Assumptions and complacency are two of safety’s worst enemies. If the following story hits uncomfortably close to home for you, it will have served its purpose.

In January 1993, two employees were killed and three were seriously injured by an electrical arc flash at a Texas power station.

The dead employee—an operator in a full flash suit—had racked-in a circuit breaker and sent the close command, but the breaker failed to close. With the breaker indicator showing that it was still open, the operator began racking-out the breaker to troubleshoot the problem with his supervisor looking on. Neither knew that the breaker had received the close command. The breaker had unlatched and begun to close, but the mechanical problem prevented it from completing the operation.

As a consequence, the operator attempted to rack out the breaker even as it was receiving a close command. When he was finally able to move the breaker, the mechanical bind was relieved and the breaker closed while it was partially racked-out. The result was an electrical arc flash and explosion that critically burned the operator and the supervisor and slammed them into wall. Both were killed. The arc flash then traveled around a corner and burned three other workers. All of this carnage occurred within milliseconds.

The operator, in addition to wearing a full-body flash suit while racking the breaker, did everything that National Fire Protection Association (NFPA) and Occupational Safety and Health Administration (OSHA) regulations required. Although the suit provided some protection, it did not save the operator’s life. The moral of this story: If the arc flash burn doesn’t get you, the high-energy blast effects will. Entergy learned the lesson quickly; immediately after this accident, the company began retrofitting all of its rackable breakers rated at 2,300 volts or higher with remote racking systems (see box).

The price for safety is small, but the costs are enormous

Wolf Creek Generating Station, near Burlington, Kansas, is a single-unit 1,200-MW Westinghouse four-loop pressurized-water reactor that went commercial in 1985. The plant is owned and operated by Wolf Creek Nuclear Operating Corp., which in turn is jointly owned by Westar Energy (47%), Kansas City Power & Light Co. (47%), and Kansas Electric Power Cooperative Inc. (6%).

The Wolf Creek plant completed its 15th and shortest refueling and maintenance outage last November, after completing a plant-record continuous run of 506 days on October 7. One of the tasks completed during this outage was the retrofitting of several 13.8-kV and 4,160-V switchgear cubicles with breaker racking systems to protect personnel from the effects of an arc flash incident. Many arc flash accidents involving medium-voltage switchgear are caused by mechanical failure of the breaker racking mechanism.

In the old days

If you’ve been in the power generation industry for more than 20 years, you may remember using very little safety equipment—perhaps a pair of rubber gloves, overalls, hard hat, and safety glasses—to rack a breaker. Thermal protective suits with a hooded helmet were adopted in the late 1990s, but the air supply was limited to the volume of the suit. Today, although our understanding of the dangers of arc flash has improved, the quality and quantity of safety gear haven’t kept pace in many plants. But there are new options many plants are adopting for worker safety.

Members of the Electrical Safety Program at Wolf Creek have studied the arc flash phenomenon for several years and adopted all the standard protective gear available on the market—including bulky 65 calorie/cm2 suits. Staff engineer calculations based on the IEEE Std 1584-2002 and NFPA 70E tables (at the time, the tables stopped at 40 calories) showed that this suit was an appropriate choice for working on 13,800-V metal-clad switchgear. To put these arc flash energy readings into perspective, realize that standard cotton apparel will ignite at about 4 cal/cm².

Defining the problem

The problem facing Wolf Creek’s plant safety committee was that no one person or organization really understood the blast effects or the amount of thermal energy released during a high-voltage switchgear arc flash. In fact, plant engineers, when accounting for the relay and breaker response time, recalculated the worst-case energy release at 67 to 77 calories for the 13.8-kV switchgear. That was significantly more than any protective suit could handle—not to mention the equally deadly blast effects for which there is no protection.

Plant management realized that safer work practices and buying higher-rated safety gear was no longer the answer: Somehow, technicians and bystanders had to be safely separated from the source of the arc flash. The calculated blast/flash distance ranges from 112 ft to 215 ft with the switchgear cabinet door open, but it is just 30 feet with the door closed. Remember to think in three dimensions—the boundary extends from the switchgear cabinet front inside a cone-shaped impact zone. If your switchgear cabinet is sitting on the turbine deck supported by metal grating, then the blast zone could conceivably extend down or up one or more levels and would certainly extend into normal work areas.

Keeping the maximum distance from the source is the best safety practice when racking or working on live switchgear. The Electrical Safety Program staff looked at a variety of options and settled on the latest remote breaker racking option that gave technicians the option to move well outside the "danger zone" when racking in a breaker. The next step was to convince the plant’s Leadership Council that spending several hundred thousand dollars to virtually eliminate the vulnerability to breaker arc flash would be worth it.

Estimating the cost

The $300,000 price tag for upgrading ninety 13.8-kV and 4,160-V breakers is peanuts compared with the human and lost-generation costs of a single arc flash incident. There may be no better way to loosen the purse strings of a stingy accountant than to have him or her watch a video of the surprising toll of an arc flash incident.

With approvals in hand, the Electrical Safety Program staff began their evaluations of alternative remote racking systems. As an intermediate measure, technicians are donning flash suits rated for the calorie rating of the work activity and establishing a flash boundary when any breaker is racked in or out. In addition, all operators at the Wolf Creek Plant are now required to wear fire-retardant work clothes during any electrical work on equipment 50 V or greater.

Making the upgrades

A comprehensive review of available remote-racking technologies revealed the usual range of similarities and differences. All of the designs investigated use some variation of a motorized racking mechanism, which allows the technician to remain well outside the arc flash and blast hazard boundary. Some designs require the cubicle door to remain open while racking the breaker. This approach provides minimal protection for anyone in the arc flash blast boundary but no added protection against mechanical malfunctions such as breaker alignment or shutter problems. Another design allows "door closed" remote racking but does not provide protection for a misaligned breaker or the failure of shutters to open—either of which could produce a fault that could trigger an arc flash blast.

After completing its evaluations, Wolf Creek chose the Safe-T-Rack (STR) remote racking system (Figure 4) from Switchgear Solutions (www.safe-t-rack.com). One of the key selection criteria was that the system required no internal switchgear cabinet rewiring. Because those cabinets are part of the plant’s safety systems, the time and cost to design, approve, and update hundreds of drawings could exceed the capital cost of the STR system. At Wolf Creek, it had already taken several months to accumulate the data and twice that long to complete all the calculations and evaluations. Adding another year or two to the schedule by making changes to the plant’s safety systems was out of the question.

4. Easy storage. A single tether and controller can be used for multiple STR-equipped switchgear cubicles. Source: POWER magazine

5. More than arm’s length. Wolf Creek’s Craig Barbee demonstrates how to rack a breaker with the Safe-T-Rack system while keeping a safe distance. Source: POWER magazine

6. Safety in a box. The Safe-T-Rack system control panel has overcurrent protection for the racking motor should the shutters not open. As the breaker stabs make contact with the malfunctioning shutters, motor current increases and the control panel trips power to the racking motor. Source: POWER magazine
 

7. Square deal. The LimiTilt is a device that attaches magnetically to the front of the breaker cover to detect if the breaker is out of level. This protects the worker should the ground stab hang up or if one side of the racking mechanism isn’t working properly. Source: POWER magazine

The STR system is operated by a remote controller tethered to a small junction box mounted on the inside of the cubicle (Figure 5). The full-function controller keeps all of the needed controls at the operator’s fingertips, including open and close as well as breaker charge. Aircraft-style fittings (Figure 6) ensure longevity in the harsh power plant environment and prevent connections from being made incorrectly. A LimiTilt attached to the breaker (Figure 7) monitors the pitch and roll of the breaker during racking.

The STR’s controls tap directly into the limit switch circuits, eliminating the need to modify control wires or original breaker controls. Unlike competing systems, this one doesn’t require the cubicle door to be opened to attach or operate the remote controller. A typical STR cubicle kit can be installed in a few hours or less by an experienced electrician without having to de-energize the bus.

By the end of the 15th refueling outage, 38 of the Wolf Creek plant’s 90 high-voltage switchgear units had been retrofitted with the STR; the rest are scheduled to be upgraded by the next refueling outage. Low-voltage switchgear (rated at 480 V and lower) will be upgraded for safety in the future.

—Dr. Robert Peltier, PE

Electricity’s dangerous downside

Few electrical phenomena are as dramatic as arc flash. When current leaves its assigned path and short-circuits through the air between conductors, or between a conductor and ground, temperatures in the resulting arc can soar well beyond that of the sun’s surface (about 10,000F) in just a fraction of a second. The host of hazards produced includes electromagnetic radiation, superheated plasma, a sound pressure wave, and shrapnel.

Each arc flash event can cost millions of dollars in medical expenses, equipment replacement and repair, lost work hours and production, lawsuits, and higher insurance premiums. The human toll from arc flash burns and other injuries is especially acute.

A 10-year study by Électricité de France found that arcing caused 77% of all recorded electrical injuries. Similarly, one corporation noted that up to 80% of its electrical injuries involve thermal burns due to arcing faults. The U.S. National Institute of Occupational Safety and Health (NIOSH) tabulated 44,363 electrical-related lost-workday cases from 1992 to 2001. Arc flash burns (Figures 1 and 2) accounted for 17,101 (or about 39%) of those cases.

1.  In your face. Flame-resistant shields and hoods protect the head, face, and eyes against the ultraviolet, infrared, radiant, and convective energies unleashed in an arc flash event. The damaged hood (bottom) protected a worker from serious injury during such an incident. The intense thermal energy produced by the flash caused the fabric to break open and char and the shield to bubble and blacken. Courtesy: Ferraz Shawmut High Power Test Laboratory

2. Hard head. A hard hat worn by an electrician exposed to an arc flash prevented injury from shrapnel propelled by the event. Flame-resistant clothing protected him from thermal burns. Courtesy: DuPont

Although arc flash phenomena have been of concern for a century or more, there are surprising gaps in what is known about them and how to protect workers from their effects. In an effort to fill this knowledge gap, the IEEE and NFPA have joined forces to explore the many aspects of arc flash and accompanying arc blast phenomena and determine how to ensure worker safety when arc flashes occur. This article first reviews these phenomena and then details what the IEEE/NFPA program hopes to accomplish.

Rogues’ gallery

The dramatic release of energy as electrons pass across a narrow arc through air takes many forms. Extreme heat is just one: The temperature at the point of flashover and in the arc can rise to between 5,000F and 30,000F, depending on the current level and other factors.
The energy packed into just a few tenths of a second also generates an explosion that sends pressure waves, debris, molten metal, and a plasma containing toxic gases and particulates expanding outward at hundreds of miles per hour (Figure 3). As a result, workers in the vicinity of an arc flash are at risk of multiple traumas, such as severe burns, smoke inhalation, eye damage, hearing loss, collapsed lungs, fractures, puncture wounds, loss of consciousness, and even death. Survivors may endure permanent disabilities and a reduced quality of life.

3. Bright as the sun. Photos taken at 1-microsecond intervals at the Ferraz Shawmut High Power Test Laboratory in Newburyport, Mass., illustrate the development of an arc flash as a jet of hot, ionized gas elongates from the sides of vertical conductors terminating in an insulator block. The conductors are part of a three-phase system energized at 480 V and 42 kA. The jet, which had its farthest reach in image "d" (shown impacting the vertical plates of a calorimeter), is shown collapsing in "e." Courtesy: Ferraz Shawmut High Power Test Laboratory

The sheer amount of thermal energy released in arc flashes and transferred via radiation, convection, and conduction makes burns the most frequent injury in these events. The extent of burns depends on the duration and temperature of the flash and its distance from the victim(s).

Laboratory measurements of the concussive forces associated with arc flash have found sound levels of 1,400 decibels and pressure levels of 2,160 psi 2 feet from the flash. As fans of heavy-metal music know, hearing is damaged by just 140 dB. Injuries associated with high sound pressure waves usually involve hearing loss but can also include burst eardrums and dislocated ossicles (bones). Elsewhere in the body, the waves can break alveoli in the lungs, producing respiratory distress, and/or cause profuse intestinal bleeding, a ruptured spleen or liver, and progressive peritonitis. They also can cause injuries if they are strong enough to knock personnel down or throw them for a distance.

The burst of energy in an arc flash expels hot gases containing, in part, copper vaporized from conductors, metal vaporized from enclosure walls, smoke from the incineration of insulation, paint and other materials, and copper oxides formed in an arc. The vaporized copper content can be significant: Just 2 to 3 inches of vaporized conductor can yield more than 75 ft³ of copper vapor. The toxic and corrosive plasma has its greatest impact on lungs, skin, eyes, and other mucus membranes.

The electromagnetic radiation produced by an arc flash is highly variable and can occur over a wide bandwidth that includes radio, microwave, infrared (IR), visible, ultraviolet (UV), and X-ray frequencies. Experience indicates that the IR, visible, and UV waves do the most damage.

IR radiation near the visible spectrum (just beyond red wavelengths) can injure the cornea and lens, whereas longer IR wavelengths are absorbed as heat. Excessive visible radiation can damage the skin and cause partial or total loss of sight. UV radiation also can harm the eyes. Ionizing radiation (shorter wavelengths, such as higher-frequency UV and X-rays) can strip electrons from atoms, creating waves of frequencies that can damage DNA and cause cancers.

As the blast wave expands outward at supersonic speed, it turns equipment, tools, and objects in its path into high-velocity projectiles. Equipment enclosures can affect the path and intensity of the energy released in arc flashes. For instance, the energy of an arc blast surging through an open enclosure door is focused through a relatively small opening. Many factors affect the impact of an arc flash event, including the size and shape of the enclosure, the size and placement of its door, and the equipment inside.

A physics-based approach

Any company that generates, transmits, distributes, or uses electricity at high, medium, or even low voltages will find it difficult to completely protect its workers from arc flash and arc blast hazards. The task is even more difficult for users of direct-current systems that cannot be de-energized. The biggest obstacle to protection is the inability to predict the type and magnitude of arc flash effects in a way that would allow the optimal use of personal protective equipment (PPE) and work practices.

The need to quantify arc flash and blast has led to a call for a physics-based approach capable of accurately describing the phenomena. Given their complexity, any description would likely involve an algorithm containing many equations. A key goal of the IEEE-NFPA joint research and testing initiative, which is just getting under way, is to gather data on all of the expressions of energy during arc flash and arc blast events. This data then could be used to help solve the equations. The data also could serve as the foundation of practical safeguards against arc flash and blast for those who work on or near electrical equipment.

The IEEE/NFPA Arc Flash Collaborative Research Project is a multiyear effort encompassing more than 2,000 test protocols. So far, over $1.8 million of the $6.5 million needed to fund the program has been contributed by power companies, manufacturers, and test and certification organizations. Major donors to date include HydroOne, Bruce Power, Underwriters Laboratories, and Ferraz Shawmut.

The initial phase of the project will explore published and unpublished information on arc flash and blast to build a coherent picture of what is known about these phenomena. This will lead to a research and test plan that seeks to tie the electrical characteristics of equipment to arc-fault hazards. The program will go well beyond what was done in the past. For instance, most arc fault studies to date have involved controlled conditions and stabilized arcs between opposing electrodes. In real life, turbulent arcs often occur between parallel electrodes and vary by several orders of magnitude along their length and with time.

The extreme temperatures in arcing faults make burns the predominant injury they cause. As a result, the program will focus on thermal mechanisms. Testing will build on current methods for measuring how arcing faults transfer thermal energy via convection, conduction, and radiation and how these processes affect human tissue and clothing. This includes understanding how much of the electromagnetic spectrum released in arc faults falls into IR wavelengths, as well as evaluating the thermal energy conducted through PPE when a plasma cloud passes over a face shield or clothing. This can involve a significant amount of heat because copper vapor, which boils at 4,500F, makes up much of this cloud.

The program will survey existing protocols for measuring the thermal effects of arcing faults on a human body and, when needed, create new protocols that encompass such variables as arcing time, working distance, frequency, electrode gap, voltage, current, and the number of phases. It will refine existing test methods and instrumentation, such as measuring heat flux and relating it to burns and upgrading calorimeter specifications.

In addition, it will review a NIOSH electrical injury database to identify measures that could have limited injury and create a template for obtaining future electrical incident information. The program also will investigate low-current (1 kA to 10 kA), long-duration arcs, because the energy transfer in such events is often so great that available PPE cannot protect against it.

Beyond thermal testing

The joint initiative also will break new ground by studying nonthermal phenomena associated with arc flash and the hazards they create. It will look to create an initial arc flash energy balance that includes radiation, pressure, and plasma. Following are some of the approaches planned to reach that goal.

The program will measure the intensity of IR, visible, UV, and other potentially injurious electromagnetic energies in an arcing fault and how they affect the body. It will explore such areas as retinal damage due to visible and UV light and may involve placing detectors behind safety goggles and other eye safety gear during arc flashes. It may also evaluate the effects of brief exposure to ionizing radiation in arc flashes.

Because little test or injury data on the high-pressure fronts created in arc blasts are available, the program will seek basic data on their intensity and consequences. It will record sound pressures and frequencies at various distances, angles, and times from a flash source, as well as the injury potential of these waves and how well shields and hoods attenuate them. Other tests will include comparing pressure waves in open areas and enclosures and determining blast pressures in arcs interrupted in one cycle or less. The program will also seek to relate the substantial understanding of pressure waves created by explosives to those formed in arc blasts.

The program will evaluate the shrapnel and molten metal ejected during arc faults. It will create a database for arc flash shrapnel injuries, to allow the frequency and severity of these injuries to be defined. It also will formulate arc flash shrapnel test protocols and use them to relate shrapnel mass and velocity to electrical parameters.

The program will also conduct pioneering research into the toxic and corrosive particles and vapors present in arc flash plasmas and how they impact personnel and PPE. This will involve determining the feasibility of generating toxic materials in laboratory-generated arc flashes and finding ways to define the amount and type of materials generated. This is a challenging area of study, given the variability of the mass, composition, and temperature of arc flash plasma clouds.

The program will evaluate arc flash events in direct current systems, which are commonplace in some industries and which are increasing as the demand for battery back-up energy in information technology systems increases. This part of the program also calls for developing a method to evaluate arcing faults in DC systems.

Finally, enclosure testing will seek to expand current understanding of how enclosures, their doors, and the equipment they contain affect arc flash properties and the potential for injury. Testing of "arc in a box" events will consider such factors as enclosure size and shape, door construction, arc location and orientation, electrical system capacity, grounding, protective-device bus orientation and spacing, and the components contained in the box.

Current PPE and work practices tend to focus on the thermal and radiative effects of arc flash, so they may not protect well against pressure waves, plasma, or shrapnel. An underlying, ongoing theme of the joint initiative will be to understand how well gloves, safety goggles, flame-resistant clothing, safety shoes, and other PPE items protect workers against the full range of arc flash and blast hazards.

Who will benefit

Electricians, mechanics, HVAC personnel, and other plant workers are at risk from arc flash when they work on panel boards, motor control centers, switchgear, transformers, and many other power system elements. Although lockout/tagout programs help prevent arc flash events, they do not eliminate them.

Arcing faults can occur through human error, especially during voltage testing and system shutdown and start-up. Bear in mind that a circuit may remain energized even after it is believed to be shut down because of inaccurate signs and labels, worker confusion, faulty wiring, or poor work practices. Arcing faults might result from damaged insulation, accidental contact with energized components, equipment with incorrect short-circuit ratings, and corrosion and impurities that provide a current path.

Because arc flash and arc blast are ever-present hazards for anyone working on or near electrical equipment, the IEEE/NFPA Arc Flash Collaborative Research Project will be an intensive effort to gain deeper insight into these phenomena. This knowledge will then help create better industry safety standards, work practices, PPE, and equipment to help reduce electrical injuries, fatalities, and property damage. The effort will benefit all stakeholders in the electric power industry, from utilities to equipment and PPE vendors to regulatory agencies and insurers.

The data and recommendations from the joint initiative will help the IEEE and NFPA strengthen arc flash safety standards—specifically, IEEE 1584, "Guide for Performing Arc-Flash Hazard Calculations," and NFPA 70E, "Standard for Electrical Safety in the Workplace." IEEE 1584 provides methods for determining the flash hazard area and how much energy workers can potentially be exposed to when working on or near electrical equipment. NFPA 70E, which is widely cited by OSHA, covers work practices such as employee training, risk evaluation, safe work conditions, approach boundaries, and PPE use. In addition, it is anticipated that the data and recommendations will lead to improvements in electrical equipment and power system designs, enhancements in maintenance tools, instruments, and work practices, and advances in personal protection technology.

The more-refined understanding of arc flash phenomena, and the gains in on-the-job safety that it will make possible, will help companies boost operating efficiency and productivity. It will enable more effective PPE, safer equipment, and better training. Better protection from arc flash also promises to decrease expenses for injuries and property damage, lower insurance claims and premiums, and reduce litigation and workers’ compensation claims.

—Those interested in learning more about, or contributing to, the IEEE/NFPA Arc Flash Collaborative Research Project should contact Sue Vogel at IEEE (732-562-3817, [email protected]) or Mark Earley at NFPA (617-984-7400, [email protected]). A prospectus on the project is available online at http://standards.ieee.org/esrc/arcflash/index.html.