Mention arc flash and thoughts often turn toward extreme heat, burn injury and arc-rated clothing and personal protective equipment (PPE). However, an arc flash has nonthermal hazards as well.
The word “flash” is part of the description for a reason. Back in the 1990s, when the term “arc flash” was first being considered, one of its chief proponents told me how people wanted to use the term “electrical explosion.” He was adamant that not every event was an explosion, but every event had an arc and a resulting (sometimes blinding) flash. His persistence led to the introduction of the term arc flash in the 1995 edition of NFPA 70E.
The flash is a high-intensity light that can damage the eye and cause temporary or permanent blindness. I had the (unexpected) opportunity to experience this several years ago while conducting a series of arc flash tests. Normally, I have the lab technician count down from three so I can look away before the arc flash. On this occasion, I was busy focusing my camera on the equipment to be tested, and without warning—BOOM! He forgot to count down. Quite stunned, I blinked my eyes several times, trying to shake it off, only to realize that my field of view directly in front was now a bright purple. It took about 20–30 minutes for my vision to return, and fortunately, there were no long-lasting effects; although, I was surprised and a bit worried at the time.
Not everyone is so lucky.
How bright is bright? Light can be measured in units known as lux, which is a measurement of illuminance and equal to 1 lumen per square meter. To put it into perspective, a typical overcast day is about 1,000 lux. Direct sunlight is upwards of 100,000 lux. The light intensity of an arc flash 3 meters from the source is over 1 million lux, and a more recent arc flash test recorded 13.1 million lux—approximately 130 times brighter than direct sunlight!
To help protect the eyes from this hazard, face protection with ultraviolet light filtering is used to reduce the possibility of eye injury.
During an arc flash, the surrounding air is superheated, causing it to rapidly expand and resulting in air pressure shock waves (think lightning and thunder). In addition, when conducting material such as copper changes state from solid to vapor, it rapidly expands to 67,000 times its volume. If the pressure, sometimes known as an arc blast, is significant enough, responses such as workers being pushed backward or knocked off ladders or platforms can occur. In one case, a worker was knocked off a roof and died.
Over the years, a few predictive models have been developed and published in technical papers, as well as more recent tests as part of the IEEE/NFPA research. Research continues in this area.
The sound in an arc flash video does not begin to capture the intensity of the actual event. The blast can also produce a significant sound pressure that damages a person’s hearing.
The volume of a given sound is related to the amplitude of the pressure wave striking the ear and is expressed in decibels (dB), which is a logarithmic—not linear—scale. As such, the actual dB numbers can be deceiving. For reference, conversational speech is approximately 60 dB. A loud rock band can result in a pressure level around 120 dB (I recall during my concert-going era my ears ringing for days afterward). Doubling the dB value from 60 to 120 does not double the sound pressure. It actually increases by a factor of 1,000!
OSHA 1910.95(b)(2) states, “Exposures to impulsive or impact noise should not exceed 140 dB peak sound pressure level.” Arc flash testing typically results in sound pressures exceeding 140 dB, and a few arc flash tests have recorded sound pressure upwards of 170 dB, well beyond the OSHA limit of 140 dB and above where mechanical damage to the ear can occur.
To protect a worker’s ears from this hazard, NFPA 70E states hearing protection shall be worn whenever working within the arc flash boundary.
More than burn injury
Next time you think about the arc flash hazard in terms of burn injury and arc-rated PPE, remember there are other hazards, too. These hazards can cause additional injuries, such as eye damage, blindness and hearing loss.