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.
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
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-
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
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.
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.
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.
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
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.
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.
Figure 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.
0 Comments