Selecting arc-rated clothing and personal protective equipment for protection from an arc flash hazard can be a confusing task. NFPA 70E lists two different PPE selection methods that can be used as part of an arc flash risk assessment. Depending on which method is chosen, the process could include using various tables or performing complex calculations and analysis. It is enough to make your head spin.
To begin, identify any arc flash hazards that may exist. The likelihood of an arc flash increases when energized electrical conductors are exposed, or if there is interaction with equipment that may result in an arc flash. NFPA 70E Table 130.5(C) lists many common electrical tasks that factor in equipment condition, and it correlates each with the likelihood of an occurrence. During normal operating equipment, as outlined by NFPA 70E, it is considered that an arc flash is not likely to occur.
If an arc flash hazard has been identified and the risk cannot be eliminated by using the hierarchy of risk-control methods, then arc-rated clothing and PPE may be required. The protection characteristic of the PPE is rated in terms of calories per square centimeter (cal/cm2), known as the arc rating, and the value is determined based on testing according to specific ASTM standards.
To select arc flash PPE, NFPA 70E 130.5(F) lists two methods that may be used incident- energy analysis and arc flash PPE category method. Proper selection of the arc flash PPE requires having an arc rating sufficient for protection from the hazard. It should be noted that arc-rated PPE is designed for thermal protection and not protection from flying debris, blast pressure and other hazards.
PPE category method
Hazard risk category tables were first introduced in the 2000 edition of NFPA 70E. It listed common equipment and tasks to define PPE requirements based on six categories from –1 to 4. The higher the number, the more protection is required. With each subsequent edition of NFPA 70E, the tables continued to evolve. The 2015 edition changed them to PPE category tables to align with the risk assessment procedure. The tables now only list equipment without tasks and use four categories to define the arc-rated clothing and PPE requirements.
Adding a bit of complexity to the tables, each listed equipment type and corresponding PPE category has additional parameters of maximum available fault current, maximum fault-clearing time and minimum working distance that must be considered.
The incident-energy analysis method requires performing complex calculations to determine the prospective incident-energy exposure in terms of cal/cm2. Arc-rated clothing and PPE are selected to protect the worker based on this value. The incident energy is dependent on a specific distance between the prospective arc source and the worker’s face and chest area, which is known as the working distance.
Incident-energy calculations are performed using equations from IEEE 1584 and IEEE Guide for Performing Arc-Flash Hazard Calculations. First published in 2002, the second edition of this standard was published in late 2018 and contains all new equations. Some major variables required for these calculations include the bolted-fault current, arc flash duration and the working distance, which is similar to the additional parameters required when using the arc flash PPE category tables. Other variables such as enclosure size, bus gap and electrode configuration are necessary.
The IEEE 1584 equations are valid for three-phase arc flash calculations with fault currents up to 106,000 amps (A) for 208–600-volt (V) systems and fault currents up to 65,000A for voltages from 601 to 15,000V. But what if the voltage falls outside of these ranges and/or a single-phase model is needed for systems such as transmission and distribution lines? Although IEEE 1584 does not provide guidance for these cases, there are a few other options.
For higher voltages and single-phase arc flash calculations, the National Electrical Safety Code IEEE C2 Standard and OSHA 1910.269 Annex E provide tables for selecting protective clothing and clothing systems. Although not referenced by NFPA 70E, these tables are based on a series of extensive calculations for open-air phase-to-ground arcs typical of transmission and distribution systems. An additional option for these cases is a commercially available program that is also listed in OSHA 1910.269 Annex E.
Regardless of which method is used, both make sure that the person is protected if an arc flash occurs. However, PPE alone is not enough. Having a good electrical safety program, using safe work practices and proper training are all a necessary part of the overall protection strategy.