Electrical Construction & Maintenance Magazine (EC&M )recently reported that there are five primary factors that determine the severity of an injury from an electrical arc:
- Distance from the arc
- Absorption coefficient of the clothing worn
- Arc temperature
- Arc duration
- Arc length
Let’s take a closer look at each one of these items so you can better understand how to protect your workers or yourself.
By James White, Shermco Industries | Electrical Construction and Maintenance
White is director of training for Shermco Industries in Irving, Texas. He can be reached at email@example.com.
Distance from the Arc
The heat of an electrical arc is referred to as the “incident energy.” This is because the heat is made up of the radiated heat (infrared) and convection heat (heat flow through air). Incident energy decreases by the inverse square of the distance as a person moves away from an arc source. A simpler way to state this is that as a person moves away from an arc, the heat will decrease rapidly. This aspect is critical to understanding how to protect oneself from an arc.
Body position is a primary factor to consider when performing energized work. A person should stand as far from the device as practical, while still being able to perform the work effectively. Standing closer than necessary will increase the incident energy that person will receive if there is an arc flash event.
If incident energy decreases by the inverse square of the distance moving away from an arc source, it will increase by the square of the distance as the distance decreases. It only takes a small change in the distance to make a large change in the incident energy. The standard working distance for work on systems operating at less than 600V is typically 18 in., while 2.4kV to 15kV power systems typically have a 36 in. working distance.
Absorption Coefficient of Clothing Worn
The type and fabric weight of clothing being worn affects the heat that is transferred to the body. NFPA 70E recommends wearing either flammable, non-melting clothing as underlayers (cotton, wool, or silk) or arc-rated underlayers for additional protection. The general rule of thumb is that each layer of clothing worn under arc-rated clothing reduces the heat to the body by approximately 50%. Flammable underlayers do not increase the arc rating of a clothing system, but will reduce the probability of a burn underneath arc-rated clothing.
The temperature of an electrical arc is mostly determined by the megawatts of power being consumed by the arc. Megawatts (watts x 1,000,000) is a tremendous amount of energy. A 3-phase, 480V fault with 50,000A of short circuit current will consume 23MW of power. The heat from an electrical arc vaporized the copper in this circuit breaker.
Keeping overcurrent protective devices calibrated reduces the duration of the arc. The arc duration is the second most-critical injury factor in an arc flash event. Incident energy is proportional to time. If a person is exposed to an arc flash for 0.08 sec, they would receive twice the incident energy as an arc of the same magnitude that lasted 0.04 sec. This is why the NFPA 70E Technical Committee added Sec. 205.4 to the standard, which states: “Overcurrent protective devices shall be maintained in accordance with the manufacturers’ instructions or industry consensus standards.”
Poorly maintained circuit breakers and other overcurrent protective devices (OCPDs) are unreliable. If an OCPD malfunctions, it will increase the time it takes to clear and extinguish the fault. NFPA 70B, Recommended Practice for Electrical Equipment Maintenance, and ANSI/NETA MTS-2015, Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems, should also be consulted to develop an acceptable maintenance program.
The arc length becomes a factor at higher voltages (i.e., greater than 600V). It has been demonstrated that with all other factors being the same a longer arc creates more incident energy than a shorter arc. Low-voltage power systems less than 208 V cannot normally sustain an electrical arc, as arc resistance causes a voltage drop of approximately 75V per inch to 100V per inch. Even though high-voltage electrical systems present the greatest risk of an arc, low-voltage systems can also suffer catastrophic failures.
The following Forensic Engineering “Case Study” was Excerpted from The “Complete Guide” to CONSULTING ENGINEERING © 2015 John D. Gaskell. Used with permission of Professional Value Books, Inc. All rights reserved. Order at http://www.TheEngineersResource.com. Use discount code “paperback” and save.
This accident occurred when two electricians were servicing a fan motor. They discovered a blown fuse in a 480-volt motor control center (a freestanding enclosure containing both fused switches and motor starters). They installed new fuses, closed the compartment door, tightened the screws holding the door and closed the switch. An arc-fault occurred in the compartment, which blew the door off, injuring both electricians; one had third degree burns over 50 percent of his body. Both electricians were covered by “workman’s compensation insurance,” so could not sue their employer. The more severely injured man sued the manufacturer of the motor control center. We were hired by his attorney to investigate.
First we spent a lot of time looking through two big boxes of paperwork. Next we did online research on the switchgear manufactures website and on the website of Underwriters Laboratory (UL). Then we visited the site, examined the switchgear, and interviewed both electricians.
We observed that the spacing of the bus bars (live parts) within the enclosure appeared to be minimal and later compared this spacing to the details of other manufacturers of the same type of equipment, which confirmed our observation. We found numerous testing reports of the model switch at issue and all ended in failures. We could not find any reports of switches that passed. However, we did assume that it likely passed some test, because it was UL listed. We pointed out these findings in a verbal report to our attorney-client.
Our “very happy” attorney-client called us two weeks later to tell us the case had settled. You don’t necessarily need to testify to provide a valuable service.