Every year, more than 2,000 workers are treated in burn centers with severe arc flash injuries. The flash is immediate, but the results can cause severe injuries that last months, years – even a lifetime. In some cases, they may cause death. Fortunately, arc flash hazards can be reduced by following safety precautions and using the proper Arc Flash Protection recommended for each application.
What is Arc Flash?
Arc Flash is the result of a rapid release of energy due to an arcing fault between a phase bus bar and another phase bus bar, neutral or a ground. During an arc fault the air is the conductor. Arc faults are generally limited to systems where the bus voltage is in excess of 120 volts. Lower voltage levels normally will not sustain an arc. An arc fault is similar to the arc obtained during electric welding and the fault has to be manually started by something creating the path of conduction or a failure such as a breakdown in insulation.
The cause of the short normally burns away during the initial flash and the arc fault is then sustained by the establishment of a highly-conductive plasma. The plasma will conduct as much energy as is available and is only limited by the impedance of the arc. This massive energy discharge burns the bus bars, vaporizing the copper and thus causing an explosive volumetric increase, the arc blast, conservatively estimated, as an expansion of 40,000 to 1. This fiery explosion devastates everything in its path, creating deadly shrapnel as it dissipates.
The arc fault current is usually much less than the available bolted fault current and below the rating of circuit breakers. Unless these devices have been selected to handle the arc fault condition, they will not trip and the full force of an arc flash will occur. The electrical equation for energy is volts x current x time. The transition from arc fault to arc flash takes a finite time, increasing in intensity as the pressure wave develops. The challenge is to sense the arc fault current and shut off the voltage in a timely manner before it develops into a serious arc flash condition.
Did you know?
The temperature of an arc flash can reach 35,000 degrees Fahrenheit – about four times as hot as the surface of the sun.
Arc Flash Personal Protection Equipment (PPE)
Personal protective equipment, or PPE is designed to protect employees from serious workplace injuries or illnesses resulting from contact with chemical, radiological, physical, electrical, mechanical, or other workplace hazards. Besides face shields, safety glasses, hard hats, and safety shoes, PPE also includes a variety of devices and garments such as goggles, coveralls, work gloves, vests, earplugs, and respirators.
In order to select the proper PPE, incident energy must be known at every point where workers may be required to perform work on energized equipment. These calculations need to be performed by a qualified person such as an electrical engineer. All parts of the body that may be exposed to the arc flash need to be covered by the appropriate type and quality of PPE. Proper PPE for welding can include Flame Resistant clothing, arc flash kits, helmet or headgear, face shield, safety glasses, gloves, shoes, etc. depending upon the magnitude of the arc energy.
Industry Standards – Four separate industry standards establish practices for the prevention of electrical explosion incidents:
OSHA 29 Code of Federal Regulations (CFR) Part 1910, Subpart S
This regulation states, in part, “Safety related work practices shall be employed to prevent electric shock or other injuries resulting from either direct or indirect electrical contacts” .OSHA also addresses the qualification of workers exposed to electrical shock hazards and the provision for protective equipment appropriate for the work to be performed. OSHA enforces safety practices and cites to the NFPA requirements.
NFPA 70e-2004, National Electrical Code
Section 110.16 requires that companies place a warning label on electrical equipment likely to constitute an electrical safety hazard. This field marking can be generic or very specific, whichever the company selects. Future revisions of the NEC standard may require more detailed information on this label.
NFPA 70E-2000, Standard for Electrical Safety Requirements for Workplaces
NFPA 70E can be considered the “how to” standard behind OSHA enforcement. It provides the detailed actions companies must take to be in federal compliance.
■Safety program with defined responsibilities
■Calculations for the degree of electrical safety hazard
■Training for workers
■Tools for safe work
■Warning labels on equipment
IEEE Standard 1584-2002, Guide for Electrical Safety Regulation
In order for the warning labels to carry enough information to show the danger zone for electrical safety conditions, companies must determine that area within which only qualified workers should enter – the protection boundary. IEEE 1584 provides a method to calculate the incident energy in order to specify the level of PPE required for workers.