak) versus prospective current (r.m.s.): ```text I_cut-off (kA peak) │ ╱ no cut-off line │ ╱ (peak = 2.5 × Irms) │ ╱ ___ 200 A fuse │ ╱ __─‾ ___ 100 A fuse │ ╱_─‾_──‾ └─────────────────► prospective (kA rms) ``` - Each fuse rating has a curve; above a certain prospective current, cut-off occurs and the curve falls below the "no cut-off" line. - Smaller fuse ratings give lower cut-off currents. **Importance:** it gives the maximum peak current (electromagnetic force) and let-through energy that the downstream cables, busbars and switches must withstand, and allows smaller equipment and cheaper switchgear to be used. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What do the two T-I curves of a fuse represent? A. Pre-arcing (melting) time and total operating time; gap between is arcing time B. Voltage and current C. Cut-off and rated D. Min and max ambient Answer: A. Total = pre-arcing + arcing. (Key point: T-I curves) 2. What happens below minimum fusing current? A. Fuse blows instantly B. Fuse does not operate at all C. Fuse arcs D. Fuse resets Answer: B. The characteristic is inverse with a lower limit. (Key point: Minimum fusing) 3. What is plotted in the cut-off characteristic? A. Time vs current B. I2t vs time C. Cut-off (peak) current vs prospective r.m.s. current for each rating D. Voltage vs time Answer: C. Curves fall below the no-cut-off line (peak = 2.5 x rms) above a certain prospective current. (Key point: Cut-off graph) 4. What is the importance of the cut-off characteristic? A. Gives rated current B. Gives arc voltage C. Gives cost D. Gives maximum peak current and let-through energy that downstream equipment must withstand Answer: D. It allows smaller, cheaper switchgear. (Key point: Importance) ## Related in Switchgear and Protection - [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 - [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 - [Fuse time-current factors and coordination](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Afuse-tcc-factors-coordination) — 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 - [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 ## Related in other subjects - [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 - [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 and single-mode range, 4×2 cm](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Awaveguide-4x2cm-cutoff-single-mode-range) — Electromagnetism, chapter 6