---
title: "Short note: Drop-out fuse"
subject: "Switchgear and Protection"
chapter: 3
key: "semester_6/switchgear_and_protection:short-note-dropout-fuse"
source: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/short-note-dropout-fuse
site: BCT Engineering
---

# Short note: Drop-out fuse

Switchgear and Protection · Chapter 3: Fuses

- Asked in: 2075 Bhadra, 3 marks
- Video: [drop out fuse working principle 11000 voltage](https://www.youtube.com/watch?v=P6lQQmjZ0JA) — Amir Hossain Learning 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/switchgear_and_protection/short-note-dropout-fuse
- Whole chapter: https://www.bctengineer.com/md/semester_6/switchgear_and_protection/chapter-3

## Question

Write a short note on drop-out fuse.

## Short answer

A drop-out fuse is an outdoor expulsion fuse used on 11–33kV lines. It has a porcelain insulator and a fiberglass tube with a fuse link; on fault, the link melts, arc-extinguishing gases expel the arc, and the tube drops down, providing visible isolation. It is cheap, simple, and used for pole transformer protection.

## Answer

A **drop-out fuse (expulsion fuse cut-out)** is an outdoor fuse widely used on 11 kV and 33 kV distribution lines and pole-mounted distribution transformers in Nepal.

**Construction:**
- A porcelain or polymer insulator with upper and lower contacts, mounted on a cross-arm.
- A **fuse tube (carrier)** made of fibre-glass with an inner lining of horn fibre or boric acid, containing a tin or copper **fuse link**.
- The fuse link holds the tube in position against a spring at the top contact.

```text
   upper contact (latch)
        ┌┐
        ││ fuse tube with
        ││ fuse link inside
        ││
       ─┘└─ hinge at lower contact
   tube drops (swings) down
   when link melts
```

**Operation:**
1. On overcurrent or fault, the fuse link melts and an arc forms inside the tube.
2. The arc heat decomposes the fibre lining, producing de-ionising gases (expulsion action) that blow the arc out through the open bottom end at current zero.
3. Melting of the link releases the latch; the tube **drops out** and hangs down by gravity, giving a visible isolation gap and clear indication from the ground.

**Advantages:** cheap, simple, visible indication of operation, provides isolation, easy re-fusing with an operating (hot) stick.

**Limitations:** noisy expulsion of gases and flame, limited breaking capacity, not current limiting, needs manual replacement of link.

**Applications:** protection of distribution transformers, spur lines and capacitor banks in rural/urban feeders.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does a drop-out fuse extinguish the arc?
   A. Sand cools the arc inside a sealed tube
   B. Vacuum interrupter removes ionisation
   C. Arc heat decomposes the fibre lining, and de-ionising gases blow the arc out the open bottom
   D. Magnetic blow-out coils lengthen the arc
   Answer: C. Gas evolved from the lining at current zero expels the arc through the open end. (Key point: Expulsion action)

2. What happens to the fuse tube when the link melts?
   A. It stays in place and a striker pops out
   B. The latch is released and the tube drops and hangs down, visible from the ground
   C. The tube explodes to isolate the line
   D. The tube slides sideways into a spare holder
   Answer: B. Melting releases the latch so the tube swings down by gravity, giving visible isolation. (Key point: Drop-out indication)

3. Which is a limitation of a drop-out fuse?
   A. It is current limiting
   B. It gives no visible indication
   C. It cannot be re-fused
   D. Limited breaking capacity and noisy gas/flame expulsion
   Answer: D. Expulsion fuses are noisy, have limited breaking capacity and are not current limiting. (Key point: Limitations)

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