Short note: Drop-out fuse

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.

   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.