The drop-out fuse (also called an expulsion fuse cutout) is the most common protective device on overhead 10–15 kV distribution laterals and on transformer primaries. When the fuse link melts, the spring-loaded holder swings down and the link falls clear, producing an obvious visible break that also isolates the faulted section. Its simplicity, low cost and visible operation are why it remains in use on rural and suburban networks worldwide.
How It Interrupts a Fault
The fuse link carries a calibrated element inside a fibre or composite tube. On a fault the element melts and an arc forms; the heat of the arc decomposes the tube wall and releases a blast of de-ionising gas that extinguishes the arc at a current zero. The whole action takes only a few half-cycles, after which the holder drops open under gravity and spring tension.
Typical Ratings (RW12 Series)
Parameter
Typical value
Rated voltage
15 kV
Rated current
100 A / 200 A
Breaking capacity
up to 12.5 kA (asymmetrical)
Power-frequency withstand
45 kV
Lightning impulse withstand
110 kV
Creepage distance
≥ 380 mm (heavy-pollution version higher)
Choosing the Fuse Link
Continuous rating: size the link at 1.5–2 times the transformer full-load current for a distribution transformer, so that normal magnetising inrush does not operate it.
Coordination: the link must clear a transformer-secondary fault before the upstream recloser or breaker operates, and must not operate on the inrush of a downstream transformer.
Speed class: “K” links are fast, “T” links are slow. Use the class specified in the coordination study; mixing classes silently destroys selectivity.
Common Faults and Causes
Repeated operation without a fault: usually an undersized link, a loose contact, or corrosion raising the contact resistance. Check the link rating and clean the contacts.
Holder fails to drop: dirt, ice or a bent hinge pin. Clean and lubricate the hinge, and check that the holder is free before re-energising.
Flashover across the insulator: pollution or a chipped shed. Wash or replace the cutout.
Safe Replacement Procedure
Confirm and isolate the upstream source; treat the line as live until proved dead with an approved detector.
Wear the specified insulating gloves and use an insulated operating rod.
Open the cutout with the rod, then remove the spent link and check the tube and holder for arc damage — replace them if the tube wall is burnt through.
Fit the correct new link, close the cutout positively, and record the link rating and date.
After restoration, inspect the installation for a compliant phase-to-phase and phase-to-earth clearance, especially where the holder swings down.
FAQ
Can I re-use a fuse link that has not operated? If it is undamaged and the rating is correct, yes — but never re-use a link that has carried a fault current, even if it looks intact.
How much clearance does the drop-out action need? Allow enough space below the cutout for the holder to swing down fully; a blocked swing means the faulted section is not isolated.
