HV-E - High Voltage Safety Operating Procedures
This manual covers the basic procedures in working safely on high voltage systems.
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Chapter 1: Basic Safety Requirements and Procedures
Basic Safety Requirements and Procedures
This chapter provides information on the basic safety requirements and procedures for high voltage electrical installations. Various types of electrical hazards are explained and their prevention is discussed.
- Develop an understanding for the necessity of safety for HV electrical systems
- Learn basic safety requirements and procedures for electrical systems
- Study electric shock hazards and arcing hazards
- Understanding Electrical insulation and its maintenance
Electricity is essential to modern life and all people are dealing with electricity directly or indirectly. Electricity is high-grade energy and working in the proximity of high voltage equipment involves danger. While commercial electricity has been around for over 100 years, the most common hazard of electricity has been electric shock or electrocution. As commercial electric systems grew, other hazardous effects such as arc-flash and arc-blast began to surface. The initiation, escalation, effects, and prevention of electrical arcs have been analyzed and researched since the early 1960’s. Human errors and equipment malfunctions contribute to the initiation of an electrical arc. Engineering design and construction of arc resistant equipment as well as requirements for safe work practices are continuing to target the risk of electrical arc-flash hazard. As the demand for electricity increases, transmission and distribution utility systems are being upgraded. Transformers are being upgraded or replaced with higher KVA ratings and lower impedances at both the utility and industrial/commercial level. Also, as the demand for higher reliability also increases, transformers are being operated in parallel by closing a tie breaker. All of these modifications to the system can cause dramatic increases in the available fault current. More electrical energy throughput is a result of these modifications; however the downside is an increase in the electrical current to feed a fault to existing equipment in industrial and commercial facilities that may now be under-rated to interrupt available fault current. This increase in available fault current can wreak havoc on under-rated and/or improperly maintained equipment. Some of the facts pertaining to electrical hazards in the US are mentioned in the below:
1.2 International standards for safety
Electrical safety standards and guidelines have been developed worldwide to keep pace with the ever growing requirements of electricity. Some of the important standards developed in this regard are:
- Occupational Safety and Health Administration (U.S.)
- National Fire Protection Act NFPA70E Standard for Electrical Safety Requirements for Employee Workplaces
- Institute of Electrical and Electronic Engineers (IEEE)
- American Society for Testing and Material (ASTM)
- Underwriters Laboratory (UL)(Standards and testing requirements for equipments)
- Factory Mutual (FM) (Standards and testing requirements for equipments)
- International Electrotechnical Commission (IEC)
- American National Standards Institute (ANSI) (standards for materials)
- Health and Safety (HSE) UK
- The Electrical Safety Council (UK) is an independent charity committed to reducing deaths and injuries through electrical accidents at home and at work. They are supported by all sectors of the electrical industry as well as local and central government and work to promote safety and good practice.
- Electricity Safety Act 1998 provides for the safety of electricity supply and use and the efficiency of electrical equipment.
- Indian Electricity rules 1956 (India)
1.3 High Voltage Electrical Hazards
High Voltage: Any voltage exceeding 1000 V rms or 1000 V dc with current capability exceeding 2 mA ac or 3 mA dc, or for an impulse voltage generator having a stored energy in excess of 10 mJ. (IEEE Trans Power App. Sys PAS-97, no. 6, 2243, Nov. 1998)
Moderate Voltage : Any voltage exceeding 120 V rms (nominal power line voltage) or 120 V dc, but not exceeding 1000 V (rms or dc), with a current capability exceeding 2 mA ac or 3 mA dc.
A number of factors influence the human body resistance, but IEC has provided 1 kΩ as value. Note: @50 V, body currents are 50 mA. Anything over 50 V must be considered High Voltage. Voltages over approximately 50 volts can usually cause dangerous amounts of current to flow through a human being touching two points of a circuit
An alternating current (ac) with a voltage potential greater than 550 V can puncture the skin and result in immediate contact with the inner body resistance. A 110-V shock may or may not result in a dangerous current, depending on the circuit path, which may include the skin resistance. A shock greater than 600 V will always result in very dangerous current levels. The most severe result of an electrical shock is death. Some of the life threatening effects of current are given below:
- Currents in excess of a human's "let-go" current (>16 mA at 60 Hz) passing through the chest can produce collapse, unconsciousness, asphyxia, and even death.
- Currents (>30 mA at 60 Hz) flowing through the nerve centers that control breathing can produce respiratory inhibition, which could last long after interruption of the current.
- Cardiac arrest can be caused by a current greater than or equal to 1 A at 60 Hz flowing in the region of the heart.
- Relatively high currents (0.25-1 A) can produce fatal damage to the central nervous system.
- Currents greater than 5 A can produce deep body and organ burns, substantially raise body temperature, and cause immediate death.
Serious burns or other complications can cause delayed reactions and even death.
So it is necessary to take proper precautions while working with high voltage systems.
Hazards from electrical equipment can be any of the following:
- Electric shock and associated effects such as:
- Internal organ damage due to passage of electricity through body
- Burns on skin at point of contact
- Injuries by electric shock combined with fall
- Arc flash causing external burns and injuries by explosive expansion of air due to the arc.
- Fall from heights
- Corrosive liquids used in electrical equipment
- Hazards resulting from explosive atmosphere
- Hazards due to high temperature
Although low voltage does not mean low hazard; high voltage can cause higher level of hazards and more severe shocks.
1.4 Electric shock
The main hazard from electrical equipment is, naturally, the danger from electric shock. Electric shock can be a result of contact with live parts such as electrical conductors or with parts of equipment which are not normally live (such as enclosures) but become life due to failure of electrical insulation.
Electric shock is thus a result of the following conditions:
- Exposure to live parts (Direct contact)
- Exposure to parts that accidentally become live (Indirect contact)
- Potential difference between different points in the earth under certain conditions
The last named is similar to indirect contact except that it does not involve contact with any electrical equipment (either a live part or enclosure). Electric shock or electrocution can cause many problems in a human body. Electric current flowing through body results in muscular contraction. If the current flows through heart muscles it can cause stoppage of heart by a condition called fibrillation. Even if an electric shock is not fatal, it can cause other problems such as internal organ damage due to excessive heating of body tissues, burns at the point of contact of the skin with live conductors, loss of consciousness or loss of balance resulting in fall while working at a height.
In some instances an electric shock may not by itself cause an injury, but a resulting fall from a height can as described in a subsequent paragraph.
There are three basic pathways electric current travels through the body:
- Touch Potential (hand/hand path)
- Step Potential (foot/foot path)
- Touch/Step Potential (hand/foot path)
Figure 1.1 illustrates these groups and the path of current through the body:
- 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.
- 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.
- 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. Even though there may be no external signs from the electrical shock, internal tissue or organ damage may have occurred. Signs of internal damage may not surface immediately; and when it does, it may be too late. Any person experiencing any kind of electrical shock should seek immediate medical attention. Using the correct personal protective equipment (PPE) and following safe work practices will minimize risk of electrical shock hazards.
Figure 1.1: Touch and step potential
Electricity and the human body
The effects of electricity on the human body have been widely studied and researched by the IEC and its findings have been published in several reports (IEC publication 479-1, 1984 and 479-2, 1987). The reports contain a number of useful graphs, showing the effects of both AC and DC currents and also the influence of frequency, and are recommended for further reading. From the reports and other sources, it may be observed that an electrical shock, whilst not always sufficiently serious to cause death, can still have a long term adverse effect on a person’s health. Much of the data generated refers to adult people in good health at the time of the shock, but if the victim is a child or a person in poor health, the effects can be more serious. The degree of risk depends not only on current, but also on time – the higher the current or the longer the time of shock, the greater the danger. The effects of current are typically observed in Table 1.1. Table 1.2 lists voltage levels of concern for human beings as per the various published standards.
Table 1.1: Effects of electric current on body
Table 1.2: Summary of Published Contact Voltage Levels of Concern for Humans
The severity of injury is determined by the voltage, current intensity, types of current, the current pathway, the duration of exposure, the resistance of the tissues, contact surface, the extent of multisystem involvement, and the circumstances surrounding the incident.
The high voltage direct current (DC) electrocution tends to cause a single muscle contraction, throwing its victim from the source. These patients tend to have more blunt trauma. Generally, the longer the duration of contact with high voltage current, the greater the degree of tissue destruction. This is true until the tissue becomes carbonized and resistance develops to the current flow. Current is often concentrated at the contact and ground points of the body, where the greatest amount of tissue damage occurs. However, extensive damage may occur between the contact and ground point. Electrical injuries occur from direct contact or as an indirect contact with a power source. Indirect contacts can be separated into arc, flash, thermal, and blunt trauma injuries. An arc injury is the most destructive of all of the indirect mechanisms.
An electrical current will produce an array of injuries if the current passes through the body. Most of the damage is beneath the skin surface and therefore the actual injury can easily be underestimated. There are often several possible components to the injury.
- The first component is the injury caused by the electrical current itself. The current (the current of injury) generates intense heat often in excess of 2000°F along its path through the body, which can lead to severe muscle, nerve and blood vessel damage.
- The second component is the injury from "arcing". Ionization of air particles associated with a voltage drop is called arcing. The heat generated in the arc can be as high as 4,000°C and can vaporize metal. This process frequently causes a patient’s clothing to ignite and cause flame burns. A form of explosion dissipates excess energy from the arc.
- The third component is the skin burn caused by a flash. A flash can result from the power source or from the ignition of clothing or surroundings. A flame burn can occur without underlying tissue injury.
- The fourth component is traumatic injury caused by the intense muscle spasm with the current or from a fall. There is also a variety of cardiac, lung muscle, nerve and internal organ injuries which can occur, some being immediately life threatening.
The pathway of current can be somewhat unpredictable, but, in general, current passes from a point of entry through the body to a grounded site, i.e., a site of lower resistance to flow compared with air, which is a poor conductor. Extremely high voltage sources usually exit in multiple areas in an explosive fashion. Current passing from hand to hand or hand to thorax has a high risk of producing cardiac fibrillation compared to hand to foot passage. Passage through the head is likely to cause an initial respiratory arrest and subsequent severe neurologic impairment.
Burns and their classification
As noted in the foregoing sections, the purpose of arc flash studies and prevention strategy is to ensure that dangerous burns (second degree) can be avoided. Burns are classified in the following manner according to the severity of the injury they cause on human skin.
Burn classification-First degree
- Red and very sensitive to touch,
- Skin will appear blanched when light pressure is applied
- Involve minimal tissue damage
- Involve the epidermis (skin surface)
- Affect the outer-layer of skin causing pain, redness and swelling
- Sunburn is a good example of a first-degree burn
Burn classification-Second degree
- Affect both the outer-layer (epidermis) and the underlying layer of skin (dermis)
- Cause redness, pain, swelling and blisters
- Often affect sweat glands, and hair follicles
Burn classification-Third degree
- Affect the epidermis, dermis and hypodermis
- Cause charring of skin or a translucent white color
- Burn areas may be numb
- Any pain is usually because of second-degree burns
- Very slow healing due to the skin tissue and structures being destroyed Usually results in extensive scarring
Common Complications of Electric Burns
- Ventricular Fibrillation
- Muscle necrosis
- Respiratory arrest
- Renal failure
- Retinal detachment
- Limb loss
- Mental changes
- Cataract (delayed)
Contact point with High Voltage Source
Figure 1.2: Burn from 10,000 Volts
Please refer to Figure 1.2.Injury is from 10,000 volts. There is obvious mummification or total destruction of the hand and the wrist is fixed in flexion as the tendons and muscles of the forearm have been destroyed. The loss of tissue water shortens the now dead tissue. The wound at the elbow crease resulted from the heat of the current as it traveled up the arm.
Electrical burns more closely resemble a crush injury than they do a thermal burn. The damage below the skin where the current passes is usually far greater than the appearance of the overlying skin would indicate. The immediate damage to muscle is caused by the heat, which is usually patchy in distribution along the course of the current, often most severe near the bones.
Within minute of injury the dead muscle releases its red pigment, myoglobin, into the blood stream. The muscle rapidly swells compressing local nerves and blood vessels. An incision through the overlying layers will be necessary to release the pressure (called a fasciotomy).
1.5 Arc Flash
Apart from electric shocks caused by contact with parts that are (or become) live, another major danger for those who work on electrical equipment is the hazard due to arc faults. Such faults are often caused by the affected workers themselves, when they work on or in the vicinity of live equipment and cause a short circuit fault inadvertently. In fact, arc faults in equipment and their potential dangers are subjects of extensive study and have given rise to standards such as IEEE 1584 (Guide for Performing Arc-Flash Hazard Calculations).
Arc flash can also result when safe clearances between a live part and earth are compromised during work. This can result in the intervening air space breaking down and initiating an arc. This is particularly true of exposed overhead equipment such as switchyards.
The most serious hazard of an arc flash is burn injuries resulting from the arc with the seriousness of injury dependent on the following factors:
- Fault energy as given by the fault level of the system (VA)
- Time of fault clearance
For example, the arc energy in an MV system short circuit fault is usually much higher compared to an LV mains circuit fault, which in turn has a much higher energy compared to a branch circuit fault in the same system. The longer an arc fault is allowed to persist, higher the damage. Faults, which are cleared much faster, are therefore much less dangerous from viewpoint of injury.
High-energy faults will also cause melting of components such as copper/aluminum conductors or steel parts of enclosure. Copper is particularly dangerous because it can result in deposition of toxic copper salts on the skin. Internal injuries and also hearing damage can result from the blast pressure and damage to eyes can happen as a result of the bright light of the arc flash.
Sometimes, the sudden expansion of air due to an arc fault within an enclosed space may dislodge mechanical parts such as terminal covers with a great force. Documented cases of such accidents causing injury or even death are on record. It is common practice in design of equipment such as HV switchgear to provide vents or flaps, which open in the event of explosive arc faults thus avoiding damage to the enclosure. They also help to direct the arc products way from an operator who may be stationed nearby. The newer versions of switchgear are built to be arc resistant in which an internal arc is unlikely to cause injuries to operating personnel in the vicinity and the energy is contained within the arc resistance enclosure.
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. Since energy equals power multiplied by time, and power (wattage) is volts X amps, we can see that calories are directly related to amperes, voltage, and time. The higher the current, voltage and time, the more calories produced. To define the magnitude of an Arc-Flash and the associated hazards, some basic terms have been established: 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). However, some calculation methods express the heat energy in Joules/cm2 and can be converted to calories/cm2 by dividing by 4.1868. 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. NFPA 70E, Standard for Electrical Safety in the Workplace categorizes Arc-Flash Hazards into five Hazard Risk Categories (HRC 0 through 4) explained in Table 1.3.
Table 1.3: Incident Energy and Arc Hazard Risk Categories
Figure 1.3: Arc flash hazard category
Studies show that many industrial Arc-Flash events produce 8 cal/cm2 (HRC 2) or less, but other accidents can produce 100 cal/cm2 or more (exceeding all HRC). It is important to remember that it only takes 1.2 cal/cm2 (HRC 0) to cause a second degree burn to unprotected skin.
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)
When a severe enough Arc-Flash occurs, the overcurrent protective device (fuse or circuit breaker) upstream of the fault interrupts the current. The amount of incident energy a worker may be exposed to during an Arc-Flash is directly proportional to the total clearing ampere-squared seconds (I²t) of the overcurrent protective device during the fault. High current and longer exposure time produces greater incident energy. The only variable that can be positively and effectively controlled is the time it takes for the overcurrent protective device to extinguish the arc. A practical and significant way to reduce the duration of an Arc-Flash and thereby the incident energy is to use the most current-limiting OCPD’s throughout the electrical system. 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 vapor. 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 (please refer to Figures 1.3 a and 1.3b). 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
Figure 1.3 (a): Arc flash light and sound effects (photos taken from e-web engineering)
Figure 1.3(b): Arc flash 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.
Effects of Arc Flash
Arcs created by a fault do not remain stationary. The interaction between an arc and the electromagnetic field caused by the fault current will cause the arc to move away from the source point with the arc behaving very much like a conductor placed in a magnetic field. The arc also causes sudden heating of the air in its immediate vicinity causing a violent expansion much like an explosion. This can result in the dislocation of loose components around the fault point and their being thrown like projectiles outwards from the arc. Following are some important effects of arc flash:
- Electric arcs produce some of the highest temperatures known to occur on earth up to 35000 degree Fahrenheit. This is four times greater than of the temperature of the sun’s surface.
- The intense heat from arc causes a sudden expansion of air. This results in a blast with a very strong air pressure.
- All known materials are vaporized at this temperature. When materials vaporize they expand in volume. The air blast can spread molten metal to great distances with force.
- For a low voltage system, a 3 to 4 inch arc can become stabilized and can persist for an extended period of time.
- Energy released is a function of arc voltage, fault current and fault duration.
- Arcs in enclosures, such as Motor Control Centers or switchgears, magnify the blast and energy is transmitted towards the worker as the blast is forced to the open side of the enclosure.
Figure 1.4 is a model of an arc fault and the physical consequences that can occur.
Figure 1.4: Arc fault model
Arc flash hazards
Total arc energy (incident energy) is the instantaneous arc energy multiplied by the arc duration. Conductive vapors help to sustain the arc and the duration of the arc is primarily determined by the time it takes for the overcurrent protective devices to open the circuit. Current-limiting fuses for example may open the circuit in 8.3 ms (1/2 cycle) or less while other devices may take 100 ms (6 cycles) or more to open. We will discuss the ways of reducing arc energy in detail later.
Some of the hazards of arcing fault are:
- Objects: Arcs spray droplets of molten metal at high-speed pressure. Blast shrapnel can penetrate the body. Tools, loose nuts and bolts, and similar items in the path of an arc blast may become projectiles.
- Clothing can be ignited from several feet away. Clothed areas can be burned more severely than exposed skin.
- Pressure: The total force on the worker standing in front of the enclosure may exceed 1000 pounds. Such forces may crush a worker’s chest breaking bones, puncturing lungs or other organs or even propel workers into equipment, walls, windows, etc, causing additional trauma.
Direct and Secondary Burns due to Intense Heat
The electrical current flowing through the ionized air creates tremendously high levels of heat energy. This heat is transferred to the plasma, which rapidly expands away from the source of supply.
Tests have shown that heat densities at typical working distances can exceed 40 cal/cm². Even at lower levels, conventional clothing ignites, causing severe, often fatal, burns. A heat density of only 1.2 cal/cm² on exposed flesh is enough to cause a second-degree burn, in a typical arc fault lasting for less than one second. Even workers not in the plasma can be severely burned from the intense heat radiated beyond typical working distances.
Injuries due to arc flash are known to be very severe. According to statistics from the American Burn Association the probability of survival decreases with an increase in the age of arc flash victims.
The effects of an arcing fault can be devastating on a person. The intense thermal energy released in a fraction of a second can cause severe burns. Tissue damage is directly proportional to time and skin temperature. Studies show that skin temperatures above 205° F for 0.1 second results in irreversible tissue damage, defined as an incurable burn. It should be noted that these are skin temperatures and not the temperature of the source which will be a lot higher. Burns can happen at relatively modest skin temperatures which are primarily determined by the intensity of the flash, the distance from the arc, and the exposure time.
Table 1.4 shows effects for other temperatures and duration times.
Table 1.4: Effect of temperature on body
Effect on skin
Cell breakdown begins
Complete cell destruction
Molten metal is blown out and can burn skin or ignite flammable clothing. One of the major causes of serious burns and deaths to workers is ignition of flammable clothing due to an arcing fault. Synthetic fibers such as nylon and polyester may melt and adhere to skin, resulting in secondary burns. Figure 1.5 shows arc flash burns, which occur during arc flash.
Figure 1.5: Arc Flash burn
Vision and Hearing Injuries
Even with regular safety goggles or glasses, arc flash may cause severe damage to vision and could even in blindness. Intense ultraviolet (UV) light created by arc flash can damage the retina in the eye. Exposure to UV can cause a feeling of grit in the eye, blurred vision, burning sensations, eye tearing, and even headaches. The pressure created from arc blasts can also compress the eye severely, thereby damaging vision. If proper eye protection is not worn, ejected materials and flying particles can come in contact with the eye and cause further damage.
Hearing can be affected by the loud noises and extreme pressure changes created by arc blasts. Sound and noise levels are commonly measured in decibels (dB). OSHA defines the permissible exposure limit (PEL) at 90db. Workers who are exposed to average levels of 85 dB or higher are required to use hearing protection. If the sound increases by 3 dB, it is equivalent to the sound level doubling. Published test data has shown arc blasts to exceed 140 dB, which is equal to an airplane taking off. Sudden pressure changes exceeding 720 lbs/ft² for 400 milliseconds can rupture eardrums. Even at lesser pressures, serious or permanent damage to hearing may occur.
The arc-blast pressure depends on the fault current and the distance from the arc and not to the arc clearing time. This force is significant and can cause falls and injuries to the worker which can result in being more serious than burn injuries. The trauma from pressure waves may not be readily diagnosed in triage because of the absence of external wounds. The tremendous pressure blast from the vaporization of conducting materials and superheating of air can fracture ribs, collapse lungs and knock workers off ladders or blow them across a room. The pressure blast can cause shrapnel (equipment parts) to be hurled at high velocity (possibly in excess of 700 miles per hour).
At high fault current levels, plasma jets are formed at the electrodes. Vaporized and molten electrode material is ejected at high velocity from these jets, reaching distances of several feet away. Since the molten metal is typically over 1000° C, it is a potential ignition source for conventional clothing.
The force of the explosion also causes a significant amount of shrapnel to be accelerated away from the source. These particles can impact a nearby worker at high velocity, resulting in physical trauma.
As the arc is established, an extremely bright flash of light occurs. The light can cause immediate vision damage and increase the potential for future vision deterioration.
Also expelled into the atmosphere are toxic combustion byproducts and copper oxides formed when the cooling copper vapor combines with oxygen.
Electrical Arc Model
The most common cause of Arc-Flash and related 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
The severity and causes of electrical hazards are varied, but the best protection is to deenergize equipment before working on it. No one has ever been killed or injured from an Arc-Flash while working on deenergized equipment. If equipment cannot be de-energized, electrical workers must be “qualified”, trained, wear appropriate personal protective equipment (PPE), and follow all applicable OSHA and NFPA standards. It is important to remember that proper selection and application of overcurrent protective devices (OCPD) will also substantially reduce the hazards. The opening time for various OCPDs is depicted in Table 1.4.
Table 1.4: Opening time of OCPDS
Both OSHA and NFPA 70E require an Electrical Hazard Analysis prior to beginning work on or near electrical conductors that are or may become energized. The analysis must include all electrical hazards: shock, Arc-Flash, Arc-Blast, and burns. NFPA 70E Article 110.8(B)(1) specifically requires Electrical Hazard Analysis within all areas of the electrical system that operate at 50 volts or greater. The results of the Electrical Hazard Analysis will determine the work practices, protection boundaries, personal protective equipment, and other procedures required to protect employees from Arc-Flash or contact with energized conductors. Shock Hazard Analysis NFPA 70E Articles 110.8(B)(1) and 130.2(A) require a Shock Hazard Analysis. The Shock Hazard Analysis determines the system voltage to which personnel can be exposed, the protection boundary requirements as established in NFPA 70E Table 130.2(C), and identifies personal protective equipment (PPE) required to minimize shock hazards. Approach Boundaries
NFPA 70E has established three shock protection boundaries (please refer to Figure 1.6):
- Limited Approach Boundary
- Restricted Approach Boundary
- Prohibited Approach Boundary
Limited Approach Boundary
The Limited Approach Boundary is an approach boundary to protect personnel from shock. A boundary distance is established from an energized part based on system voltage. To enter this boundary, unqualified persons must be accompanied by a qualified person and use PPE.
Restricted Approach Boundary
The Restricted Approach Boundary is an approach boundary to protect personnel from shock. A boundary distance is established from an energized part based on system voltage. Only qualified persons are allowed in this boundary and they must use PPE.
Prohibited Approach Boundary
The Prohibited Approach Boundary is an approach boundary to protect personnel from shock. Work in this boundary is considered the same as making direct contact with an energized part. Only qualified persons are allowed to enter this boundary and they must use PPE. Shock protection boundaries are based on system voltage and whether the exposed energized components are fixed or movable. NFPA 70E Table 130.2(C) defines these boundary distances for nominal phase-to-phase system voltages from 50 Volts to 800kV. Approach Boundary distances may range from an inch to several feet. Please refer to NFPA 70E Table 130.2(C) for more information. In summary, a Shock Hazard Analysis is performed to reduce the potential for direct shock. It will establish shock protection boundaries and determine PPE required for protecting workers against shock hazards.
Figure 1.6: Arc Flash Boundaries
IEEE 1584 Arc-Flash Hazard Calculation
The Institute of Electrical and Electronic Engineers (IEEE) publishes the IEEE 1584 “Guide for Performing Arc-Flash Hazard Calculations.” It contains detailed methods and data that can be used to calculate Arc-Flash Hazards for the simplest to the most complex systems. The Petroleum and Chemical Industry committee of the IEEE spent many years developing these methods. They are based on empirical testing of Class RK1 and Class L fuses, Low Voltage Molded Case Circuit Breakers, Insulated Case Circuit Breakers and Low Voltage Power Circuit Breakers as well as theoretical modeling. Included in 1584 are spreadsheet programs that simplify the calculation of incident energy and flash-protection boundaries. There are many practices that will help reduce Arc-Flash and other electrical hazards while conforming to OSHA and NFPA 70E regulations and guidelines. Circuit designers and electrical maintenance engineers should carefully consider each of the following recommendations:
Design a safer system
When designing a safer system the following goals and factors should be considered:
- Provide maximum protection to personnel, equipment, and property.
- Meet all applicable code requirements (OSHA, NFPA, Building and Insurance codes, etc.)
- Utilize current-limiting overcurrent protective devices to minimize Arc-Flash hazards.
- Utilize “touch-safe” components to minimize exposure to energized components
- Utilize fuses with blown fuse indication to minimize exposure to energized components while trouble-shooting the circuit.
- Provide selective coordination (only the area where the fault occurs is shut-off)
- Provide a system that is safe to service and maintain.
Use and upgrade to current-limiting overcurrent protective devices
The incident energy from an Arc-Flash depends on the magnitude of the current and the time it is allowed to flow. Within their current-limiting range, current-limiting devices reduce the peak fault current. Current-limiting fuses have much faster clearing times when operating within their current-limiting range than standard circuit breakers. The faster the overcurrent protective device clears the fault, the lower the I²t and incident energy will be. If current-limiting fuses are used, the incident energy and the Hazard Risk Category may be reduced significantly.
Implement an Electrical Safety Program
Electrical Safety Programs protect both employees and employers and provide goals, procedures and work practices to insure safety. NFPA 70E Article 110.7 requires employers to establish an Electrical Safety Program that must be documented and include the minimum following components:
- Scope of the Program
- Company Philosophy
- Establishment of a Safety
- Team or Committee
- Written Procedures
- Work Instructions
- Identification of Industry Codes & Standards to be adhered to
- Establishment of a Documented Training Program
- Establishment of Assessment and Audit Requirements
- Company Policies and Enforcement Increased safety will be possible with the implementation and vigorous enforcement of a well-designed and documented Electrical
- Safety Program.
- Observe safe work practices
Safe maintenance practices and procedures include properly training employees in the knowledge of the equipment and tools necessary for maintenance and repair. Test equipment as well as hand tools are often overlooked and must be insulated and rated for the voltage of the circuits where they will be used. All tools and equipment used for maintenance must also be periodically inspected to ensure they are not damaged (i.e. torn insulation) and are still in good working condition.
Operating a damaged disconnect switch, whether it’s a fusible switch or circuit breaker, can be dangerous. Serious injury could occur if someone is standing in front of a faulty switch or circuit breaker while opening or closing the device. If the handle is on the right hand side of the device, stand to the right, use your left hand to grasp the handle, turn your face away and then operate it. If the handle is on the left side, reverse the procedure. Use special caution while operating circuit breakers. If closed into a fault, circuit breakers will trip, drawing an internal arc. The gases from the arc are very hot, and vent through openings in the breaker. These hot gases often vent around the handle and can cause burns unless proper protective equipment is used.
Proper Service or Repair of All Equipment or Devices
- Locate the equipment where work is to be performed. If equipment is running, follow manufacturer’s shutdown procedures being sure that all unit switches are off. Do not open any enclosures. Determine if there is adequate working space and that it is clear of obstructions.
- Locate all disconnecting means providing power to the equipment, including all sources of emergency, alternate, and control power. This must include discharging capacitors and other sources of stored energy. Turn all disconnecting devices to the OFF position and apply lockout/tagout devices as required by company’s Electrical Safety Program.
- While wearing proper personal protective equipment, open the enclosure door or access panels. Test the voltage meter to be used on a known energized source to be sure it is working properly. Test all exposed wires, contacts and other components likely to be energized insuring that the equipment is in an electrically safe work condition.
Equipment containing fuses
- If it is suspected there is one or more opened fuses, remove fuses from the circuit using the proper size fuse puller.
- Place fuses on a non-conductive surface and measure fuse resistance across the ends (endcaps/blades) of the fuse with a meter. If the fuses have knife blades be sure to test from blade to blade since some types of fuses have insulated end caps and will give a false reading. High resistance indicates that the fuse may be open.
- Investigate the circuit to identify the cause of any blown fuses. Look for loose connections or signs of overheating. Correct the problem.
- Verify the proper fuse class, voltage, ampere, and interrupting ratings before installing replacement fuses. (Caution: because fuse characteristics may vary between manufacturers and fuse classes, fuses should be of the same manufacturer and class for each application.)
- Examine fuse clips or mountings for signs of corrosion, overheating, or loss of tension. Service if necessary. Install the replacement fuse with the proper size fuse puller.
Equipment containing circuit breakers
- After following steps as mentioned above, look for circuit breakers and examine to see if any are tripped. Examine the circuit breaker(s) to see if the case or surrounding area shows signs of severe venting indicating a serious fault.
- Investigate the circuit for the causes of circuit breaker tripping. Correct the problem. If breaker is protecting motor starters, especially IEC or single-purpose type, test the motor starters to be sure they are still functional. If the motor starters have heaters (resistance coils) in the overloads, test the resistance across the heaters to insure they are still functional.
- Test resistance across the poles of the open circuit breaker to be sure all poles are open and there are no shorts between poles. Close the circuit breaker and measure resistance across the closed poles to insure resistances are within tolerances and are equal from pole to pole.
Placing equipment in service
- Following manufacturer’s instructions, close all internal switches and circuit breakers and other procedures necessary for start-up.
- Close enclosure door(s) and access panels and check the area for other personnel. Remove lockout/tagout devices following safety program procedures.
- Restore power standing to the side of the switch enclosures.
- Restart equipment following manufacturer’s instructions and exercising caution until satisfactory operation is insured.
Lockout / Tagout Procedures
OSHA requires that energy sources to machines or equipment must be turned off and disconnected isolating them from the energy source. The isolating or disconnecting means must be either locked or tagged with a warning label. While lockout is the more reliable and preferred method, OSHA accepts tagout to be a suitable replacement in limited situations.
- Make necessary preparations for shutdown
- Shut down the machine or equipment
- Turn OFF (open) the energy isolating device (fuse/circuit breaker)
- Apply the lockout or tagout device
- Render safe all stored or residual energy
- Verify the isolation and deenergization of the machine or equipment
Removal of Lockout / Tagout Devices
- Inspect the work area to ensure that nonessential items have been removed and that machine or equipment components are intact and capable of operating properly. Especially look for tools or pieces of conductors that may not have been removed.
- Check the area around the machine or equipment to ensure that all employees have been safely positioned or removed.
- Make sure that only the employees who attached the locks or tags are the ones that are removing them.
- After removing locks or tags, notify affected employees before starting equipment or machines.
Use Personal Protective Equipment (PPE)
The proper selection and use of Personal Protective Equipment will significantly reduce the risk of Arc-Flash and other electrical hazards to personnel working on energized equipment. Employees working in areas where there are potential electrical hazards shall be provided with, and shall use, electrical protective equipment that is appropriate for the specific parts of the body to be protected and for the work to be performed.
A variety of PPE is available from numerous manufacturers. The most common types of protective gear include:
- Nonconductive flame-resistant head, face, and chin protection (hard hats, full face shields, switching hoods, etc.)
- Eye protection (face shields, safety glasses, goggles)
- Body protection resistant to flash flame (shirts, pants, jackets, coveralls)
- Hand and arms protection (insulating gloves and sleeves with leather protectors)
- Foot and leg protection (insulated leg and footwear)
- Insulating blankets or mats
Selection of PPE is dependent on the task to be performed. NFPA provides guidance for the selection of personal protective equipment to be used for specific tasks and hazard levels. The Table of PPE requirements below provides typical clothing requirements for Hazard Risk Categories from 0 through 4.
Note: Hazard Risk Category 0 still requires some level of protective clothing or equipment. Manufacturers have also developed tables and selection guides based on NFPA 70E recommendations. It is important to note that the level of PPE recommended by NFPA 70E is:
“intended to protect a person from arc-flash and shock hazards”. Even with PPE, some arc-flash conditions may result in burns to the skin or include arc blast pressures, toxic vapors, and propelled particles and materials. PPE that is selected should be rated for, or greater than, the minimum Arc-Flash rating required for each Hazard Risk Category.
Common Personal Protective Equipment Terms and Definitions
Arc Thermal Performance Exposure Value (ATPV):
The incident energy level (in cal/cm²) that can cause the onset of a second-degree burn as defined in ASTM F 1959 Standard Test Method for Determining the Arc Thermal Performance Value of Materials for Clothing. Personal Protective Equipment will be labeled with a calorie rating (Example: 11 cal/cm²).
Tools and gloves rated and tested for the line-to-line voltage at the area where the work is to be performed.
Flame Resistant (FR):
“The property of a material whereby combustion is prevented, terminated, or inhibited following the application of a flaming or nonflaming source of ignition, with or without subsequent removal of the ignition source.”
Breakopen Threshold Energy (EBT):
The incident energy level which does not cause flame resistant (FR) fabric breakopen and does not exceed second-degree burn criteria, as defined in ASTM F 1959. Standards such as OSHA also specify that protective gear must be maintained and periodically inspected to ensure that it remains in a safe and reliable condition. It is also extremely important to avoid contamination of PPE material. Contact with grease, solvents, and flammable liquids may destroy the protection. Typical protective clothing systems for various hazard risk categories are shown in Figure 1.6 .
Table 1.6: Typical Protective Clothing System for Various Hazard Risk Category
Table 1.7: Insulating Glove Application
Details of insulating glove application for various voltage grades is provided in Table 1.7.
List of applicable standards for PPE is shown in Table 1.8
Table 1.8: Standards for PPE
Use Warning Labels
The National Electrical Code recently recognized Arc-Flash hazards and developed a warning label requirement. NEC Article 110.16 states:
Switchboards, panelboards, industrial control panels, meter socket enclosures, and motor control centers that are in other than dwelling occupancies and are likely to require examination, adjustment, servicing, or maintenance while energized shall be field marked to warn qualified persons of potential electric Arc-Flash hazards. The marking shall be located so as to be clearly visible to qualified persons before examination, adjustment, servicing, or maintenance of the equipment.” While the overall requirement is very comprehensive, the required label format can be very generic. However, if a complete electrical hazard analysis is performed, the preferred approach would be to include the Hazard Risk Category, Flash Protection Boundary, Incident Energy available, level of PPE required, system voltage, and shock protection boundaries on labels. See Figure 1.7 for examples of typical warning labels.
Figure 1.7: Warning Label for Arc Flash
The use of detailed warning labels not only increases safety, but also minimizes the time required to identify minimum levels of PPE. Other types of warning labels should also be used to include information about proper fuse replacements, location of disconnects and other sources of power, etc. Warning labels can be applied directly to pieces of equipment or on enclosure doors. Computer programs and adhesive blank labels make it easy to create labels for almost every purpose.
Avoid Hazards of Improperly Selected or Maintained Overcurrent Protective Devices
Whether in the design or maintenance of an electrical system, hazards exist if the proper overcurrent device is not selected and applied. Circuit breakers and other electrical equipment must be maintained and serviced regularly to ensure that they will operate properly when needed. Unfortunately, in many industries and especially during economic turndowns, the tendency is to limit or eliminate regularly scheduled maintenance on circuit breakers and other electrical equipment. However, the potential costs associated with OSHA violations, liability lawsuits, workers compensation, equipment replacement, and lost production far exceeds the costs of regular testing and maintenance of circuit breakers and other electrical equipment.
Most employers and employees understand the analysis of electrical shock hazard but very few understand the electrical arc-flash hazard let alone how to properly perform an analysis. There are many pieces to this puzzle but after we analyze each of the pieces carefully we will find that they all fit together in a manner that provides electrical workers the protection they deserve.
1.6 Other hazards
Table 1.9: Electrical hazards in different equipment
Type of equipment
Electric shock, arc flash, mechanical hazards
Electric shock, arc flash, fire hazard, fall from heights
Overhead Transmission/distribution lines
Electric shock, arc flash, fall from heights
Electric shock, arc flash, fire hazard
Electric shock, arc flash, thermal hazard, fall from heights
Electric shock, arc flash, thermal hazard, fire hazard, mechanical hazard
Electric shock, arc flash, thermal hazard, mechanical hazards
Electric shock, arc flash, thermal hazard
Electric shock, arc flash, thermal hazard, fall from heights
Uninterrupted power supplies with battery
Electric shock, arc flash, hazards from corrosive liquids and explosive gases
Additional hazards include:
- induction from other circuits and communications equipment such as radio transmitters
- build up of static charges due to weather conditions
- feedback from secondary or tertiary systems
- stored energy in high voltage capacitor banks
- working under or over other live conductors.
1.7 Electrical accidents and safety measures
We will briefly discuss in this section about why electrical accidents happen and how we can avoid them. These points will be elaborated in subsequent chapters in further detail. Electrical accidents happen mostly as a result of the following:
- Failure to isolate or inadequate or insecure isolation of live parts
- Poor maintenance and faulty equipment
- Insufficient information about the system being worked on
- Carelessness and lack of safety procedures
Isolating normally live equipment before starting any work on it can improve safety substantially in any system. We must however bear in mind that there are certain kinds of equipment where live work is possible and certain kinds of activities where work in the vicinity of exposed live parts is unavoidable. But such work must be carried out according to well laid safety procedures.
The other major cause of accidents is faulty equipment (which can include both poorly designed or improperly operating equipment). Unless safety is built into the design of the equipment, it can result in accidents and injury. Similarly, improperly maintained equipment too can result in failures and thereby cause accidents. Insufficient knowledge of operating personnel, lack of familiarity with equipment and system etc. too can result in unsafe situations. Absence of proper operational safety procedures and violations of existing procedures can both result in accidents.
The following are the general safety measures, which need to be adopted to reduce the possibility of accidents in electrical equipment.
- Safe design/installation of plant and equipment as per applicable codes and regulations
- Safe operating and maintenance practices established through documented procedures and instructions
- Appropriate knowledge on the part of workers by proper training and certification
- Posting clear warning signs at points of hazard
- Use of equipment/sensors to warn incipient problems with automated hazard containment measures
- Proper periodic inspection and prompt repairs
- Use of personal safety equipment mandated in safety procedures
- Creating an organizational safety structure to handle safety issues, lapses and accidents
- Create safety awareness among the workforce
We will discuss these measures in detail in the ensuing chapters
Remember the ‘safety clearance’
Definition: Safety clearance is the minimum distance any, part of a persons body or any work tool may encroach to any unearthed, bare LV conductor or to any unearthed and unscreened MV/HV conductor. Table 1.10 mentions the rated voltage and the corresponding safety clearance.
Table 1.10: Safety clearance
Up to 11 KV
Exceeding 11 KV but not exceeding 33-KV
Exceeding 11 KV but not exceeding 132-KV
Exceeding 11 KV but not exceeding 275-KV
1.8 Basic safety requirements
Basic safety requirements can be summarized as follows:
- Plan the job from start to finish, & think about the things could go wrong.
- Use procedures, drawings & other documents as tools to help you do the job properly.
- Don't work alone - in the event of an emergency another person's presence may be essential.
- Know the emergency procedures to follow in case of an accident i.e. put any emergency buzzer available or call for ambulance etc.
- Never enter alone into an area containing exposed electrical energy sources.
- Use only the test instruments, and insulated tools rated for the voltage and current specified.
- Always keep one hand in your pocket when anywhere around a powered line-connected or high voltage system.
- Wear rubber bottom shoes or sneakers.
- Don't wear any jewelry or other articles that could accidentally contact circuitry and conduct current, or get caught in moving parts.
- Set up your work area away from possible grounds that you may accidentally contact.
- Know your equipment.
- Don't attempt repair work when you are tired. Not only will you be more careless, but also your primary diagnostic tool - deductive reasoning - will not be operating at full capacity.
- Finally, never assume anything without checking it out for yourself! Don't take shortcuts!
1.9 Basic safety procedures
- De-energize the equipment at least twice prior to beginning work. Make sure that the controls applied will prevent operation of the equipment and that all hazardous energy, including residual or stored energy, is blocked, discharged, or relieved prior to starting work.
- Open and lockout the main input power circuit breaker.
- Check for auxiliary power circuits that could still be energized.
- Inspect automatic shorting devices for proper operation.
- Short the power supply with