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SHORT CIRCUIT STUDY IN ELECTRICAL TECHNOLOGY
A Technical seminar Report Submitted in partial fulfillment of the requirement for the degree of
BACHELOR OF TECHNOLOGYIn
ELETRICAL AND ELECTRONICS ENGINEERING
BY
K.V.V.S.CHAKRADHAR (06J21A0241)
Under the esteemed guidance ofCH.VENKAT, B.TECH
Asst.prof,EEE- Dept.JBREC.
Department of Electrical and Electronics Engineering
JOGINPALLY B.R. ENGINEERING COLLEGE
Yenkapally, Moinabad Mandal, R.R.District
Affiliated to J.N.T. University, Hyderabad
1
JOGINPALLY B.R. ENGINEERING COLLEGE
Yenkapally, Moinabad Mandal, R.R.DistrictAffiliated to J.N.T. University, Hyderabad
BONAFIDE CERTIFICATE
This is to certify that the Technical seminar report
entitled “SHORT CIRCUIT STUDY IN
ELECTRICALTECHNOLOGY” is being submitted by
K.V.V.S.CHAKRADHAR (06J21A0241), in technical
fulfillment for the award of the degree of Bachelor of
Technology in Electrical and Electronics Engineering to
the Jawaharlal Nehuru technological university, as a
record of bonafide work carried out by him under my
guidance and supervision. The result embodied in the
Technical seminar report has not been submitted to any
other University or Institute for the award of any degree.
PROJECT GUIDE HEAD OF THE DEPATRMENT
CH.VENKAT M.Kondalu,M.Tech,(Ph.D),Asst.prof, EEE-Dept.JBREC HOD, EEE, JBREC
EXTERNAL EXAMINER
2
ACKNOWLEDGEMENT
We express our profound sense of gratitude for the administration
of JOGINPALLY B R ENGINEERING COLLEGE for giving
us an opportunity to take up the TECHNICAL SEMINAR work in their
organization
We express our great pleasure to have opportunity to take up the
TECHNICAL SEMINAR work under the guidance of D.N.RAO M.E,
Ph.D PRINCIPAL of JOGINPALLY B R ENGINEERING
COLLEGE whose invariable references, suggestions, and
encouragement have immensely helped us in the successful completion of
this TECHNICAL SEMINAR.
We express our sincere thanks and gratitude to Mr.
KONDALU M.Tech (Ph.D) Associate Professor and HEAD OF
THE DEPARTMENT of ELECTRICAL AND
ELECTRONICS ENGINEERING for valuable help and
encouragement throughout the TECHNICAL SEMINAR work.
We are very much thankful to Mr.CH.VENKAT, B.TECH
EEE DEPARTMENT for his excellent guidance AND
encouragement throughout the TECHNICAL SEMINAR work.
3
We thank all of teaching and non-teaching staff members of EEE
department for their extended cooperation.
INDEX
CONTENT : Pg.No
1. ABSTRACT……………………………………………..5
2. INTRODUCTION………………………………………7
3. WHAT IS SHORT CIRCUIT?........................................8
4. SHORT CIRCUIT TYPES …………………………….9
5. REASONS OF SHORT CIRCUIT…………………….13
6. EFFECTS OF SHORT CIRCUIT…………………….17
7. PREVENTION METHODS ………………………….19
8. ADVANTAGES OF SHORT CIRCUIT STUDY…….27
9. CONCLUSION………………………………………..29
4
ABSTRACT
Short circuit study in electrical technology
Short Circuit (Fault Current) studies are required to insure
that existing and new equipment ratings are adequate to
withstand the available short circuit energy available at
each point in the electrical system. Fault currents that
exceed equipment ratings are capable of extensive
equipment damage and are a serious threat to human life
On large systems, short circuit studies are required to
determine both the switchgear ratings and the relay
settings. No substation equipment, motor control centers,
breaker panels, etc. can be purchased without knowledge
of the complete short circuit values for the entire power
distribution system.
The short circuit calculations must be maintained and
periodically updated to protect the equipment and the
lives. It is not necessarily safe to assume that new
equipment is properly rated.Fires from electrical cords or
from wiring devices are increasing in these years. The
reason is supposed to be the increase of electrical power
dissipation for domestic use.
DELIVERABLES OF SHORT CIRCUIT STUDY
A typical short circuit study includes:
5
Short circuit calculations, which highlights any
equipment that is ascertained to be underrated as
specified
Suggested modifications to rectify the underrated
equipment; (trip sizes within the same frame, the
time curve characteristics of induction relays, CT
ranges, etc.).
The Protective Device Setting and Coordination Study is
the suggested follow on analysis to develop the
coordination curves, highlighting areas lacking
coordination. Presentation of a protective device study
would include a technical evaluation with a discussion of
the logical compromises for best coordination
The building/facility may not be properly protected
against short-circuit currents. These currents can damage
or deteriorate equipment. Improperly protected short-
circuit currents can injure or kill maintenance personnel.
Recently new initiatives have been taken to require
facilities to properly identify these dangerous points
within the power distribution of the facility.This is the
main reason for requirement of study about short circuit.
6
INTRODUCTION
Fires from electrical cords or from wiring devices are
increasing in these years. The reason is supposed to be the
increase of electrical power dissipation for domestic use.
A circuit breaker is used to protect the circuit from
overcurrent or short-circuit. If the conductors do not touch
directly and they are shorten by an arcing along the
carbonised insulating material, the current flows
intermittently.
This is the reason why a circuit breaker does not cut off
the current more than 100 Amps and fire hazardous
sparking continues.
Peak value of the short-circuit current is limited by the
resistance of the circuit. In typical conditions the circuit
breaker does not cut off the intermittent current under
about 200 Amps in peak value. This condition is easily
7
made when an extending cord of about 10 meters long is
used.
India has approximately 60000 fires a year. The number
of fires related to electrical cause or electrical appliances
are shown in table 1. Although insulating materials have
been increased their quality, the number of electrical fires
have not been decreased.
Table 1: Loss of lives in Andhra Pradesh due to fires
Year No.of fire accidents No.of lives lost
1996 12741 79
1997 13569 184
1998 12459 58
1999 14456 81
2000 16987 123
2001 12584 58
2002 12999 78
2003 18456 156
2004 16271 249
8
2005 15631 183
2006 17452 129
2007 18975 149
Short circuit arcing was made intentionally in this study.
Although insulating materials have been increased their
quality, fires are still caused by these failures. It is
necessary to doubt about their safety and to make a study
on the mechanism of the beginning of short circuit arcing
in order to prevent these fires.
A short circuit (sometimes abbreviated to short or s/c) in
an electrical circuit is one that allows a current to travel
along a different path from the one originally intended.
The electrical opposite of a short circuit is an "open
circuit", which is an infinite resistance between two nodes.
It is common to misuse "short circuit" to describe any
electrical malfunction, regardless of the actual problem.
What is short circuit?
A short circuit is a fault. It means there is a very low
resistance conducting path from one side of a component
to the other. For example a wire might have come loose
which connects two sides of a circuit together. Or perhaps
there's some moisture on the surface of a component that
means current can bypass it.
The wire or the moisture 'shorts' the circuit because the
length of the conducting path back to the battery has
decreased.
9
Simple activity showing the effect of shorting out a bulb.
A short makes the circuit behave as if the component
wasn't there. The component stops working (it isn't there,
after all) and the current everywhere in that circuit will
increase, which can damage other components or, in
extreme cases, cause a fire.
So how can we explain shorts? A very misleading way of
explaining them is to say that current takes the easiest path
Shorting out a single component
which is in series with others
When you connect a wire across the terminals of a bulb
you're effectively creating a little parallel circuit. The
thing with parallel circuits is that the effective resistance is
less than the smallest resistance. In this case the smallest
resistance is just the wire, and this has a very low
resistance indeed.
10
Simulation A full explanation of what happens when a
bulb is shorted out.Now you've reduced the resistance of
the series circuit and so the current everywhere increases.
The current through the unshorted bulb increases and so it
gets brighter. But brightness isn't just a function of current.
You have a low resistance (the shorted bulb) in series with
a higher resistance (the unshorted bulb) and this changes
the way the voltage is shared around the circuit. The
bigger resistance takes a bigger share of the total voltage.
So the second bulb is bright for these two reasons, bigger
current through it AND bigger voltage across it.
Remember that voltage and current are connected. The
current through the second bulb can only increase because
the voltage across it is bigger.
In the same way the shorted bulb has a very low voltage
across it so the current through it is very small and that's
why it's out. The wire doing the shorting has the same
voltage across it as the bulb but it also has a very low
resistance so the current through the wire is big. The
current through the wire and the current through the bulb
add up to the current through the unshorted bulb.
Shorting out a whole circuit
This is exactly the same as saying that the power supply is
shorted out. In this case the explanation of why the bulb
goes out is slightly different
11
Animation showing different ways of shorting out a whole
circuit by connecting one terminal of a battery directly to
the other.
Again we've introduced a parallel circuit and the parallel
circuit has an effective resistance of a little bit less than
the wire. This means that there isn't really any resistance
anywhere in the circuit and so the current supplied by the
battery becomes very big. So the battery has to work very
hard.
When you work very hard you sweat a lot and this is
similar to what the battery does. The chemical reactions
in the battery take place very rapidly and lots of the energy
released gets turned straight into heat rather than being
given to the charges in the circuit. This means the voltage
is a lot less than it should be. The voltage across the
components is very low and so none of them work. This
type of short can cause a battery to get very hot. It may
even explode!
How does a short circuit happen?
When the cables of the electrical appliances are worn out
or it's not connected properly a short circuit may occur. A
short circuit has a very low resistance that almost all
electric current flow through it. It'll affect the operation of
12
the electrical appliances. Owing to the heating effect the
excess electric current would produce a large amount of
heat without a fuse or a circuit breaker a fire may be
occured when there's a short circuit.
A short circuit is an abnormal low-resistance connection
between two nodes of an electrical circuit that are meant to
be at different voltages. This results in an excessive
electric current (overcurrent) limited only by the Thevenin
equivalent resistance of the rest of the network and
potentially causes circuit damage, overheating, fire or
explosion. Although usually the result of a fault, there are
cases where short circuits are caused intentionally, for
example, for the purpose of voltage-sensing crowbar
circuit protectors.
In circuit analysis, the term short circuit is used by
analogy to designate a zero-impedance connection
between two nodes. This forces the two nodes to be at the
same voltage. In an ideal short circuit, this means there is
no resistance and no voltage drop across the short. In
simple circuit analysis, wires are considered to be shorts.
In real circuits, the result is a connection of nearly zero
impedance, and almost no resistance. In such a case, the
current drawn is limited by the rest of the circuit.
Examples
An easy way to create a short circuit is to connect the
positive and negative terminals of a battery together with a
low-resistance conductor, like a wire. With low resistance
in the connection, a high current exists, causing the cell to
deliver a large amount of energy in a short time.
13
A large current through a battery can cause the rapid
buildup of heat, potentially resulting in an explosion or the
release of hydrogen gas and electrolyte, which can burn
tissue and may be either an acid or a base. Overloaded
wires can also overheat, sometimes causing damage to the
wire's insulation, or a fire. High current conditions may
also occur with electric motor loads under stalled
conditions, such as when the impeller of an electrically
driven pump is jammed by debris; this is not a short,
though it may have some similar effects.
In electrical devices, unintentional short circuits are
usually caused when a wire's insulation breaks down, or
when another conducting material is introduced, allowing
charge to flow along a different path than the one
intended.
In mains circuits, short circuits may occur between two
phases, between a phase and neutral or between a phase
and earth (ground). Such short circuits are likely to result
in a very high current and therefore quickly trigger an
overcurrent protection device. However, it is possible for
short circuits to arise between neutral and earth
conductors, and between two conductors of the same
phase. Such short circuits can be dangerous, particularly as
they may not immediately result in a large current and are
therefore less likely to be detected. Possible effects
include unexpected energisation of a circuit presumed to
be isolated. To help reduce the negative effects of short
circuits, power distribution transformers are deliberately
designed to have a certain amount of leakage reactance.
The leakage reactance (usually about 5 to 10% of the full
load impedance) helps limit both the magnitude and rate
of rise of the fault current.
14
A short circuit may lead to formation of an arc. The arc, a
channel of hot ionized plasma, is highly conductive and
can persist even after significant amount of original
material of the conductors was evaporated. Surface
erosion is a typical sign of electric arc damage. Even short
arcs can remove significant amount of materials from the
electrodes.
A short circuit is an accidental path of low resistance
which passes an abnormally high amount of current. A
short circuit exists whenever the resistance of a circuit or
the resistance of a part of a circuit drops in value to almost
zero ohms. A short often occurs as a result of improper
wiring or broken insulation
Reasons for short circuit occurs
A short circuit is simply a low resistance
connection between the two conductors supplying
electrical power to any circuit. This results in excessive
current flow in the power source through the 'short,' and
may even cause the power source to be destroyed. If a fuse
is in the supply circuit, it will do its job and blow out,
opening the circuit and stopping the current flow.
A short circuit may be in a direct- or alternating-current
(DC or AC) circuit. If it is a battery that is shorted, the
battery will be discharged very quickly and will heat up
due to the high current flow.
Short circuits can produce very high temperatures due to
the high power dissipation in the circuit. If a charged,
15
high-voltage capacitor is short circuited by a thin wire, the
resulting huge current and power dissipation will cause the
wire to actually explode.
Arc welding is a common example of the practical
application of the heating due to a short circuit. The power
supply for an arc welder can supply very high currents that
flow through the welding rod and the metal pieces being
welded. The point of contact between the rod and the
metal surfaces gets heated to the melting point, fusing a
part of the rod and both surfaces into a single piece.
How do we locate short circuit?
Along a wire, there should be a place where some
insulation is burnt where short occur. Some times you
might also want to locate a place where there is a lapse of
electrical connection, or where there is a break along the
wire. To locate wires that are broken you can measure the
resistance with a multimeter. First shut off all power
16
to item and wires that you are going to test. Set to measure
resistance at the highest setting. Place one lead at the
beginning or end of the wire and then place the other lead
the other end of the wire. With the measurement of infinite
resistance or very high resistance, that means that there is
a break in that wire. With zero or very little resistance, that
means the wire is good. Safety is a large concern. The
following images show some steps of the setup. There are
areas available for double-checking setup before taking
actual measurements. The motor contactor below is used
to cause the transformer to short circuit to show a fault.
17
Below (Fig 2) is the short circuit part of the test circuit.
On the left side is a current transformer that was used to
measure the current through the shorting circuit. The fuse
box on the right has fuses which we blew to stop the short
circuit. The small black wires coming off of the side of the
contactor are connected to a switch. When the switch is
flipped, the short circuit is then working. After a few
cycles (or 1/10 of a second), the fuses blow and the short
circuit is no longer conducting current. This prevents
damage to the transformer. The other small black wires
connect to the relay and as soon as the short circuit turns
on, the relay starts collecting data.
Figure 2
18
A diagram of the shorting circuit is below.
19
Figure shows Normal and short circuit conditions.
A short is caused by improper wiring. Note the effect on
current flow. Since the resistor has in effect been replaced
with a piece of wire, practically all the current flows
through the short and very little current flows through the
resistor. Electrons flow through the short (a path of almost
zero resistance) and the remainder of the circuit by passing
through the 10-ohm resistor and the battery. The amount
of current flow increases greatly because its resistive path
has decreased from 10,010 ohms to 10 ohms. Due to the
excessive current flow. the 10-ohm resistor becomes
heated. As it attempts to dissipate this heat, the resistor
will probably be destroyed.
EFFECTS OF SHORT CIRCUIT
Short circuit currents play a vital role in Influencing the
design and operation of equipment and power
system and could not be avoided despite careful planning
20
and
Design, good maintenance and thorough operation of the
system.
This paper discusses the short circuit analysis conducted in
KSO
Briefly comprising of its significances, methods and
results. A result sample of the analysis based on a single
transformer is
detailed in this paper. Furthermore, the results of the
analysis and
its significances were also discussed and commented.
Home electrical circuits may have a number of problems:
Too many lamps or appliances on one circuit;
Faulty wiring within the house;
Defective wall switches or receptacles;
Defective cords or plugs;
Defective circuits within appliances.
Short circuits happen when a hot wire touches a
neutral or ground wire; the extra current flowing
through the circuit causes the breaker to trip or
fuse to blow.
Although it's often easy to tell when you have a short or
overloaded circuit—the lights go dead when you plug in
the toaster oven—it isn't always as simple to tell where in
the system this has occurred.
Start by turning off all wall switches and unplugging all
lights and appliances. Then reset the circuit breaker. Pull
21
the lever to off and then to on again to reset a circuit
breaker with a lever switch. If a fuse is blown, it must be
replaced. Unscrew the fuse to replace it with one with
exactly the same amperage rating (both circuit breakers
and fuses should be sized according to the wire used in the
circuit they protect).
- If the breaker trips immediately: the problem may be a
short circuit in a receptacle or switch.
- If the breaker does not trip again, turn on each switch
one at a time and check if and when the breaker trips
again.
- If turning on a switch causes the breaker to trip, there's a
short circuit in a fixture or receptacle controlled by the
switch.
-If turning on the switch makes no difference, the problem
is in one of the appliances connected to the switch.
- If the circuit went dead when you plugged in the
appliance, the problem is probably in the cord or plug.
- If the circuit went dead when you turned on the
appliance, the appliance itself is defective.
Table 2
Causes and effects of short circuit :
Causes Effects
22
1. Over temperatures due to
excessively high over current.
2. Disruptive changes causes by over
voltage.
3. Arcing caused by moisture together
with impure air especially on
insulators.
1. Power supply interruptions.
2. System components damage or
destruction.
3. Development of unacceptable
mechanical & thermal stresses in
electrical operational equipments.
HOW CAN WE PREVENT SHORT
CIRCUITS?
Whenever you are working with electricity, the proper use
of safety precautions is of the utmost importance to
remember. In the front of all electronic technical manuals,
you will always find a section on safety precautions. Also
posted on each piece of equipment should be a sign listing
the specific precautions for that equipment. One area that
is sometimes overlooked, and is a hazard especially on
board ship, is the method in which equipment is grounded.
By grounding the return side of the power transformer to
the metal chassis, the load being supplied by the power
supply can be wired directly to the metal chassis. Thereby
the necessity of wiring directly to the return side of the
transformer is eliminated. This method saves wire and
reduces the cost of building the equipment, and while it
solves one of the problems of the manufacturer, it creates
a problem for you, the technician. Unless the chassis is
physically grounded to the ship's ground (the hull), the
chassis can be charged (or can float) several hundred volts
above ship's ground. If you come in contact with the metal
chassis at the same time you are in contact with the ship's
23
hull, the current from the chassis can use your body as a
low resistance path back to the ship's ac generators. At
best this can be an unpleasant experience; at worst it can
be fatal. For this reason Navy electronic equipment is
always grounded to the ship's hull, and approved rubber
mats are required in all spaces where electronic equipment
is present. Therefore, before starting to work on any
electronic or electrical equipment, ALWAYS ENSURE
THAT THE EQUIPMENT AND ANY TEST
EQUIPMENT YOU ARE USING IS PROPERLY
GROUNDED AND THAT THE RUBBER MAT YOU
ARE STANDING ON IS IN GOOD CONDITION. As
long as you follow these simple rules, you should be able
to avoid the possibility of becoming an electrical
conductor.
TESTING :
There are two widely used checks in testing electronic
equipment, VISUAL and SIGNAL TRACING. The
importance of the visual check should not be
underestimated because many technicians find defects
right away simply by looking for them. A visual check
does not take long. In fact, you should be able to see the
problem readily if it is the type of problem that can be
seen. You should learn the following procedure. You
could find yourself using it quite often. This procedure is
not only for power supplies but also for any type of
electronic equipment you may be troubleshooting.
(Because diode and transistor testing was covered in
chapter 1 and 2 of this module, it will not be discussed at
this time. If you have problems in this area, refer to
chapter 1 for diodes or chapter 2 for transistors.)
24
BEFORE YOU ENERGIZE THE EQUIPMENT, LOOK
FOR: SHORTS - Any terminal or connection that is close
to the chassis or to any other terminal should be examined
for the possibility of a short. A short in any part of the
power supply can cause considerable damage. Look for
and remove any stray drops of solder, bits of wire, nuts, or
screws. It sometimes helps to shake the chassis and listen
for any tell-tale rattles. Remember to correct any problem
that may cause a short circuit; if it is not causing trouble
now, it may cause problems in the future.
DISCOLORED OR LEAKING TRANSFORMER - This
is a sure sign that there is a short somewhere. Locate it. If
the equipment has a fuse, find out why the fuse did not
blow; too large a size may have been installed, or there
may be a short across the fuse holder.
LOOSE, BROKEN, OR CORRODED CONNECTION -
Any connection that is not in good condition is a trouble
spot. If it is not causing trouble now, it will probably cause
problems in the future. Fix it.
DAMAGED RESISTORS OR CAPACITORS - A resistor
that is discolored or charred has been subjected to an
overload. An electrolytic capacitor will show a whitish
deposit at the seal around the terminals. Check for a short
whenever you notice a damaged resistor or a damaged
capacitor. If there is no short, the trouble may be that the
power supply has been overloaded in some way. Make a
note to replace the part after signal tracing. There is no
sense in risking a new part until the trouble has been
located.
25
ENERGIZE THE EQUIPMENT AND LOOK FOR:
SMOKING PARTS - If any part smokes or if you hear
any boiling or sputtering sounds, remove the power
immediately. There is a short circuit somewhere that you
have missed in your first inspection. Use any ohmmeter to
check the part once again. Start in the neighborhood of the
smoking part. SPARKING - Tap or shake the chassis. If
you see or hear sparking, you have located a loose
connection or a short. Check and repair.
If you locate and repair any of the defects listed under the
visual check, make a note of what you find and what you
do to correct it. It is quite probable you have found the
trouble. However, a good technician takes nothing for
granted. You must prove to yourself that the equipment is
operating properly and that no other troubles exist.
If you find none of the defects listed under the visual
check, go ahead with the signal tracing procedure. The
trouble is probably of such a nature that it cannot be seen
directly-it may only be seen using an oscilloscope.
Tracing the ac signal through the equipment is the most
rapid and accurate method of locating a trouble that cannot
be found by a visual check, and it also serves as check on
any repairs you may have made. The idea is to trace the ac
voltage from the transformer, to see it change to pulsating
dc at the rectifier output, and then see the pulsations
smoothed out by the filter. The point where the signal
stops or becomes distorted is the place look for the
trouble. If you have no dc output voltage, you should look
for an open or a short in your signal tracing. If you have a
low dc voltage, you should look for a defective part and
26
keep your eyes open for the place where the signal
becomes distorted.
Signal tracing is one method used to localize trouble in a
circuit. This is done by observing the waveform at the
input and output of each part of a circuit.
Let's review what each part of a good power supply does
to a signal, as shown in figure 4-51. The ac voltage is
brought in from the power line by means of the line cord.
This voltage is connected to the primary of the transformer
through the ON-OFF switch (S1). At the secondary
winding of the transformer (points 1 and 2), the scope
shows you a picture of the stepped-up voltage developed
across each half of the secondary winding-the picture is
that of a complete sine wave. Each of the two stepped-up
voltages is connected between ground and one of the two
anodes of the rectifier diodes. At the two rectifier anodes
(points 4 and 5), there is still no change in the shape of the
stepped-up voltage-the scope picture still shows a
complete sine wave.
Complete power supply (without regulator)
27
However, when you look at the scope pattern for point 6
(the voltage at the rectifier cathodes), you see the
waveshape for pulsating direct current. This pulsating dc
is fed through the first choke (L1) and filter capacitor (C1)
which remove a large part of the ripple, or "hum," as
shown by the waveform for point 7. Finally the dc voltage
is fed through the second choke (L2) and filter capacitor
(C2), which remove nearly all of the remaining ripple.
(See the waveform for point 8, which shows almost no
visible ripple.) You now have almost pure dc.
No matter what power supplies you use in the future, they
all do the same thing - they change ac voltage into dc
voltage.
Component Problems
The following paragraphs will give you an indication of
troubles that occur with many different electronic circuit
components.
TRANSFORMER AND CHOKE TROUBLES. - As
you should know by now, the transformer and the choke
are quite similar in construction. Likewise, the basic
troubles that they may develop are comparable.
A winding can open.
Two or more turns of one winding can short
together.
A winding can short to the casing, which is usually
grounded.
28
Two windings(primary and secondary) can short
together.
This trouble is possible, of course, only in
transformers.
When you have decided which of these four possible
troubles could be causing the symptoms, you have definite
steps to take. If you surmise that there is an open winding,
or windings shorted together or to ground, an ohmmeter
continuity check will locate the trouble. If the turns of a
winding are shorted together, you may not be able to
detect a difference in winding resistance. Therefore, you
need to connect a good transformer in the place of the old
one and see if the symptoms are eliminated. Keep in mind
that transformers are difficult to replace. Make absolutely
sure that the trouble is not elsewhere in the circuit before
you change the transformer. Occasionally, the shorts will
only appear when the operating voltages are applied to the
transformer. In this case you might find the trouble with a
megger-an instrument which applies a high voltage as it
reads resistance.
CAPACITOR AND RESISTOR TROUBLES. - Just
two things can happen to a capacitor:
29
It may open up, removing the capacitor completely
from the circuit.
It may develop an internal short circuit. This
means that it begins to pass current as though it
were a resistor or a direct short.
You may check a capacitor suspected of being open by
disconnecting it from the circuit and checking it with a
capacitor analyzer. You can check a capacitor suspected of
being leaky with an ohmmeter; if it reads less than 500
kilohms, it is more than likely bad. However, capacitor
troubles are difficult to find since they may appear
intermittently or only under operating voltages. Therefore,
the best check for a faulty capacitor is to replace it with
one known to be good. If this restores proper operation,
the fault was in the capacitor.
Resistor troubles are the simplest. However, like the
others, they must be considered.
A resistor can open.
30
A resistor can increase in value.
A resistor can decrease in value.
You already know how to check possible resistor troubles.
Just use an ohmmeter after making sure no parallel circuit
is connected across the resistor you wish to measure.
When you know a parallel circuit is connected across the
resistor or when you are in doubt disconnect one end of
the resistor before measuring it. The ohmmeter check will
usually be adequate. However, never forget that
occasionally intermittent troubles may develop in resistors
as well as in any other electronic parts. Although you may
observe problems that have not been covered specifically
in this chapter, you should have gained enough knowledge
to localize and repair any problem that may occur. The
continuous rating of the main components such as
generators, transformers, rectifiers, etc., therefore
determine the nominal current carried by the busbars but
in most power systems a one to four second short-circuit
current has to be accommodated. The value of these
currents is calculated from the inductive reactance of the
power system components and gives rise to different
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maximum short-circuit currents in the various system
sections.
DAMAGES
Damage from short circuits can be reduced or prevented
by employing fuses, circuit breakers, or other overload
protection, which disconnect the power in reaction to
excessive current. Overload protection must be chosen
according to the prospective short circuit current in a
circuit. For example, large home appliances (such as
clothes dryers) typically draw 10 to 20 amperes, so it is
common for them to be protected by 20–30 ampere circuit
breakers, whereas lighting circuits typically draw less than
10 amperes and are protected by 15–20 ampere
breakers. Wire gauges are specified in building and
electrical codes, and must be carefully chosen for their
specific application to ensure safe operation in conjunction
with the overload protection.
In an improper installation, the over current from a short
circuit may cause ohmic heating of the circuit parts with
poor conductivity (faulty joints in wiring, faulty contacts
in power sockets, or even the site of the short circuit
itself). Such overheating is a common cause of fires. An
electric arc, if it forms during the short circuit, produces
high amount of heat and can cause ignition of combustible
substances as well.
ADVANTAGES OF SHORT CIRCUIT STUDY
Short Circuit Analysis uses the point-to-point method to
calculate fault currents at various points in an electrical
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system up to 600V. Calculations can be made from the
utility transformer secondary to the utilization equipment
in an electrical system. Printouts are available for each
calculation, and the help system guides you through the
calculations.
Benefits :
Save time by easily obtaining the short circuit
magnitude at each point in the power system.
Design safer systems by comparing the calculated
fault current to the ratings of installed equipment.
Increase design reliability by supporting proper
selection of circuit protection equipment for
protection and coordination.
Reports ac and dc current for 4 user defined times.
Reports zero crossing time of total current.
How to perform short-circuit
calculations.
Reduce the risk a facility could face and help avoid
catastrophic losses.
Increase the safety and reliability of the power
system and related equipment.
Evaluate the application of protective devices and
equipment.
Identify problem areas in the system.
Obtain recommended solutions
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Performing short-circuit calculations requires an
understanding of various system components and their
interaction.
It's very important to understand the meaning of the term
"short-circuit fault." Basically, a short-circuit fault in a
power system is an abnormal condition that involves one
or more phases unintentionally coming in contact with
ground or each other. Thus, short-circuit protection is
necessary to protect personnel and apparatus from the
destructive effects of the resulting excessive current flow,
which is caused by the relatively low impedance of the
short-circuit fault connection.
To provide the required protection, we must determine the
extent of short-circuit current at various points of our
power distribution system. This determination requires a
calculation.
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CONCLUSION
Finally from this short circuit study, We have to learn
about short circuit And we can clarify from some
doubtable question like, what is short circuit?, how it
happens?, when it happen?, why it happens?. And also
learn prevention methods of short circuit. We can know
the advantages about short circuit study.
Short Circuit (Fault Current) studies are required to insure
that existing and new equipment ratings are adequate to
withstand the available short circuit energy available at
each point in the electrical system. Fault currents that
exceed equipment ratings are capable of extensive
equipment damage and are a serious threat to human life.
Recently, 23000 fire complaints a year from all over state.
There are lot of fire accidents are occurred due to these
short circuit. To protect the buildings from these short
circuit, we have to learn about this short circuit
phenomenon.
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REFERENCE
1. www.google.com \ short circuit phenomenon
2. www.howstuffworks.com \ electrical stuff
3. IEEE journals on short circuit phenomenon
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