Putting Arc-Flash Calculations into Perspective

Putting Arc-Flash Calculations into Perspective

Developing a plan for protecting personnel against arc flash requires close attention to OSHA and NFPA documents.

The number of arc flash incidents in the United States is greater than many engineers realize since most accidents don't make the daily news. Chicago-based Capelli-Schellpfeffer, Inc. reports that five to 10 arc-flash injuries that result in hospitalization occur every day.

Severe arc-flash burns can cause a slow, painful death, but even when they aren't lethal, they can do serious damage. Hot gases can injure lungs and impair breathing. Even curable burns can result in painful skin and tissue injury that can take weeks or months to heal. However, not all arc-flash injuries are physical. Psychological effects like depression, job apprehension, and family tension can also manifest themselves. Therefore, avoiding any burn is important in terms of time, money, and a person's well being.

In the hopes of improving electrical safety and informing electrical technicians of the burn hazards of electrical arcs, Code-Making Panel 1 added text to the 2002 NEC that can be summarized in two concepts: Flash protection is required when examining, adjusting, servicing, or maintaining energized equipment, and the equipment shall be field marked to warn qualified persons of potential electric arc-flash hazards.

Although OSHA doesn't directly state what to do about arc-flash hazards, OSHA 29 CFR 1910.132(d)(1) requires employers to evaluate the workplace for hazards. And based on these assessments, the employer must select and require the use of appropriate personal protective equipment (PPE) for its employees.

In conjunction with the requirements set forth in the NEC, Art. 130.3 of NFPA 70E - 2004, Electrical Safety Requirements for Employee Workplaces, states, “A flash hazard analysis shall be done in order to protect personnel from the possibility of being injured by an arc flash. The analysis shall determine the Flash Protection Boundary and the personal protective equipment that people within the Flash Protection Boundary shall use.” In addition, IEEE Std. 1584 - 2002, Guide for Performing Arc-Flash Hazard Calculations, provides details of the calculation methods to determine the incident energy level a worker could be exposed to in calories/cm2.

Annex D in NFPA 70E offers the same incident energy level equations that appear in IEEE Std. 1584, but the latter goes into more detail. NFPA 70E doesn't specify that the IEEE Std. 1584 method has to be used. The incident energy level could be determined using one of the several arc-flash software programs that are currently available on the market.

The bottom line is that the data and requirements presented in these three documents should be viewed as a working package.

For those who aren't technically inclined, NFPA-70E presents a table for determining the necessary personal protection equipment (PPE) to be used in reference to the risk hazard. Table 130.7(C)(9)(a) requires you to know the bolted fault level and approximate fault clearing time of the equipment on your system. Keep in mind, though, Table 130.7(C)(9)(a) can suggest PPE levels quite different from those established through the calculation methods of IEEE Std. 1584. In addition, the NFPA 70E table doesn't cover all possible fault or clearing times. For example, when the fault current on a 240V panelboard is greater than 25kA, PPE isn't specified in the table. In such cases, it's a good idea to increase the recommended PPE level for the task at hand.

Addressing the hazard at hand

NFPA-70E focuses on safety and the way in which a worker plans and executes a task. When it's necessary to work on energized equipment, written work permits that include a description of the work to be done and the safety hazards involved should be issued. However, wearing the proper safety equipment for the risk hazard involved doesn't guarantee that a worker will remain free from injury or burns. Its purpose is to reduce deaths and life threatening burns to the chest and face areas. Burns to hands and arms aren't considered life threatening.

The focus of IEEE Std. 1584 is the radiated heat or incident energy produced by an arcing fault that falls on a given surface. A bolted fault doesn't produce any radiated flash energy since no arc is involved. A value of 1.2 calorie/cm2 (1.2 calorie/cm2=5.02 Joules/cm2=5.02 Watt-sec/cm2) for a clearing time of 0.1 second is the incident energy level generally used as a guide to restrict the flash hazard to a second-degree or curable burn. In order to maintain that same level of injury, if the clearing time were increased to 1.0 second, the energy level would have to be reduced to 0.12 calorie/cm2.

To properly estimate the exposure hazard, it's necessary to have the maximum bolted short-circuit current, the arcing fault current, and the operating time of the interrupting device at the arcing fault current. The incident energy should be calculated at maximum and at 85% of maximum arc fault current levels. Due to the inverse nature of protective devices, such as fuses and relays, a longer operating time at lower arcing currents can result in a higher energy exposure.

IEEE equations and test results for open-air arcs

It should be noted that there isn't a 100% accurate method for calculating the degree of exposure that workers may face. The equations given in IEEE Std. 1584 are based on experimental 208V to 15kV laboratory tests. Three sets of equations are provided for the three distinct voltage ranges: 208V to 650V; 1,001V to 15,000V; and greater than 15,000V. While the empirical equations given in this standard for voltages up to 1,000V tend to yield higher limits of radiated energy than those from the test arcs, the actual radiated energy could be higher than the values yielded from the equations. The environment in which the arc takes place affects the arc-flash energy level. Factors like humidity, contaminants, temperature, enclosure type, and material consumed in the arc will affect the radiated energy level. In addition, other factors like power factor, the length and impedance of the arc, and the duration of the arc also come into play.

Fig. 1 shows a plot of the IEEE Std. 1584 reference equations and 600V test data for an open-air arc. An open-air arc is one that radiates the heat in all directions. A fault on a cable in an open tray could be considered an open-air arc. The calories/cm2 incident energy levels in this figure are based on a surface located 24 inches (61 centimeters) away from an arc that lasts 1.0 second. The line labeled “IEEE 1584 Equations” is derived from IEEE Std. 1584 equations (1 to 6) for an arc gap of 1.25 inches (32 centimeters). IEEE Std. 1584 also provides an optional equation based on Ralph Lee's method given in a 1982 IEEE IAS Transactions paper, “The Other Electrical Hazard: Electric Arc Blast Burns.” This equation is used for voltage levels greater than 15kV until future tests are done at higher voltages. The line labeled “IEEE Lee's Method” is shown for comparison to the IEEE 1584 Equations and test data. Lee's method is simpler because it calculates the maximum incident energy without knowing the arc details. The IEEE 1584 equations calculate an estimated arc current from the bolted short-circuit current and arc spacing. These values are then used to calculate the incident energy level. At 600V, the IEEE 1584 Equations and IEEE Lee's Method lines are actually higher than the incident energy test values.

Fig. 2 and 3 show the relationship of the IEEE 1584 Equations and IEEE Lee's Method lines for two additional voltage levels (4,160V and 13.8kV). In each case, some of the test data points are significantly above the IEEE 1584 Equations line. The IEEE Std. 1584 working group is aware of the apparent discrepancy and plans to make modifications to the equations at higher voltage levels once additional testing is complete. In the meantime, those engineers who use IEEE Std. 1584 equations may consider increasing the Cf constant in Equation 6 to at least 2.2 for added safety. This will increase the incident energy level by a factor of 2.2, which may result in higher-level PPE clothing requirements.

System grounding

The test results by the IEEE Std. 1584 working group showed a difference in the incident energy level depending on the type of grounding system used on the power distribution system. The following incident energy equation includes a value, K2, that changes based on the type of grounding system used on the power distribution system:

Normalized incident energy=10 K1 + K2 + 1.081*Log10(Arc kA) + 0.0011G

However, the wording in the standard and its accompanying spreadsheet (Table 1) aren't in 100% agreement. Note the difference in values for a low-impedance grounded system.

Therefore, to be conservative on low-impedance grounding systems, you should treat them as an ungrounded system. Typically an ungrounded system results in the incident energy level that's about 30% greater than that of a solidly grounded system.

Personal protective equipment

The purpose of running through the incident energy calculations is to determine the appropriate PPE that will limit the possible thermal energy exposure to the critical body parts, such as face and chest areas. Usually, the calculations give the heat-exposure in calories/cm2 or Joules/cm2. Once you know the heat exposure level, you can choose the protective clothing to best protect your employees. Table 2 (click here)which is based on NFPA 70E data, provides this cross reference.

In addition to the clothing requirements noted in Table 2, insulated tools, face shields, and gloves rated for the voltage class are required for some work tasks around energized equipment. NFPA 70E provides guidelines for PPE required for different work tasks. While companies make clothing with a calorie/cm2 rating greater than 40, there are other factors than flash hazard that may be of concern. For example, at high energy levels, the blast pressure from an arcing fault can be rather strong.

Arc blast pressure

The blast energy or pressure isn't presently addressed in NFPA 70E or IEEE Std. 1584, but future plans by the IEEE arc-flash working group call for measuring these forces. They can be significant and can blow workers away from the arc, possibly limiting burns, but causing falls and injuries that may be more severe than the burns themselves.

In Ralph Lee's second IEEE paper, “Pressures Developed by Arcs,” published in 1987, he cites several case histories. In one, an electrician was somersaulted 25 feet away from the arc when an approximate 100kA bolted fault occurred on a 480V system he was working on. Using the data in Lee's paper, the approximate initial impulse force at 24 inches (100kA bolted fault, about 42kA arc) was calculated to be about 260lb/ft2 using the equation below.

Pounds/ft2=(11.5×kA arc)÷(distance from arc in feet)0.9

Because the chest and face area for most workers consists of nearly 2 square feet of surface area, one can see that the worker could easily be blown off his feet during such an event.

Being forced away from the arc reduces the electricians' exposure to the heat radiation and molten copper, but can subject them to serious falls or impact injuries.

Limiting arc exposure

Incident energy increases with time and fault current. You can reduce the incident energy level through system design or operating procedures. Although it's best to work on de-energized equipment, this isn't always possible. Therefore, the following tips can reduce the incident energy level for your employees:

  • On new or retrofitted breakers with electric close and trip control, place the close/open control switch on a remote or nonbreaker panel.

  • If possible, use a remote or longer operating arms when racking in or opening/closing breakers.

  • Review protective devices to see if they can be lowered in time and pick-up.

  • When working on/with double-ended load centers or substations with a normally closed tie breaker, open an incoming breaker or the tie breaker to reduce the available fault level.

  • Review protective fuse sizes. Smaller fuses reduce the exposure time. This can be significant when the arcing current or 85% of arcing current isn't in the fuse current limiting range.

  • Change relay settings when working on live equipment. For many load centers of both high and low voltages, the feeders have instantaneous-set protective devices that operate and clear the fault in one to eight cycles, thereby reducing the exposure time. In order to be time coordinated with the feeders, the incoming main breaker generally won't have an instantaneous setting. The main fault clearing time could be in the range of 0.2 to 1.0 seconds. This long clearing time greatly increases the arc exposure time and amount of radiated energy a worker would experience if the arc blast pressure isn't high enough to propel the worker away from the fault.

  • One way to limit the arc exposure on buses where the protective devices are time coordinated is to order the main breaker with an instantaneous protective device and a safety switch. Normally the instantaneous protection wouldn't be functional due to the open contact of the safety switch. However, when work is being done on the energized equipment, the safety switch would be turned on, thus limiting the arc exposure time to the worker should an arcing fault accident occur. In the interest of safety, the time-selective system would be eliminated for the duration of work.

  • While not a way to reduce arc incident energy levels, it's good work practice to use the buddy system. This way, in the event an accident should occur, a second worker should be available to call for help in a very short time frame.

Calculation means

Arc-flash incident energy levels and boundary distances can be calculated a number of ways. It's up to you to decide which method is most appropriate for each project. One option is to use the equations in IEEE Std. 1584. This requires you to obtain the bolted 3-phase short-circuit current and clearing times of the circuit breakers on the system. IEEE has also made available a spreadsheet program for about $500 that you can use in conjunction with these equations. The program calls for you to enter the fault level, voltage, clearing time, and distance from an expected arc to the worker. It then calculates the incident energy level and boundary distance for this specific location on the system. You would use this data to create the labels required to be placed on the electrical equipment.

Your other option is to use off-the-shelf software. Companies that provide industrial-based electrical system analysis software packages have arc-flash hazard packages integrated with their short-circuit and protective device packages. At least one software vendor has modified the IEEE Std. 1584 equations for the 1.0kV to 15kV class of equipment to make the calculated incident energy level equal to or greater than the laboratory test results noted earlier.

Editor's Note: Copies of the NEC, NFPA 70E, and IEEE Std. 1584-2002 can be purchased from their parent organizations.

St. Pierre is president of Electric Power Consultants, LLC, Schenectady, N. Y.

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