Arc Flash – It’s Dangers and Precautions
4/11/2011
Prior to 1982, it was assumed that electric shock was the major risk associated with live electrical work. However, it was during this year that Dr. Ralph Lee first identified arc flash, in a paper titled “The Other Electrical Hazard: Electric Arc Blast Burns.” According to this paper, as many as 80% of electrical injuries result from arc flash rather than from shock.
An arc flash is an electrical breakdown (A large, abrupt rise in electric current) of the resistance of air resulting in an electric arc (discharge of current) which can occur when there is sufficient voltage in an electrical system and a path to ground or lower voltage. In other words, an arc flash is a short circuit through air that flashes over from one exposed live conductor to another conductor or to ground. When this happens the surrounding air ionizes to form a superheated plasma (ionized gas). Temperatures can reach 5000-35,000°F in a few thousandths of a second. At this point conductors melt and vaporize, exploding from the arc as toxic gases and a spray of molten metal. The sudden expansion of air from the heating causes a pressure wave carrying shrapnel and hitting surfaces with forces of several hundred lbs/in2. Workers and equipment can be thrown several yards by the blast. This type of event is sometimes referred to as an arc blast. A 10,000 arc at 480 volts is equivalent to 800 MW or approximately 8 sticks of dynamite. Pressure on the chest can be as high as 2000 lbs/sq foot, causing lung collapse. The sound associated with blast can exceed 160 dB. The sound of jet engine only 145 db!
Arc flash incidents are common and costly, and the frequency of reported accidents is increasing – making arc flash a very hot topic (excuse the pun) within OSHA and the overall safety industry. Of the approximately 350 persons killed in the work place by electricity last year, roughly 50% were related to arc flash. A report compiled by an independent safety consultant estimates that five to 10 arc flash explosions happen in the USA every day, resulting in 1 to 2 deaths per day. That figure only takes into account incidents where victims were sent to special burn centers. The number does not include cases sent to regular hospitals or clinics, nor unreported cases or near misses.
A hazardous arc flash can occur in any electrical device, regardless of voltage, in which the energy is high enough to sustain an arc. Potential places where this can happen include:
Panel boards and switchboards Motor control centers
Metal clad switch gear Transformers
Motor starters and drive cabinets Fused disconnects
Any place that can have equipment failure
Who is at risk? Some of the employees at risk from arc flash hazards include mechanics, electricians and HVAC personnel. The most dangerous tasks include:
- Removing or installing circuit breakers or fuses
- Working on control circuits with energized parts exposed
- Racking circuit breakers in and out of switch gear
- Applying safety grounds
- Removing panel covers
- Low voltage testing and diagnostics
Therefore, an arc flash is a possibility any time a conducting material approaches exposed live parts. Many arc flash incidents are caused by human error. Dropped tools, accidental contact with live parts, and even loose tools left in the equipment by previous workers have caused arc flash incidents. As a matter of fact, just being within a close proximity to a high-amp source with a conductive object can cause the electricity to flash over. A few conditions that can result in arc flash include:
- Equipment failure due to use of substandard parts, improper installation, or even normal wear and tear.
- Breaks or gaps in insulation.
- Dust, corrosion or other impurities on the surface of the conductor.
Serious burns are not uncommon even at a distance of 10 feet. Arc flash can cause the following injuries:
- Skin burns by direct heat exposure
- Metal is vaporized at this temperature. Droplets of molten metal can be propelled over great distances, causing serious burns or igniting clothing.
- High-intensity flash can also cause damage to eyesight.
Since 1990, OSHA, as well as other organizations involved with electrical safety (NFPA and IEEE) have been actively promulgating standards and guidelines to protection employees from arc flash. Below are four published industry standards.
- OSHA 29 Code of Federal Regulations (CFR) Part 1910, Subpart S
- NFPA 70-2002, National Electrical Code
- NFPA 70E-2000, Standard for Electrical Safety Requirements for Employee Workplaces (Fig. 1).
- IEEE Standard 1584-2002, Guide for Performing Arc Flash Hazard Calculation
These standards and guidelines provide detailed information for the below-listed protective measures:
Warning labels to inform employees of potential hazards,
Lockout/Tagout devices: to prevent unexpected re-energization of equipment while work is being performed, and
Personal Protective Equipment: One source for PPE for protection against arc flash can be found in NFPA 70E. Table 130.7(C)(9)(a) lists a number of typical electrical tasks are various voltage levels and recommends the category of PPE that should be worn.
While safety products are readily available, many companies make the mistake of thinking that just by purchasing them and providing them to their work force, their employees shall be properly protected. However, having the proper PPE available may be useless unless workers understand how to use them as well as knowing their limitations. Similarly, a warning label means little if employees don’t have the training to treat hazards with the care they require. Therefore when in doubt, use a SAFETY PAUSE and contact your S&H representative. He/she may not know the answer, but they will find the right subject matter expert who can provide the appropriate information. Electricity and arc flash are serious hazards and should never be taken lightly.
In the confrontation between the stream and the rock, the stream always wins – not through strength, but through persistence.
Buddha