A fuse is a current-interrupting device that breaks or opens a circuit by melting its element when the current in the circuit surpasses a specific value. Fuses are predominantly categorized into two types: high-voltage fuses and low-voltage fuses. Low-voltage fuses can be further divided into two subtypes: semi-enclosed or rewirable fuses, and fully enclosed cartridge fuses.
Rewirable Fuses
Rewirable fuses, commonly referred to as kit-kat fuses, are widely utilized in residential wiring and small-current circuits. These fuses feature a porcelain base with fixed contacts, through which live wires are connected. The fuse carrier, an independent component, can be easily inserted into or removed from the base.
The fuse element is typically crafted from lead, tin, copper, or a tin-lead alloy. The current required to melt the fusing element is usually twice the normal operating current. When multiple (more than two or three) fuse elements are employed, they must be adequately spaced apart. The de-rating factor of the fuse element ranges from 0.7 to 0.8. In the event of a fault, the fuse element melts, interrupting the circuit.
Once the fuse element has melted, it can be removed and replaced with a new one. By reinserting the fuse into the base, the electrical supply can be restored. Rewirable fuses offer the advantage of safe fuse element replacement at a minimal cost.
However, rewirable fuses also have several drawbacks:
- Unreliable Operation: Selecting an appropriately sized element is crucial for the fuse's proper functioning. The element can deteriorate due to oxidation, which occurs as a result of continuous heating.
- Low Rupturing Capacity: The breaking capacity of rewirable fuses is limited. For instance, a fuse with a normal current capacity of 16A may have a breaking capacity of 2kA, while a 200A fuse might have a breaking capacity of 4kA.
- Slow Operation Speed: These fuses have a relatively long arcing time, and they lack additional means for arc extinction.
- Flame and Fire Risk: The rewirable feature may lead to the use of inexpensive wires for protecting devices against short circuits and overloads, increasing the risk of fire.
Enclosed or Cartridge Type Fuses
In enclosed or cartridge type fuses, the fuse element is housed within a sealed container, held in place by metal contacts. These fuses are further classified into D-types and link types. Link type cartridge fuses can be sub - categorized into knife blade or bolted designs.
D - Types Cartridge Fuse
D - types cartridge fuses are non - interchangeable. The main components of this fuse include the fuse base, adapter ring, cartridge, and fuse cap. The cartridge slides into the fuse cap, which is then attached to the fuse base. The fuse element makes contact with the tip of the fuse base, completing the circuit via the fuse link.
The standard ratings of fuses include 6, 16, 32, and 63 amperes. The breaking capacity of fuse elements is 4kA for 2A and 4A fuses, and 16kA for 6A or 63A fuses. This type of fuse has no reported disadvantages and offers highly reliable operation.
Link Type Cartridge or High Rupturing Capacity (HRC) Fuses
The fuse frame is constructed from steatite (a powdered mineral) or ceramic materials, chosen for their excellent mechanical strength. Brass caps secure the fuse element within the ceramic body, fastened with specialized force to withstand high internal pressure during fault conditions.
End contacts are welded to the metallic caps, ensuring robust electrical connectivity. The space between the fuse element and the cartridge body is filled with quartz powder, which serves as an arc-extinguishing medium. This powder absorbs heat generated by short-circuit currents, converting into a high-resistance state that suppresses restriking voltage and rapidly quenches arcs, enhancing the fuse’s rupturing capability and reliability.
The fuse element is constructed from silver or copper and connected via a tin joint, which regulates the fuse’s temperature during short-circuit events. Silver has a melting point of 980°C, while tin melts at 240°C. During a system fault, short-circuit current first passes through the tin joint, which restricts current flow through the silver element.
The fusing factor of the link fuse is 1.45, though some specialized fuses may have a lower fusing factor of 1.2. Common types include knife-blade and bolted designs.
