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

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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

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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.

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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

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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:

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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.

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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

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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

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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.

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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.

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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

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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

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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.

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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.

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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,

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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

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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.

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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

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A diagram of the shorting circuit is below.

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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

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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

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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

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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

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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.)

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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.

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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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