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UNIT I u2013 ELECTRICAL HAZARDS
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KONGUNADU COLLEGE OF ENGINEERING AND TECHNOLOGY (AUTONOMOUS) 20BM801PE – ELECTRICAL SAFETY AND QUALITY ASSURANCE T.Muthukumar, Assistant Professor/BME, Kongunadu College of Engineering and Technology.
UNIT I – ELECTRICAL HAZARDS Review of Electrical concept, Electrostatic Electro magnetism - Electrical Hazards- Energy leakage - Clearance and insulation - Current surges - Electrical causes of fire and explosion Human interface with electricity, Human resistance to electricity
REVIEW OF ELECTRICAL CONCEPT Electric current • Electric Current is defined as rate of flow of electric charge. i = dq/dt ampere Alternating Current (ac) • The type of electric current which reverses at regularly recurring intervals of time and which has alternately positive and negative values. Direct Current (dc) • The type of electric current in which the electrons move continuously in one direction through the conductor.
Voltage • An atom has two types of charges based on its structure. Those are positive and negative charges. A certain amount of energy (work) is required to overcome the force and move the charges through a specific distance. All opposite charges possess a certain amount of potential energy, because of the separation between them. • The difference in potential energy of the charges is called Potential difference. Potential difference in electrical terminology is known as Voltage and it is denoted by V (or) E.
Energy • Energy can be defined as “it is the capacity to do work”. Energy may be in many forms like Mechanical, Electrical, Chemical and so on. Power • Power is defined as “the rate of change of Energy”. It is denoted by P. Resistance • The resistance of a circuit is the property by which it opposes the flow of current. • The resistance of a conductor depends on (a) its length, (b) the cross- sectional area, (c) the material of the conductor (d) the temperature.
Resistivity • The electrical resistivity is the electrical resistance per unit length and per unit of cross-sectional area at a specified temperature. ρ = E/J Electrical Conductance • The reciprocal of resistance is called conductance. Its unit is Siemen and its symbol is G. G = 1/R • Similarly, the reciprocal of resistivity is called conductivity. Its symbol is σ. σ = 1/ρ
Ohm’s law • Ohm's Law states that the current flowing in a circuit is directly proportional to the applied potential difference and inversely proportional to the resistance in the circuit. V = IR Limitations of Ohm’s law • Ohm’s law does not apply to all non-metallic conductors • It is not applicable to non-linear devices such as Zener diode, vacuum tubes etc., • Ohm’s law true for metal conductors at constant temperature. If the temperature changes, the law is not applicable.
Impedance • The total opposition offered to the flow of an alternating current. It may consist of any combination of resistance, inductive reactance, and capacitive reactance. Inductance • Inductance is the ability of an inductor to store energy and it does this in the magnetic field that is created by the flow of electrical current. Self-inductance: • Self inductance is the property of a circuit, often a coil, whereby a change in current causes a change in voltage in that circuit due to the magnetic effect of caused by the current flow. Mutual-inductance: • Mutual inductance is an inductive effect where a change in current in one circuit causes a change in voltage across a second circuit as a result of a magnetic field that links both circuits. This effect is used in transformers.
Electromagnetism • The magnetic effect created when an electric current flows in a conductor. This magnetic effect surrounds the conductor only while current is flowing. Electromotive Force (EMF) • The electrical force caused by a difference in potential between two points. EMF is measured in volts. Fault • An insulation failure that exposes electrified conductors, causing current to leak and possibly resulting in electric shock. Fuse • A protective device which allows a piece of metal to become part of a circuit. The metal melts under heat created by excessive current, thereby interrupting the circuit and preventing the flow of electricity from exceeding the circuit's current-carrying capacity.
Ground • A conducting connection, intentional or unintentional, between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth. Ground-fault • A fault, or insulation failure, in the wire used to create a path to ground. Grounding • To prevent the buildup of hazardous voltages in a circuit by creating a low-resistance path to earth or some other ground plane. Guarding • Placement of live parts of electrical equipment where they cannot accidentally be contacted, such as in a vault, behind a shield, or on a raised platform, to which only qualified persons have access. Insulation • Non-conductive materials used to cover or surround a conductor, permitting it to be handled without danger of electric shock.
Insulator • Any material, such as glass or rubber, that prevents the flow of electric current. Overcurrent • Any current in excess of the rated capacity of equipment or of a conductor. High Voltage • A term that normally implies a voltage higher than 600 volts. Arc Blast • A release of mechanical, acoustical, thermal, and optical energy from an electric arc. Arc Flash • A release of thermal energy from an electric arc by the vaporization and ionization of materials, reaching temperatures up to 35,000 °F. Exposure to these extreme temperatures both burns the skin directly and causes ignition of clothing.
Electrical Hazard • A dangerous condition such that contact or equipment failure can result in electric shock, arc-flash burn, thermal burn, or blast. Flashover • A flashover is an electric discharge over or around the surface of an insulator. This happens when the ignition of smoke or fumes from surrounding objects causes the unexpected and rapid spread of fire through the air. Shock • Electrical shock happens when current passes through the body. Electricity travels through closed circuits, and people, sometimes tragically, can become part of the circuit. When a person receives a shock, electricity flows between parts of the body or through the body to a ground. This can happen if someone touches both wires of an energized circuit, touches one wire of the circuit while standing unprotected or touches a metal part that has become energized.
Electrocution • Electrocution refers to the injury or lethal dose of electrical energy. Electricity can also cause forceful muscle contraction or falls. The severity of injury depends on the amount of current flowing through the body, the current's path through the body, the length of time the body remains in the circuit and the current's frequency. Fire/Explosion • Electrical fires may be caused by excessive resistance that generates heat from any of the following: • Too much current running through wiring where overcurrent protection fails or does not exist • Faulty electrical outlets resulting in poor contact or arcing • Poor wiring connections and old wiring that is damaged and cannot support the load • An explosion can occur when electricity ignites a flammable gas or combustible dust mixture in the air. Ignition from a short circuit or static charge is possible.
Lockout/Tagout • When servicing and maintenance tasks involve electricity and electrical equipment, you must prevent the unexpected startup of equipment. Electrical Earthing • The process of transferring the immediate discharge of the electrical energy directly to the earth by the help of the low resistance wire is known as the electrical earthing. The electrical earthing is done by connecting the non-current carrying part of the equipment or neutral of supply system to the ground. Circuit breakers • Circuit breakers are devices that keep circuits safe from excessive electrical energy. Electricians install circuit breakers throughout buildings to limit the amount of electricity that flows throughout a circuit. Breakers typically determine the maximum flow that a device can handle.
ELECTROSTATICS • Electrostatics is a branch of physics that studies electric charges at rest. • Electrostatics involves the buildup of charge on the surface of objects due to contact with other surfaces. Although charge exchange happens whenever any two surfaces contact and separate, the effects of charge exchange are usually only noticed when at least one of the surfaces has a high resistance to electrical flow. • This is because the charges that transfer are trapped there for a time long enough for their effects to be observed. These charges then remain on the object until they either bleed off to ground or are quickly neutralized by a discharge.
Coulomb’s Law • Coulomb’s law is experimental law formulated in 1785 by Charles Augustin de Coulomb, then a colonel in the French army. • It deals with the force a point charge exerts on another point charge. By a point charge we mean a charge that is located on a body whose dimensions are much smaller than other relevant dimensions. • For example, a collection of charges on a pinhead may be regarded as point charge. • The polarity of charges may be positive or negative; like charges repel, while unlike charges attract. Charges are generally measured in coulombs (C). • One coulomb is approximately equivalent to 6 x 1018 electrons; it is very large unit of charge because one electron charge e = -1.6019 x 10-19 C.
Coulomb’s law states that the force F between two point charges Q1 and Q2 is: • Along the line joining them • Directly proportional to the product Q1 Q2 of the charges • Inversely proportional to the square of the distance R between them. • The force is along the straight line joining them. If the two charges have the same sign, the electrostatic force between them is repulsive; if they have different signs the force between them is attractive.
Electric flux • Electric flux is the measure of the electric field through a given surface, although an electric field in itself cannot flow. It is a way of describing the electric field strength at any distance from the charge causing the field. Electric Field Intensity • The electric field intensity (or electric field strength) E is the force that a unit positive charge experiences when placed in an electric field. Electric flux density • Electric flux density is the electric flux passing through a unit area perpendicular to the direction of the flux.
Electrostatic potential • As the electric field is irrotational, it is possible to express the electric field as the gradient of a scalar function, called the electrostatic potential (also known as the voltage). • An electric field E, points from regions of high electric potential to regions of low electric potential.
Electrostatic generator • The presence of surface charge imbalance means that the objects will exhibit attractive or repulsive forces. This surface charge imbalance, which yields static electricity, can be generated by touching two differing surfaces together and then separating them due to the phenomena of contact electrification and the triboelectric effect. • Rubbing two nonconductive objects generates a great amount of static electricity. This is not just the result of friction; two nonconductive surfaces can become charged by just being placed one on top of the other. Since most surfaces have a rough texture, it takes longer to achieve charging through contact than through rubbing. • Rubbing objects together increases amount of adhesive contact between the two surfaces. Usually insulators, e.g., substances that do not conduct electricity, are good at both generating, and holding, a surface charge.
Electrostatic induction • If a positively charged object is brought near an uncharged metal object, the mobile negatively charged electrons in the metal will be attracted by the external charge, and move to the side of the metal facing it, creating a negative charge on the surface. • When the electrons move out of an area they leave a positive charge due to the metal atoms' nuclei, so the side of the metal object facing away from the charge acquires a positive charge. These induced charges disappear when the external charge is removed. Induction is also responsible for the attraction of light objects, such as balloons. • The surface charges induced in conductive objects exactly cancel external electric fields inside the conductor, so there is no electric field inside a metal object. This is the basis for the electric field shielding action of a Faraday cage. Since the electric field is the gradient of the voltage, electrostatic induction is also responsible for making the electric potential (voltage) constant throughout a conductive object.
Gauss's law • Gauss's law constitutes one of the fundamental of electromagnetism. • Gauss's law states that the total electric flux through any closed surface is equal to the total charge enclosed by that surface.
ELECTRO MAGNETISM • The first indication of a relationship between magnetism and electricity was discovered by Hans Christian Oersted in 1820. He connected a wire between the two terminals of a battery and held the wire over a magnetic compass needle. When the wire was held parallel to the compass needle, the needle deflected from its normal position. • When current flows through a conductor, a magnetic field is formed outside of the conductor. The current direction determines the direction of the magnetic field. • Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electrically charged particles. • Electromagnetism is a fundamental force in nature consisting of the elements electricity and magnetism. It is also referred to as electromagnetic force.
The electromagnetic force is carried by electromagnetic fields composed of electric fields and magnetic fields, and it is responsible for electromagnetic radiation such as light. • It is one of the four fundamental interactions (commonly called forces) in nature, together with the strong interaction, the weak interaction, and gravitation. • At high energy the weak force and electromagnetic force are unified as a single electroweak force. • In Faraday's law, magnetic fields are associated with electromagnetic induction and magnetism, and Maxwell's equation describe how electric and magnetic fields are generated and altered by each other and by charges and currents.
Magnetic Field Intensity and Magnetic Flux Density • Magnetic field intensity (H) at any point in the magnetic field is defined as the force experienced by the unit north pole at that point. In simple terms, it is a measure of how strong or weak any magnetic field is. The SI unit of magnetic field intensity is Ampere/meter (A/m). The magnetic flux density (B) is the magnetic moment developed per unit volume of a material when placed in a magnetizing field. Magnetic flux density is the measure of the number of magnetic lines of flux that passes through a point on a surface. It is measured in tesla and is a vector quantity. • The relation between the magnetic flux density and the magnetic field intensity is given by: B = μH
Electromagnetic or magnetic induction • Another phenomenon related to electromagnetism is electromagnetic induction. By moving a wire through a magnetic field (or in between the north and south poles of a magnet), electromagnetic effect occurs and then creates an electric current. By changing the strength of the magnetic field, the magnitude of current is also changed. • It is the production of an electromotive force across an electrical conductor in a changing magnetic field. • Electromagnetic induction has found many applications, including electrical components such as inductors and transformers, and devices such as electric motors and generators.
Biot-Savart’s Law • Biot-Savart’s law states that the differential magnetic field intensity dH produced at a point P by the differential current element is proportional to the product and the sine of the angle between the element and the line joining P to the element and is inversely proportional to the square of the distance r between P and the element.
Applications of Biot-Savart’s Law: • Calculate magnetic responses even at the atomic or molecular level. • Used in aerodynamic theory to calculate the velocity induced by vortex lines. Importance of Biot-Savart’s Law: • Biot-Savart’s law is similar to the Coulomb’s law in electrostatics. • The law is applicable for very small conductors too which carry current. • The law is applicable for symmetrical current distribution.
Ampere’s Circuital Law • Magnetic field intensity around a closed path is equal to the current enclosed by that path.
ELECTRICAL HAZARDS • An electrical hazard is a dangerous condition where a worker can or does make electrical contact with energized equipment or a conductor. It can either be "state" or "dynamic". • Electrical injuries can be divided into four types: fatal electrocution, electric shock, burns, and falls caused by contact with electric energy. • Electrocution is one of the major hazards on construction sites. Electrocution is death or severe injury by electric shock, electric current passing through the body. The word is derived from "electro" and "execution", but it is also used for accidental death.
Electric Shock • Electric shock occurs when the human body becomes part of a path through which electrons can flow. • Direct: Injury or death can occur whenever electric current flows through the human body. Currents of less than 30 mA can result in death. • Indirect: Although the electric current through the human body may be well below the values required to cause noticeable injury, human reaction can result in falls from ladders or scaffolds, or movement into operating machinery. Such reaction can result in serious injury or death. • When personnel come in contact with energized conductors they receive a shock with current flowing through their skin, muscles and vital organs. • The severity of the shock depends on the current’s path through the body, the current intensity, and the duration of the contact.
There are three basic pathways electric current travels through the body; 1) Touch Potential (hand/hand path) 2) Step Potential (foot/foot path) 3) Touch/Step Potential (hand/foot path)
1) In a touch potential contact, current travels from one hand through the heart and out through the other hand. Because the heart and lungs are in the path of current, ventricular fibrillation, difficulty in breathing, unconsciousness, or death may occur. 2) In a step potential contact, current travels from one foot through the legs, and out of the other foot. The heart is not in the direct path of current but the leg muscles may contract, causing the victim to collapse or be momentarily paralyzed. 3) In a touch/step potential contact, current travels from one hand, through the heart, down the leg, and out of the foot. The heart and lungs are in the direct path of current so ventricular fibrillation, difficulty in breathing, collapse, unconsciousness, or death may occur.
Burns • Burns can result when a person touches electrical wiring or equipment that is improperly used or maintained. Typically, such burn injuries occur on the hands.
Arc-Flash and Arc Blasts Arc-Flash • An Arc-Flash is an unexpected sudden release of heat and light energy produced by electricity traveling through air, usually caused by accidental contact between live conductors. • Temperatures at the arc terminals can reach or exceed 35,000 degrees Fahrenheit (F), or four times the temperature of the sun’s surface. Arc Blasts • The air and gases surrounding the arc are instantly heated and the conductors are vaporized causing a pressure wave called an Arc Blast. • Personnel directly exposed to an Arc-Flash and Arc-Blast events are subject to third degree burns, possible blindness, shock, blast effects and hearing loss. • Even relatively small arcs can cause severe injury. The secondary effect of arcs includes toxic gases, airborne debris, and potential damage to electrical equipment, enclosures and raceways.
The high temperatures of the arc and the molten and vaporized metals quickly ignite any flammable materials. While these fires may cause extensive property damage and loss of production, the hazards to personnel are even greater. • Any energized electrical conductor that makes accidental contact with another conductor or with ground will produce an Arc-Flash. • The arcing current will continue to flow until the overcurrent protective device used upstream opens the circuit or until something else causes the current to stop flowing. The arc current can vary up to the maximum available bolted fault current.
Arc-Flash Metrics • In order to determine the potential effects of an Arc-Flash, we need to understand some basic terms. An Arc-Flash produces intense heat at the point of the arc. Heat energy is measured in units such as BTU’s, joules, and calories. • The amount of instantaneous heat energy released by an Arc-Flash is generally called incident energy. It is usually expressed in calories per square centimeter (cal/cm2) and defined as the heat energy impressed on an area measuring one square centimeter (cm2 ) • If we place instruments that measure incident energy at varying distances from a controlled Arc-Flash, we would learn that the amount of incident energy varies with the distance from the arc. It decreases approximately as the square of the distance in feet.
Just like walking into a room with a fireplace, the closer we are, the greater the heat energy. Tests have indicated that an incident energy of only 1.2 cal/cm2 will cause a second-degree burn to unprotected skin. • A second-degree burn can be defined as “just” curable. For the purpose of understanding the potential effects of an Arc-Flash, you must determine the working distance from an exposed “live” part. • Most measurements or calculations are made at a working distance of 18 inches. This distance is used because it is the approximate distance a worker’s face or upper body torso may be away from an arc, should one occur. • Some parts of a worker may be less than 18 inches away, but other work may be performed at greater distances. The working distance is used to determine the degree of risk and the type of personal protection equipment necessary to protect against the hazard.
Hazard Risk Categories • NFPA 70E, Standard for Electrical Safety in the Workplace categorizes Arc-Flash Hazards into five Hazard Risk Categories (HRC 0 through 4) based on the amount of energy that can be released at a certain working distance during an Arc-Flash event.
What determines the severity of an Arc Flash? • Several groups and organizations have developed formulas to determine the incident energy available at various working distances from an Arc-Flash. • In all cases, the severity of the Arc-Flash depends on one or more of the following criteria: • Available short circuit current • System voltage • Arc gap • Distance from the arc • Opening time of overcurrent protective device (OCPD)
Arc Blast Effect • During an Arc-Flash, the rapidly expanding gases and heated air may cause blasts, pressure waves, or explosions rivaling that of TNT. The gases expelled from the blast also carry the products of the arc with them including droplets of molten metal similar to buckshot. • For example, the high temperatures will vaporize copper, which expands at the rate of 67,000 times its mass when it changes from solid to vapour. Even large objects such as switchboard doors, bus bars, or other components can be propelled several feet at extremely high velocities. • In some cases, bus bars have been expelled from switchboard enclosures entirely through walls. Blast pressures may exceed 2000 pounds per square foot, knocking workers off ladders or collapsing workers’ lungs. These events occur very rapidly with speeds exceeding 700 miles per hour making it impossible for a worker to get out of the way.
Light and Sound Effects • The intense light generated by the Arc-Flash emits dangerous ultraviolet frequencies, which may cause temporary or permanent blindness unless proper protection is provided. • The sound energy from blasts and pressure waves can reach 160 dB, exceeding the sound of an airplane taking off, easily rupturing eardrums and causing permanent hearing loss. For comparison, OSHA states that decibel levels exceeding 85 dB require hearing protection.
Explosions • Explosions occur when electricity provides a source of ignition for an explosive mixture in the atmosphere. • Ignition can be due to overheated conductors or equipment, or normal arcing (sparking) at switch contacts. • OSHA standards, the National Electrical Code and related safety standards have precise requirements for electrical systems and equipment when applied in such areas.
Fires • Electricity is one of the most common causes of fire both in the home and workplace. • Defective or misused electrical equipment is a major cause, with high resistance connections being one of the primary sources of ignition. • High resistance connections occur where wires are improperly spliced or connected to other components such as receptacle outlets and switches. This was the primary cause of fires associated with the use of aluminium wire in buildings during the 1960s and 1970s.
Common Causes The most common cause of Arc-Flash and other electrical accidents is carelessness. No matter how well a person may be trained, distractions, weariness, pressure to restore power, or overconfidence can cause an electrical worker to bypass safety procedures, work unprotected, drop a tool or make contact between energized conductors. Faulty electrical equipment can also produce a hazard while being operated. Electrical safety hazards such as exposure to shock and Arc-Flash can also be caused by: • Worn or broken conductor insulation • Exposed live parts Loose wire connections • Improperly maintained switches and circuit breakers • Obstructed disconnect panels • Water or liquid near electrical equipment • High voltage cables • Static electricity • Damaged tools and equipment
