Fuse And Its Characteristics

A fuse can be defined as a device that opens a circuit with a fusible part, which is heated and severed by current flowing through it. The fusible part of the fuse is called as the Element. When there is current flows in a fuse then heat is generated and due to this heat generation, the temperature of element rises. If the current is less or equal to its continuous rated value, then the steady-state temperature is such that the fuse does not melt and the circuit continues to be closed circuit.

If the current has a large magnitude, it will cause the fuse element to melt before the steady-state temperature conditions are achieved. After melting, an arc may be struck and it opens the circuit. The fault current will be finally interrupted when the arc is de-ionized or when there is no arc after few times. Thus, fuse operation involves two-stage i.e. one is melting and another one is a current interruption.

Location of Fuse In Distribution System

The above figure shows location of fuses in a distribution system. Each transformer and capacitor bank has fuse protection to selectively disconnect the device in case of a fault in the device. Transformer fuses can also provide overload protection. The sectionalizing fuses are used to divide the system into smaller sections which can be then isolated from the rest of the system.

The above figure shows the location of fuses in a distribution system. Each transformer and capacitor bank has fuse protection so as to make selectively and safely disconnect the device in case of a fault in the device. Transformer fuses can also be used for overload protection. Fuse can also be used by dividing them into the section which can be then isolated from the rest of the system.

For the fault F1 or F2 fuse A has the responsibility to operate. Thus, the customers connected to this line are affected by the fault. In the absence of fuse A, fuse B would have to be operated but due to fuse B more customers suffers an interruption in the service.

Fuse Characteristics

Fuses are characterized by thermal and interrupting characteristics. Thermal characteristic is natural and relate to the following:

Thermal Characteristics

As the current increases, the melting time reduces as the heat increases in the element due to the flow of hight magnitude of the current. It should be natural that when a larger magnitude of current flow then it leads to higher power dissipation ( I2R ) in the fuse and a hence faster rise in temperature of the element of the fuse. This indicates that the melting time of the fuse element should be inversely proportional to the magnitude of the square of the current. The relationship between the magnitude of the current that causes the melting of fuse element and the time needed for it to melt element of the fuse is given by the fuse’s melting time-current characteristics (TCC).

As the magnitude of the current increases, the melting time reduces. It should be obvious that a larger magnitude currents will lead to higher power dissipation ( I2R ) in the fuse and hence faster rise in temperature of the element. This would imply that the melting time of the fuse should be inversely proportional to the magnitude of the square of the current. The relationship between the magnitude of the current that causes melting and the time needed for it to melt is given by the fuse's melting time-current characteristics (TCC).

The current on the x-axis is the symmetrical current. It does not include dc offset current. Further, the fuse does not carry the initial current and the ambient temperature of the fuse element is between 200 C and 300C (IEEE Standard). Since the melting time varies in a range to the range of fuse as per the requirement, you can see the minimum melting time curve is plotted as shown in the above figure.

The fuse element is a main consequence of thermal effect. It does not depend upon mechanical forces or any stress developed, inertia, etc. Thus there is no limit on how short the melting time can be or how much time it take to melt. This extremely small melting (fast operation) of a fuse at very high currents tends to discriminate it from most other protective devices.

Interrupting Characteristics

Melting of a fusing element is not sufficient to interrupt the current supply in the circuit. Consequently, there is always some arcing before the current is interrupted in the circuit where it installed. During this period, the fuse must resist immediate transient voltage conditions and subsequent steady-state recovery voltage. The addition of melting time and this arcing overhead is called as the total clearing time.

Melting of a fusing element is not sufficient to interrupt the current. Consequently, there is always some period of arcing before the current is interrupted. During this period, the fuse must withstand any immediate transient voltage condition and subsequent steady-state recovery voltage. The addition of melting time and this arcing overhead gives the total clearing time.

The total clearing TCC curve shown in the above fig describes this information. For the mall value of currents melting time can be large and arcing time small because of lower stored energy \frac { 1 }{ 2 } LI^2 in induction circuit. However, for large currents, melting time is small but the arcing time is large. Hence, TCC for melting time and total clearing time separate as “I” increases.

Very Inverse Melting Characteristic

The rate of heat generated in the element is low due to small overcurrent and only slightly higher than the rate of dissipation. As a consequence temperature of the element increases gradually.

As the current increases, the melting time reduces at a rate that is more than regarded as an increased rate of heat generation (I2R). This is because, heat which is generated in reduced cross-section of the element, cannot be removed as fast as it is produced due to the low heat dissipation area. This gives fuse very inverse characteristics. At a very short period of melting time, no heat is lost from the smaller cross-section of the fuse element.

Voltage Rating

However, the fuse has a maximum rated voltage. It is the highest voltage at which fuse is designed to operate and a fuse should not interrupt current above this voltage. Faults can be line to ground or line to line or any other short-circuit fault. When applied phase to ground on three-phase systems, the voltage rating of the fuse should equal or greater than the phase to ground system voltage. When applied in the line on the same system, the stable or protective approach is to choose the fuse voltage to be equal to the system phase to phase voltage. Sometimes, for a fuse both maximum and minimum interrupting currents are specified.

Leave a Comment