For cutout fuse construction, two tube designs are common: the moisture-proof fuse tube and the horn fiber fuse tube. Each suits a different current class and environment, and choosing the right one reduces liner expansion that can cause interruption problems.
Two Tube Types, Two Duties
- Moisture-proof fuse tube: standard on all 100 A designs. It is made from aramid fiber, polyester fiber, and epoxy resin, which makes it suitable for harsh environments such as cold, damp, and hot conditions. It is reusable, which lowers operator replacement cost and fuse cost and improves efficiency.
- Horn fiber fuse tube: used on the 200 A design, where a flared design is common. It consists of a horn fiber liner inside a glass-wound housing.
Both designs are durable and can withstand the rigors of high fault current interruption. The tubes are painted to help protect them during outdoor service.
The 100 A and 200 A Split
The two designs are not interchangeable, and the reason comes down to current and to how much gas the liner must generate to clear a fault. Higher current means more energy to absorb and a larger bore to carry it, which in turn means a liner that can release gas fast enough without losing its shape.
| Item | Moisture-proof tube | Horn fiber tube |
|---|---|---|
| Standard pairing | 100 A designs | 200 A designs |
| Liner construction | Aramid fiber, polyester fiber, and epoxy resin | Horn fiber liner in a glass-wound housing |
| Interface with housing | Straight bore, close fit along the length | Flared bore at the exhaust end |
| Moisture behaviour | Low pickup, dimensionally stable when damp | More sensitive to moisture, hence the painted housing |
| Reuse | Designed to be reused within its rating | Replaced as a set more often |
| Typical service | Coastal, humid, and cold inland lines | Standard overhead distribution |
Moisture Uptake and Why It Ends Interruption
Liner expansion is a leading cause of interruption problems in cutouts. The mechanism is worth spelling out, because it explains why the moisture-proof liner is standard on the smaller current class rather than an upgrade.
Fiber liners are hygroscopic. When one absorbs moisture, the wall swells, and because the liner is constrained by the housing it can only swell inward. The bore narrows. A narrower bore changes two things at once. First, the gas channel between the liner and the fuse element becomes tighter, so the gas has less room to build pressure before it must escape. Second, the arc root sits closer to the wall, so the liner ablates unevenly and the erosion is concentrated. The result is a tube that either fails to clear the fault or clears it late, and both outcomes damage the cutout.
A liner made with aramid fiber, polyester fiber, and epoxy resin resists that uptake, so the bore stays where the design intended. That is why it is specified as standard for 100 A, and why it is reused rather than replaced every cycle.
What the Paint Layer Does
Both tubes are painted, and the paint is not decoration. On an overhead line the tube sees sun, rain, and airborne contamination. A sound coating slows moisture ingress into the fiber, keeps surface contamination from forming a conductive film that encourages tracking, and protects the glass-wound housing from UV-driven resin breakdown. When you inspect a used cutout, a chalked or flaking coating is a signal that the liner behind it has been exposed and should be checked before the tube goes back in service.
Field Checks Before Re-Energizing
- Bore condition: look for a chalky or glazed surface and for channelling in the direction the arc ran.
- Bore diameter: compare against a new tube of the same rating. Visible swelling means the liner has taken on moisture.
- Flare integrity on horn fiber tubes: the flare must be intact and even, since it shapes the exhaust gas path.
- Coating: reject tubes with bare patches, deep scratches, or flaking coating.
- End faces: square, crack-free, and clean where the contacts seat.
Specifying a Replacement Cutout Tube
Send us the cutout model or drawing, the voltage class, the current rating, and the tube type currently fitted. If the tube is unbranded, a dimensioned sketch of the bore, the length, and the end configuration is enough for us to match it. Tell us the environment as well, because that is what decides between a standard fiber liner and the moisture-proof construction.
Why Source From a Direct Manufacturer
Tianze Electrical supplies ABB-style fuse tubes in both moisture-proof and horn fiber designs, with ISO 9001 certification and SGS material testing available. Direct supply lets us match the liner and housing to your cutout and fault level, rather than asking you to adapt a catalogue item to a design it was not made for. Our arc extinguishing measures on fuse cutouts article covers the interruption side in more detail.
Frequently Asked Questions
Q: Can I use a horn fiber tube on a 100 A design?
A: The standard pairing is moisture-proof for 100 A and horn fiber for 200 A. Follow the design’s specification.
Q: Is the moisture-proof tube reusable?
A: It is designed to be reusable within its rating, which reduces replacement cost in the field.
Q: Why is a flared bore used on the 200 A tube?
A: The flare shapes the exhaust path so the gas and arc products leave the tube in a controlled direction instead of bursting the wall.
Q: How often should the paint be reapplied?
A: Repaint when the coating chalks, flakes, or shows bare fiber. Frequency depends entirely on UV exposure and local pollution, so inspection interval matters more than a fixed calendar.
Request a cutout tube specification from sale@fusestube.com, or see the fuse tube range.
