When I conduct electrical safety classes, the first question I ask the class is this: “Show of hands. How many of you have been shocked?” Hands immediately go up for 90 to 95% of participants. When I ask the same question about arc flash, only about 10 to 15% of participants raise their hands. With arc flash, I often see several hands go halfway up because the person is not sure whether they have been in an arc flash event. Many people who work around live electricity are not completely sure what arc flash is and how it is different from shock. Let’s clear up the confusion.
Shock occurs when current runs through a person’s body or on the surface of the skin. When someone is shocked, they become part of the circuit. The severity depends on the amount of current and the path the current takes through the body. Severe shock can result in ventricular fibrillation, internal burns to organs, external burns to skin, and even death, which is known as electrocution. Ventricular fibrillation can be caused by only 0.2 amps for one second.
Arc Flash and electric shock are very different. In an arc flash event, the person does not become part of the circuit. The danger lies with being at ground zero of a hot and violent electrical explosion. In an arc flash event, air is the conductor of electricity. For many reasons, air can become ionized enough to conduct electricity causing an arc that short circuits between phases or between phase and ground. This results in a heated wave of plasma that can quickly reach temperatures up to 35,000 degrees Fahrenheit. To put that in perspective, the surface of the sun is about 10,000 degrees Fahrenheit.
When temperatures reach these levels, every known material on earth is vaporized, including copper conductors. When copper transforms from solid to vapor, it expands by 67,000 times. This can happen in a fraction of a second in an arc flash event. The rapid expansion leads to a tremendous dissipation of energy. The pressure from 100kA arc can reach 400 lb.ft2 of pressure, which is ten times the wind resistance typical walls are rated to withstand. In many cases, an arc flash event is similar to a bomb detonation with the worker being at ground zero.
As you have learned, shock and arc flash are different in many ways. As a result, different types of protection are required in the form of Personal Protective Equipment (PPE). Shock PPE consists mainly of insulating equipment in the form of rubber material that is voltage rated. Voltage-rated gloves and insulated tools are the most common PPE used to protect a worker from shock, although many other types of equipment may apply depending on the task.
Arc Flash protection is mainly focused on protecting a worker from a massive amount of heat. Remember, an arc flash can reach temperatures in the tens of thousands of degrees. PPE for arc flash is designed to protect a worker from severe burns, UV light, and very loud sound. Arc flash PPE does not typically provide insulation from shock. In addition, arc flash PPE is limited in the kind of protection it can provide from the pressure wave of the arc blast.
Details of implementing a shock and arc flash PPE program are beyond the scope of this article, but will be addressed in future articles. Suffice it to say that PPE programs designed to protect workers from electrical hazards can be complicated because the worker must protect himself or herself from two different hazards that have very different characteristics and levels of risk depending on the work being performed. The worker must assess the risk of both shock and arc flash, and the risks can vary for each of these hazards.
Shock and arc flash are vastly different events and are usually independent of each other. Although a shock can trigger an arc flash and vice versa, the two events are usually exclusive of one another. The graphic below summarizes the different characteristics and different PPE requirements of each event.
In future articles, I'll go into more detail about teach o the hazards. In the meantime, keep this in mind:
Shock and arc flash pose no risk whatsoever if you shut off the power before working on electrical equipment.
Bobby Lindsey – CESCP
Mitchell & Lindsey – President
M: (502) 836-4217
email: blindsey@mitchellandlindsey.com
