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Practical Articles and Presentations for Electrical Engineers

R. K. Sinha

www.power-publishers.com

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Copyright: R. K. Sinha

Published by: Power Publishers,

www.Power-Publishers.com

9/1 Motijheel Avenue,

Kolkata 700074

Printed from: Ray Dot Com,

College Street, Kolkata 700073, India

Cover Design: Pinaki Ghosh

Editor: Sanchaita Roy

First Published: April 2011

ISBN number: 978-93-81205-20-4

Price: `300

Available from: www.Power-Publishers.com

www.Flipkart.com

Printed on recycled paper. No trees were cut for the production of this book.

All rights reserved. No part of this publication may be reproduced, stored in a

retrieval system, or transmitted in any form or by any means electronic,

mechanical, photocopying, recording or otherwise without the prior

permission of the publisher and author. The views expressed in this book are

those of the author. The publisher is not in any way responsible for the views

expressed in this book. All legal actions are subject to the jurisdictions of

courts of Kolkata, India.

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Dedication

This collection of my Articles and Presentations is dedicated to my respected parents who have been a source of constant inspiration, to my loving wife Poonam who has always encouraged and supported me in all my endeavours and my loving children Suraj and Rachna who are my life !

R K Sinha

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Foreword

I am a practicing electrical engineer by profession. During my more than 25 years of working in the industry I have had the opportunity of handling various assignments in Maintenance Management, Contract Management, Basic Work Scope, Design and Detailed Engineering reviews, Installation, Testing and Commissioning activities of Projects, Procurement of Electrical Equipments and Spares etc. I have worked in the Oil & Gas sector where hazards are too many and also in the harsh marine environment of offshore Oil Fields. These helped a lot in improving my safety consciousness in selection of correct electrical equipments according to the area classification and their proper operation and maintenance. The experience of projects provided me the opportunity of getting familiar with various national and international codes and standards. The offshore oil field equipments and systems are technologically advanced and the best in the world. Proper Operation and Maintenance of the same is a challenge in itself and it has been a journey of continuous learning for me. I have always believed in Knowledge Sharing by whatever means possible such as Discussions, Presentations, Articles etc. During my working life in the industry I have time and again searched for relevant practical books on various aspects of Electrical Engineering but have mostly drawn blanks as far as Indian Publications are concerned. Most of the Indian books are academic in nature and suited as text books for students of electrical engineering. The imported books are too costly for most of us. In keeping with the spirit of knowledge sharing , I have made an effort to bring out this collection of Practical Articles and Presentations for Electrical Engineers. I hope the effort will be embraced and welcomed by the practicing electrical engineering fraternity of India.

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INDEX OF ARTICLES

Sl. No. Title

1 Area Classification & selection of

Electrical equipments 08

2 Circuit Breakers 17

3 Earthing 24

4 Earth Leakage Currents & ELCBs 33

5 Electric Shock 39

6 Electrical Engineering Softwares 52

7 Fault Levels 60

8 Fuses 66

9 Maintenance Management 73

10 Electrical Motors 79

11 Offshore Electrical System 90

12 Partial Discharge Analysis 96

13 Power Factor Improvement 100

14 Relay Coordination 106

15 Solar Photo - Voltaic 112

16 Static Electricity 117

17 Safety against Arc Flash 124

18 Safe Usage of Multimeters 130

19 Variable Frequency Drives 134

INDEX OF SLIDES

Sl. No. Title

1 Induction Motors 138

2 Area Classification 150

3 Cables 168

4 Concepts of Earthing 178

5 ELCB 184

6 Electric Shock 191

7 Generator Protections 204

8 Harmonics 212

9 Lighting 219

10 Motor Protections 233

11 Partial Discharge Analysis 248

12 Power Factor Improvement 250

13 Relay Coordination 258

14 Static Electricity 264

15 Arc Flash 280

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PART 1

ARTICLES

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AREA CLASSIFICATION AND SELECTION OF ELECTRICAL EQUIPMENTS

In most of the industrial plants such as Chemical plants, Refineries,Crude Oil & Gas processing plants, hazardous areas exist due to the presence of flammable gases . While designing such plants the first step is to finalise the layout of facilities and equipments. Area Classification drawings are then developed based on the layouts. The purpose of the area classification drawings is to clearly identify the hazardous and safe areas in the plant . For safe operations all electrical equipments must be selected keeping in view their area of application in the plant.It goes without saying that electrical equipments designed for hazardous areas are much costlier than the ordinary ones and hence the plant facility layout has a considerable bearing on the capital expenditure. SOME DEFINITIONS We know that for a fire to take place three things are essential :

1. Presence of fuel such as flammable gas 2. Presence of oxygen 3. Presence of heat ie. sufficient ignition energy to ignite

the flammable mixture While discussing hazardous areas ,it is useful to have the following relevant definitions in mind :

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• Ignition Temperature : The lowest temperature at which ignition occurs in a mixture of explosive gas and air.

• Flash Point : The temperature at which the liquid gives so much vapour,that this vapour,when mixed with air,forms an ignitable mixture.

• Explosive Limits : The extreme values for the concentration of a flammable gas or air under atmospheric conditions,which can be ignited by an electric arc or spark.

STANDARDS Most countries have developed their own standards and codes for Area Classification but internationally two main standards are being followed.These are :

• The North American standards produced by the API and NFPA.

• The International Electrotechnical Commission (IEC) standards.

• API 500 is used in the United States whereas IEC 79 is popular in Europe.

Area classification basically covers two aspects :

1. The probability of flammable atmosphere in the area

2. The type of gases / chemical vapours involved The North American API RP 500 defines the above two aspects as Divisions and Gas Groups whereas in IEC 79 they are termed as Zones and Gas Groups. NORTH AMERICAN METHOD CLASS : Is used to provide a general definition of the physical characteristics of the hazardous material with which we are dealing. The three classes are : CLASS 1 : Gases, Vapours and Liquids that can be present in explosive or ignitable mixtures. Examples : Gasoline , Natural Gas. CLASS 2 : Combustible dust that can be present in amounts that could produce potentially explosive mixtures or dust of an electrically conductive nature. Examples : Flour or cornstarch. As a compact mass these may only burn or smolder but when finely distributed in air , the mixture becomes explosive.

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Metallic dust such as aluminium or magnesium have several dsangerous properties.

(i) They are electrically conductive. (ii) They can burn very violently even when not finely

distributed in air. (iii) When finely distributed in air they can be violently

explosive. CLASS 3 : Fibers or flyings that are easily ignitable but are not apt to be suspended in air in such amounts to produce ignitable mixtures. Example : Rayon, Nylon, Cotton, Sawdust etc. DIVISIONS

DIVISION 1A Hazardous concentrations exist continuously or intermittently under normal operating conditions.

DIVISION 1B Hazardous concentrations may exist frequently due to leakage.

DIVISION 1C The breakage or faulty operations of equipment or process which might release hazardous concentrations of flammable gases and might also cause simultaneous failure of electrical equipments.

DIVISION 2A Hazardous volatile liquids,vapous or gases are normally confined within enclosed containers or closed systems from which they can escape only in the case of accidental rupture or breakdown.

DIVISION 2B Hazardous concentration are normally prevented by positive ventilation but might become hazardous through failure of the ventilation system.

DIVISION 2C Hazardous concentrations of the gases or vapours might occasionally be communicated because of their proximity to Division 1 areas.

GAS GROUPS

GROUP A Atmospheres containing acetylene.

GROUP B Atmospheres containing hydrogen

GROUP C Atmospheres containing ethyl ether vapours,ethylene or equivalent gases

GROUP D Atmospheres containing gasoline,naptha,propane,acetone,natural gas or equivalent

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EUROPEAN METHOD ZONES

ZONE 0 An area in which hazardous atmosphere is continuously present.In oil industry such a condition exists in confined spaces, such as the vapour space of closed process vessels,storage tanks etc.

ZONE 1 An area where explosive gas and air mixture is continuously present for a long period or is likely to occur in normal operation.

ZONE 2 An area in which explosive gas and air mixture is likely to occur only under abnormal operating conditions. For example : Gas Turbine enclosures.

NON-HAZARDOUS AREA Areas not falling under Zone 0,1 or 2 are considered as safe areas. Oil and gas pipelines laid in the open outside hazardous areas and that do not have any flange joints,which cannot become loose, are considered safe area.

GAS GROUPS

GROUP 1 Covers gases produced in coal mines (mainly fire damp methane).

GROUP 2A Atmospheres containing acetone, ethane, hexane, ethyl acetate, ammonia, benzene, butane, diesel, propane etc.

GROUP 2B Atmospheres containing ethylene, town gas, ethyl ether etc.

GROUP 2C Atmospheres containing hydrogen, acetylene, ethyl nitrate, carbon disulphide.

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In addition to the zones ( defining probability of occurrence of flammable mixture) and Gas Groups ( defining type of flammable gas) , the European Standard also has a Temperature Classification .

• The external surfaces of explosion proof equipment must not exceed the temperature whereby they may be liable to become source of ignition for the surrounding atmosphere.

• According to ignition temperature gases and vapours are divided into six temperature classes as follows :

T1 450 deg C

T2 300 deg C

T3 200 deg C

T4 135 deg C

T5 100 deg C

T6 85 deg C

The ignition temperature of natural gas is approximately 480 deg Celsius.

TYPES OF PROTECTION FOR ELECTRICAL EQUIPMENTS Electrical equipments are designed and manufactured with the following types of protections :

Type ‘d’ (Flameproof) The enclosure will withstand an internal explosion of the flammable gas,which may enter it,without suffering damage and without communicating the internal flammation to the external flammable atmosphere through any joints or structural opening in the enclosure.

Type ‘e’ (Increased safety) A method of protection by which additional measures are applied to provide increased safety against the possibility of excessive temperatures and of the occurrence of arcs or sparks during the service life of the apparatus.

Type ‘i’ (Intrinsically safe) A protection technique based upon the restriction of electrical energy within apparatus and

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the interconnecting wiring,exposed to a potentially explosive atmosphere, to a level below that which can cause ignition by either sparking or heating effects.Devices whose electrical parameters do not exceed any of the values 1.2 V,100mA,20 J or 25 mW.

Type ‘p’ (Pressurized) A method of protection using the pressure of a protective gas to prevent the ingress of an external flammable atmosphere to a space which may contain a source of ignition.

Type ‘n’ (Non-sparking) A type of protection applied to an electrical apparatus such that,in normal operation,it is not capable of igniting a surrounding explosive atmosphere , and a fault capable of causing ignition is not likely to occur.

Type ‘o’ (Oil Immersed) A method of protection where the enclosure is made safe by oil-immersion in the sense that flammable gases or vapours above the oil or outside the enclosure will not be ignited.

Type ‘q’ (Sand filled) A method of protection where the enclosure of electrical apparatus is filled with a powdery material such that, if an arc occurs, it will not be able to ignite the external flammable atmosphere.

Type ‘s’ (Special) Special methods of protection which may be a combination of above methods

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SELECTION OF ELECTRICAL EQUIPMENTS

Zone 0 Only Type ‘ia’ (Intrinsically safe) and some special type ‘s’ allowed.

Zone 1 Apparatus suitable for Zone 0 and Intrinsically Safe ‘ib’,Flameproof (‘d’),Oil Immersed (‘o’),Pressurised (‘p’), Sand Filled (‘q’).

Zone 2 Apparatus for Zone 0 & 1 plus Increased Safety (‘e’),Non-sparking (‘n’) allowed.

INGRESS PROTECTION To complete the subject it is also worthwhile to touch upon the topic of Ingress Protection for enclosures of electrical equipments and switchgear. Ingress protection defines the level of protection of the enclosure against the ingress of solids and liquids. It is denoted as IP followed by two digits eg. IP 55. Here the first digit specifies protection against ingress of solids whereas the second digit specifies protection against ingress of liquids.The following tables provides the details : SOLIDS

0 No protection

1 Protected against solid objects upto 50 mm (eg. Hands)

2 Protected against solid objects upto 12.5 mm (eg. Fingers)

3 Protected against solid objects upto 2.5 mm (eg. Tools)

4 Protected against solid objects over 1 mm (eg. Wires)

5 Protected against dust (No harmful deposits)

6 Totally protected against dust.

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LIQUIDS

0 No protection

1 Protected against vertically falling drops of water

2 Protected against water spray upto 15 deg from the vertical

3 Protected against water spray upto 60 deg from the vertical

4 Protected against water spray from all directions

5 Protected against water jets from all directions

6 Protected against strong water jets from all directions

7 Protected against immersion upto 1 Mtr depth.

8 Protected against lengthy immersion under pressure.

SOME IMPORTANT AGENCIES

API AMERICAN PETROLEUM INSTITUTE

NFPA NATIONAL FIRE PROTECTION ASSOCIATION

IEC INTERNATIONAL ELECTROTECHNICAL COMMISSION

UL UNDERWRITERS LABORATORY

CENELEC EUROPEAN COMMITTEE FOR ELECTROTECHNICAL STANDARDIZATION

BASEEFA BRITISH ASSOCIATION OF SAFETY ON ELECTRICAL AND ELECTRONICS FLAMEPROOF APPARATUS

DGMS DIRECTORATE GENERAL OF MINES SAFETY

CMRS CENTRAL MINING RESEARCH STATION SOME IMPORTANT STANDARDS

API RP 500

Recommended practice for Classification of Locations for Electrical Installations at Petroleum facilities Classified as Class 1,Division 1 and Division 2.

API RP 505

Recommended practice for Classification of Locations for Electrical Installations at Petroleum facilities Classified as Class 1,Zone 0,Zone 1 and Zone 2.

API Recommended Practice for Design and Installation of

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RP 14 F

Electrical Systems for Offshore Petroleum Facilities.

IEC 79-10

Classification of Hazardous Areas

IS 5572

Indian Standard for Classification of Hazardous Areas

It is imperative that all of us engaged in the pursuit of oil and gas clearly understand the disastrous implications of an electrical equipment wrongly selected or improperly maintained. Even if we select world class equipment/apparatus ,it will not reduce the risks if not well maintained. So the onus is on the electrical maintenance engineers ( a thankless and much hassled community) to beware of each missing bolt from a flameproof equipment or a worn gasket in a field junction box. And my dear brethren (read electrical maintenance engineers) do we not know that 90% of the fires in the industry are thrust in our laps as caused by electrical short-circuits. After all, any fire burns out cables and wires also ,doesn’t it ? Very difficult to establish the cause and the effect later. So why give the world a chance ? Let us resolve to mitigate the risks with our knowledge, skills and commitment.

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CIRCUIT BREAKERS

A circuit breaker is a switching and current-interrupting device in a switchgear.The circuit breaker serves two basic purposes :

1. Switching during normal operating conditions. 2. Switching during abnormal conditions such as short

circuits and interrupting the fault currents. The first function mentioned above is quite simple as it involves interrupting normal currents.However,the second function is tough as the fault currents are very high and they should be interrupted within a few cycles to save the connected circuit components and equipments from the disastrous effects of the thermal and dynamic stresses. As a fault occurs in a power system,the current increases manifold due to the low fault impedance.During the fault,the current and the voltage undergo a continuous change and the phenomena observed are called ‘transient phenomena’.the word ‘transient’ refers to a ‘temporary state’ which lasts for a short duration of time.The fault current varies with time. During the first three cycles,the fault current is very high but decreases very rapidly.This zone is known as the ‘Sub-transient State’.after the first few cycles the decrease in current is less rapid.This region of slow decreases in the short-circuit current is

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called the “Transient State’.The transient state lasts for a few cycles.After the transient state,’Steady State’ is reached.During the steady state the RMS value of the short-circuit current remains almost constant. THE CIRCUIT BREAKERS OPERATE DURING THE TRANSIENT STATE. THE FAULT CLEARING PROCESS The protective relays are connected in the secondary circuits of current transformers and/or potential transformers.The relays sense abnormal conditions and generate a trip command for the circuit breaker.The circuit breaker opens its contacts.As the contacts separate an arc is drawn between them.The arc is extinguished by suitable medium and technique.The stresses occurring on the circuit breaker while interrupting the arc,can be analysed by studying the following transient phenomena :

• Transient variation of the short-circuit currents

• Transient variation of the voltage after final arc interruption (transient recovery voltage)

• The arc extinguishing phenomenon After final arc extinction, a high voltage wave appears across the circuit-breaker contacts tending to re-establish the arc.This transient voltage wave is called Transient Recovery Voltage (TRV).The TRV comprises a high frequency transient component superimposed on a power frequency recovery voltage. These phenomena have a profound influence on the behaviour of the circuit-breakers. In circuit-breakers the mode of arc extinction is either ‘high resistance interruption, or ‘ zero-point interruption’. High Resistance Interruption : In this technique the resistance of the arc is increased by lengthening and cooling it to such an extent that the system voltage is no longer able to maintain the arc and the arc gets extinguished.This method is used in air-break circuit breakers and DC circuit breakers. Zero Point Interruption : In this technique the arc gets extinguished at the natural current zero of the AC wave and is prevented from restriking again by rapid buildup of dielectric strength of the contact space.This principle is applied in almost all AC circuit-breakers.

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CLASSIFICATION OF CIRCUIT-BREAKERS Circuit-Breakers are generally classified as follows according to the medium of arc extinction :

1. MCB/MCCB/Air-break circuit breakers. 2. Air blast circuit breakers. 3. Oil circuit breakers. 4. Minimum oil circuit breakers. 5. SF6 circuit breakers. 6. Vacuum circuit breakers.

MINIATURE CIRCUIT BREAKERS (MCBs) As is clear from the name,these are tiny circuit-breakers generally used in low voltage lighting and control circuits where the fault currents are of the order of a few kiloamperes(KA) only. MOULDED CASE CIRCUIT BREAKERS (MCCBs) These are used in medium voltage circuits and are available in current ratings upto 800 Amps.They provide integrated overload and short-circuit protectionsand are mainly used in 415 V motor circuits and lighting mains.These are maintenance–free units and have the following advantages over switch-fuse units :

• No time is lost in fuse replacement.

• Less chances of motor single-phasing. AIR-BREAK CIRCUIT BREAKERS (ACBs) Air-break circuit-breakers utilize air at atmospheric pressure for arc extinction.These are generally used indoors in medium voltage DC and AC circuits.These circuit breakers employ the high resistance arc extinction process.The arc is rapidly lengthened by means of arc runners and arc chutes and the resistance of the arc is increased by cooling,lengthening and splitting the arc.The arc resistance increases to such an extent that the supply voltage is no longer able to sustain the arc and it extinguishes.These circuit breakers are equipped with thermal,magnetic and shunt trips and are now available upto 12 KV,500MVA ratings. AIR BLAST CIRCUIT BREAKERS (ABCBs) In Air Blast Circuit Breakers high pressure air is forced on the arc through a nozzle at the instant of contact separation.The ionized medium between the contacts is blown away by the air

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blast.After the arc extinction the chamber is filled with high pressure air which prevents the arc from restriking. These circuit breakers were developed during 1930’s.Nowadays they are becoming obsolete and being replaced by Vacuum and SF6 breakers. OIL CIRCUIT BREAKERS (OCBs) The oil circuit breaker was developed around 1885.In these dielectric oil is used for insulation and arc extinction.Contact separation takes place in steel tanks filled with oil.These breakers have become obsolete because of the following demerits :

• Large quantity of oil is required in the tank to provide insulation between the live parts and the earthed steel tank.

• These are bulky and difficult to maintain.

• The entire oil needs checking and replacement due to deterioration.

MINIMUM OILCIRCUIT BREAKERS (MOCBs) In minimum oil circuit breakers the contact separation takes place inside oil filled interrupters made of insulating material such as porcelain or ceramic.MOCBs were developed for a voltages from 3.6 KV to 145 KV. The heat of the arc causes decomposition of the dielectric oil,the products of decomposition being hydrogen gas and other gases like acetylene. DISADVANTAGES OF OIL

1. The decomposed products of dielectric oil are inflammable and explosive.If the oil circuit breaker is unable to break the fault current,the pressure in the tank may rise above safe limit and explosion may occur.This does not happen in other types of breakers.

2. The oil absorbs moisture easily.Its dielectric strength reduces by carbonization which occurs during arcing.The oil needs replacement after certain breaker operations.It needs regular maintenance.

3. Oil is not a suitable medium for breakers which have to operate frequently because the oil deteriorates.

SULPHUR HEXAFLOURIDE (SF6) CIRCUIT-BREAKERS SF6 is an inert,heavy gas having excellent dielectric and arc extinguishing properties.The dielectric strength of SF6

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increases with pressure and is more than that of dielectric oil at a pressure of 3 Kg/cm2.SF6 circuit breakers .These breakers were developed around 1970 and are available for rated voltages from 3.6 KV to 760 KV. AT 15Kg/cm2 SF6 gas starts liquefying at 10 deg Celcius.Hence this gas is not suitable for pressures above 15 Kg/cm2. Physical properties of SF6 Gas

• Colourless

• Odourless

• Non-toxic

• Non-inflammable

• State – Gas at normal temperature and pressure.

• Density – Heavy gas, density 5 times that of air at 20 deg C and atmospheric pressure.

VACUUM CIRCUIT BREAKERS (VCBs) Vacuum circuit breakers were developed during 1960’s and are being used extensively upto 36 KV.Here the contacts separate in vacuum which has good dielectric strength.A vacuum level of 10

-6 to 10

-10 bar is used.

ADVANTAGES OF SF6 CB OVER VACUUM CB

• Probability of leakage is less in SF6 because of lower pressure difference from atmosphere.

• Even after leakage SF6 being heavier than air maintains a pressure of 1 bar and is still safe.However, a VCB is not even fit for breaking normal currents after a leakage.

• For inductive and capacitive loads VCBs give rise to large switching overvoltages and hence surge arrestors are required in conjunction.This is not required in case of SF6.

TECHNICAL PARTICULARS OF A CIRCUIT BREAKER A circuit breaker ratings consist of the following particulars :

• Type of medium for arc extinction.

• Rated voltage.

• Rated breaking current.

• Type of operating mechanism.

• Total break time eg. 2 cycles,3 cycles etc. Breaking Capacity :A circuit breaker must be capable to break the fault current on occurrence of a fault.This is known as its

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Breaking Capacity.If the current is symmetrical it is referred as symmetrical breaking capacity, whereas if the current is asymmetrical it is termed as asymmetrical breaking capacity. Symmetrical breaking current is the RMS value of the AC component of the short circuit current at the instant of contact separation.The asymmetrical breaking current is the RMS value of total current comprising the AC as well as DC components at the instant of contact separation. Making Capacity : A circuit breaker must be capable of being closed on to a fault.It must be able to withstand the extremely high electromagnetic forces under these conditions.The making current of a circuit breaker is the peak value of the maximum current wave (including the DC component) in the first cycle of the current after the circuit is closed by the circuit breaker.The rated making capacity of a circuit breaker is approximately 2.55 times the symmetrical breaking capacity. TRANSIENT RECOVERY VOLTAGE In AC circuit breakers ,the current interruption takes place at the natural zero of the current wave.Before the contacts separate the voltage between them is zero.After the contact separation the voltage across contact increases.In fact this voltage is the voltage drop across the arc during the arcing period.The voltage across the arc is in phase with the current since the arc is resistive.Finally when the arc is extinguished a high frequency voltage transient appears across the contacts which is superimposed on the power frequency system voltage.This high frequency transient voltage tries to restrike the arc and is known as the Transient Recovery Voltage (TRV).The TRV causes high dielectric stress between the circuit breaker contacts.If the dielectric strength of the medium between the contacts does not build up faster than the rate of rise of TRV ,breakdown takes place causing re-establishment of the arc.Hence for the circuit breaker to interrupt the fault currents successfully it is extremely important to build up the dielectric strength of the medium between the contacts.This is an important feature of any circuit breaker design.

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HRC FUSES AND CIRCUIT BREAKERS : A COMPARISION 1. HRC fuses possess high rupturing capacity

compared to the circuit breakers in the medium voltage range.Fuses with breaking capacity of 100 KA are available.Breakers with corresponding capacity will be too bulky.

2. HRC fuses are cheaper than the circuit breakers.

3. HRC fuses take less space and are simpler to install and maintain.

4. In case of severe faults,HRC fuses open a circuit in less than half cycle,ie. before the current peak is reached.As against this,circuit breakers take 2-3 cycles.

5. There is no deterioration in case of HRC fuses while circuit breakers are prone to become sluggish in operation with lapse of time.The worn latches,contacts etc. can cause mal-operation of circuit-breakers.

6. It takes some time to replace fuses whereas a circuit breaker can be switched on quickly to resume operations.

7. Fuses need replacement after every fault clearing.

Hence,we see that circuit breakers are protectors of the electrical system.However, circuit breakers must be selected properly for the intended application to provide maximum safety and protection.

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CONCEPTS OF EARTHING

INTRODUCTION Earthing is an integral part of any electrical installation.Earlier,very little was known about the requirements of earthing.Even now,when a wealth of information is available about the necessities and methodology of earthing systems,it still remains a complex and confusing subject.It is so common,yet so little understood.This article is an attempt to brush up the subject of earthing in the minds of electrical engineers and inspire them to dig further in the subject. SOME FUNDAMENTALS Earth : From electrical engineering point of view Earth is defined as the conductive mass of the earth,whose electric potential at any point is conventionally taken as zero.Being electrically neutral and at zero potential,the earth provides a common reference for voltage measurements. Earthing : An electric connection to the general mass of earth,whose dimensions are very large in comparision to the electrical system being considered. The terms ‘Ground’ and ‘Grounding’ are synonymous with ‘Earth’ and ‘Earthing’ and are more prevalent in some countries like North America. Equipotential Bonding : Electrical connection putting various exposed conductive parts and extraneous conductive parts at a substantially equal potential. Exposed Conductive Part : A conductive part of an equipment which can be touched and which is not a live part but which may become live under fault conditions. Extraneous Conductive Part : A conductive part liable to transmit a potential including earth potential and not forming part of the electrical installation. NEED FOR EARTHING Earthing is of utmost importance for safety of plant,equipment,property and human as well as animal life.In

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the absence of a well designed effective earthing system,earth fault conditions may lead to tremendous loss of property and lives.The main objectives of earthing are as follows :

• To ensure safety of life and property from hazards of electric shock and electric fires.

• To ensure that system voltages on healthy lines remain within reasonable limits under fault conditions thereby prevending insulation breakdowns.

• To provide a low impedance path to facilitate the satisfactory operation of protective devices under fault conditions.

• To minimize arcing burn downs as in an earthed system arcing fault would produce a current in ground path thereby providing an easy means of detecting and tripping against phase to earth arcing fault breakdowns.

• To provide an equipotential platform on which electronic equipments can operate.

• To provide an alternative path for induced current and minimize the electrical noise in cables.

EARTHING CATEGORIES There are two types of earthing.

1. SYSTEM EARTHING : This is primarily concerned with the protection pf electrical equipmen by stabilizing voltages with respect to ground.

2. EQUIPMENT EARTHING : This is primarily concerned with the protection of personnel from electric shock by maintaining the potential of non-current carrying equipment at or near ground potential.

SYSTEM EARTHING This is basically achieved by earthing the neutral of the supply system.Ungrounded neutral supply systems are not in use mainly for the following disadvantages :

1. Under a single line to earth fault the voltage to earth of the two healthy phases rises from phase value to line value ie. Root 3 times.

2. The capacitive current in the two healthy phases increases to root 3 times the normal value.

3. The capacitive current in the faulty phase is 3 times its normal value.

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4. The capacitive fault current gives rise to arcing ground condition.Under such condition the phase voltage may rise to 5 to 6 times its normal value causing insulation breakdown.

5. It is difficult to detect and isolate a single phase to earth fault.

Modern supply systems operate with their neutral points grounded.The advantages are :

1. There is no voltage rise in the healthy phases during phase to earth fault.

2. There are no unbalanced voltages with respect to earth. 3. Persistent arcing grounds are eliminated. 4. Earth fault currents can be utilized to operate protective

relays to disconnect the fault. METHODS OF NEUTRAL EARTHING

1. Solid or Effective Grounding : The use of solid grounding is limited only to systems where the normal circuit impedance is sufficient to prevent very high value of fault current.This is necessary to avoid excessive damage at the fault location.Experience shows that the combined impedance of the equipment,circuit and earth return path in systems, operating at voltages below 2.2 KV and above 33 KV, is sufficiently high so as to limit the value of fault current to a safe value.

2. Resistance Grounding : Neutral earthing is done through a resistor to limit the earth fault current.Neutral earthing resistors are normally designed to carry their rated current for a short period ,usually 30 seconds.Resistance grounding is normally employed on systems operating at voltages between 2.2 KV and 33 KV, when the total power source capacity exceeds 5000 KVA,as the current characteristics of such systems usually give rise to excessive currents under ground fault conditions.

3. Reactance Grounding : Another method of neutral grounding wherein the fault current can be limited is through reactance grounding.The reactance connected between neutral and earth provides a lagging current which neutralizes the capacitive current.Reactance grounding is preffered for circuits where high charging currents are involved such as transmission lines,underground cables etc.

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4. Arc Suppresson Coil Grounding : Sometimes neutral earthing is done with an arc suppression coil.It is provided with tappings and the reactance of the coil can be tuned depending upon the length of the transmission line and the capacitance to be neutralized.

EQUIPMENT EARTHING Under fault conditions the non-current carrying metal parts of an electrical installation such as frames,enclosures,supports,fencing etc. may attain high potential with respect to ground so that any person or stray animal touching these or approaching these will be subjected to potential difference which may result in the flow of a current through the body of the person or the animal of such a value as may prove fatal. Safe value of current in amp(rms) which a human body can tolerate is I = 0.165/root t for t < 3 secs And I = 9 mA for t > 3 secs where ‘t’ is time duration in seconds of the flow of current. To avoid this the non-current carrying metal parts of the electrical system are connected to the general mass of earth by means of an earthing system comprising of earth conductors to conduct the fault currents safely to the ground.The object of earthing is to ensure safety by discharging the electrical energy to the earth.The conductors may be in the form of a grid (also called mat) or multiple electrodes in the form of rods,plates,pipes etc. Here,it is important to understand the concept of STEP and TOUCH voltage. Touch Voltage : It is the potential difference between a grounded metallic structure and a point on the earth’s surface ,separated by a distance equal to the normal maximum horizontal reach,approximately one metre. Step Voltage :It is the potential difference between two points on the earth’s surface ,separated by a distance of one pace,that will be assumed to be one metre in the direction of maximum potential gradient.

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Consider that damage to a terminal bushing on a transformer (see fig.) has resulted in the faulting of the respective phase to the transformer body which is earthed by a pipe electrode driven into the earth.The earth fault current will flow from the phase conductor to the transformer body and through the earth electrode to earth.Around the electrode the current will flow outward in all directions. As the current flows through a constantly increasing volume of earth ,its density drops as the distance from the electrode increases.The highest potential is at the electrode which is the same as that of the transformer tank.As the distance from the electrode increases,the less is the difference in earth surface potential between two points per unit length. Now if a person happens to touch the transformer tank,the potendial difference between his hands and feet will be

E(Touch) = V1-V2 Where E(Touch) is termed as the Touch Potential . It is the voltage that exists between the hand and both feet of the person.

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On the left side of the figure a person is shown walking towards the transformer tank.At any time,the earth surface potential between his feet will amount to E(Step) = V3-V4 Where E(Step) is termed as the Step Potential and is the voltage between the two feet of a person. It can be seen that E(Step) = (Rk + 2Rf) Ik volts And E(Touch) = (Rk + Rf/2) Ik volts Where Rk is the resistance of the body, Rf is the grounding resistance of one foot in ohms,taken for all practical purposes to be equal to 3 times the resistivity of the soil near the surface of ground in ohm-meter, And Ik is the current in amps(rms) flowing through the body. When a grounding system is installed,the objective is to obtain as low values of E(Step) and E(Touch) as possible in order to ensure full safety for human beings and stray animals. CODES AND STANDARDS The following are some of the important codes and standards which describe the earthing requirements and methods:

• Indian Electricity Rules

• National Electrical Code (India)

• IS:3043 (Code of Practice for Earthing)

• API RP 14F (Design and Installation of Electrical Systems for Fixed and Floating Offshore Petroleum Facilities)

• IEEE Std. 142 (Recommended Practice for Grounding of Industrial and Commercial Power Systems)

WHAT THE CODES SAY Following are some excerpts from relevant codes : INDIAN ELECTRICITY RULES For Low and Medium Voltages :

• Rule 61(1) states that neutral conductor of a 3phase,4 wire system shall be earthed by not less than two separate and distinct corrections with a minimum of two different earth electrodes or such large number as may be necessary to bring the earth resistance to a satisfactory value.The earth electrodes so provided,may be interconnected to reduce earth resistance.

• Rule 61(2) states that the frame of every generator,motor and the metallic parts (not intended as

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conductors) of all transformers and any other apparatus used for regulating or controlling energy and all medium voltage energy consuming apparatus shall be earthed by two separate and distinct connections with earth.

• Rule 61(4) states that all earthing systems shall – a) consist of equipotential bonding conductors capable of

carrying the prospective earth fault current and a group of pipe/rod/plate electrodes for dissipating the current to the general mass of earth without exceeding the allowable temperature limits in order to maintain all non-current carrying metal works reasonably at earth potential and to avoid dangerous contact potentials being developed on such metal works.

b) Limit earth resistance sufficiently low to permit adequate fault current for the operation of protective devices in time and to reduce neutral shifting.

c) Be mechanically strong,withstand corrosion and retain electrical continuity during the life of the installation.

For HV/EHV Systems Rule 67(1) states that all non-current carrying metal parts associated with HV/EHV installation shall be effectively earthed to grounding system or mat in order to :

a) limit the touch and step potential to tolerable values. b) limit the ground potential rise to tolerable values so as

to prevent danger due to transfer of potential through ground,earth wires,pipe lines etc.

c) maintain the resistance of the earth connection to such a value as to make operation of the protective device effective.

d) Rule 67(1A) states that the neutral point of every generator and transformer shall be earthed by connecting it to the earthing system as defined in Rule 61(4) by not less than two separate and distinct connections.The neutral point of a generator may be connected to the earthing system through an impedance to limit the fault current to the earth. Additional Precautions to be Adopted in Mines and Oil-Fields

• Rule 116(1) states that in the interest of safety,appropriate switchgear with necessary protective equipment shall be suitably placed for automatically disconnecting supply to any part of the system where a fault including an earth fault occurs.

• Rule 117(5) states that no switch,fuse or circuit-breaker shall be inserted in any earth conductor.

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NATIONAL ELECTRICAL CODE (INDIA)

• Clause 2.0.2 states that the object of an earthing system is to provide, as nearly as possible, a surface under and around a station which shall be at a uniform potential and as nearly zero or absolute earth potential as possible.The purpose of this is to ensure that all parts of apparatus,other than live parts,shall be at earth potential,as well as to ensure that operators shall be at earth potential at all times.Also by providing such an earth surface of uniform potential under and surrounding the station,as nearly as possible,there can exist no diffrence of potential big enough to shock or injure an operator when fault conditions occur.

• Clause 2.0.3 states that earthing associated with current-carrying conductor is normally essential to the security of the system and is generally known as system earthing,while earthing of non-current carrying metal work is essential to the safety of life and property and is generally known as equipment earthing.

• Clause 2.0.9 states that each earth system shall be so devised that the testing of individual earth electrode is possible.It is recommended that the value of any earth system resistance shall not be more than 5 ohms,unless otherwise specified.

• Clause 2.0.10 states that a drawing shall be prepared for each installation showing the main earth connection and earth electrodes.

• Clause 2.0.11 states that no addition to the current-carrying system shall be made which will increase the maximum available earth fault current or its duration until it has been ascertained that the existing arrangement of earth electrodes,earth bus-bar etc. is capable of carrying the new value of earth fault current.

• Clause 2.0.11 states that no cut-out,link or switch other than a linked switch arranged to operate simultaneously on the earthed or earthed neutral conductor and the live conductors shall be inserted on any supply system.

• Clause 2.1.1.2 states that the earth system resistance should be such that when a fault occurs against which the earthing system is designed to give protection,the protective gear will operate to isolate the faulty portion of the plant and render it harmless.

API RP 14 F

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Clause 6.10.3 states the following : Grounding of electrical equipment on fixed and floating offshore petroleum facilities in a positive manner is of particular importance because personnel standing on steel decks or in contact with steel framing present a low impedance path to ground ,effectively grounded.In addition,the dampness and salt deposition contribute to the breakdown of insulation and the possibility of leakage on the surface of insulators and similar devices.It is recommended that all metal equipment,such as skids,vessels etc. be grounded to the steel structure.Exposed,noncurrent-carrying metal parts of fixed equipment that may become energized because of any condition shall be grounded.The physical contact obtained when equipment is bolted to a steel structure is not necessarily an adequate effective ground because of paint and possible corrosion. To provide the desired safety ,equipment grounding should accomplish the following :

a) Grounding shall limit the voltage (normally to 42 V maximum) that may be present between the equipment in question and any other grounded object with which personnel may be in contact at the same time.

b) For solidly grounded systems ,grounding should present a low impedance path for short circuit current to return to the source of power,thus opening a fuse or tripping a circuit breaker.This requires that the equipment ground be bonded to the system ground.

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EARTH LEAKAGE CURRENTS AND ELCBs

Poor insulation in electric devices and equipments is the cause of earth leakage currents.Earth leakage currents are a major source of two very common electrical hazards :

• Risk of fire

• Risk of electrocution In addition to the above, continuous undetected earth leakage currents also result in waste of electricity.

Protective devices like fuses/MCBs seldom offer protection against earth leakage currents.More often the magnitude of these are much below the operating level of the fuse or MCB thus rendering them ineffective.

The only effective protection against earth leakage currents is provided by Earth Leakage Circuit Breakers(ELCBs) which are also known as Residual Current Circuit Breakers (RCCBs). An MCB + ELCB combination provides complete protection against not just shocks and electrocution,but also fires due to overloads,short-circuit and earth leakage.It also saves valuable energy. EFFECT OF ELECTRIC CURRENT ON HUMAN BODY The effect of electric current passing through a human body depends upon the current strength,the time for which the current passes through the body as well as the organs involved in the path of current.

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Current Effect

1 ~ 10 mA Mild sensation

10 ~ 20 mA Human body remains stuck to the conductor

20 ~ 30 mA Muscle contraction

70 ~ 100 mA The heart begins to vibrate

500 mA Death – Cardiac Arrest / Nervous breakdown

Safe value of current in amperes(rms) which a human body can tolerate is given by : I = 0.165/ sq. root of t for t < 3 secs And I = 9 mA for t > 3 secs Where ‘t’ is the time duration in seconds of the flow of current. The current values generally considered as safe limit are : 30 mA for time < 1 sec. 300 mA for time < 50 ms. 500 mA for time < 30 ms. The above facts and figures clearly bring out the importance of ELCBs.

Table 1: Human resistance to electrical current

Body Area Resistance (ohms)

Dry skin 100,000 to 600,000

Wet skin 1,000

Internal body (hand to foot) 400 to 600

Ear to ear ~100

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The approximate effects of electric current on the human body:

AC Current

DC Current

Effect

10 milliamps

1 milliamp Threshold of sensation

>60 milliamps

>10 milliamps

Strong involuntary muscle contractions.

You may not able to release a live wire. You may be thrown across the room by the contraction!

>500 milliamps

100 milliamps

Cardiac Arrest!!!!

The amount of current through the human body for a given voltage depends on the resistance of the body. The interior of the human body is a good conductor due to the abundance of ions in the body fluids. The main barrier to current flow is the skin. The resistance of the skin decreases significantly when it is wet. A new rule 61A, introduced in July 1985 in the Indian Electricity Rules,makes the use of Earth Leakage Protective Devices mandatory for all medium voltage installations and all low voltage (240 V) installations above 5 KW.

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FIVE GOOD REASONS FOR USING ELCBs

1. PROTECTION AGAINST INDIRECT CONTACT : Due to internal fault or insulation failure metal enclosures of electric appliances can become live and cause electric shock to unwary persons touching them.ELCB trips instantaneously and thus exclude possible risk from dangerous indirect contact.

2. PROTECTION AGAINST DIRECT CONTACT : Accidental contact with live parts of electric appliances cause earth leakage currents to flow through the human body resulting in shock that may be fatal.ELCB trips immediately under these circumstances and saves human life.

3. PROTECTION AGAINST ELECTRIC FIRES : Earth leakage current of 500 mA and above cause electric sparks which can spread into major fires.Such leakages are quickly detected by ELCBs which isolate the faulty circuit.

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4. ENERGY SAVINGS : Non-productive earth leakage current is measured by the energymeter as part of the total energy consumption.The ELCB detects these leakages and trips,helping us to rectify the fault and thus save energy.

5. PROTECTION OF EARTHING AGAINST CORROSION : Earth leakage currents of higher magnitude cause corrosion through electrolysis.The ELCB ensures that the earthing remains intact.

ELCBs are generally available in the sensitivity range of 30 mA/100 mA /300 mA. ELCBs of 30 mA sensitivity offer adequate protection against even direct touch with medium voltage and are meant for personal safety,whereas ELCBs of 300 mA sensitivity are suitable for the prevention of fire hazards and are crucial for protection of property. 30 mA RCCB The current passing through human body is likely to be between 80 ~ 240 mA in case of direct contact at 230 V.The protective device should operate within 50 ms at 240 mA and 240 ms at 80 mA to be within zone-2 (safe) of the IEC curves.Both these conditions are fulfilled by the 30 mA RCCB and therefore it provides a very high degree of protection against electrocution. OPERATING PRINCIPLE AND COMMON FEATURES OF ELCBs ELCBs are current operated devices which operate on the principle of measurement of differential(residual) current using a current balance transformer and tripping a switching device through an electromagnetic tripping relay. Some outstanding features of ELCBs are as below :

1. Being current operated device,ELCB is totally independent of the mains voltage for tripping and can operate at nominal voltage less than 10 volts.

2. The mechanism is trip-free ensuring that the ELCB cannot be reclosed/reset if the earth leakage/fault persists.

3. They have a very long operational life of over 20,000 operations.

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4. A test button is provided to check the correct operation of the unit.

5. ELCBs are capable of withstanding starting inrush currents of motors upto 4 to 8 times the rated current.

6. The ELCBs have excellent short circuit withstand capability ensuring that there is no damage to the device itself,till the back-up protection fuse or another overcurrent device clears the fault.

CONCLUSION : Thus we see that the money invested in ELCBs in industry as well as our home goes a long way in saving our lives and property.The electrical consultants and common users shall ,therefore, adopt this wonderful device to safeguard against the hazard of electric shock and fire.

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ELECTRIC SHOCK : PREVENTION AND

TREATMENT

In today’s world , it is difficult to imagine life without electricity. Whichever way we glance we will find gadgets, appliances and machines working on electricity. At home starting from the simple bulbs, tubelights and fans, we enjoy the convenience of electric press, mixers, refrigerators, TV, washing machines, geysers, water purifiers, microwave ovens and many other appliances. And the industrial world is full of electrical machines ranging from hand lamps, portable drill machines, portable grinders, welding machines, alternators and so on and so forth. So each one of us, in whichever profession we are, is handling electricity everyday. And thus each one of us is susceptible to an electric shock. In understanding the phenomenon of electric shock, let us begin from the basics. We know that in an electric circuit there are three basic entities viz. Voltage ( V in Volts), Current ( I in Amperes) and Resistance (R in Ohms). According to the fundamental Ohm’s law of electricity : Current (I) = Voltage (V) / Resistance (R) The voltage is responsible for current flow in a circuit and the magnitude of current will depend on the resistance. If a human being accidentally comes in contact with a live conductor or a body at higher potential , current flows through the body and the

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person is said to have experienced an electric shock. The magnitude of current depends on the voltage and body resistance. And one thing we shall clearly understand that it is the current which kills and not the voltage. Of course, higher the voltage , higher will be the current for the same body resistance. The following three things are crucial in case of an electric shock :

1. Magnitude of current 2. Path of current 3. Duration of exposure

Magnitude of Current : Electric current has the following three harmful effects on the body :

1. It harms or interferes with proper functioning of the nervous system and heart

2. It subjects the body to intense heat resulting in burns 3. It causes the muscles to contract.

50 / 60 Hz AC Current Effect

1 mA ~ 2 mA Threshold of sensation

5 mA ~ 10 mA Mild sensation

10 ~ 15 mA Pain

15 ~ 50 mA Muscle paralysis (can’t let go)

50 ~ 100 mA Ventricular Fibrillation

> 100 mA Stop heart or breathing

However, the above figures are approximate only and vary from person to person depending on physical conditions. Currents greater than 100 milliamperes can

• Completely stop the heart

• Inhibit breathing in two ways 1. by causing the chest muscles to contract

thereby preventing the lungs from expanding. 2. by blocking the nerve center of the brain that

controls the breathing function. Path of Current : When a human body comes in contact with a current carrying conductor or an object which is at greater potential than earth due to insulation failure etc. , a current flows through the human body from the part touching the object/wire (normally one hand) to ground (normally through the feet) .The path of the current is very important. And we all know that current always takes the

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path of least resistance. Let us consider the following two common cases : Case 1 : One hand comes in contact with the shock giving object and the other hand is in contact with the ground then current will pass from one hand to other through the heart The shock may prove to be fatal due to heart seizure. To minimize the risk of current flow through the heart the ‘hand-in-the pocket’ technique is popular with skilled electrical personnel.While working on electrical systems they keep one hand in pocket or behind their back thus avoiding the two hands completing a current circuit through the heart. Case 2 : Victim standing on ground.The right hand comes in contact with a shock giving object.Current follows a path from the right hand to right feet without touching the heart. The shock may not be fatal in this case. Duration of Exposure to Current : This also has a very important bearing on fatality due to electric shock. Safe value of current in amperes(rms) which a human body can tolerate is given by the Dalziel’s Formula: I = 0.165/ sq. root of t for t < 3 secs And I = 9 mA for t > 3 secs Where ‘t’ is the time duration in seconds of the flow of current. Effect of AC Current (mains frequency 50 / 60 Hz) is depicted in the following graph in IEC 479 and IS 8437.

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Body Resistance : The resistance offered by the body is mainly the resistance of the skin. The interior of the human body is a good conductor due to the abundance of ions in the body fluids. The body resistance is not constant but varies under different skin conditions.The two extreme conditions are depicted below :

Condition Resistance

Dry skin 1,00,000 Ohms

Wet skin 500 Ohms

In U.S.A. the domestic supply voltage is 110 Volts, 60 Hz and in India it is 230 Volts , 50 Hz. Now let us examine the current for 110 V under the different skin conditions :

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Dry Skin Case : Current = 110 * 1000 / 100000 milliamperes (mA) = 1.1 mA Wet Skin Case : Current = 110 * 1000 / 500 mA = 220 mA Similarly we can calculate current magnitudes for Indian domestic supply voltage of 230 Volts under different skin conditions : Dry Skin Case : Current = 2.3 mA Wet Skin Case : Current = 460 mA With this we can clearly understand that even voltages as low as 50 Volts may prove to be fatal under certain conditions such as the current path is through the heart and the skin is wet.Also if the victim is not immediately released, the skin resistance goes down due to burns. So we can realize that the same voltage may only give a mild shock if the skin is dry but may prove to be lethal after a swim, in a bathtub or if we are sweating after a game of tennis or some hard work in the sun. EFFECT OF CURRENT ON HEART If the path of current flowing through a human body is through the heart , it may prove to be fatal in most of the cases. Mains frequency AC ie 50/60 Hz is the most dangerous as it interferes more with the heart’s electrical pacemaker, leading to ventricular fibrillation and subsequent cardiac arrest. To examine this we have to briefly understand the working of human heart.

Our heart is divided into four chambers with separation walls. It has doors (valves) which let the blood flow in and out. The two upper chambers in our heart are called the atria. The atria are the receiving chambers of our heart. When blood flows into our heart from the body or lungs, it always flows into either the right or left atrium—never anywhere else. The two lower chambers in our heart are called ventricles. The ventricles are the pumping chambers of our heart. When blood leaves our heart, it is always pumped out from the ventricles—never from anywhere else. The ventricles are very strong because they have to pump hard enough to push blood through your lungs and entire body.

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The heart produces its own electricity.The electrical system in the heart consists of a natural pacemaker which generates 60 to 100 electrical pulses per minute which makes out heart beat.The heart also has a bundle of specialized cells between the upper chambers (Atria) and the lower chambers (Ventricles) that conduct the heart’s electrical signals.The electrical system in the heart makes our ventricles contract and relax in a rhythmic pattern which results in pumping of blood to the lungs and other body parts.The heart’s electrical activities can be monitored and recorded with the help of Electro-Cardio-Gram (ECG).

A problem in the heart’s electrical system can disrupt its natural rhythm. Any kind of abnormal rhythm or heartbeat is dangerous as the heart is not able to deliver enough blood to the body organs. It is normal for our heartbeat to speed up or slow down during the day as our activity level changes, but it is not normal for the heart to beat out of rhythm. An electric current greater than 50 mA at 50/60 Hz through the heart muscles results in a condition known as Ventricular Fibrillation. Under this condition the ventricular muscles quiver instead of contracting and

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relaxing and the heart is not able to pump blood to the body organs. This leads to death due to sudden cardiac arrest. The current range of 100- to 200-mA. is particularly dangerous because it is almost certain to result in lethal ventricular fibrillation. Victims of high-voltage shock usually respond better to artificial respiration than do victims of low-voltage shock, probably because the higher current clamps the heart and hence prevents fibrillation. AC VERSUS DC CURRENT Another very important question is : Which is more dangerous AC or DC ? Alternating current (AC) is four to five times more dangerous than direct current (DC). For one thing, AC causes more severe muscular contractions. For another, AC can lower skin resistance and thereby increase the shock-current. The skin resistance goes down rapidly with continued contact because sweating is stimulated and the skin oils and even the skin itself are burned away. Consequently, it is extremely important to free the victim from contact with the current as quickly as possible before the current increases to the fibrillation-inducing level. Also, the frequency of the AC influences the effects on the human body. Unfortunately, the standard electrical power frequency of 50/60 Hertz is in the most harmful range. At this frequency, as little as 30 volts can kill. On the other hand, people have withstood 40,000 volts at a frequency of a million Hertz or so without fatal effects. “Let Go” Current : Electric shock causes contraction of the muscles and many times the victim is unable to free himself/herself from the current source.The maximum current that can cause the flexors of the arm to contract but that allows a person to release his hand is termed as the ‘let-go’ current. For DC, the let-go current is about 75 mA for an average heathy person. For AC current, the let-go current is about 15 mA, depending upon muscle mass. RELEVANT STANDARDS Indian Standard IS 8437 : PART 1 & 2 (1993) AND IEC Publication No. IEC 479 – 1 (1984) : Effects of current passing through human body. PREVENTION It is said that “ Prevention is better than cure”.The following preventive measures shall be adopted by all of us to safeguard against the lurking danger of electric shocks :

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At Home and Office:

• Educate children and other family members about

electrical safety. Make them aware of the dangers so that they always treat electricity with respect.

• Know the location of the main switch so that power can be switched off immediately in case of need.

• Never operate switches with wet hands.

• Install a mains Earth Leakage Circuit Breaker (ELCB). An ELCB with 30 mA sensitivity protects even in the case of direct contact with 230 V AC.

• Earthing is the best safeguard.Even in case of insulation failure of some appliance like a toaster, it will not allow the body to aquire a higher potential. Always use a three pin plug (having an earthing connection) for home appliances.

• Install miniature circuit breakers (MCBs) instead of fuses. These are convenient and trip off in case of circuit fault. In case of old house wirings having fuses , very few people use the correct type and rating of fuses thereby nullifying the protection.

• Always use good quality wires and plug-sockets .Never compromise on these. The cheaper ones may be an invitation to death.

• Get condition of wiring checked every five years or so. Replace if necessary.

• Do not go on adding loads on the same old wiring. Plan your gadgets and get the wiring suitably done to meet the new requirements.

• Do not overload the sockets by using multi-plugs at will. The overheating will spoil the insulation.

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• Install sockets with automatic protection covers to minimize the risk to children.

• Always hire a qualified and competent electrician for electrical works at home. A novice may install a switch in neutral instead of phase connection which may be very dangerous.

• Buy only gadgets and appliances of standard reputed brands. That cheap hair dryer may prove to be very costly someday.

• Invest in making electricity your friend at home. It is like investing in a life insurance scheme.

In Industrial Plants / Factories / At Construction Sites : It goes without saying that electricity is the heart of industry. Normally any well designed and properly commissioned industrial plant / factory has a safe electrical set-up. The safety requirements are Permit-to-Work system, Lock-out / Tag-out system , proper earthings, skilled and competent operation and maintenance staff etc. However, the main danger is during expansion / modification works or at new construction sites. The following machinery is commonly deployed during constructon jobs :

• Welding machines

• Power saws

• Portable drilling machines

• Portable grinding machines

• Hand lamps for temporary lighting

• Temporary switchboards As the use is temporary in nature, often the electrical safety norms are not adhered to in the use of the above mentioned machinery. Many a times electrical shock incidents do happen and sometimes combined with the wet conditions at construction sites they prove to be fatal. Also there are many reported cases of secondary injuries such as fall from a height due to electric shock. Hence the importance of adhering to electrical safety during construction jobs must be enjoined upon all concerned.The following areas are important in this regard :

• Proper earthing to all switchboards and power distribution boards.

• Proper earthing of welding machines

• Use of good quality cables .

• Using 3 pin plugs and 3-core cables for all portable machines and lighting.

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• Use of only good quality machines.

• Skilled and competent electricians

• Use of hand gloves. TREATMENT

In spite of our best efforts , electrical shock incidents do occur.Hence we must know how to respond to such situations.If we learn some basic first aid procedures for electric shock., we may be able to save the life of a person in case of electric shock.

Electric shocks can result in:

• Slight shocking sensations

• Muscle spasms

• Seizures

• Interrupted breathing

• Irregular heart beats

• Third degree burns (at the spots where the electricity enters and exits the body)

• Unconsciousness

• If a person does suffer a severe shock, it is important to free the victim from the current as quickly as can be done safely. Switch off the current source immediately. Do not touch the person until the electric power is turned off. You cannot help by becoming a second victim.You may use a dry insulating object such as wooden stick to free the victim.

• Call for ambulance/medical help immediately.

• Check for heartbeat and breathing. Feel for a pulse along the neck, under the earlobe, on the chest or on the wrist. Watch the rise and fall of the chest to see if the person is breathing. If there is no heartbeat and no breathing, do CPR. The ABCs of Cardio-pulmonary-resuscitation (CPR) are Airway, Breathing and Circulation.

• Airway : Place victim on back on a hard surface.

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a. Tilt head back. b. Lift chin. c. Clear out mouth.

• Breathing : If victim is not breathing, begin artificial respiration.

a. Pinch nostrils. b. Give two full breaths. c. Watch for chest rise and fall. d. Give one breath every 5 seconds.

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• Circulation : Check victim’s pulse. If pulse is absent, begin CPR immediately.

a. Depress breast bone by 1 to 2 inches. b. Give 15 compressions at the rate of 80 per minute,

with two full breaths every time 15 compressions are completed.

c. Continue till pulse is established or medical help arrives.

• If the person is breathing and pulse is established, put him in the recovery position.

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Adjust the top leg so that both the hip and knee are bent at right angles. Gently tilt the head back to keep the airway open. Keep the person warm until medical help is obtained. If breathing or circulation stops at any time, roll the person back on to his or her back and begin CPR.

• In some advanced countries ambulances are equipped with an external defibrillator. It is a device which sends controlled electric current through the heart to reverse the process of ventricular fibrillation.The heart’s natural pacemaker then takes over and restores heartbeat.

It is truly said that “ Electricity is a good friend but it can be a very dangerous enemy”. The onus is on us to befriend it and reap its benefits

through enhanced awareness and adherence to safe appliances and safe practices.

ALWAYS TREAT ELECTRICITY WITH RESPECT.

SUMMARY

Electric current damages the body in three different ways: (1) it harms

or interferes with functioning of the nervous system and heart; (2) it

subjects the body to intense heat; and (3) it causes the muscles to

contract. Electrical shock can be lethal. The hazards must be

understood and general safety rules must be followed.

• It’s the current that kills. Voltage is not a reliable indication

of danger because the body's resistance varies so widely it is

impossible to predict how much current will result from a

given voltage.

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AN INTRODUCTION TO ELECTRICAL ENGINEERING SOFTWARES

The various aspects of electrical engineering design involve a lot of complex calculations.Earlier these calculations were done manually.The process was quite time consuming and the results were approximate. However, today computer based design solutions have made inroads in every discipline of engineering. A number of softwares are now available encompassing all areas of electrical design.These softwares make the life of electrical designers and consultants comfortable,enhance productivity and provide more accurate results.In India,however, the use of electrical softwares is in a nascent stage and a very large percentage of working electrical engineers lack awareness about the same.This subject also remains largely uncovered by electrical journals and computer magazines. This article is an attempt to introduce the electrical engineers to several useful electrical softwares. Electrical system design mainly consists of the following:

• Layouts

• Single line diagrams

• Cost estimations

• Short circuit analysis

• Transient stability analysis

• Harmonics analysis

• Protection relay coordination

• Cable selections

• Lighting calculations and layouts

• Selection of motors

• Transformer sizing

• Battery bank sizing

• Ground grid design

• Following is a glimpse of some useful electrical engineering software :

1. ETAP (www.etap.com) : This is a comprehensive electrical engineering software for the design, simulation, and analysis of generation, transmission, distribution, and industrial power systems.

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ETAP has been designed and developed by engineers for engineers to

handle the diverse discipline of power systems in one integrated

package with multiple interface views such as AC & DC networks,

cable raceways, ground grid, GIS, panels, protective device

coordination & selectivity, and AC & DC control system diagrams.

ETAP is a fully integrated analysis tool used by thousands of

engineers in diverse companies around the world to design, maintain,

and operate electric power systems.

ETAP utilizes real-time data to perform power system studies and

playback. Operators and managers use ETAP to monitor, control, and

optimize power systems.

2. SynerGEE Electric : Offers a complete design package.

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SynerGEE® Electric is a simulation software package

used to model and analyze electric distribution systems. It is brought to you by Advantica, the global leader in engineering simulation technology.

SynerGEE provides a full suite of powerful analysis tools, including:

• Radial and network load-flow • Radial and network fault calculations • Load and phase balancing • Contingency studies • Capacitor placement recommendations • Harmonic analysis • Predictive reliability calculations

3. SKM (www.skm.com) :

Power*Tools for Windows (v. 5.0)

Power*Tools is an integrated set of programs written

for engineers who design and analyze commercial, light

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and heavy industrial, institutional, utility, and petro-

chemical sites and facilities. A simple graphical

interface and a powerful object oriented database

insures that Power*Tools for Windows will efficiently

assist you in the design, analysis, and implementation

of your power system.

In addition to Power*Tools for Windows, SKM Systems

Analysis offers PTW-LT, a version of the Power*Tools

software that is designed for engineers who work on

smaller electrical systems.

Power*Tools for Windows Study Modules

PTW-LT - A light version of Power*Tools designed for

smaller applications.

Arc Flash -- Incident energy and arc flash boundary for

each bus in a system.

A_FAULT -- ANSI fault analysis.

CAPTOR -- Computer Aided Plotting for Time

Overcurrent Reporting.

CABLE 3-D -- Cable Pulling Tension Calculation.

DAPPER -- Distribution Analysis for Power Planning

Evaluation and Reporting.

DC System Analysis -- Includes battery sizing, DC load

flow, DC short circuit (ANSI & IEC)

Equipment Evaluation -- Protective equipment ratings.

GroundMat -- Electrical Grounding Analysis

HI_WAVE -- Harmonic Investigation, Wave Analysis,

and Voltage Evaluation.

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IEC_FAULT -- IEC fault analysis.

IEC_61363 -- IEC short circuit study module models

current that flows under abnormal conditions.

PTW - I*SIM -- Industrial Simulation and Transient

Stability.

Reliability -- System design reliability.

TMS -- Transient Motor Starting.

Unbalanced Studies -- Simulate systems with single-

phase, two-phase and unbalanced three-phase load

conditions 4. Amtech (www.amtech-power.com) :

AMTECH is a leading software developer producing quality software specifically for the electrical industry.

Supplying solutions for Low Voltage Design, Test and Inspection, and High Voltage analysis, AMTECH has become the clear choice for thousands of users world-wide.

5. CYMFAULT (www.cyme.com) : Offers fault analysis solutions and other modules as below :

• CYMSTAB : For transient stability analysis

• CYMLINE : For single line diagrams

• CYMHARMO : For harmonic analysis

• CYMGRD : For substation grounding

• CYMCAB : For cable ampacity calculations 6. CAPE (Computer Aided Protection Engineering) : Is a world class productivity tool ,developed by Electrocon, consisting of various modules. The programs of the Computer-Aided Protection Engineering (CAPE) series were designed to serve you the protection engineer with the most powerful, easy-to-use software tools we could devise. CAPE is a world-class productivity tool developed by Electrocon under the initial sponsorship of ten major U.S. electric utilities. CAPE consists of a series of core and optional

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modules for analysis and reporting, linked by a general-purpose database. Modules

• Database Editor

• Short Circuit

• One-Line Diagram

• Coordination Graphics

• Relay Setting

• System Simulator

• Relay Checking

• Line Constants

• Order Production

• Short Circuit Reduction

• Power Flow

• Breaker Duty Analysis

• Settings Transfer Utilities

7. Smartdraw (www.smartdraw.com) : It is an excellent software for making electrical diagrams in a jiffy. SmartDraw is the easy drawing software that helps you create perfect electrical circuit diagrams and schematics in minutes.

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SmartDraw has an extensive collection of electrical symbols and templates to help you create professional diagrams for:

Electronic Circuits Automotive Wiring Circuit Schematics

Electrical Wiring Digital Circuits

Parallel Circuits and Much More

8. LightCalc (www.lightcalc.com)

Summary of features:

• Determines the overall reflectance in the room.

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• Finds the proper footcandle level for general, task, and art lighting.

• Adjust the footcandle level for dark to light rooms and client age.

• Determines the proper spacing needed.

• Uses both Inverse Square Law and Lumen methods.

• Suggests a grid layout for general lighting.

• Art Lighting - See the light right on artwork!

9. AGI32 (www.lsa.com.au) : A comprehensive lighting design software package with advanced features.

10. ALADAN : It is a lighting design software package from GE Lighting system. GE Lighting Systems' ALADAN® software package can be run on a MS DOS or Windows based personal computer. This easy to use set of programs provides a quick assist to anyone laying out a lighting job. Simply by entering the parameters of the job, invoking photometric data and selecting a GE Lighting Systems product, anyone can produce a bill of materials, lighting layout, detailed light level array, and other valuable information for producing a top-quality lighting result. Applications include roadway, indoor facilities, area lighting, sports lighting, etc.

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11. Light Table (www.powerangle.com) : Software for Protective Relay Coordination and Fault Analysis from Power Angle Software.

HVAC Software : Carmel Software offers a complete range of HVAC (Heat,Ventilation and Air Conditioning) design software. Demo version of many of these software can be downloaded for free to get a feel and assess their utility. The electrical engineers involved in design and consultancy can evaluate the software and then buy it if found useful. Besides, the design softwares other packages are also available for Computerized Maintenance Management ,Inventory Management etc.A comprehensive maintenance management software covers all activities in the maintenance spectrum such as :

• Planning and scheduling

• Equipment history

• Spare parts management

• Reporting Going for software based electrical system design and maintenance activities can be of great help in better and faster design solutions, enhanced productivity and improved quality. All of us (the electrical engineers of India) shall ,therefore, endeavour to introduce the right software in our sphere of working.

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FAULT LEVELS IN ELECTRICAL SYSTEMS Faults in electrical power systems occur due to insulation failures. Type of fault may be one of the following :

1. Single phase to ground 2. Phase to phase 3. Two-phases to ground 4. Three-phase fault All these faults cause heavy currents to flow in the system known as short-circuit or fault currents.These short-circuit currents are much higher than the normal currents flowing in a healthy power system. Single phase – to – ground faults are the most common whereas the three-phase short-circuit are the most severe faults. These short-circuit currents give rise to tremendous thermal and electromagnetic stresses which are highly destructive in nature.Hence,for every power system design it is essential to carry out proper fault studies and select all components viz. bus-bars,circuit-breakers,current-transformers etc. to have a fault withstand capacity higher than the maximum fault currents in the system for a certain duration (normally 1 sec) till the fault is cleared by a combination of protective relays and circuit-breakers.

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SOURCES OF FAULT CURRENTS The generators connected to the power system are the main sources of fault currents.Under short-circuit conditions, a drop in frequency and voltage are common and under these conditions synchronous machines feed back into the system. Also, large induction motors, having considerable flywheel energy , act as generators in the event of reduced frequency. So large induction motors shall also be included in the short-circuit studies.The KVA can be assumed to be equal to the HP rating of the motor for the purpose of fault calculations.

LIMITATIONS TO FAULT CURRENT The short-circuit current s which flow in a power system are only limited by the circuit impedance upto the point of fault.The circuit impedances include the generator and transformer reactances.

FAULT CALCULATIONS For the purpose of fault calculations, a detailed single line diagram of the power system is drawn followed by an impedance diagram. From these fault levels are calculated for various fault locations and switchgear is selected accordingly. With the modern power systems growing more and more complex, today fault studies are invariably carried out by specialized computer software. However, details of the same are outside the scope of this article.

A TYPICAL CASE STUDY At one of the offshore Oil and Gas processing plants, the power generation system comprised 3 installed gas turbine – generator sets each rated 18 MVA. The main voltage levels were 11 KV and 415 V (achieved by means of step-down transformers ) for catering to all connected load. The electrical arrangement was as depicted below :

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As shown in the figure, according to the short-circuit studies carried out during plant design , the 11 KV bus-bars and the circuit-breakers were rated for a fault current of 36.1 KA for 1 sec. After about 20 years of operation, a new plant was planned to cater to the future oil and gas production profile.This new plant is to be an extension of the existing plant there electrical systems were to be integrated. These systems were to be designed so as to be capable of operating either in synchronized or in island mode via an interconnector arrangement as shown in the following diagram :

However, the electrical design engineers were facing a typical problem. The fault studies of the integrated system revealed that the 11 KV busbars and circuit breakers of the existing plant (having a fault current rating of 36.1 KA ) would become highly underrared for the new fault ratings in the range of 50 ~ 55 KA for the combined power system. The electrical design engineers had two options.

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Option 1 : Replace the entire 11 KV switchboard of the existing plant with a new one suitable for higher fault rating. Option 2 : Install fault-current limiters in appropriate locations. Option 1 was rejected as , besides high cost involved , it would require total plant shutdown for a considerable time resulting in huge production losses. Hence, the design team concentrated on the second option and the following integrated electrical scheme was finalized :

Here, fault currents within the CB (circuit-breaker) ratings will be cleared by the respective CBs but for higher fault currents ,the fault limiters will isolate the power circuits immediately to bring down the fault level within the rated capacity of switchgear. The fault limiters distinguish between major and minor short-circuits by detecting the instantaneous current level and the rate of current rise. It trips only when both set responses are reached.

FAULT CURRENT LIMITERS Leading electrical switchgear manufacturing companies like ABB , have pioneered the concept of short-circuit current limiters and are offering them upto 36 KV,140 KA systems. They limit the short-circuit current within the very first current rise ( in less than 1 millisecond ) and never allow the short-circuit current to reach its peak value.The effect is depicted in the following diagram :

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These fault current limiters can be refurbished and re-used after clearing a major short-circuit. Hence, we see that fault-current limiters can provide low cost and very effective design solutions to electrical power system expansions.

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