The operation and construction of fuses

Switch Gear & protection
Chapter-07
(The operation construction of Fuses)
    -Asiqur Rahman Milon,EETE-DIU,EEE-IEB(Con)

# What is fuse?
- Electric fuse an electric fuse is a device that is used to protect electric circuits and electric appliances against high current caused by short - circuiting or overloading due to withdrawal of large current. A fuse is a short piece of wire made of a material of high resistance and low melting point. The fuse wire is an alloy of lead and tin. The two types of fuse is that are most commonly used are plug fuse and cartridge fuse. Plug fuse The plug fuse consists of two parts namely the porcelain casing and the porcelain grip. Porcelain casing It consists of hollow rectangular block of porcelain in which there are two rectangular brass terminal T1 and T2. The casing is fixed to a wooden board and the live wire is connected to the terminal T1 and there is no direct connection between the T1 and T2 and hence no current will flow from T1 to T2. Porcelain grip It consists of solid porcelain block. Two rectangular clamp made of copper are fixed on the porcelain block as shown here. These clamps can fit tightly on the terminals T1 and T2. The fused wire is fixed in between the copper clamps. When the grip is inserted into the casing the current flow from the terminal T1 to terminal T2 through the fuse wire. The fuse wire that completes the circuit melts. If the circuit gets overloaded or short circuited, this stops the flow of current in the electric circuit by inserting a new fuse wire in the fuse griped the circuits gets completed. Cartridge fuse It consists of two metallic terminals fixed to a holder. A thin length of fused wire is stretched between these terminals. This type of fuse is used to protect the electrical appliances that are highly sensitive to voltage fluctuations. If by any chance a high current passes through the electric appliance the fuse wire melts. Hence, an electric fuse is described as a safety device.

Electrical Fuse
To understand the working principle behind a fuse, two critical concepts should be kept in mind
1.    Current flows in a loop
2.    Heating effect of current
Electric current can flow through a conductor only when the circuit formed is complete. If there is a break in the loop, electric charges cannot flow through. This is also how switches operate. For example, when you put on the light switches at home, the lights come on because you have just completed the circuit allowing charges from the power source to flow through and power your lights.
When current passes through a conductor, the different electrical components of the circuit like the devices attached or even the wire itself, offer resistance to the current flow. The work done to overcome this resistance presents itself in the form of heat. This is a simple explanation of the “heating effect” of current.
Electrical Fuse
Working Principle of Electrical Fuse
The primary use of an electric fuse is to protect electrical equipment from excessive current and to prevent short circuits or mismatched loads. Electrical fuses play the role of miniature circuit breakers. Apart from protecting equipment, they are also used as safety measures to prevent any safety hazards to humans.
The fuse wire in an electrical fuse is selected in such a way that it does not face any damage when the normally stipulated amount of current flows through the circuit. Under normal conditions, the fuse wire is a part of the circuitry, contributing to a complete loop for charges to flow through it. However, when an excessive amount of current flows through the fuse wire, the heating effect of current causes the fuse wire to melt. This is because the fuse wire is chosen such that it has a low melting point. This causes the loop to break thereby stopping the flow of charges in the circuit.
It is important to select a fuse that is properly specified for the circuit in consideration. For example, if the fuse that is used is underrated, then it will fail even under normal current conditions, unnecessarily breaking the circuit loop. If it is overrated, then it will not break the circuit when required and cause equipment damage and failure and may even present itself as a safety hazard.

#What is the advantages and dis advantages of uses fuse?
-Advatages-
1) Fuses are cheap.
2 ) Most fuses are self protecting and they extinguishing the arc.
3 ) The fuse element is change easily.
4) Fuse need zero maintenance.
5) Operation of fuse is simple and no complexity is involved
6)  Fuse has the ability to interrupt enormous short circuit current without producing noise, flame, gas or smoke
7)The operation time of fuse can be made much smaller than operation of circuit breaker. It is the primary protection device against short circuits
8) It affords current limiting effect under short-circuit conditions
9) Fuse inverse time current characteristic has the ability to use for over-load protection

Disadvantages-
1) Low breaking capacity
2) Protection of fuse is not reliable.
3)Slow speed
4) It’s not suitable for overload.
5) It’s can’t bear the surge current in case of motor starting.
6) During short circuit or overload once fuse blows off replacing of fuse takes time. During this period the circuit lost power
7)  When fuses are connected in series it is difficult to discriminate the fuse unless the fuse has significant size difference

#Where is the application of uses?

-Uses and Applications of Fuses

Electronic Fuses can be used in all types of electrical and electronic applications including:
·       Motors
·       Air-conditions
·       Home distribution boards
·       General electrical appliances and devices
·       Laptops
·       Cell phones
·       Game systems
·       Printers
·       Digital cameras
·       DVD players
·       Portable Electronics
·       LCD monitors
·       Scanners
·       Battery packs
·       Hard disk drives
·       Power convertors


#What are the types of Fuses?

-Types of Fuses



The fuse is the current interrupting devices which break or open the circuit by fusing the element and thus remove the faulty device from the main supply circuit. The fuses are mainly classified into two types, depends on the input supply voltages they are the AC fuses and the DC fuses. The different types of the fuses are shown in the image below.

DC Fuse

The DC fuse opens or breaks the circuit when the excessive current flow through it. The only difficulty with the DC fuse is that the arc produced by the direct current is very difficult to extinct because there are no zero current flows in the circuit. For reducing the DC fuse arcing the electrodes are placed more distance apart due to which the size of the fuse increases as compared to AC fuse.

AC Fuses

The AC fuses are categorised into two types they are the low voltage fuses and the high voltage fuses. The frequency of the AC fuses changes it amplitude from 0º to 60º in very one second. Thus, the arc extinction in the AC circuit can be done easily as compared to the DC circuit.
types-fuses-1
The low voltage fuses can be further divided into four classes shown below in the image Semi-enclosed or rewirable type and totally enclosed, or cartridge type switches are the most commonly used switches.
types-of-fuses-2

Rewirable Fuses

This type of circuit is mostly used in the small current circuit or for domestic wiring. The fuse case and the fuse carrier are the two main parts of the rewirable fuse. The base of the fuse is made up of porcelain, and it holds the wires which may be made up of lead, tinned copper, aluminium or alloy of tin-lead. The fuse carrier can be easily inserted or taken out in the base without opening the main switch.

Totally Enclosed or Cartridge Type Fuses

The fuse element is totally enclosed in an enclosed container, and it has metal contacts on both sides. These fuses are further classified as D-type cartridge fuses and the Link type cartridge fuses.
types-of-fuses-3
D-Type Cartridge Fuses
The main parts of the D-type fuse are the base, adapter ring, cartridge and a fuse cap. The cartridge is kept in the fuse cap, and the fuse cap is fixed to the fuse base. The cartridge tip touches the conductor when it is completely screwed to the base and thus completes the circuit through the fuse links.
Link Type Cartridge or High Rupturing Capacity
In such type of fuses, the fuse element carries the fault current for a long duration. If the fault is not clear, then the fuse element will melt and open the circuit. The major advantage of HRC fuse is that it clears the low as well as a high fault current.
HRC fuse has the high-speed operation and also does not require maintenance. But the fuse element of the HRC fuses needs to be replaced after each operation, and it also produced the heat during the faults which will affect the operations of the nearby switches.
The enclosure of the HRC fuse is filled with powdered pure quartz, which acts as an arc extinction medium. The silver and copper wire is used for making the fuse wire. The fuse wire has two or more sections which are joint by using tin-joint. The tin-joint reduces the temperature under overloaded condition.
For increasing the breaking capacity of the fuses two or more silver wire is joined in parallel with each other. These wires are adjusted in such a way so that only one wire will melt at a time. The HRC fuse is of two types
types-of-fuses-4
In knife blade type switches the fuse wire is replaced with a live circuit with the help of fuse puller.The bolted type HRC fuses have two conducting plates which are bolted to the fuse base. This fuse requires the additional circuit for taking out the switch without getting a shock.

Dropout Fuse

The melting of fuse causes the fuse element to drop out under gravity about its lower support. Such type of fuse is used for the protection of outdoor transformers.

Striker Fuse

It is a mechanical device having enough force and displacement which can be used for closing tripping/indicator circuits.

Switch Fuse

Such type of switches is used for low and medium voltages circuit. The rating of the fuse unit is in the range of 30, 60, 100, 200, 400, 600, and 800 amperes. The fuse unit is available as 3-pole and 4-pole unit. The making capacity of such type of fuses is up to 46 kA. They can safely break depending upon rating currents of the order of 3 times the load current.

High Voltage HRC Fuses

The main problem of the high voltage fuses is that of the corona. Therefore the high voltage fuses have the special design. They are mainly classified into three types.
types-of-fuses-5

Cartridge Type HV HRC Fuse

The fuse element of the HRC fuse is wound in the shape of the helix which avoids the corona effect at the higher voltages. It has two fused elements placed parallel with each other, one of low resistance and the other is of high resistance. The low resistance wire carries the normal current which is blown out and reducing the short circuit current during the fault condition.

Liquid Type HV HRC Fuse

Such type of fuses is filled with carbon tetrachloride and sealed at both the ends of the caps. When the fault occurs then the current, exceed beyond the permissible limit, and the fuse element is blown out. The liquid of the fuse acts as an arc extinguishing medium for the HRC fuses.They may be employed for the transformer protection and the backup protection to the circuit breaker.

Expulsion Type HV Fuse

Expulsion type fuses are widely used for the protection of feeders and transformer because of their low cost. It is developed for 11kV, and their rupturing capacity is up to 250 MVA. Such type of fuses comprises a hollow open-ended tube made of synthetic resin-bonded paper.
The fuse elements are placed in the tubes, and the ends of the tubes are connected to suitable fittings at each end. The arc producing is blown off in the inner coating of the tube, and the gases thus formed extinguish the arc.
#What is Fusing current?
-Fusing current is the current required to melt the wire which is inside the fuse. ... When current flows more than its rating wires of fuse melts and it causes currentinterruption. The minimum value of current which cause to melt fuse is calledfusing current.
#What is fusing factor?
-A "fusing factor" is simply a slang term (which has it's origin in the days of fuses) for the multiple of In at which disconnection will occur in conventional time. Every device has a fusing factor, but for common devices this will be 1.45 x In.
Fusing factor=Fusing current/current rating
#What is cut off current?
-Cut off current is short circuit current or fault current highest point reach before fuse cut connection it’s called cut off current.
#



Characteristics of a Fuse

There are different types of Fuses available in the market for different types of applications like residential, industrial, automotive, etc. All the fuses are often characterized by the following characteristics.
·       Current or Ampere Rating
·       Melting Time
·       Voltage Rating and
·       Interrupting Rating or Breaking capacity
·       I2T Value of the Fuse
·       Packaging
·       Temperature

Fuse Operation and Principle

Fuse is a fundamental component used to protect electronic and electrical circuits from excessive current or short circuit condition. Install the fuse in series to the circuit that you want to protect such as in Figure 2a. If the subject circuit has several branches (different current paths), be sure to connect the fuse in the section where the sum of all the currents is flowing as depicted by Figure 2b. Fuse must protect the circuit under abnormal condition by quickly opening the circuit. This is the ultimate purpose of the fuse that should not be defeated, thus it is very important to select a correct fuse rating.



Fuse Operation and PrincipleFigure 2 – a) Fuse location with circuit that has one current path. b) Location of the fuse for several current paths. A fuse is can be inserted to any branch in Figure 2b as well for the reason to protect the devices on the specific branches.


With the location of the fuse depicted in Figure 2b, the circuit total current is surely covered. In the event of short circuit or abnormal increase in circuit current, the fuse will quickly open and the large current cannot flow anymore to the circuit. When a fuse blows, do not replace the fuse with another rating or value instead maintains the rating as this is practically designed for the circuit. Replacing the fuse rating to a higher one will put the circuit in danger since it will not blow on a specified current and time. On the other hand, if a fuse rating has been replaced by a smaller one, the circuit will keep on opening even the current is not yet reaching the specified trip level. You can also install fuse to any branch in Figure 2b as needed. Be sure to realize the purpose of the fuse.
There are some important parameters to consider in selecting a fuse. These are the Current Rating, Ampere Square Seconds, Breaking Capacity and Voltage Rating. More information on these parameters will be discussed below, just keep reading.
A useful design practice in selecting a fuse which is also recommended by fuse suppliers is to consider the 75% factor. A 75% factor means that the continuous circuit current should only be 75% of the fuse continuous current rating. The purpose of this is to compensate the environment temperature effect since at high ambient temperature a fuse breaking point will decrease. For instance, a 10 ampere total circuit current should use a 13 amperes fuse. However, the designer should ensure that the circuit can withstand the 13 amperes of current in a short duration until the fuse open up.


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