The pros and cons of using the NFPA 70E table method vs. an arc flash hazard calculation analysis for PPE selection
When working on energized equipment, electricians and maintenance technicians can find themselves in a catch-22 situation: Wearing too much personal protective equipment (PPE) can impair movement or slow down work, but wearing too little can lead to serious injury or even death. OSHA and NFPA standards require employers to identify hazards and determine the types and ratings of PPE workers must use to protect themselves from potential arc flash hazards.
To guide workers in their PPE selection, NFPA 70E defines hazard risk categories (Table 1). For example, if the potential incident energy is less than 1.2 calories per centimeter squared, the hazard risk category (HRC) is 0. Incident energy levels greater than 1.2 calories per centimeter squared can produce second-degree burns; therefore, flame-resistant garments are needed to protect a worker within the flash protection boundary (FPB).
The NFPA 70E standard defines two acceptable methods for determining the arc flash risk category. One is to perform an arc flash hazard analysis and calculate the potential heat energy a worker would be exposed to in the event of an arc flash at a specific piece of equipment. The second is to use NFPA Table 130.7(C)(9)(a), commonly know as the “table method,” which provides guidelines for selecting PPE based on the type of electrical equipment and task being performed (Table 2) (click here to see Table 2).
Generally, company managers might prefer the table method because it's simpler than the arc flash calculation method. This mindset can create problems. Unless they properly apply the table, managers may unknowingly be putting workers and assets at risk. They may think the table method is acceptable simply because it's published by NFPA. However, there are several footnotes to the table that limit its application — footnotes that are often overlooked.
Ultimately, it's an employer's responsibility to determine the HRC and PPE by determining the potential arc flash energy at each piece of equipment. If there is an injury resulting from an arc flash event, OSHA will expect the employer to prove that a detailed analysis was performed or that all the conditions associated with using the table method were verified. To help avoid these pitfalls, the following sections present the pros and cons of using NFPA 70 Table 130.7(C)(9)(a), compared with using an arc flash hazard calculation analysis described in Art. 130.3 to determine incident energy levels, HRCs, and flash protection boundaries.
The table method is a task-based method to determine the HRC and PPE required for specific tasks. NFPA 70E Table 130(C)(9)(a) is a listing of typical tasks that might be performed on energized equipment while a worker is within the flash protection boundary. The 2004 edition of NFPA 70E, 130.3(A) essentially states that for systems rated at or below 600V, the default flash protection boundary (FPB) is 4 feet. However, the FPB could be greater than or less than 4 feet, based on a more thorough analysis. Any tasks performed on or near energized systems within the FPB will expose a worker to potential arc flash burns.
Table 130(C)(9)(a) describes work, such as voltage testing or replacement of a fuse or circuit breaker, while the panelboard or equipment is energized. The table has eight sections, beginning with tasks on panelboards rated 240V and below, and goes on to cover tasks on equipment rated up to 7.2kV. For example, the table assigns HRC 1 to replace a fuse or circuit breaker on a live panelboard rated 240V.
It's important to adhere to the footnotes below Table 130 (C)(9)(a). For example, an asterisk beside hazard/risk category 2 (last line of Table 2) means that a double-layer switching hood and hearing protection are required for this task, in addition to the other category 2 PPE requirements listed in NFPA Table 130.7(C)(10). Moreover, other footnotes to the table list limitations or conditions on its use. For instance, the excerpt listed in Table 2 references Notes 1 and 3, which state:
Note 1: Maximum of 25kA short-circuit current available, 0.03 second (2 cycle) fault clearing time.
Note 3: For <10kA short-circuit current available, the hazard/risk category required may be reduced by one number.
In this example, if the conditions in the footnotes cannot be verified, an arc flash hazard calculation analysis is required. For arc flash calculations to be valid, an up-to-date short-circuit study and protective device coordination analysis are required.
Even when the footnotes are properly applied, the table method is not as accurate as an arc flash hazard calculation analysis. In fact, NFPA and IEEE have begun a multi-year arc flash collaborative research project to gain a better understanding of arc flash hazards and how to protect electrical workers. One concern is that the table method may not accurately reflect the true hazard, depending on the geometry of conductors involved during an arc flash event. This could lead workers to wear unnecessarily cumbersome PPE, or worse, wear inadequate protective gear. The new collaborative research is intended to improve the accuracy of both the table and calculation methods. In addition, the committee responsible for updating NFPA 70E recently revealed it may amend the Table Method Notes in the 2009 edition of NFPA 70E.
The principal advantages and disadvantages of using the table method are:
It's fast, potentially low-cost, and can be done by an employer or worker (if qualified).
It may be a good solution for those facilities that have recently completed short-circuit and protective device coordination studies.
It can be used as a guide for workers or contractors who are collecting data for a detailed arc flash calculation analysis.
It may result in under- or over-specifying PPE, but tends to over-specify.
It assumes the flash protection boundary is 4 feet, when it could be much greater.
It may require more specific worker training that has workers routinely using NFPA 70E tables to determine PPE requirements by task.
It does not identify other short-circuit and coordination hazards that may be greater than the arc flash hazards list in the table method.
Employers are required to make risky assumptions regarding tasks not listed in the table, and/or are faced with doing engineering studies to comply with the table's notes.
Table 130.7(C)(9)(a) covers most, but not all, types of equipment, voltage levels, etc.
Any tasks on equipment outside of those listed in the table, or those that have greater than the noted available short-circuit current and fault clearing times, require an arc flash calculation.
In summary, the use of the table method is limited to specific tasks, equipment types and voltages, and verification of available fault currents and fault clearing times. If the table method is used, it's the employer's responsibility to verify and document that the conditions and limitations associated with the table apply to the specific equipment and task being performed. If the work to be performed is not listed in the table — or the conditions cannot be verified — the only acceptable alternative is to perform an arc flash hazard calculation analysis.
An engineering assessment provides more accurate results in determining arc flash hazard/risk categories and the resulting PPE selection. This method avoids a major shortcoming of the table method, which is the overlooking of the table's notes that require the verifying of the available fault currents and the fault clearing times. The actual fault current and clearing time may be higher or lower than that assumed by the table. Higher fault current increases the temperature of an arc flash. Lower current can lower the temperature but increase the duration of the event, increasing energy levels. So both situations can increase the incident energy and risk of burn injuries to a worker.
An arc flash hazard calculation analysis begins by determining a worker's safe working distance from Sec. 130.3(B) of NFPA 70E. Next, the incident energy level is calculated based on knowledge of the electrical system and analysis of over-current protective device clearing times. Generally, this requires circuit analysis to determine the available bolted fault current. The arc duration (fault clearing time) depends on the circuit protective device — typically less than 0.0083 seconds for a current-limiting fuse, or 0.10 seconds for a molded-case circuit breaker. If a transformer is involved, its impedance is needed. The potential incident energy is then calculated at the working distance, which is generally assumed to be 18 inches for 600V systems and below.
NFPA 70E references IEEE Standard 1584, which provides additional equation alternatives for calculating incident energy from these variables. There are several software packages (some available as freeware) that make it easy to calculate the incident energy.
The pros/cons of performing arc flash hazard calculation analysis include:
It is a more involved and costly process requiring specialized engineering analysis and calculations.
It usually involves creating or updating one-line drawings and a circuit/equipment analysis.
It provides more accurate fault current and clearing time values.
The use of readily available engineering software simplifies and reduces calculation time.
It may be part of a more comprehensive electrical hazard assessment that companies must do to fully comply with OSHA and NFPA 70E standards.
It provides a more accurate flash protection boundary to warn both qualified and unqualified workers.
It results in more appropriate PPE and boundaries, which can reduce downtime and speed up maintenance.
Engineering and calculation analysis allows for precise modeling of system settings and devices so that hazard levels and PPE can be reduced, thus reducing the cost of PPE while providing improved safety.
The combination of engineering studies and an arc flash hazard calculation assessment begins by building a software computer model that determines available short-circuit current flow and coordination analysis. This provides for the development of updated system drawings, or one-line drawings. The typical analysis should include contingencies for all possible values of available short-circuit current. This is because the available short-circuit energy can vary based on changes to the utility supplying electrical power to the plant, any additional sources of power, backup power, large motor contribution, and modes of operation, etc.
Whether the table method or an arc flash hazard calculation analysis is used to select PPE, employers should observe all other precautions and responsibilities. For example, NFPA 70E requires an energized electrical work permit for work on systems operating above 50V. When determining HRCs, recognize that all incident energy calculations are estimates. Therefore, it is wise to err on the conservative side. Remember that PPE may not protect against arc blast pressures, which can be fatal. If possible, work should be done on de-energized circuits using proper lock-out/tag-out procedures. However, all voltage and arc flash PPE must be used when verifying circuits are indeed de-energized.
Altmayer is POWR-GARD services manager, and Cybart is senior technical sales engineer, Littelfuse, Inc., Des Plaines, Ill. They can be reached at laltmayer@littelfuse.com and kcybart@littelfuse.com.