May 26, 2016

Characteristic Parameters and Laws of Fuse



A fuse can conduct continuously at maximum current without interrupting the circuit.

Speed:

The fuse blown speed depends upon the current flows through the circuit and type of material used to made the fuse. The operating time of the fuse is not a fixed interval, whereas it decreases when the current increases. Operating time of the fuses have different characteristic when compared to current, characterized as fast-blow or time-delay, according to the time respond to an over current condition. Normally fuse requires double the time of its rated current to blows in .1 sec, and a slot blow fuses are requires double of its rated current for 10 sec to blow.  

The selection of fuse may depend on the characteristics of load. Fast or ultra-fast fuses are used in semi conductor devices, because it heat rapidly when an over current flows. In most of the responsive electrical equipment requires fastest blowing fuses, since a short exposure to an overload current could damage the electrical machines highly. These kinds of fuses are used for general purpose. The slow blow fuse (time-delay fuse) are designed to permit the current through fuse above the rated value for a short interval without the blowing the fuse. These kind of fuses are use in motors, which can draw larger the rated currents quite a few seconds while coming up to its rated speed.

I2T Value:

The quantum of energy depleted by the fuse element to clear the electrical faults. This expression is normally used in short circuit conditions and the values are used to perform coordination studies in an electrical network. I2T parameters are provided by chart in manufacturer data sheets for every fuse. The fuse co-ordination operations with upstream or downstream devices, both melting I2t and clearing I2T are specified. The melting I2T is proportional to the sum of energy required to begin melting the fuse element. The clearing I2T is proportional to the overall energy permit through by the fuse once clearing a fault. The energy is primarily reliant on current and time for fuses as well as the available fault level and system voltage. Since the I2T rating of the fuse is proportional to the energy it allows through, it is a measure of the thermal damage and magnetic forces that will be produced by a fault.

Breaking Capacity:

The breaking capacity is the maximum current that can safely be interrupted by the fuse. Generally, this should be higher than the prospective short circuit current. Miniature fuses may have an interrupting rating only ten times of their rated current. Some fuses are designated HRC (High Rupture Capacity) and are regularly packed with sand or a related material. Fuses for small, low voltage, residential wiring systems are normally rated, to interrupt 10,000 amperes. Likewise for larger power system fuses have higher interrupt ratings, with some low voltage current limiting interrupting fuses rated for 30,000 amperes. Fuses for high voltage equipment up to 1, 15,000 volts are rated by the total apparent power of the fault level on the circuit.

Rated Voltage:

The fuse voltage rating must be greater than or equal to what would become an open circuit voltage. For instance, a glass tube fuse rated at 32V would not reliability interrupt current from a voltage source of 120V or 230V. If a 32volt fuse attempt to interrupt the 120V or 230V supply an arc may appear. Plasma inside the glass tube fuse may keep on conducting current until current ultimately so diminishes that plasma reverts to an insulating gas. Rated voltage should be larger than the maximum voltage source it would have to disconnect. Rated voltage remains same for any one fuse, still when related fuses are connected in series. Connecting fuses in series does not increase the rated voltage of the combination.

Voltage Drop:

A voltage drop across the fuse is frequently provided by its manufacturer. Resistance of the fuse element may vary when it becomes hot due to the dissipation of energy while conducting higher currents. This resulting voltage drop should be taken into account, predominantly when using a fuse in low voltage application. Voltage drop frequently is not important in more traditional wire type fuses, but can be significant in other technologies such as resettable fuse (PPTC) type fuses.

Voltage de rating:

Ambient temperature will change fuses operational parameters. A fuse rated 1Amps at 25oC may conduct up to 10% or 20% more current at 40oC and may open at 80% of its rated at 100oC. Operating values will vary with each fuse family and are provided in manufacturer data sheets.  

Fuse Materials:

Fuses are manufactured in different sizes and styles to provide in many applications, manufactured in standardized package layouts to make them easily interchangeable. Fuse bodies may be made of ceramic, glass, Plastic, fiber glass, Molded mica laminates or molded compressed fiber depending on application and voltage class.

Laws of Fuse:

It determines the current carrying capacity of a fuse wire. At stable condition the fuse carry usual current without rising its normal temperature to the melting limit. That means at this condition, heat generated due to current through fuse wire is equal to heat dissipated from it.
Heat generated     = I2
                             = I2ρ (l/a)
                             = 4I2Ρl / πd2
Since a= πd2/4
Where
           R - is the resistance of the fuse wire.
           ρ - is the resistivity,
           l  - is the length and
           a  - is the cross sectional area of fuse wire.
                             = I2K1(l/d2)  -------->1
Where K1 is a constant. Heat lost ∝ surface area of fuse wire ∝ πdl.
 
∴ Heat Loss = k2dl  ----------->2
 
Equating 1 & 2 we get
                             I2K1(l/d2) = k2dl
                             I2 = Kd3
 
Where K =K2/K1
                              I = Kd3/2
                              I = Kd1.5
 
This is known as fuse law.


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