Electrical Fuse
What is electrical fuse ?
In normal working condition of electrical network, the electric current flows through the network is within the rated limit. If fault occurs in the network mainly phase to phase short circuit fault or phase to ground fault, the network current crosses the rated limits. This high electric current may have very high thermal effect which will cause a permanent damage to the valuable equipments connected in the electrical network. So this high fault current should be interrupted as fast as possible. This is what an electrical fuse does. A fuse is a part of the circuit which consists of conductor which melts easily and breaks the connection when electric current exceeds the predetermined value.
An electrical fuse is a weakest part of an electrical circuit which breaks when more than predetermined current flows through it.
Fuse wire
The function of fuse wire is to carry the normal current without excessive heating but more than normal current when pass through fuse wire, it rapidly heats up and melts.
Materials used for fuse wires
The materials used for fuse wires are mainly tin, lead, zic, silver, antimony, copper, aluminium etc.
Fuse wire rating
The melting point and specific resistance of different metals used for fuse wire
| Metal | Melting point | Specific Resistance |
|---|---|---|
| Aluminium | 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.
1. Fuse it is already defined earlier.
2. Fuse wire, it is also defined earlier
3. Minimum fusing current: It is minimum value of current due to which fuse melts.
4. Current rating of fuse: It is maximum value of current due to which fuse does not get melt.
5. Fusing factor: This is the ratio of minimum fusing current and current rating of fuse.
Therefore, fusing factor = Minimum fusing current / current rating of fuse.
The value of fusing factor is always more than 1.
6. 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.
7. Melting time of fuse or pre arcing time of fuse: This is the time taken by an fuse wire to be broken by melting. It is counted from the instant, the over current starts to flow through fuse, to the instant when fuse wire is just broken by melting.
8. 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 arc being extinguished is known as arcing time of fuse.
9. Operating time of fuse: When ever over rated current starts to flow through a fuse wire, it takes a time to be melted and disconnected, and just after that the arcing stars between the melted tips of the fuse wire, which is finally extinguished. The operating time of fuse is the time gap between the instant when the over rated current just starts to flow through the fuse and the instant when the arc in fuse finally extinguished. That means operating time of fuse = melting time + arcing time of fuse.
Current Carrying Capacity of fuse wire
Current carrying capacity of a fuse wire depends upon numbers of factors like, what material used for it, what are the dimension of it, i.e. diameter and length, size and shape of terminals used to connect it, and the surrounding.
Fuse Law
Fuse law determines the cuyrrent carrying capacity of a fuse wire. The law can be established in the following way. At steady state condition that is when fuse carry normal current without increasing its temperature to the melting limit. That means at this steady state condition, heat generated due to current through fuse wire is equal to heat dissipated from it.
Heat generated = I2.R
Where R is the resistance of the fuse wire
| I2.ρ. | l |
| a |
Where ρ is the resistivity, l is the length and a is the cross sectional area of fuse wire
| I2.ρ. | l |
| πd2/4 |
Where d is the diameter of fuse wire
| I2.K1. | l | .............(i) |
| d2 |
Where K1 is a constant
Heat lost ∝ surface area of fuse wire ∝ πd.l
Therefore, heat lost = K2.d.l .............(ii)
Where K2 is a constant
Now, equating (i) & (ii), we get,
| I2.K1. | l | = K2.d.l |
| d2 |
⇒ I2 = K.d3
| Where K = | K2 | is another costant |
| K1 |
⇒ I = K.d3/2
⇒ I = K.d1.5
This is known as fuse law
| Metal | value of K when d is measured in mm |
|---|---|
| Aluminium | 59 |
| Copper | 80 |
| Iron | 24.6 |
| Lead | 10.8 |
Rewirable or Kit kat fuse unit
This is most commonly used fuse in our day to day life. This fuse has mainly two parts. The unit in which the incoming and outgoing line or phase wire connected permanently is known as fuse base. The removable parts which hold a the fuse wire and fits into the base, is known as fuse carrier. The fuse carrier is also known as cutout.
cartridge fuse
In cartridge fuse the fuse wire is enclosed in a transparent glass tube or bulb, the whole unit is sealed off. In case the fuse blows, it is to be replaced by new one as the cartridge fuse can not be rewired due to its sealing.
Lead – tin Alloy Fuse Wire or Eutectic Alloy Fuse Wire
For small value of current interruption lead – tin alloy fuse wire has been used in past. The most preferred lead – tin alloy for fuse wire containing 37% lead and 63% tin. This alloy fuse wire is also known as known as Eutectic Alloy Fuse Wire. This type of alloy has some specific characteristics due to which this is preferred as fuse wire.
1. It has the high brinnel hardness and has less tendency to spread over.
2. The alloy metal is quite homogeneous.
3. If the fusing characteristics of eutectic alloy and other composition of alloys is studied there is only one arrest point in 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 : - The minimum length of the fuse wire used must be 2.5 to 3.5 inches. The values in the above table are true only when the fuse wire does not touches the fuse grip body because when the fuse wire comes in contact with porcelain or other the value of fusing current increases as the heat dissipation rate from the current carrying fuse wire, is increased. Hence precaution should always be taken during rewiring a fuse wire on a fuse grip so that it should not touch the fuse grip body.
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