Fuse is a circuit interruption device. It is the simplest device for the interruption of circuits and it is installed in series with the circuit which is needed to be interrupted. It is a short piece of metal which melts when an excess amount of current is passed through it. The operating time of fuse depends on the amount of current passed through it. It follows inverse time-current characteristics which means, the higher the current the lower the time of operation. Hence it can be used for over current protection.
Fuse is the cheapest of all circuit interruption devices and requires no maintenance. Its operation time is even less than a Circuit Breaker.
Fuse is just a piece of metal. The selection of this metal depends on its usage. Commonly, Copper, Tin, Silver, lead and alloys of these metals are used for fuse element. The fuse element should have low melting point and high conductivity. Low melting point ensures the interruption of circuit in short time. When excess amount of current is passed through the fuse element, the temperature of fuse rises and due to Low melting point, it melts and interrupts the circuit quickly. High conductivity is necessary because fuse element has to carry the load current at normal condition. Therefore, it should not cause any hindrance to the normal current.
The coefficient of expansion is also a main concern. The metals which have small coefficient of expansion doesn’t expand under normal condition which increases the life of fuse. Oxidation of the metal causes deterioration of the metal. Therefore, fuse element should be less prone to it. If oxidation occurs, the life and performance of fuse is affected.
Silver is the best metal which has very less deterioration rate and its conductivity is good as well. The main problem is the cost. Cost of silver is comparatively very higher than other fuse element materials. But it is still used for suitable applications. Copper can undergo oxidation but it can be tinned to protect it from oxidation. Tin and Lead are used for low current ratings while copper and silver are used for larger currents.
General Characteristics of a Fuse:
Fuses are distinguished on the basis of its characteristics. There are many types of fuses available and all of them possess certain characteristics. These characteristics does not depend on fault current magnitude. The main characteristics are given below,
- Current Rating of Fuse element
- Fusing Current
- Fusing Factor
- Breaking Capacity
- Melting Integral
Current Rating of Fuse element:
The current which can be passed by the fuse without heating of the fuse element is referred as Current Rating. It is written on the fuse. If you visit an electric store and ask for a 10A fuse, then this 10A is the Current Rating. It means fuse can easily pass 10A just like a conductor without behaving abnormally.
It is the minimum current at which fuse operates. At this current the fusing element melts due to overheating. It is always greater than the Rated Current of Fuse. For instance, we have a fuse of rating 10A, it means there will be no overheating at 10A and its fusing element will melt if the current is 11A or above. Here, 11A is the Fusing Current.
The ratio of Fusing Current and Current rating of fuse element is known as Fusing Factor. It value is always greater than 1.
If the value of fusing factor is near to one, then the fuse will allow little overloading. Let’s say we have a fuse of 10A Current Rating and 12A Fusing Current. The fusing factor will be 1.2 which is near to 1. It will allow only 2A of overloading. The negative impact of this will be deterioration of fuse element as there is a little difference between Current Rating and Fusing Current. The fuse element can undergo heating at nominal current as well.
The r.m.s. value of maximum fault current (when passed through conductor) which can be handled by the fuse is known as Breaking Capacity. Suppose the rms value of a fault is 1000A. The fuse will operate before it reaches to 1000A. It means that the fuse will safely operate at this fault. If the fuse cannot handle the fault current above 1000A then it will be the maximum fault current which can be handled by the fuse. Hence we can say that 1000A is the Breaking Capacity of the fuse.
The point to be noted here is that the Breaking Capacity is defined as a particular voltage level.
The Energy required to melt the fuse is referred as Melting Integral. This energy depends on the square of the current and time. It is often called as I2t and its value depends on the fuse element. The melting of fuse depends on the peak of current and the time for which it occurs. Both of these together will decide whether the fuse will melt or not. It is defined for every fuse. If the fuse Melting integral is attained, then the fuse will melt.
Characteristics of Fuse dependent on Fault Current:
There are some characteristics of fuse which depends on the magnitude of fault current. These characteristics are given below,
- Prospective Current
- Cut-off Current
- Pre-arcing Time
- Arcing Time
- Operating Time
Before discussing the above topic, have a look on the image below,
I have considered a fault current and the fuse has interrupted the circuit safely.
The r.m.s. value of the first loop (from the occurrence of fault till the first zero crossing) of fault current obtained when the fault current is passed through a conductor without hindrance is known as Prospective Current. Generally, at starting the fault current is quite severe and its magnitude decreases with time. Therefore, the impact of fault is maximum soon after its occurrence. Hence Prospective Current is obtained from the first loop.
It is the current at which the fuse element melts. In the figure above the Cut-off current is the value of fault current at ‘b’. Fuse operates before the peak of fault current is achieved which saves the circuit from experiencing the fault current.
The arcing starts when the fuse melts. The time from the occurrence of fault (point ‘a’ in figure) to the melting of fuse (the instant where the Cut-off Current is obtained which is point ‘b’ in the figure) is known as Pre-arcing Time.
The time from the melting of fuse (point ‘b’ in the figure) till the extinction of arc (point ‘c’ in the figure) or the complete interruption of circuit is called Arcing Time. This time depends on the arc sustainability. If there is filling powder such as Chalk, dust and quartz then the arc extinguishes rapidly.
The sum of Pre-arcing Time and Arcing time is known as Operating Time. It is the time from the occurrence of fault, till the complete interruption of fault.
There are some factors which should be considered while selecting and operating the fuse.
- Ambient Temperature
- Voltage Rating
- Normal Operating Current or Rerating
- Pulses or Surge Currents
The temperature of air which is surrounded by the fuse is known as Ambient Temperature. The room temperature is not always the ambient temperature of the fuse. Fuse is normally located near devices which can produces heat due to losses, such as Transformer. So the ambient temperature is the temperature of air which is surrounding the fuse.
Ambient Temperature should be considered because fuse is a temperature sensitive device. If the temperature is higher than the test condition temperature of the fuse, then the at Rated Current fuse can undergo overheating which decreases the life of fuse.
Fuses are rated for particular voltage level. The fuse can ensure the safety of such circuits which operate below and near the Rated Voltage of Fuse. The common rated voltages of fuse are 32V, 63V, 125V, 250V and 600V or 220, 380 and 500 for Low Voltage fuses and 3.6kV, 7.2kV, 12kV, 24kV and 36kV for High Voltage fuses. It is also the main concern whether the fuse is for AC or DC.
Normal Operating Current or Rerating:
Fuse manufacturers recommend to use fuse at 25% less than its Current Rating (at 25˚C ambient temperature) due to errors. Therefore, the Normal Operating Current is 75% of the Current Rating. If fuse is operated at 100% then its life can be affected.
Surge Current (Pulse Current):
Pulses generally means transients. Due to transients such as inrush current and start-up current, there is a deviation of current wave as compared to its steady state. Pulses are important for fuses because for every application there are different kind of pulses. Fuse is to be selected on the basis of surges or pulses it can bear before its opening which depends on the Melting Integral of the fuse. For instance, the fuse element will melt if the surge has enough current for specific time. If it the value of current is high but for less time or vice versa then the fuse will not blow, it will withstand the pulse. Consider the following picture,
We have a pulse of rectangular shape, then its Melting Integral of Pulse can be given as,
If the Melting Integral of the pulse is greater or equal to the Fuse Melting Integral, then the fuse will melt.
Relation between Fusing Current and Diameter of Fusing Element:
The flow of current depends on the diameter (or cross-sectional area) of the wire. The wider the wire, the less hindrance will be provided to current. Fuse Element is a very thin as compared to the conductor used in circuit.
As current is directly proportional to diameter,
k is the fuse constant which is defined for every fusing element and I is the Fusing Current.