|Home | Articles | Forum | Glossary | Books|
This Guide focus on: 1. Electrical engineering--Safety measures. 2. Electricity--Safety measures. 3. Electric apparatus and appliances--Safety measures.
[Note: "Tables" and various equations (denoted by "e.") are not yet avail., but coming soon.]
From the beginning of my career in electric power engineering, safety has been an important topic. The first training provided by my new employer, after we had completed the paperwork and received an introduction to the company and its many products and services, was a series of safety training talks and films.
The graphic nature of this material can cause some discomfort, but it was looked on as the only way to communicate the severity of the problem.
When arc flash protection became the law in the United States, instituted in a peculiar fashion by OSHA, which meant that industry had to follow the private industry consensus standard NFPA 70E, all of this changed. Soon I found myself toting two large duffel bags of safety gear to the arc flash hazard and electrical safety classes I was teaching to electricians, engineers, and managers in industry. The most telling moments of the course were in the showing of a video, "The Mark Emdee Story", which was a somber moment in the proceedings, after which I had to pause and let people reflect on what they had just witnessed. This was the story of a man who was going to work on energized high voltage electrical equipment, when he was injured by a severe arc flash, receiving second- and third-degree burns over 40% of his body. After months of excruciating treatment and rehabilitation in a burn center, he recovered fully, and was able to tell his story. Mark Emdee is now an electrical safety speaker and trainer, spreading the message of electrical safety. After that video, the course covered many aspects of electrical safety, shock hazards, and arc flash protection. Teaching the course was the beginning of my interest in the field, which led to this guide.
There was, in fact, an incident where I worked which bore many similarities to the Mark Emdee story, and I visited one of my colleagues in a burn unit, where he was swathed in bandages, lying in a hospital bed, and unable to speak. He also, has since recovered and returned to his electrical career. But many are not so lucky, and the number of fatalities is still unacceptably high.
Another episode, of which I was aware, was when an engineer went to measure the voltage and current of energized electrical equipment.
Here, one attaches voltage leads to the "hot" conductors, for example, putting an alligator clip around the end of a bolt and clamp-on current probes around a conductor. The current probes had an iron core, and when they were opened, the conductive iron was exposed, and an arc occurred to an energized conductor, causing severe burns, and sending the engineer to the hospital.
A third example was the case of the motor control centers (MCCs). Here, an experienced engineer and a newly hired engineer were doing troubleshooting of some low voltage, 480 V, MCCs. This work involved taking measurements with a digital voltmeter (DVM) of the voltages on the equipment. The lead engineer had to step out for a minute to answer a phone call, and the younger engineer continued working, taking more measurements.
When the first engineer returned to the room, he found the other engineer knocked down on the floor, and severely burned. This was because the next MCC was a 4160 V high voltage unit. This tragedy should never have happened. The first mistake was inadequate preparation and planning. The tasks should have been clearly laid out, the equipment to be worked on identified, and safety procedures put in place. All personnel who work on specific equipment are required to be trained in that equipment in addition to their general safety training. This training did not occur for the high voltage MCC, because it was not part of the work scope. When the first engineer left the room, all work should have stopped. The rule is never to work alone on electrical equipment. No measurements should be taken on any equipment unless the expected voltage level is known, and the appropriate test equipment is used. In this case, VOMs should never be used on high voltage circuits.
Electrical accidents have been relatively common in the industry. Incidents such as these are readily preventable, but it takes knowledge and organization to provide effective protection. With the advent of more comprehensive safety programs, safer equipment, greater awareness, and improved arc flash protection, they are fortunately becoming rarer.
Electrical safety is an often-neglected area of electrical engineering. There has been a wide-ranging and pervasive set of changes taking place in attitudes toward electrical safety. Beginning with the Institute of Electrical and Electronics Engineers (IEEE) annual Electrical Safety Workshops, and with new and updated safety standards, the process of changing the electrical safety culture has been changing the world. The earlier attitude toward electrical safety was that industrial production took priority and that if it was necessary to take risks by working on live equipment, this went with the job. This was compounded by a lack of safe work procedures, inadequate safety equipment, and unawareness or indifference to the terrible human cost of industrial accidents.
It has become clear now that electrical injuries are not acceptable. People's lives and health should not be sacrificed for the sake of production. An occupational health and safety policy (AIHA, 2012) should commit the organization to "protection and continual improvement of employee health and safety." The ultimate goal of electrical safety is "prevention by design," which is designing or redesigning equipment and systems such that they are safe to work with in the first place.
The word electricity is derived from the Greek "elektron," for amber. This substance, a fossil tree resin, produces static electricity when rubbed on cloth or fur. Everybody is familiar with the "tingle" of electricity when touching a household conductor, 120 V or higher. Children have been electrocuted while playing with electrical outlets and sticking objects into the openings. Electricity has been the cause of innumerable fires, in homes and elsewhere, which are surely also electrical accidents.
Electricity is a hazardous substance, just as arsenic is hazardous, or any of hundreds of other materials which cause injury on exposure, contact, or ingestion. This has not always been considered to be the case, because electricity is invisible, odorless, and colorless. Electricity travels through solid materials, as well as through gas and liquids, and even vacuum.
Furthermore, electricity consists of two parts, a physical flow of charged particles and a nonphysical flow of energy in force fields. So the entire concept of electricity as a substance is nebulous. But it is a substance which has its own precise definition, its characteristics, and its very definite hazards. Exposure to electricity can cause injury and death just as surely as exposure to more conventional hazardous substances. The same methodology of hazard analysis and risk assessment, preventive and protective measures should be followed with electricity as with other dangerous materials (Mitolo, 2009a). The complexity and ubiquitous nature of electromagnetic phenomena, however, put them in a different category than other hazards, and warrant their special treatment.
Electricity has always been known to be hazardous and to have significant biological effects. The early experiments of Volta with frog's legs are known to all. The muscular contractions caused by the flow of electricity through living tissue are a significant cause of injury and death. The reaction from somebody touching an energized conductor can cause them to jerk their arm and be bruised or cut or throw them across the room. Internal muscular contractions can cause invisible injuries which only show up much later or they can cause cessation of breathing or of the heartbeat, resulting in immediate death. Protection against contact with energized electrical conductors is an essential safety practice.
The well-known experiments of Franklin showed that lightning is the flow of electricity in the air, and the electrical energy can be collected for scientific analysis and human use.
Lightning has been the major source of electrical injury and death throughout all of human history. Lightning has first of all and most dramatically caused death by direct strike to the person. A direct strike will first of all kill by the flow of a large current, often thousands of amperes, through the body. At this level of current, muscular contractions are not an issue.
The flow of current causes heating, as it does in any conductor, causing severe burns, both internal and external. While there have been many stories of miraculous escapes, lightning can, and does, do to people what it does to trees. Who has not seen the burned and charred remnants of a direct stroke on a tree, usually damaging only part of it, causing the trunk to split and branches to fall off? The tree may live, with partial remnants of living tissue giving continuing life to some branches. What is more rarely seen is the death and destruction of a tree. The tree is totally burned, inside and out, leaving a forlorn and blackened stick. This can and does happen to people as well as trees.
Fires caused by lightning, both in forests and in human structures, have probably killed far more people than the electricity itself.
Lightning contains many high frequency components, and tends to travel along the surface of objects, easily jumping from one conductor to another. Owing to the high potential, materials which are not normally good conductors of electricity will nonetheless conduct large amounts of electricity. The dangers associated with lightning are now brought to nearly every home and workplace through the ubiquitous electrical power and communication systems in place. Lightning currents flow through the earth as well, once the stroke has hit the ground. Anybody standing on the ground or touching an object may find current passing through their body.
Protection against lightning and its effects is a major part of electrical safety.
Sometimes relegated to mandatory safety training courses and routine safety meetings, nothing has a greater effect on the electrical workers health and well-being than electrical safety practices. Electricians, power line workers, electrical and electronic technicians, laboratory scientists and researchers, students and instructors in teaching laboratories, field engineers, and many others are exposed to the hazards of electricity on a daily basis. This extends to nonelectrical workers who may be exposed because the electrical work is in or close to their work area. The general public is also at risk, both in their homes and with outdoor conductors such as power and communication lines. The results of an electrical accident can be catastrophic, ranging from shock injury, through electrical burns to arc burns, pressure waves, and shrapnel injury.
Yet this subject is rarely taught in colleges and universities, and has but a small literature. The electrical safety measures used in industry are not always applied in the electrical engineering laboratories of educational institutions. The majority of books on electrical safety are for the practicing engineer, and not of much use to the average student. It is a certainty that while standards and industry guidelines, and the law itself, are important in many ways, the foundations of electrical safety both in the electrical theory and the physical effects on the body are in science and engineering. These principles do not change, while technology and regulations are constantly changing. Electrical safety has a firm foundation in science and this should be understood. Unfortunately, it sometimes seems that a subject is not considered serious or important unless it is treated in a rigorous manner, working from the fundamentals of physics and mathematics.
Safety is such an important topic that it must be a respectable subject at any level. This guide will give all due respect to scientific fundamentals and their application to real life. It is the aim of this guide to introduce the subject of electrical safety to a wider audience, and have it become part of the preparation of every engineer.
Bioelectricity has many effects other than the hazards discussed here. Devices such as pacemakers and defibrillators have saved countless lives. The measurement of the electrical characteristics and electrical activity of the human body have proved essential in ECG, EEG, and other techniques. The uses of electricity and electromagnetic effects in health care are immense and are only going to grow in the future. As such, this guide is not intended to be an in-depth treatment of the medical and forensic aspects of electricity; these topics are more than adequately covered elsewhere.
The sections of this guide are designed to provide an introduction to theory followed by a series of practical applications. Following this introduction, the second Section provides an introduction or review of the mathematics used in analyzing electricity and magnetism dynamically in three dimensions. While three-dimensional partial differential equations are an extremely difficult concept to visualize and understand, it is crystal clear that they are essential to understanding the flow of electricity through space and the human body. Directly related to the mathematical background, and connecting to the physical world are the fundamental physical equations. Just as Newton's laws are the foundation of dynamics, providing the tools to analyze the motion of solid bodies through three-dimensional space, Maxwell's equations are the foundational description of electromagnetism in physics. As any physicist will tell you, Maxwell's equations are not a true description of electromagnetism any more due to advances in fields such as relativity and quantum mechanics. For the human scale world in which we live, their accuracy is unquestioned.
The next three Sections examine the electrical fundamentals of resistance, inductance, and capacitance as applied to the human body.
Resistance, covered in Section 3, is the electrical analog of friction, opposition to current flow. While this may seem a simple manner, and we are all familiar with the algebraic formulation of Ohm's law: V = IR, when considered in a three-dimensional body with electrical and magnetic fields of varying frequency and intensity, resistance becomes a complex matter. The material in which the resistance exists is a conductor and has the property of conductivity or its inverse, resistivity. Since the human body is amorphous, unlike a well-defined electrical conductor or resistor, the flow of current is ubiquitous and changing, necessitating a more broad-based approach to resistance. In addition to current flow, resistance also concerns heating, the result of the dissipation of power, producing energy flow. This in itself will make resistance a crucial aspect of electrical safety, as we have mentioned the deleterious effects of current causing electrical burns when the body dissipates excessive electrical power.
Capacitance, covered in Section 4, is the capability of "space" to store electrical energy.
This energy can be put into space and returned by moving electrical potentials. These potentials are usually considered as voltages on conductors. When this stored energy is returned to a conductor, the possibility of electrical injury occurs. Capacitance is the measure of the amount of electrical energy which can be stored in a given physical situation. Capacitance may exist in the storage of a physical charge as in static electricity.
Capacitance also exists in relation to electrical conductors, where the charge in the conductors may be either fixed or in motion. The material, or dielectric, in which the capacitance exists, affects the amount of energy storage. The most common safety hazard of capacitance is the discharge of a discrete, fixed capacitor, causing electric shock, burns, sparking, and arcing.
Capacitance exists in other electrical equipment besides fixed capacitors. Power lines and cables are an example of what is called a distributed capacitance, where the energy storage is spread out over a distance. What is a few microfarads per meter may result in a large and dangerous energy storage device. Capacitance must be considered in the analysis of electrical circuits which people may come in contact with.
Capacitance exists within the human body and between the human body and other objects, which will affect the occurrence and magnitude of electrical shocks. Capacitance in "space" is a fundamental part of electromagnetic waves, which propagate everywhere and can have harmful effects.
Inductance, which is examined in Section 5, is the capability of "space" to store magnetic energy. Normally, we think of magnetism as a fixed quantity, such as produced by a bar magnet. For the magnet to produce its physical effects of attraction and repulsion, magnetic energy must travel from object to object where there may be no physical medium.
Electromagnetism is magnetism produced by the flow of electrical current, both constant and changing. As with capacitance, energy can be stored in "space" and returned by currents (instead of potentials) in conductors. The material in which magnetism exists is measured by the property of permeability, or the "resistance" to the flow of magnetic energy.
Inductance, the measure of how much magnetic energy can be stored, applies both to discrete inductors, and to distributed inductance, as in cables and transmission lines. The magnetism has effects on the human body, and on conductive objects which we may come in contact with, causing induced currents which may be harmful. Magnetic fields may also have microscopic effects, affecting the more elusive effects of electrical shock. As with capacitance, the magnetic energy storage in "space" is an integral part of the electromagnetic wave phenomenon, which can have harmful effects.
The calculation of inductance is a difficult problem; indeed, even the definition of inductance can be problematical owing to its complex physical dimensions.
Electromagnetism causes the physical motion of conductors which are exposed to it, just as does fixed magnetism. The hazardous physical effects of stored magnetic energy include the motion of conductors, such as power lines, and the destruction of objects, such as fixed inductors or coils, by the forces from excessive current flow.
Section 6 concerns the electrical properties of the human body, and how they affect the propensity for electrical injury. On the simplest level, the human body can be modeled as an electrical equivalent circuit, consisting of discrete resistance, inductance, and capacitance in the various human body parts. The most important and complex of these is the skin, through which electric current must pass to enter into the body. Internally, the current will spread throughout the body in proportion to its internal electrical properties. This level of analysis is used in determining, for example, the effect on the heart for electrical contact by hands, feet, and other body parts. On a deeper level, the human body is considered as a grouping of regions of varying degrees of resistivity, dielectric strength, and magnetic permeability in which electromagnetic fields interact. These interactions, in turn, can affect the operations of the internal organs.
This is followed by an analysis of the effects of current on the human body. Section 7 extends the electrical properties of the human body to the effects of current on the human body. This will range from an unfelt shock all the way up to death. As the saying goes "current kills." The effects are generally arranged into amount of current (mA or A) versus effect. The flow of current through the human body is analyzed, with especial emphasis on the International Electrotechnical Commission (IEC) methodology.
Safety in substation grounding is the first practical application, focusing on step and touch potentials. Section 8 examines the design and analysis of ground grids. These are primarily underground structures which are installed to mitigate the effects of electrical hazards for people in electrical substations and other locations where hazards exist from the flow of electricity in the earth. Electrical hazards in substations are mainly caused by ground faults, which are occur when an energized conductor comes in contact with the earth, whether by insulation failure, through arcing, or broken conductors. The ground fault current will disperse depending on the earth resistivity and the presence of underground metal objects. One form of danger is the "step potential" where the voltage drop across the earth is sufficient to cause current flow through the feet and body of a person standing on the ground. Another significant danger is the "touch potential" where a person standing on the ground touches a conductive object, such as a metal support, and is exposed to a dangerous potential between their hands and feet. Ground grids, usually a network of copper conductors in a rectangular array, reduce the resistance across space within the grid, and with remote earth through ground rods, will, if designed, installed, and maintained properly, lower the step and touch potentials, increasing worker safety.
Section 9 examines the effect of high fault currents, both short circuits between current-carrying conductors and ground faults, on electrical equipment and the potential hazards this presents to people in the vicinity.
Typical effects include bending of busbars, shattering of insulators, and failure of protective equipment, such as circuit breakers.
When high fault currents enter a ground grid, they can cause failure of connections and conductors, and the drying of soils, which increases earth resistivity.
The multigrounded distribution system and the effect of ground return currents are examined next. Section 10 goes into stray currents, which are usually continuous currents, not caused by high current faults and which flow outside their intended path. A current should go from the power source to the load and return through the intended insulated conductors, not exposing any persons to the current flow. Current which flows through unintended conductors such as safety grounding conductors, electrical conduits, conductive water pipes, or through the earth itself may cause harm to both people and animals, particularly in agricultural situations. These current flows, while elusive at times, and difficult to measure, may, in fact, be analyzed in many cases, and their levels and routing predicted. Remediation for stray currents is specific to each situation. Improper grounding and bonding (interconnection connection of noncurrent-carrying conductive objects), inductive and capacitive coupling, incorrect wiring of neutral circuits, and degradation of insulation systems are some of the causes of stray currents. Often, more than one factor may be at play in a particular case.
Section 11 covers the topic of arc flash hazards.
Arc flash hazard analysis is based on industry standard procedures set forth in NFPA 70E (NFPA, 2014) and IEEE 1584-2002 (IEEE, 2002b) (IEEE, 2004). Arc flash has come into prominence as an electrical hazard in recent years, as the seriousness of the hazard has become apparent. When a short-circuit current flows through air, the initial spark can become an arc. The arc escalates to a high temperature plasma (many thousands of degree Celsius). The great heat is first transferred outward through radiation. This can cause severe burns, second degree and higher, on unprotected or insufficiently protected skin. Heat will then be transferred by convection caused by expansion of the hot gases. The rapid heating of the air will produce an acoustic pressure wave capable of causing total hearing loss. The pressure of the expanding air can bend the steel walls of an electrical cabinet, blow doors off their hinges, send shrapnel in all directions, and throw people through the air for long distances. The vaporizing of the conductors where the arc occurs will result in molten metal globules traveling at high velocity, which produce severe burns and other injuries. Because of the great number of arc flash-related injuries, mostly severe burns, standards have been put in place to reduce arc flash hazards and provide improved personal protection in case one is exposed to an arc flash.
Sections 12 through 15 deal with the effects of short-circuit currents on various portions of the electrical power system. Section 12 deals with the effect on protection and metering. Section 13 deals with the effects of high short-circuit currents on circuit breakers. When the systems which are in place to protect against short circuits fail to operate correctly, either due to measurement errors or due to equipment failure, a significant hazard is created. Failure to interrupt a short circuit can cause fires and explosions which damage persons and property.
The first half of Section 13 covers high voltage circuit breakers and their failure modes when attempting to interrupt short-circuit currents.
The second half of Section 13 provides a survey of a variety of international standards for the testing of low voltage circuit breakers. The intention is to provide a narrative and coherence to the sometimes complex and difficult-to-read standards documents. It is not intended to provide all the information on all the tests, but to help the user to select the right standard and provide a brief introduction to its methods. Finally, a short section introduces the testing of high voltage circuit breakers. Section 14 deals with the mechanical forces caused by high short-circuit currents, particularly on substation equipment. Section 15 covers the effect of high short-circuit currents on transmission lines, conductors, and insulators.
Section 16 is an introduction to the effects of transients, in particular those caused by lightning, in electrical power systems. High transient voltages and currents are a significant cause of electrical injuries, including both shock and fire hazards.
Writing a guide of this nature is a monumental task that has progressed over the course of many years. The list of people who have helped and encouraged me is very long, and I sympathize with the actors at an awards ceremony who must thank everybody who has helped make their success possible. Although I have never met him, I must begin with Mark Emdee, whose safety training video really opened my eyes and put me on the trail of explaining electrical safety to the world. It is impossible to name all of the many people I have worked with over the years that were instrumental in my learning the safety procedures and great hazards of electricity.