Without warning, smoke rolled out from under the tires as they squealed against the pavement with the brakes locked up. The big truck seemed to come from nowhere. It felt like an eternity; although it was really only a matter of seconds, then … CRASH! As the driver and passenger began to absorb what had happened, they looked at the wreckage and saw that the car’s airbags had deployed and the seatbelts kept them in one place. Often taken for granted, the car’s “PPE” worked. Fortunately, most people will never experience a wreck like this and find out first hand.
Electrical personal protective equipment (PPE) often follows the same storyline. PPE is worn to protect against an arc flash hazard; yet, like the auto accident, most people will never have that horrific arc flash incident and see how the PPE performs. But, if an arc flash does occur, the PPE and arc-rated clothing can make the difference between how serious the injuries are and when (or if) you recover.
What to wear?
It took decades of research and evolution leading up to today’s automobile PPE. Airbags, seatbelts and other safety components are either automatic or require nothing more than a simple “click.”
However, when it comes to the arc flash hazard, protection is not automatic, and careful decisions need to be made. I am often asked, “What do I wear?” in reference to arc flash protection. To provide a suitable answer, the first step is to properly identify the hazard’s severity.
The main objective of arc flash protection is to minimize the likelihood of burn injury by providing an adequate thermal barrier that will limit the energy exposure at a person’s skin to no more than 1.2 calories per centimeter squared (cal/cm2). This value is commonly regarded as the energy sufficient to produce the onset of a second-degree burn, so it is important to recognize that there is still the possibility of some burn injury.
Who says so?
Who or what defines protection requirements for the arc flash hazard? The requirements actually come from several different sources. At the federal level, the Occupational Safety and Health Administration’s 1910.335(a)(1)(i) defines the protective clothing and equipment requirements with the following language: “1910.335(a)(1)(i) 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.”
Simple enough. Just wear what is appropriate. However, what does the word “appropriate” mean? Is a 4 cal/cm2 arc-rated shirt appropriate? Is an arc-rated face shield required, or is a 40 cal/cm2 “moon suit” needed?
Since OSHA’s mandates are federal law, a violation could result in large fines and citations. The actual language only provides general guidance and does not give specific detail about what is appropriate. Instead, OSHA relies on what are known as industry consensus standards, such as NFPA 70E, Standard for Electrical Safety in the Workplace, to provide the details.
As a consensus standard, NFPA 70E provides many details for the selection process of arc-rated clothing and PPE. This standard not only defines the selection process but also when protection must be used. According to Article 130.5(B), Protective Clothing and Other Personal Protective Equipment (PPE) for Applications with an Arc Flash Hazard Analysis, where it has been determined that work will be performed within the arc flash boundary, one of the following methods shall be used for the selection of protective clothing and other PPE:
• Incident energy analysis—This method requires the incident energy exposure of a worker to be calculated and arc-rated clothing and PPE to be selected with an arc rating suitable for the calculated value. The calculations are usually based on IEEE 1584–IEEE Guide for Performing Arc Flash Hazard Calculations. (See “IEEE 1584 Update—The Best Is Yet to Come,” on page 50.)
• Hazard/risk category classification table—Arc-rated clothing and PPE can be selected by using tables found in NFPA 70E that identify a hazard/risk category (HRC) based on the equipment type and task being performed. Depending on the equipment type and task, a HRC number of 0 through 4 is assigned. The higher the HRC, the more protection is required.
Each of these methods is based on the worker’s head and chest being located a specific distance from the prospective arc flash, known as the working distance. Since the incident energy increases exponentially as distance decreases, more protection may be necessary for parts of the body that are closer than the working distance.
Regardless of whether incident energy calculations are performed or HRC tables are used, the severity of a prospective arc flash is a function of many variables. The three most important variables include short-circuit current, arc flash duration and distance from the arc flash.
Playing with fire
The intensity of an arc flash is directly related to the short-circuit current. When an arc flash occurs, the greater the short-circuit current, the greater the energy per time and the more intense the event will be. Duration also is very important. The longer the arc flash lasts, the greater the total energy exposure could be for the worker. A simple way to describe the relationship between short-circuit current and duration is by using a candle as an example.
I remember when I was around 10 years old, learning the trick of waving my finger through a candle flame without being burned. (Yes, I played with fire.) This was possible because the finger is in the flame for a very short time and the skin surface does not have enough time to heat up and burn. Of course, next I attempted to see how long I could hold my hand above the flame. It only took a few seconds before it began to hurt. Even though the skin was not directly in the flame, the longer exposure time was enough for the skin to heat up to the point where it hurt.
Incident energy follows this same concept and is a function of both intensity (short-circuit current) and the duration. An arc flash occurring from a low short-circuit current with a long duration can result in a similar burn injury as a large short-circuit current with a short duration.
Learn not to burn
Among the many injuries resulting from an arc flash, exposure to the thermal energy can result in severe burns to a person as well as ignition of his or her clothing. The severity of the burn injury is often due to a person’s clothing igniting and burning long after the actual arc flash is over.
Many of us recall when a fireman came to school to show us how to “learn not to burn.” One phrase drilled into us is “stop, drop and roll.” Great advice, but few people faced with burning clothing actually do it. From my experience, a more common response is exclaiming, “Get the flames off of me!” while tearing off their clothing. Unfortunately, the average time for this can be 30–60 seconds or more; and, with burning clothing on the skin for that long, serious and often fatal burn injury can result.
Of the people that have been severely injured or killed by an arc flash, there are usually a few things in common. They were not wearing arc-rated clothing. Their clothing ignites and continues to burn on their skin. The longer the clothing burns, the more severe the burns are and the less likely it is that the person will survive.
Reduce the hazard in 3-D
Relying on the arc flash PPE is frequently referred to as the last line of defense. That being said, avoiding the arc flash hazard should always be the first line of defense. Many methods currently exist that can be used to reduce the prospective incident energy. Most of them rely on controlling one of three key variables: distance, duration and direction—the three D’s.
• Distance—Getting out of harm’s way and locating the worker farther away from a prospective arc flash will greatly reduce the incident energy should an arc flash occur. Remote operating devices and hot sticks are just a few of the methods.
• Duration—Incident energy is proportional to the arc flash duration. Making changes to protective device settings can reduce the arc flash duration resulting in reduced incident energy. This can either be a simple setting change or by adding alternative protective device schemes.
• Direction—Arc-resistant equipment is designed to contain the arc flash hazard and redirect it away from the worker.
There also is a fourth method, which is actually the best one: Avoid the hazard and only perform work on equipment and systems placed into an electrically safe working condition.
Most of us heard the phrase “learn not to burn” at a very early age. When it comes to protection against the arc flash hazard, these words are even more important today.