--- title: "Fuse selection for transformer, capacitor, lighting" subject: "Switchgear and Protection" chapter: 3 key: "semester_6/switchgear_and_protection:fuse-selection-transformer-capacitor-lighting" source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/fuse-selection-transformer-capacitor-lighting site: BCT Engineering --- # Fuse selection for transformer, capacitor, lighting Switchgear and Protection · Chapter 3: Fuses - Asked in: 2076 Bhadra, 6 marks - 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-selection-transformer-capacitor-lighting - Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-3 ## Question What are the considerations in selecting a fuse for (i) Transformer protection (ii) capacitor protection (iii) lighting loads? ## Short answer Transformer: HV fuse 1.5–2 × I_FL must withstand magnetising inrush (10–12× for 0.1 s), coordinate with LV protection, and have adequate breaking capacity. Capacitor: fuse rating 1.43–1.65 × rated current must endure switching inrush and discharge, with individual unit fuses. Lighting: 1.1–1.25 × I_FL, withstand cold‑filament/driver inrush, low fusing factor, discrimination. ## Answer #### (i) Transformer protection 1. **Full-load current:** fuse rating above full-load with allowance for permissible overload; HV side typically **1.5–2 × I_FL**, LV side **1.1–1.25 × I_FL** (next standard rating). 2. **Magnetising inrush:** the fuse must not melt on inrush of about **10–12 × I_FL for 0.1 s** (and 25 × I_FL for 0.01 s). 3. **Through-fault clearing:** the HV fuse must clear a short circuit on the LV terminals (I_FL/Z_pu) before the transformer is damaged (within thermal withstand time, e.g. 2 s). 4. **Discrimination:** HV fuse must coordinate with LV fuses or LV breaker (curves referred to the same voltage must not cross). 5. **Breaking capacity** ≥ system fault level; HRC or expulsion (drop-out) fuses on 11 kV. 6. Use striker-pin fuses with a switch to avoid single phasing. #### (ii) Capacitor protection 1. **Overcurrent due to harmonics and over-voltage:** capacitor current may be up to 1.3 × rated (IEC 60871/60831) plus tolerance, so fuse rating is about **1.43–1.65 × rated capacitor current** (commonly 1.5–1.8 ×). 2. **Switching inrush:** high-frequency inrush at energising (up to 100 × I_rated for back-to-back switching) must not melt the fuse. 3. **Discharge energy:** the fuse must withstand discharge of parallel units into a faulted unit; individual-unit fuses must clear a failed unit before case rupture (fuse curve below tank-rupture curve). 4. Use **individual (internal or external) fuses** for each unit in large banks and a group fuse/breaker for the whole bank. 5. Voltage rating to suit system, considering neutral shift in ungrounded star banks. #### (iii) Lighting loads 1. Rating just above the normal full-load current of the circuit (e.g. **1.1–1.25 × I_FL**), and not greater than the current-carrying capacity of the wiring. 2. **Switch-on surges:** incandescent lamps (cold filament ≈ 10–15 × for a few ms), discharge lamps and LED drivers (capacitive inrush) – choose a fuse that withstands these surges. 3. Lighting circuits have little overload, so a fuse with low fusing factor can be used; fast HRC or cartridge fuses/MCBs are suitable. 4. **Discrimination** with the distribution-board main fuse (ratio ≥ 1.6:1). 5. Breaking capacity ≥ prospective fault at the board; fuse must clear an earth fault quickly enough for shock protection (fault loop impedance check). 6. Power factor of discharge lamps and harmonic currents in the neutral should be considered. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which transformer consideration prevents nuisance fuse melting on energising? A. Fuse must withstand 10-12 x I_FL magnetising inrush for 0.1 s B. Fuse must have zero breaking capacity C. Fuse rating must be below I_FL D. Fuse must melt at 2 x I_FL immediately Answer: A. Inrush of about 10-12 x I_FL lasting 0.1 s must not melt the fuse. (Key point: Transformer inrush) 2. Why is a capacitor fuse rated about 1.43-1.65 x rated capacitor current? A. Capacitors have large inrush only B. Harmonics and over-voltage allow capacitor current up to 1.3 x rated, plus tolerance C. Because capacitors do not carry current normally D. To make the fuse blow on harmonics Answer: B. Allowed overcurrent of 1.3 x rated plus capacitance tolerance sets the margin. (Key point: Capacitor fuse rating) 3. Why are individual fuses used on each unit in large capacitor banks? A. To clear a failed unit before its case ruptures B. To increase bank voltage C. To avoid switching inrush D. To reduce harmonic currents Answer: A. The fuse curve must be below the tank-rupture curve so the faulty unit is isolated safely. (Key point: Individual capacitor fuses) 4. For lighting circuits, the fuse must also consider: A. Magnetising inrush of 25 x I_FL B. Switch-on surges of lamps and drivers C. Neutral shift in star banks D. Tank rupture curves Answer: B. Cold filaments, discharge lamps and LED drivers draw short surges that must not blow the fuse. 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