There are some misconceptions that exist in today’s workplace about electrical safety. For instance many people believe that arc flash can only be hazardous when working with high voltage or voltage above 600v. Statistics say otherwise. It is important to understand that one of the most critical pieces of determining your hazard is to identify the magnitude and duration of the fault current for a particular piece of equipment. The combination of the maximum fault current that is available during an arc flash incident and the duration of time that arc can sustain itself, is critical to the severity of the injury that may result.
The Hazard Risk Category (HRC) table 130.7(C)(9)(a) in NFPA 70E-2004 describes very general tasks performed within certain voltage ranges. These voltage ranges begin below 240v. The next range is 240v to 600v. The HRC table was developed to show maximum exposures in a given situation while working within a voltage range.
Let’s examine the voltage range of 240v to 600v. There are several footnotes that are found along with the voltage range identification. Footnote 1 says that the exposure rating was derived for a maximum value of 25kA for the duration of 0.03 second (2 cycle) fault clearing time. Most commonly this would be 480-volts three phase equipment. The last job description found in this voltage range is “working while energized, including voltage testing.” NFPA70E 2004 rates this job description as a 2*. This means that if an arc flash occurred while voltage testing a 480v three phase piece of equipment your exposure would not exceed 8 cal/cm². A 2 nd degree burn can be incurred by being exposed to as little as 1.2 cal/cm². In the situation just described, the work performed was below 600v and, in the industry, is known as working low voltage. NFPA70E 2004 also states that any work performed above 50 volts, while energized, requires the proper use of PPE.
Another misconception is most work is performed de-energized. The fact is people work on energized equipment at some point everyday.
Lock out tag out procedures have became an everyday practice in the workplace. OSHA 1910.147 has been identified as one of the most critical aspects of implementing an electrical safety program. Misunderstanding comes from believing that if a workplace has a complete lock out tag out program, then everyone is working on de-energized equipment at all times.
OSHA 1910.147 states the following:
OSHA 1910.147(d)(2)Machine or equipment shutdown. The machine or equipment shall be turned off or shut down using the procedures established for the machine or equipment. An orderly shutdown must be utilized to avoid any additional or increased hazard(s) to employees as a result of the equipment stoppage.
1910.147(d)(6)Verification of isolation. Prior to starting work on machines or equipment that have been locked out or tagged out, the authorized employee shall verify that isolation and de-energization of the machine or equipment have been accomplished.
Locks and tags are placed and the equipment is then proven to be de-energized without the possibility of being accidentally re-energized. The process leading up to this point is considered, and must be treated as, working with energized equipment. Therefore if you are performing lock out tag out procedure you must perform the work as though you are working on energized equipment.
Other problems can occur by not selecting the proper PPE system for a given HRC category. NFPA 70E table 130.7(C)(11) Protective Clothing Characteristics, provides an exposure value for each hazard risk category. This value is found in the far right column of the table and is in parenthesis. The exposure values are as follows; HRC 1 = 4cal/cm², HRC 2 = 8cal/cm², HRC 3 = 25cal/cm², and HRC 4 =40cal/cm².
These ratings are maximum exposures. They are not ranges. HRC 2 is not 8cal/cm² to 25cal/cm². This means that in an HRC 2 category the maximum exposure will be 8cal/cm². It would be correct to select clothing that has a value of 8cal/cm² or greater. In other words, you could wear a 40cal/cm² suit in an HRC 2 situation and meet the requirements, but it is not necessary. When selecting your PPE, the testing requirements for testing arc flash clothing should be taken into account. ASTM F1958 requires that fabric be given an ATPV or arc thermal protection value. Arc Thermal Protection Value is a protection value assigned to textile materials based on predicting 2 nd degree burn injury based on the Stoll Curve. This means if a worker is involved in an 8cal/cm² arc flash while wearing an 8cal/cm² that worker has a 50% chance of receiving a 2 nd degree burn. By wearing higher calorie rated clothing so that it exceeds the maximum exposure of an HRC category, you decrease the chances of receiving a 2 nd degree burn. For instance if a worker wore an 11 cal/cm² garment while performing a task which falls into HRC2, the worker would be less likely to receive a 2 nd degree burn than if worker had worn an 8 cal/cm² garment.
Understanding proper hand protection is extremely important when selecting your PPE. NFPA address two hazards when discussing glove usage. Both shock protection and arc flash protection are taken into account. It is easy to become confused when reading
NFPA 70E section 130.7(C)(6) which states “Employees shall wear rubber insulating gloves where there is danger of hand and arm injury due to electric shock.” The section goes on to say that hand and arm protection shall be worn where there is a possible exposure to arc flash burn and then references section 130.7(C)(13)(c). This section states “leather or FR gloves shall be worn where required for arc flash protection. Where insulated rubber gloves are used for shock protection, leather protectors shall be worn over the rubber gloves.” Many people would interpret this to mean that leather gloves, by themselves, are acceptable for working where an arc flash may occur. This also leads people to believe that if leather gloves, which offer no voltage protection at all, are acceptable then other gloves made of non flammable textiles are also acceptable. OSHA
1910.269 states when working on or near 50 volts or greater, while energized, the use of voltage rated gloves is required. NFPA70E 130.7 (C)(9)(a) requires voltage rated gloves for many of the tasks listed.
While there are some tasks where voltage may not be present, most tasks with a possibility of an arc flash involves the presence of voltage. Because there is no recognized test method for determining the ATPV value of any type of glove, NFPA 70E does not assign a cal/cm² rating for voltage gloves and leather protectors. In 2002, 2003, and most recently in 2005, Salisbury performed arc flash testing on both voltage rated gloves and leather protectors. By using the existing ASTM
F1958 test method procedures for determining ATPV values for garments, Salisbury was able to determine the approximate cal/cm² rating for voltage rated gloves and leather protectors. Class 0 gloves with a leather protector had as much as a 56 cal/cm² protection level. Class 2 gloves had as much as a 100 cal/cm² protection level. We now have a more accurate understanding of the arc flash protection of voltage gloves. The results of these tests make it much easier to select your hand protection today.
In summary, when implementing your NFPA 70E program, properly understanding the dangers of working on low voltage, and the realities of working while energized are key components to identifying the exposures that may result when performing different tasks. Understanding these exposures is vital to selecting the appropriate personal protective equipment to ensure the proper protection for the worker. For more information about personnel protective equipment and industry links, please visit www.whsalisbury.com.
