commonsense rules of thumb in mind to help make ‘hot work’ less
The Occupational Safety and Health Administration (OSHA) has always maintained that electrical work should only take place on de-energized equipment. This idealistic goal hasn't always been practical or economical in the field and is routinely overlooked by electrical workers and their supervisors. However, the National Fire Protection Association's “Standard for Electrical Safety in the Workplace, 2004 edition” (NFPA 70E) now gives OSHA a meaningful reference to categorize the risk associated with working on energized electrical equipment.
Categories of hazards range from Level 0 (little to no arc-flash risk) to Category 4 (high risk of arc flash). Each category requires workers to wear different levels of personal protective equipment (PPE) when working on or near energized equipment. Although the study of arc flash and resultant injuries is in its infancy, adhering to the requirements of NFPA 70E can make working on energized equipment safer for all workers.
OSHA and NEC requirements. Everyone involved in electrical “hot work” must study and learn about the requirements of NFPA 70E because both OSHA and the NEC require its use. The NEC's requirements in 110.16 are minimal, consisting of a vague arc-flash hazard warning label to be placed on switchboards, panelboards, industrial control panels, and motor control centers. While this label notifies personnel of the hazard's existence, it gives no indication of severity or guidance for protection.
OSHA's involvement is far more direct. They can issue citations and levy fines for non-compliance. Section 1910.333 of Subpart S states that “Safety related work practices shall be employed to prevent electric shock or other injuries resulting from either direct or indirect electrical contacts.” It doesn't refer directly to arc flash, but by including “other injuries,” it requires employers to understand its nature and the hazards involved and to protect their workers accordingly. OSHA has adopted the NFPA 70E standard as an acceptable means of compliance to meet this requirement.
Incident energy. Incident energy is one of the key terms in understanding any arc-flash hazard. NFPA 70E defines it as “the amount of energy impressed on a surface, a certain distance from a source, generated during an electrical arc event.” The incident energy level is expressed in calories per centimeter-squared and is a measure of the heat created by the electrical arc.
The key numbers to remember are 1.2 and 40. Incident energy levels greater than 1.2 calories per centimeter-squared can produce second-degree burns. Flame-resistant garments are necessary for protecting yourself against possible burns caused by energy levels above 1.2. Arc-flash energy levels above 40 can be fatal because they're accompanied by a massive pressure blast and sound pressure waves, which produce projectiles. Clothing is available for arc-flash exposures all the way up to 100 calories per centimeter-squared, but it's useless against the force of the pressure blast.
Until more research is completed on how to reduce the danger of such blast waves, you should prohibit work on live equipment with incident energy levels above 40 calories per centimeter-squared.
Fault current levels and circuit length The level of arc-flash hazard at any piece of equipment depends on the level of arc-fault current and the time it takes to trip the nearest upstream overcurrent protection device. Your local utility engineer can typically tell you what the fault current levels are on the local distribution system and on the line side of the main service at a particular facility.
But be aware that these fault current values may be based solely on the impedance of the transformer that serves this particular building. In reality there are additional impedances upstream of the transformer that will lower this number. If these additional impedances aren't included in the utility engineer's calculations, the incident energy levels you calculate in the facility may be underestimated.
Low utility fault current levels and/or long circuit lengths can have a severe effect on incident energy levels. For example, Fig. 1 shows 400A RK1 and RK5 fuses protecting 10-foot and 400-foot feeders. This sample system has a fault current level of 50,000A. The high fault current levels make this system sufficiently robust to trip the RK1 fuse quickly, regardless of circuit length. As such, the resultant hazard levels in this example are relatively low.
So what happens when the fault current level is found to be lower? As shown in Fig. 2 on page C18, a reduced fault current level of 15,000A is sufficient to quickly trip the device that protects the 10-foot feeder. However, the 400-foot feeder creates even lower fault current levels and longer trip times, elevating the arc-flash hazard to a Category 3 designation. It's also worth noting that RK5 fuses have longer trip times than RK1 fuses, which raises incident energy and hazard levels to higher values as compared to the RK1 fuse.
Fig. 3 on page C18 shows the overall relationship between fault current levels for RK1 and RK5 type devices and incident energy levels. As the fault current in the system becomes weaker the incident energy gets stronger. This relationship holds true for other fuse types and breakers. As a rule of thumb, arcing currents that are greater than 10 times the rating of the device will trip within 1 cycle or less. Since arcing faults are typically about half the bolted fault current values, the fault current levels at each overcurrent device need to be more than 20 to 30 times the rating of the protective device to trip instantaneously.
For example, a 1,000A protective device will need to be installed in an electrical system with a bolted fault current level of 20,000A to 30,000A or more to trip within 1 cycle. A 400A device will require an 8,000A to 12,000A fault current level to trip as quickly. Fault currents lower than these factors will delay the opening of the device and cause the incident energy values to rise.
Electrical distribution systems located in rural areas tend to have low available fault currents and are described as weak electrical systems. In urban areas, where utility infrastructure tends to be much closer together, fault current levels tend to be higher, incident energies are lower, and protective devices trip faster, which results in reduced arc-flash hazards. This relationship between fault current levels and protective device ratings can help you better understand where greater hazards exist.
Pitfalls behind arc-flash labels. The recent popularity of NFPA 70E has created a market for arc-flash studies. The first step in any arc-flash study is gathering a large amount of specific data, such as protective device model numbers, feeder lengths, and fault current levels. This information is then entered into a computer software program, where the incident energy levels, PPE requirements, and arcing current information are all calculated automatically. All of this information is then printed on a detailed warning label, which is to be posted on each piece of equipment in the system (Fig. 4 above).
But these labels have problems. As soon as any single piece of input data changes, the label is no longer accurate. Short-circuit current levels in electric utility systems are continuously changing, and both electricians and maintenance personnel routinely replace overcurrent devices. Fuses are often changed from the originally specified RK1 type to the RK5 variety for cost considerations. Panelboards are upgraded or added to after spaces are renovated or more equipment is purchased. All of these changes greatly affect the arc-flash energy level at various points in the system and may easily turn a Category 1 into a Category 3 overnight.
Electricians should understand that labels created from a snapshot in time may no longer be accurate. In a perfect world, the arc-flash calculations would be recomputed from scratch each time energized work is necessary. However, the time necessary to update the study and the complexity of the calculations makes this virtually impossible.
Tables 130.7(C)(9) and 130.7(C)(11). At a minimum, field workers should carry with them a copy of Tables 130.7(C)(9) and 130.7(C)(11) from Section 130 of NFPA 70E. Table 9 outlines the hazard/risk categories (HRCs) for common tasks performed by electrical workers on a daily basis. Table 11 outlines the various HRCs associated with corresponding incident energy levels and offers a brief description of the types of PPE clothing that must be worn when working under these conditions. The Table on page C20 shows a simplified version of Table 11. Distributing these tables to all employees will provide workers with basic concepts of 70E and start their understanding of the various hazard levels they face in their daily work.
Using Table 9 eliminates the time and effort required to perform a full arc-flash study while still providing an adequate level of safety. An arc-flash analysis has been completed for each task listed in the table based upon the parameters listed in the notes at the bottom of the table. However, it's important to note that if the fault clearing times on the system you're working on are longer than what's specified in the table, then you should perform a full arc-flash analysis prior to working on the system.
IEEE Standard 1584. IEEE provided significant research to produce a set of arc-flash equations that are listed in Annex D of NFPA 70E. These equations are based upon research of experimental arc-flash tests between 208V and 15kV. The experimental tests performed by IEEE and equipment manufacturers note that it's extremely difficult to sustain an arc at 240V and below for more than two to three cycles.
These results led to the language of Section 9.3.2 in IEEE Standard 1584, Guide for Performing Arc Flash Hazard Calculations. This section of the document is very important to residential and small commercial contractors who are trying to understand 70E and comply with OSHA but don't have the budget necessary for outfitting all of their employees with PPE.
IEEE says that certain voltages and ampacities aren't considered arc-flash hazards. Systems rated below 240V-to-ground and below about 400A (125kVA) fit in this category. For all practical purposes, these systems can be considered Category 0. As such, residential electrical systems, small commercial 208/120V services, and the secondary of a 480-208/120V step-down transformer (112.5kVA or less) are defined as presenting little or no arc-flash hazard. Understanding this clause allows a contractor to avoid the costs associated with outfitting their employees in PPE yet still maintain a safe work environment that meets OSHA standards.
Pick Category 2. Understanding NFPA 70E and its terms and calculations can be a daunting task. Many companies have elected to develop their own procedures to comply with the requirements set forth in this standard. Whether or not they're fully compliant is a difficult question to answer.
The instinct to require your employees to use the highest level of protection at all times is natural, but it's not always feasible. Because it's so bulky and uncomfortable, Category 4 level clothing can slow productivity, and it's expensive.
Prevailing wisdom says that 85% of all arc-flash hazards fall below Category 2 levels, or 8 calories per centimeter-squared. Therefore, at a bare minimum, contractors should outfit their personnel with Category 2 level PPE. Going this route is not only more reasonable, but less expensive than other options. The cost to outfit a field worker with Category 2 arc-flash clothing is approximately $400. This is the optimal balance between minimal cost and reasonable safety precautions.
However, for work at more hazardous energy levels, one or two Category 4 arc-flash suits can and should be kept on hand. This again would be the most economical approach to providing adequate protection for those working on or near energized equipment.
Get the word out. Operating protective devices, testing for voltage, and installing or removing breakers are common tasks that offer the possibility of an arc-flash injury. That's why it's imperative that you and your workers understand the requirements of NFPA 70E as much as you understand the NEC. This means having a clear understanding of how various fault-current levels affect the HRC rating on a system.
Most contractors acknowledge that the calculations required by 70E are too complex for the everyday work environment. That's why it's recommended that the tables in Section 130 be distributed to each and every field worker and that each worker be outfitted with a minimum Category 2 PPE. Electricians' tools should also be reviewed to make sure they comply with 70E. Just simply knowing 70E guidelines will help reduce the number of arc-flash injuries.
Ayer is vice president of Biz Com Electric, Inc. in Cincinnati and is a member of the NFPA 70E Committee.
Sidebar: Don't Forget About Shock Hazards
For the last several years NFPA 70E has focused heavily on arc flash and its associated injuries. However, you shouldn't forget that 70E requires workers to be protected against electric shock, too. Remember, only a very low amount of current flowing through the human body is necessary to cause death or serious physical harm. While contractors are obligated to inform their employees of the dangers of arc flash and how to prevent them, shock hazard education shouldn't be overlooked.
Residential and small commercial projects of 240V or less involve little to no arc-flash hazard. Yet, by performing routine tasks like removing panel covers, checking voltage, and replacing breakers, they're still exposed to shock hazards. Make sure you use insulated tools and voltage-rated gloves when working on energized systems. Off-the-shelf screwdrivers, Allen wrenches, and socket sets aren't properly rated.