- Electrical Fuse Definition: An electrical fuse is a protective device that breaks the circuit when the current exceeds a predetermined value to prevent damage.
- Fuse Wire Function: The fuse wire carries normal current without excessive heating but melts and breaks the circuit when excess current flows through.
- Materials for Fuse Wires: Common fuse wire materials include tin, lead, zinc, silver, antimony, copper, and aluminum, each with specific melting points and resistances.
- HRC Fuse: An HRC fuse, or High Rupturing Capacity fuse, can handle heavy short-circuit currents for a set period before blowing, providing reliable circuit protection.
- Operating Time of Fuse: The operating time of a fuse is the sum of its melting time and arcing time, defining how long it takes to interrupt the current flow during a fault.
Electrical Fuse
An electrical fuse is a weak conductor that melts and opens the circuit when current exceeds a set value. In normal service that current stays inside the fuse rating. A phase-to-phase or phase-to-ground fault can push it far above the rating.
That excess current can overheat equipment. An electrical fuse interrupts the fault current by melting before the rest of the circuit is damaged.
The fuse is the easiest part of an electrical circuit to melt. When current goes above the chosen value, that conductor opens the path.
Fuse Wire
The fuse wire carries its rated current without melting. Extra current heats it until it melts and opens the circuit.
Materials used for Fuse Wires
The materials used for fuse wires include tin, lead, zinc, silver, antimony, copper and aluminum.
Fuse Wire Rating
Melting point and specific resistance of metals used for fuse wire:
| Metal | Melting point | Specific Resistance |
| Aluminum | 240oF | 2.86 μ Ω – cm |
| Copper | 2000oF | 1.72 μ Ω – cm |
| Lead | 624oF | 21.0 μ Ω – cm |
| Silver | 1830oF | 1.64 μ Ω – cm |
| Tin | 463oF | 11.3 μ Ω – cm |
| Zinc | 787oF | 6.1 μ Ω – cm |
Some Important Terms need for Fuse
Fuse
It is already defined earlier.
Fuse Wire
It is also defined earlier.
Minimum Fusing Current
It is the minimum value of current due to which fuse melts.
Current Rating of Fuse
It is the maximum value of current due to which fuse does not get melt.
Fusing Factor
This is the ratio of minimum fusing current and current rating of the fuse.
Therefore, fusing factor = Minimum fusing current or current rating of the fuse.
The value of the fusing factor is always more than 1.Prospective Current in Fuse
Before melting, the fuse element has to carry the short circuit current through it. The prospective current is defined as the value of current which would flow through the fuse immediately after a short circuit occurs in the network.
Melting Time of Fuse or Pre-arcing Time of Fuse
This is the time taken by a fuse wire to get broken by melting. It gets counted from the instant; the overcurrent starts flowing through the fuse, to the instant when the fuse wire gets just broken by melting.
Arcing Time of Fuse
After breaking of fuse wire there will be an arcing between both melted tips of the wire which will be extinguished at the current zero. The time accounted from the instant of arc initiated to the instant of the arc gets extinguished is known as arcing time of fuse.
Operating Time of Fuse
Whenever overrated current starts to flow through a fuse wire, it takes time to be melted and disconnected, and just after that the arcing stars between the melted tips of the fuse wire, which finally gets extinguished. The operating time of the fuse is the time gap between the instant when the overrated current starts to flow through the fuse and the instant when the arc in the fuse finally gets extinguished. That means operating time of fuse = melting time + arcing time.
Current Carrying Capacity of Fuse Wire
The current a fuse wire can carry depends on its metal, diameter and length, the size and shape of its terminals, and the air or enclosure around it.
Fuse Law
Fuse law gives the current a fuse wire can carry from its diameter. The steps below set that relation out.
In the steady state the fuse carries its normal current without reaching the melting temperature. Heat produced in the wire then equals heat lost from its surface.
Heat generated = I2R.
R is the resistance of the fuse wire.
ρ is the resistivity, l is the length and a is the cross-sectional area of the wire.
d is the diameter of the fuse wire.
K1 is a constant.
Heat lost ∝ surface area of fuse wire ∝ πd.l.
K2 is a constant.
Equating (i) and (ii) gives
That relation is fuse law
| Metal | value of K when d is measured in mm |
| Aluminum | 59 |
| Copper | 80 |
| Iron | 24.6 |
| Lead | 10.8 |
Rewirable or Kit Kat Fuse Unit
A rewirable or Kit Kat fuse is a common domestic fuse. It has two parts. The base holds the incoming and outgoing line or phase conductors. The removable carrier holds the fuse wire and fits into that base. The carrier is also called a cutout.
Cartridge Fuse
In a cartridge fuse the wire sits in a sealed glass tube or bulb. After it blows, the whole unit is replaced. A sealed cartridge fuse cannot be rewired.
Lead – tin Alloy Fuse Wire or Eutectic Alloy Fuse Wire
Lead-tin alloy wire was used for small currents. The usual mix is 37% lead and 63% tin. That eutectic alloy fuse wire is preferred because of the properties listed below.
- It has a high brinner hardness and has less tendency to spread over.
- The alloy metal is quite homogeneous.
- If the fusing characteristics of eutectic alloy and other composition of alloys is studied there is only one arrest point in the eutectic alloy as compared to two other types of alloys.
Approximate fusing currents of lead-tin alloy fuse wire in air:
| Diameter of wire in inch | Fusing Current in A | Maximum safe Current in A |
| 0.02 | 3 | 2 |
| 0.022 | 3.5 | 2.3 |
| 0.024 | 4 | 2.6 |
| 0.028 | 5 | 3.3 |
| 0.032 | 6 | 4.1 |
| 0.036 | 7 | 4.8 |
| 0.048 | 10 | 7 |
| 0.064 | 16 | 11 |
NB: – Keep the fuse wire 2.5 to 3.5 inches long. The table values apply only when the wire does not touch the porcelain grip. Contact with the body sheds more heat and raises the fusing current. When rewiring a grip, keep the wire clear of the body.
HRC Fuse or High Rupturing Capacity Fuse
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An HRC fuse or high rupturing capacity fuse can carry a heavy short-circuit current for a known time. If the fault clears in that time the fuse stays intact. If not, the element melts. The body of an HRC fuse is glass or a ceramic compound.
The enclosure is tight enough to keep air and moisture off the fuse materials. A ceramic body has a metal cap at each end, and a silver fuse wire is welded to those caps. Filling powder packs the space around the element.
The fuse has an inverse-time curve: a large fault current opens it quickly, and a smaller overcurrent takes longer.
Operation of HRC Fuse
When current above rating flows through the element of a high rupturing capacity fuse, the element melts and vaporises.
Silver vapour then reacts with the filling powder and forms a high-electrical resistance solid that helps in quenching the arc.
