What is an Arc Flash?

Arc Flash is a massive electrical discharge, much like a lightning strike during a storm. Like a lightning bolt, it explodes with heat temperatures 4 times hotter than the surface of the sun. Even at a distance of several feet, workers can receive third degree burns and the blast from this explosion can propel fragmented components outward at a velocity in excess of 700 miles per hour. Some of the potential injuries are permanent blindness, hearing loss, shrapnel injuries, collapsed lungs, broken bones, loss of limbs and loss of life. Some burns are so serious that victims must be placed in drug induced comas for months. The continued risk of such horrendous injuries is no longer acceptable to either OSHA or the electrical industry.

OSHA Requirements        

OSHAâ Ts concern is with employees working on live equipment. An accidental short when working on live equipment can cause an arc flash. OSHA, NFPA and the IEEE have worked together to research the best methods to reduce the arc flash danger to employees. Published guidance on handling arc flash is now available to the industry and is known as NFPA 70E.

OSHA has adopted NFPA 70E as an industry standard for electrical safety ithe workplace. OSHA requires the employer to conduct an arc flash hazanalysis in accordance with OSHA 29CFR1910.132(d)(1) before workingenergized parts above 50 volts. If an Arc Flash hazard is present, or likto be present, then the employer MUST select and require employeesuse protective apparel. Employers who carry out an arc flash hazard risk assessment and require their employees to use protective clothing another equipment appropriate for the task are considered to be in compliance with OSHA regulations.

NFPA 70E Requirements The first concern in an Arc Flash hazard analysis is to determine the distance from the Arc Flash at which a worker would most likely receive second degree burns. This distance has been defined as the Flash Protection Boundary.

OSHA and NFPA 70E have provided two basic methods for determining the Arc Flash boundary distance. The first method for 600 volts or less is to simply use a distance of 4 feet. The second method is to perform calculations based on equations described in NFPA 70E.

The second concern is to determine the incident energy level. Incident energy is defined as the amount of heat on the workerâ Ts skin at a typical working distance. This heat is measured in either calories or Joules per square centimeter. The incident energy level will determine the type of personal protective equipment (PPE) required to minimize the probability of a second degree burn.

As in the case for determining the Arc Flash boundary, NFPA 70E provides two methods for determining the incident heat energy.

The first method is to refer to tables within NFPA 70E. The obvious advantage to using tables is their ready availability however, in this particular case it will almost certainly result in burdening the employee with wearing unnecessary levels of PPE for most cases and may result in too little PPE for some critical cases.

The second method is to use the equations in Appendix D (IEEE 1584) of NFPA 70E. The employer can either perform the calculations manually, with Excel sheets or use commercial integrated software. Employers may elect to have the analysis performed by engineering firms specializing in arc flash analysis. Engineering firms will use software and methods fully compliant with OSHA, NFPA, IEEE and ANSI requirements.

All of the methods using equations require that the short circuit or fault current be known or calculated. Existing fault current studies will likely not be valid for determining Arc Flash incident energy. New fault and selective coordination studies will be required that consider a fault current range and the resulting changes in overcurrent device tripping times.

In order to calculate the arc flash boundary and incident energy level, a complete electrical analysis must be made of your electrical system. This analysis involves the following series of steps:

1. An electrical one-line must either exist or be created.
2. Data must be collected on cable runs and equipment configurations.
3. The data must be entered into either manual equations, excel sheets or integrated commercial software, such as SKM.
4. Short-circuit (fault) calculations must be performed, calculating both maximum and minimum available fault current.
5. A system selective coordination study of the overcurrent devices must be performed. Clearing times must be determined for both maximum and minimum fault current values.
6. Once the fault currents and OCP clearing times have been determined, then the equations in NFPA 70E Appendix D (IEEE 1584) can be used to calculate the Arc Flash incident energy. Once the incident energy is known, the required PPE can be specified.
7. When all the calculations are complete, the relevant information is printed on an Arc Flash label to be installed on each switchboard, panel, disconnect and MCC. This label provides guidance to the worker on wPPE to wear and the level of arc flash and voltage hazard.
8. NFPA requires safety training for all employees exposed to live circuits.

In summary, Arc Flash danger is real and can result in catastrophic financial cost and human damage. OSHA and the NFPA have provided guidance and are now requiring compliance. Future columns will expand on the subjects of Arc Flash phenomena, personal protective equipment, calculation methods
and options, procedures for compliance, recommended methods and implementing an electrical safety program.

Bruce Blouin PE, President of Power Analysis Associates.