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 COLLEGEYenkapally, 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.

We thank all of teaching and non-teaching staff members of EEE

department for their extended cooperation.

3

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:

Short circuit calculations, which highlights any

equipment that is ascertained to be underrated as

specified

5

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

made when an extending cord of about 10 meters long is

used.

7

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

2005 15631 183

2006 17452 129

2007 18975 149

8

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

11

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

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!

12

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 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

13

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.

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.

14

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.

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.

15

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,

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

17

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.

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

20

system and could not be avoided despite careful

planning 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.

21

Start by turning off all wall switches and unplugging all

lights and appliances. Then reset the circuit breaker. Pull

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

22

Causes and effects of short circuit :

Causes Effects

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

23

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 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

24

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.)

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

25

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.

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

26

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 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.

28

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.

• 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

29

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:

• 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

30

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.

• 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

31

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 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

32

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

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.

33

Reports ac and dc current for 4 user defined

times.

Reports zero crossing time of total current.

How to perform short-circuit

calculations.

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 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

34

power distribution system. This determination requires a

calculation.

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.

35

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.

REFERENCE

36

1. www.google.com \ short circuit phenomenon

2. www.howstuffworks.com \ electrical stuff

3. IEEE journals on short circuit phenomenon

37