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Arc Flash Protection

Published: 26th Nov 2013 in OSA Magazine

Flame resistant clothing and arc flash hazards

Focusing on arc rated clothing, Trent Peyton addresses the levels of protection necessary to guard against electrical hazards.

In 1982, Ralph Lee presented a paper to the Institute of Electrical and Electronics Engineers’ Industry Applications Society (IEEE IAS), entitled The Other Electrical Hazard: Electric Arc Burns1. From his work in the electrical safety field, particularly in the area of arc flash hazards, several standards have been developed to help provide a new level of safety when working around electrical hazards.

The primary standard in the United States for safe electrical work, both in methods and Personal Protective Equipment (PPE), is the National Fire Protection Association (NFPA) 70E. Several other countries have developed equivalent or similar standards, such as Canada’s Z462, which is technically identical to 70E.

As of this publication we are currently working under the 2012 edition of the NFPA’s 70E standard, with the 2015 edition expected towards the end of 2014.

Arc rated versus flame resistant

In early versions of NFPA 70E, the use of flame resistant (FR) clothing was mandated to provide protection for electrical workers against electrical hazards. As the standard evolved, however, some significant changes were made. In NFPA 70E-2012, the term flame resistant for clothing or garments was changed to arc rated (AR). Although the terms seem to be synonymous, the definition of AR requires that each fabric must undergo arc flash testing (ASTM F-1958) and report the arc rating, which defines the thermal insulation provided by that brand and weight of fabric.

This change was made to ensure that arc flash PPE was truly adequate to protect electrical workers from the hazards associated with energised electrical tasks, not just to have something to make the worker simply ‘feel’ more protected. It was discovered when the standard was first applied, that there was confusion as to what a flame resistant fabric was when applied to arc flash protection. Many vendors were selling ‘flame resistant’ clothing to the United States’ market that proved to be inadequate to withstand the incident energies and resulting high temperatures associated with an arc flash.

The term arc rating is defined as: 

The value attributed to material that describes its performance on exposure to an electrical arc discharge. The arc rating is expressed in cal/cm2 and is derived from the determined value of the arc thermal performance value (ATPV) or the energy of break-open threshold (EBT) should a material system exhibit a break-open response below the ATPV value. Arc rating is reported as either ATPV or EBT, whichever is the lower value.

AR clothing or equipment indicates that it has been tested for exposure to an electric arc.

FR clothing without an arc rating has not been tested for exposure to an electric arc2.

Arc flash hazards and energised work

At times, electrical work needs to be conducted while the electrical system is in an energised condition. An example of necessary energised work is troubleshooting. This occurs when the system is energised and the worker should be concerned about the possibility of shock and arc flash. 

An arc flash is an electrical explosion that results in extremely high heat exposures for a brief period of time. These heat values are commonly in the range of 5,000 to 8,000 degrees Fahrenheit. If the electrical worker’s skin is unprotected when he/she is exposed to an arc flash, the worker will receive serious burns including the possibility of third degree burns. This type of burn can easily lead to death. 

The electrical hazards are removed by de-energising the equipment and the system can be put into an Electrically Safe Work Condition (ESWC). When an ESWC exists, the AR clothing and PPE is not required. In order to create an ESWC, the system is verified with a digital voltmeter and the hazard has been proven not to exist. Once the ESWC has been achieved, it guarantees that there is no electrical arc flash hazard and thus the protection, which is provided by AR garments, is not necessary.

The necessity of arc rated PPE

Electrical PPE must be worn any time energised electrical work is performed. Although electrical PPE is required for protection of the head, hands and feet, we are not going to address theseitems in this article. Our focus is on AR clothing that is necessary to protect the majority of the human body. 

The primary purpose of AR clothing is to ensure that the garment’s fabrics do not ignite and severely burn the worker over a large body surface area. Thus, all fabrics submitted for arc rating must also be flame resistant. The second important function of AR garments is to insulate the wearer and to reduce or eliminate second and third degree burns. 

When teaching my electrical safety classes, I often demonstrate this principle by holding the fabric over a direct flame from a lighter. As the fabric is exposed to the flame, usually for several seconds, it begins to burn and char. When I move the lighter’s flame away from the fabric, however, the fire goes out and does not continue to burn. I then point out that if the AR fabric will not continue to burn after direct exposure to a flame applied for three to five seconds, then it will not support combustion after exposure to an arc flash, since the typical arc flash duration is approximately 0.1 seconds. 

Arc flash calculations, as defined by IEEE 1584 and NFPA 70E-2012 Annex D, can be expressed in calories/cm2, which is a measurement of heat. Approximately 1.2 cal/cm2 will cause second degree burns to exposed skin, while 2 to 3 cal/cm2 s is often enough to ignite cotton clothing. 

We utilise the incident energy values in two ways. Firstly, by performing an arc flash assessment or study we can quantify the amount of heat that the electrical worker could experience. Once we know the quantity of incident energy that could be generated, AR garments and PPE can be specified to ensure that sufficient protection is achieved. In order to properly protect the worker, PPE that has ratings equal to or in excess of the calculated incident energy must be selected and worn. If the incident energy generated by the flash is greater than the garments worn by the worker, the insulating capacity of the garment will be overwhelmed, which can result in burns through the PPE. 

The selection of arc rated PPE should be determined by an engineer or safety professional who understands the implication of the study results. These results will yield incident energy ratings for each of the electrical enclosures that are evaluated. From this information, the engineer or safety professional can determine what PPE is required. 

Arc flash hazard ratings

In 2005, when we began conducting arc flash studies, we began tracking the results to see the breakdown of incident energies by NFPA 70E groupings or PPE levels. These values range from PPE level 0 to PPE level 4. In addition, any incident of energy greater than 40 cal/cm2 is grouped into a PPE level classified as dangerous. 

From the results of more than 200 studies, we found that more than 90% of electrical enclosures are rated 8 cal/cm2 or less. This shows the importance of completing the hazard assessment and labelling of the equipment. If labelling communicates the exposure levels, the electrical worker can choose and wear the PPE that is appropriate for the task. In many cases, this clothing can be as simple as an AR uniform or coveralls.

When the calculations are performed, it is critical that an appropriate working distance (WD) is selected, since the incident energy value is a function of this distance. The WD defines how far the electrical worker is away from the arc gap where the flash is initiated. If the WD is set too far away when conducting the evaluation, the resultant incident energy values will be less than that of the worker’s actual exposure. In the United States, for instance, 480VAC is the typical voltage used in industrial and commercial applications. 

Most engineers, when performing the calculations, will set the WD at 18 inches (45.72cm), as that is a typical distance that the worker stands away from the electrical equipment in relation to the work he is performing. The calculated incident energy should therefore be a value that appropriately reflects the amount of heat that the worker is exposed to, based on where he is standing.

If the worker is 18 inches away but the incident energy is calculated at 24 inches (60.96cm), the resulting incident energy reported on the arc flash labels will be less than what the worker would actually experience, and there is a high probability that garments with an insufficient AR would be utilised. 

Recently, when conducting an arc flash inspection, I noticed that the arc flash labels indicated an incident energy rating of 39.9 cal/cm2. Upon further inspection of the label, I noted that the working distance had been set at 24 inches, when at 480VAC it should actually have been 18 inches. This meant that the worker would be protected against less than 40 cal/cm2, but would in fact be exposed to something in the range of 45-50 cal/cm2. It was possible or likely that the worker would suffer some second degree burns despite the PPE. It is therefore not only important that careful consideration be given to the selection of AR garments, but also to the variables used in performing the incident energy calculations. 

In the US, many vendors offer PPE kits that include coveralls, which are rated for PPE level 2 (8 cal/cm2). Many facilities that do not perform an abundance of electrical work prefer to use this type of AR garment to cotton clothing, but, if the electrical worker is expected to enter energised enclosures or conduct troubleshooting tasks multiple times a day, a daily wear uniform consisting of AR trousers and shirts is often chosen. Most manufacturers have standard AR clothing designs of work shirts that are in the 8 to 10 cal/cm2 range and trousers in the 8 to 12 cal/cm2 range. 

When dressed in this type of PPE an electrical worker is going to have very low exposure to the possibility of being burned, unless the worker were to inadvertently enter a higher AR level enclosure of greater than 8 cal/cm2 and experience an arc flash.

Higher incident energy rated enclosures, in the range of 8 to 40 cal/cm2, are represented in the PPE charts and energised work can be conducted if necessary. The increased incident energies require not only increased AR clothing, but that clothing is worn in multiple layers. 

In enclosures classified as dangerous or in excess of 40 cal/cm2, energised work is highly discouraged. Typically, locations that yield results requiring heavy duty AR suits are switchgear, panels or switchboards fed from large step-down transformers or locations that have slow reacting upstream overcurrent protection. There are few electrical tasks conducted in these types of enclosures that are diagnostic in nature and most of these tasks can be conducted with the power turned off. 

Although flash suits are commercially available in ratings exceeding 100 cal/cm2, the worker’s exposure to heat is not the only hazard to consider. At these levels, the electrical worker may be exposed to a significant pressure wave, with the covers being blown off the equipment and a possibility of shrapnel. Many high energy flash suits have built in ballistic-rated head and chest protection for these hazards.

Importance of quality fabrics

We recommend AR fabrics supplied from manufacturers who have gone through extensive field use and testing. There are many manufacturers who make adequate AR fabrics, but it is essential that the PPE specifier exercises due diligence in ensuring that the garments and fabrics meet the minimum requirements for the application. 

This includes understanding the arc flash hazard levels that exist in the facility, along with the capabilities of the AR fabrics and garments per ASTM F1506. 

Many of the fabrics being utilised for electrical flash protection can provide protection against other heat hazards. One particular concern is the flash fire hazard, as defined in NFPA 2112, for industries such as refining. In these applications, judicious choices of AR fabrics can result in protection against multiple heat hazards2. 

Maximising protective values

It is important that AR fabrics are properly laundered. If you have an industrial clothing provider that handles the laundering of AR clothing, it is important that they do so by adhering to the fabric manufacturer’s guidelines. The use of fabric softener on AR garments is never recommended. For this reason, many companies outsource the laundering of AR clothing to professional laundering services. This type of service can make sure that the manufacturer’s laundering recommendations are followed so as to protect the integrity and protective value of the garment. There is always a concern that if the worker is allowed to clean his own AR garments that the guidelines might not be adhered to and that the protective characteristics of the fabric might be compromised by inadequate laundering.

Before using any AR apparel, whether it is a uniform, coveralls or outwear, the worker needs to inspect the garment to verify that there is no contamination or damage. Contamination from a flammable substance such as grease or oil can cause afterflame on the AR garment as the contaminant burns. If highly compromised with this type of contamination, the afterflame can cover a large enough surface area and last long enough to cause injuries to the electrical worker. Rips, tears, holes or other integrity issues can allow the flame of the flash to burn the exposed skin in the area of the garment damage. It is critical to remember that using an AR garment does not guarantee protection in a flash event if it is not clean, if damage exists or if it is not worn properly. 

When using AR clothing, shirts should be tucked in and buttoned to the top button, with sleeves rolled down and buttoned at the cuffs. The primary purpose of an AR garment is to resist ignition of the garment when exposed to a flash. Many of the most serious burn injuries from an arc flash are the result of the worker’s non-FR and AR clothing igniting and continuing to burn after the flash is over. If the clothing continues to burn, the worker often will sustain a much higher percentage of burns to the body. If the clothing does not ignite and the worker has all of his skin covered and therefore protected with the AR garments, then it is possible for the worker to have minimal or even no burn injuries whatsoever.

Over the last few years, we have seen AR garment selection increase as the market for AR has expanded. Manufacturers have now developed shirts that look and feel similar to regular cotton clothes, and base layers that have the feel of athletic shirts that wick away sweat. Other garments that are new to the market include high visibility clothing with AR reflectors, jackets and rainwear. Although AR clothing may never make the fashion runways in Milan and Paris, its general appearance and appeal for the person wearing the protective clothing has improved greatly.

For applications which require hairnets, AR hair and beard nets are also commercially available. As they are expensive they are not single use head protection; instead they can be washed and used repeatedly. We advise our clients to keep their AR hair and beard nets in a re-sealable plastic bag and store them with their voltage rated gloves. This ensures that they can be easily found when needed and that they are not exposed to any unnecessary damage or contamination. Since the AR hairnets are required in sterile environments - when working with food and electronics, for example - proper storage becomes important not only for safety reasons, but also for product quality.

It is also important when AR clothes are used that any undergarments are either cotton or some other natural, non-melting fabric. An AR fabric does not keep all heat from penetrating the garment. As the heat penetrates the outer shirt or trousers, it is possible that it would melt any undergarment made of a synthetic fabric such as Rayon, Nylon, or Spandex.

Outerwear such as jackets or raingear worn over the PPE must also possess arc ratings, since these garments would be directly exposed to the heat and molten metal associated with an arc flash.

Summary

Careful attention must be given to the hazard assessment and specification of AR garments.

Arc rated fabrics and garments are improving in comfort, appearance and protective properties. By matching AR garments and other electrical safety PPE to the hazard exposure levels of an arc flash, electrical workers can be adequately protected from injury or death. 

References

1. www.uofaweb.ualberta.ca/uofaengineer/article.cfm?article=71047&issue=70866
2. www.westex.com/understand-the-hazards

Published: 26th Nov 2013 in OSA Magazine

Author


Trent Peyton


Trent Peyton graduated from the Rose-Hulman Institute of Technology with a Bachelor’s Degree in Electrical Engineering and a specialism in Power Engineering. He has held several positions that have given him a broad background in the electrical and industrial arena. He began Peyton Technical Services in 2005, which specialises in electrical safety. It has a particular emphasis in arc flash studies, electrical safety training and plant procedures.

Trent has trained thousands of electricians and maintenance personnel in electrical safety and participates in informational seminars. In addition, he is a frequent speaker at industrial and safety conferences. He was invited to speak in Brazil in 2009 to an industrial group responsible for developing the Brazilian electrical safety standards that parallel those of the NFPA 70E standards in the United States’.


Trent Peyton

Website:
http://www.ptsarcflash.com

Email:
trent@peytontechsvc.com

Phone:
001 (812) 738-2016

trent@peytontechsvc.com
http://www.ptsarcflash.com
001 (812) 738-2016

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