Fuse selection for transformer, capacitor, lighting

(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.