An electrical fuse is the simplest current interrupting device for protection against excessive current.
As explained earlier in our previous article protective devices, an electrical installation must be safeguarded against the harmful effects of excessive current. This excessive current may be caused by overloading or short circuit faults. A protective device such as a fuse is necessary.
A high electric current leads to an excessive heat rise which, if near to inflammable material will almost certainly cause an outbreak of fire.
In all such cases, therefore it is necessary to interrupt these excessive currents before they cause any damage. This is where fuses, circuit breakers and other forms of protection come into use.
What is an electrical Fuse?
The thin layer in the fuse that melt and open the electrical circuit path under abnormal condition is called the fuse element. The rating or the capacity of the fuse depends on the type and size of the fuse element used.
Electrical fuses operate based on the heating effect (I2*R) of electric current. That is the square of the amount of current I in amperes flowing through the resistance R of the fuse element measured in ohms. This yields power loss in a form of heat which in turn causes the fuse element to melt. One common operation attribute of fuses is that they are normally wired in series with the circuit to be projected.
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The material used for the Electrical Fuse Element
Different types of metals can be used to manufacture fuse elements for different applications. However, metals with low melting points like tin, lead, zinc or lead-tin alloys are found more suitable for this purpose. Normally, lead-tin alloy wire is used below 10 amperes while the copper wire is most suitable for higher currents. The present trend, however, is to use silver for higher currents despite its higher cost as it is comparatively free from oxidation.
Types of Electrical Fuses.
In practice, there are three main types of electrical fuses. These are; the rewirable fuse, the cartridge fuse and the high – rupturing – capacity (h.r.c.) fuse.
The rewirable Electrical fuse
The rewirable electrical fuse sometimes called a semi-enclosed fuse is a simple and comparatively cheap system of over-current protective devices. The fuse element consists of a short length of fuse wire of diameter depending upon the current rating of the circuit the fuse is protecting.
The wire is threaded through a small hole in the porcelain or ceramic fuse carrier (bridge) and secured to the contacts (terminals) using screws. It also has a separate holder called the fuse base which holds the fuse carrier.
The incoming and outgoing live or phase wires are also connected permanently with the help of connecting terminals to the base. After blowing off the element during a fault condition, the carrier can be pulled out and again rewired with a new fuse wire. Thus, restoring service very quickly with negligible additional expenditure.
Applications of rewirable fuse
Commonly used in domestic installations and some circuits where very low fault currents are to be handled.
- They are relatively cheap
- They are easy to repair
- Electrical service to consumers can be restored very quickly.
- Cannot be used for high fault current. This is because the fuse bridge and base can easily get damaged under very heavy to short-circuit conditions.
- Protection is not reliable. This point is proven by the fact that the fuse element can easily be rewired with an incorrect gauge, especially by inexperienced people.
- Since the wire is exposed to air, it is subjected to deterioration due to oxidation caused by heating.
- Relatively slow in interrupting fault current (slow speed of operation).
- Risk of fire – hazards due to external flash on blowing.
- It cannot discriminate between sudden high current (as in the case of starting heavy motors) and a continuous faulty condition.
The cartridge Electrical Fuse
With the increase in fault current level, the fuse clearing the fault would be called upon to withstand extremely heavy stresses in the process. A rewirable fuse would not be able to withstand these stresses and would probably disintegrate violently. This and perhaps all the disadvantages of using rewirable fuse led to the development of the cartridge fuse.
The cartridge fuse consists of a porcelain carrier tube with metal caps to which the fuse element is attached internally. The porcelain tube is filled with silicon and oxygen (silica). Since the fuse element is enclosed, the entire fuse enclosure has to be replaced with one of the same rating (as determined by the manufacturer) when it blows. They are mostly found in modern electrical plug tops used with 13A socket outlets and inside consumer electronic machines.
They are gradually replacing the rewirable fuse in modern domestic installations. Especially power intake from the regular 13A socket. They are also used as the protective device for many modern electrical appliances such as electric iron, electric cooker, microwave oven etc.
- Being closed, there is no or less prone to deterioration of the electrical fuse element
- Protection is reliable because the blown fuse is likely to be replaced by the same rating.
- High-speed operation. They remove fault current faster
- Ability to interrupt high fault current.
- Interrupt silently and without flame, gases or smoke. Hence safe from fire hazards.
- This fuse is not recommended for use in places prone to electrical faults.
- It is expensive to replace as compared to the rewirable fuse.
- Delayed in the restoration of service if there is no available cartridge at the time of interruption.
- Overheating of the adjacent contacts is possible during the operation of the fuse.
High Rupturing Capacity (H.R.C)
Sometimes called the enclosed fuse, the high rupturing capacity is a type of cartridge fuse, specially designed for extremely rapid operation with heavy electrical loads. High rupturing capacity fuses were introduced as a result of the increased number of electrical installation networks which also increased the electricity demand.
They are particularly used in industrial installations. They are also frequently used in low voltage distribution systems.
Advantage and Disadvantage
As compared to the rewirable electrical fuse, the high rupturing capacity fuse has the same advantages and disadvantages as a standard cartridge fuse in addition to exhibiting the capacity to withstand heavy fault currents.