Arc Flash Calculator Notes
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This software program, calculations, and other information are intended to clearly present technical information that will help the user determine the thermal arc flash hazard. We reserve the right, without notice, to change this program and to discontinue or limit its distribution. We also reserves the right to change or update, without notice, any technical information contained on this website. The data and information presented in this website and/or calculator are believed to be accurate. However, any and all liability for the content, or any omissions from this website, including any inaccuracies, errors, or misstatements in such data, calculations or information, is expressly disclaimed. The software, calculation results and other information are provided without warranty of any kind, either expressed or implied, but not limited to, the implied warranties of merchantability, or fitness for a particular purpose. We disclaim any liability for the use of this software, calculations or other information.
General notes for fuses and circuit breakers
First and foremost, the information found in this calculator, or in any other calculator, is not to be used as a recommendation to work on energized equipment. This information is intended to help assist in determining the proper flash protection boundary and level of PPE to help safeguard a worker from the burns that can be sustained from an arc flash incident. The output of this calculator does not address the worker protection required against the effects of pressure, shrapnel, molten metal spray, or the toxic copper vapor resulting from an arc fault. Workers must also be protected against these hazards and therefore additional investigations and additional personal protective equipment may be required.
The calculations and data in this calculator and procedures used for determining incident energy exposure, level of PPE, and flash protection boundary are based upon IEEE Standard 1584, Guide for Performing Arc Flash Hazard Calculations (23 September 2002). The methods for determining incident energy exposure from this IEEE 1584 standard were created so that the level of PPE selected from the calculated incident energy would be adequate to protect the torso against incurable arc flash burns for 98% of arc flash incidents. In up to 2% of incidents, serious injury, incurable burns to the torso, and death could result. (Equations are also provided in IEEE 1584 to cover 95% of arc flash incidents, but this calculator utilizes the more conservative equations related to 98% of arc flash incidents.) Calculations are based upon PPE with standard ATPVs of 1.2, 8, 25, 40 and 100 cal/cm2. PPE with intermediate ATPV values can be utilized, but at the next lower standard ATPV rating.
This information is not intended to promote working on or near exposed energized parts. The intent is for those situations such as taking voltage measurement during the lockout/tagout procedures where arc flash analysis must be performed and the worker must utilize adequate PPE while the circuit is being put into an electrically safe work condition.
PPE must be utilized any time that work is to be performed on or near energized electrical equipment or equipment that could become energized. Voltage testing while completing the lockout/tagout procedure (putting the equipment into an electrically safe work condition) is considered as working on energized parts per OSHA 1910.333(b). As a general work practice, it is suggested that, at the very minimum, the worker utilize voltage rated gloves with leathers, long sleeve cotton shirt, heavy-duty cotton pants, a face shield, safety glasses and hard hat, in addition to the recommendations from NFPA 70E (even though NFPA 70E requirements do not require all these items for the lower Hazard/Risk Categories).
To use this calculator, the available three-phase (3Ø) bolted short-circuit current must be calculated at each point in the system that is to be analyzed. In some cases, using conservatively high bolted short-circuit currents may result in lower incident energy than what is possible. This is dependent upon the time-current characteristics of the overcurrent protective devices. Calculations must be conducted with both the maximum and minimum available three-phase (3Ø) bolted short-circuit current.
The equations used in creating this calculator are from IEEE Standard 1584, Guide for Performing Arc Flash Hazard Calculations (23 September 2002). The parameters for this calculator are based upon an incident energy exposure of 1.2 cal/cm2 at an 18-inch working distance, 32mm (1-1/4") arc electrode spacing, 600V 3Ø ungrounded system, three phase bolted fault current in the range of 700-106,000A, and a 20 inch X 20 inch X 20 inch box open on the one side facing the front. Parameters were selected to achieve what was considered to be the worst-case results based upon the latest testing as reported in IEEE papers available at the time. For example, an arc flash inside of a box will achieve a higher incident energy than an arc flash in open air. This is because the sides of the box will focus the arc flash energy towards the opening, whereas open air will allow the energy to dissipate in all directions. Actual results from real arc flash incidents could be different for a number of reasons, including different (1) system voltage, (2) short-circuit power factor, (3) distance from the arc, (4) arc gap, (5) enclosure size, (6) fuse manufacturer, (7) fuse type, (8) orientation of the worker, (9) grounding scheme, (10) circuit breaker manufacturer, (11) circuit breaker type, (12) circuit breaker settings, and (13) circuit breaker condition.
Incident Energy Exposure in this calculator has been set to display a minimum value of 0.25 cal/cm2 and a maximum value of 100 cal/cm2 (with appropriate warnings). Actual incident energy values may be less than 0.25 but since it is not the intent of this calculator to encourage workers to go without PPE, the .25 cal/cm2 default is utilized whenever lower values are encountered. See Note 1 and Note 4. The maximum value of 100 cal/cm2 was set because at the time the calculator was created, no PPE is available with an ATPV greater than 100 cal/cm2. See Note 1 and Note 2.
The fuse information is based upon extensive tests that were conducted at various fault currents for each Bussmann series KRP-C-(amp)SP, Class L fuse, and Low-Peak LPS-RK-(amp)SP, Class RK1 fuse. No testing has been completed for KRP-C-(amp)SP fuses greater than 2000 amps, and therefore no specific current-limiting fuse equations have been developed. However, the calculation methods in IEEE 1584 may be utilized for these larger fuses. (The IEEE 1584 calculator will determine the arcing current and fuse time-current curves can be utilized to determine the time to open. The IEEE 1584 calculator will then determine the arc flash energy.) Contact Application Engineering at (636) 527-1270 for help in determining the arc flash energy let-through by fuses larger than 2000 amps.
To create the equations used for the fuse incident energy equations used in this calculator, worst-case values from the extensive testing were used (See Note 7 and Note 8). Actual values from these tests in many cases were found to be much lower than what is shown in the calculator. (Higher values are provided for purposes of smooth curve plots.) For example, in one test at 15.7 kA, the highest result for incident energy was 1.1 cal/cm2 but the number plotted for the chart was 2 cal/cm2.
The fuse incident energy values do not go below 0.25 cal/cm2 even though many actual values were below 0.25 cal/cm2. The minimum FPB of 6 inches, which results from an incident energy exposure of 0.25 cal/cm2, was chosen to keep from encouraging workers to work on energized equipment without using PPE because of a low FPB. For example, due to the tremendous energy limitation of the Low-Peak fuses, some of the tests resulted in incident energies of 0.04 cal/cm2 and a FPB of less than 2 inches. While the resulting flash may not be very large for these situations, molten metal may still be experienced, and PPE should be utilized any time that work is to be done on live electrical equipment that includes voltage testing during the lockout/tagout procedure.
The actual tests were conducted with Bussmann series Low-Peak LPS-RK-(amp)SP and KRP-C-(amp)SP fuses. This information can also be used for equivalent or lower amp rated LPJ-(amp)SP, JJS-(amp), and LP-CC-(amp) fuses to determine the incident energy available and flash protection boundary. This is due to the current-limiting ability of these fuses yielding lower values of let-through current as well as opening in less time than that of the LPS-RK-(amp)SP fuses. Lower let-through values together with a shorter arcing time result in a lower amount of arc flash energy.
Circuit breaker notes
The source of the equations for the circuit breaker incident energy values used in this calculator are from IEEE Standard 1584, Guide for Performing Arc Flash Hazard Calculations (23 September 2002). The circuit breaker information in this standard comes from theoretical equations that are based upon how circuit breakers operate together with the arc flash equations that were developed in this standard. These arc flash equations were created so that PPE chosen as a result of the equations would be adequate for 98% of arc flash incidents. In up to 2% of incidents, incurable burns to the torso, or death, could result. This was based upon PPE with standard ATPVs of 1.2, 8, 25, 40 and 100 cal/cm2. PPE with intermediate ATPV values can be used, but at the next lower standard ATPV rating. See Note 6 and Note 7.
The values obtained from the calculator for circuit breakers are for a molded case circuit breaker with a thermal magnetic trip setting from 1-400 amp, a molded case circuit breaker with an electronic trip setting from 401-600 amp, and a low voltage power circuit breaker (air-frame) with long time delay and short time delay settings from 601-2000A. As indicated in the IEEE Standard 1584, Guide for Performing Arc Flash Hazard Calculations (23 September 2002), the instantaneous trip setting was set at 10 times. To account for manufacturing tolerance on the instantaneous trip setting of plus or minus 10% at the high setting, an additional 10 percent was added to the 10 times setting.
Personal Protective Equipment (PPE) Notes
Employees must wear and be trained in the use of appropriate protective equipment for the possible electrical hazards with which they may face. Examples of equipment could include a hard hat, face shield, flame resistant neck protection, ear protectors, Nomex™ suit, insulated rubber gloves with leather protectors, and insulated leather footwear. All protective equipment must meet the requirements as shown in the latest edition of NFPA 70E. Protective equipment, sufficient for protection against the potential electrical flash, is required for every part of the body. The selection of the required thermal rated PPE depends on the incident energy level at the point of work.
As stated previously, the common distance used for most of the low voltage incident energy measurement research and testing is at 18 inches from the arcing fault source. So what energy does a body part experience that is closer to the arc fault than 18 inches? The closer to the arcing fault, the higher the incident energy and blast hazard. This means that when the flash protection analysis results are calculated at 18 inches from the arc fault source, the incident energy and blast energy at the point of the arc fault are considerably greater. Said in another way, even if the body has sufficient PPE for an 18-inch working distance, severe injury can result for any part of the body closer than 18 inches to the source of the arc.