Electric Current through Tissue and Air - Shock and Arc Flash Hazards

Basic electrical theory is that the greater the load is, the greater the current. Normally, the current flows through conductors and various electrical components, but what if the current takes a different path? What if it flows through air or through human tissue? A high-magnitude current flowing through air can create an arc flash, possibly resulting in severe injury or death. A fatality from electric shock only takes a fraction of an amp. Let’s explore the details of both paths, and methods that can be used to reduce the risk of each.

Eliminate the hazard

With very few exceptions, OSHA and NFPA 70E require systems be placed into an electrically safe working condition before work is performed—i.e., eliminate the hazard. NFPA 70E defines the steps for establishing this condition in Section 120.5, which includes testing for absence of voltage. This test in itself can be a risk if the system remains energized for some reason, such as a missed feeder or switching failure. Also, energized work still may occur where it is permitted by NFPA 70E 110.4, Energized Work.

During my training programs, I often recall an incident where testing for absence of voltage saved two contractors. The workers opened a disconnect for switch feeding equipment that required servicing and assumed the equipment was de-energized. The lead person remembered how I explained the requirement to test for absence of voltage during a training program at their site.

Rather than proceeding with the work, he stopped and got his meter, and was surprised to find the bus was still energized when he tested it. Further inspection found that, although the switch handle was in the off position, the internal switch mechanism failed to open. Disaster avoided.

Safer absence of voltage verification methods may include devices that provide a visual indication on the outside of the equipment without exposing the electrical worker directly to the hazards, as shown in Figures 1 and 2. NFPA 70E 120.5 Exception 1 provides details regarding the use of such devices.

Current through tissue: shock and electrocution

Shock and electrocution hazards have been known since the early days of electrical power systems. The first generation of electrical workers quickly learned the hard way not to touch the “shiny parts” or it could hurt—or worse. However, not much was known about the physiological effects of electric current on the body.

This lack of understanding continued into the 1950s and 1960s. The 1953 edition of the American Electrician’s Handbook gave guidance for testing circuits by touching! It specifically states, “Electricians often test circuits for the presence of voltage by touching the conductors with the fingers. This method is safe where the voltage does not exceed 250.”

It goes on with much more information—the kind of stuff that would make a great social media meme. I can honestly say I have never intentionally tested a circuit using that method but I have talked with people over the years who either did or knew those who did.

To explore the physiological effects of electric current on the human body, Charles Dalziel conducted experiments on test subjects and published the results in his landmark 1961 paper, “Deleterious Effects of Electric Shock.” The experiments included sheep, pigs, dogs and even humans.

(It was a different time, 60 years ago.)

Dalziel’s work formed the foundation of what we know today, such as the threshold of current where a person’s muscles involuntarily contract and they can’t let go of the conductor. In addition, if the current path is across a person’s chest and vital organs, Dalziel’s research helped define the threshold of current where respiratory paralysis and ventricular fibrillation are likely and can lead to a fatal outcome.

Protection

Several protective measures are now available to help those working around electricity. Two are mentioned here:

Insulating gloves and blankets:?Isolating the electrical worker from energized conductors can be achieved through proper use of electrical insulating gloves, which are manufactured in accordance with strict standards such as ASTM D120, Standard Specification for Rubber Insulating Gloves. They are available based on several voltage classes, depending on the maximum-use voltage requirements. With few exceptions, insulated gloves are required to be used with leather protector gloves.

GFCI:?The likelihood of electric shock increases when electricity is used around wet locations. Dalziel’s work resulted in his invention of the ground-fault circuit interrupter (GFCI). The basics of a GFCI are simple: it measures the current flowing to a load on the phase conductor and compares it to the current returning on the neutral conductor. If a mismatch greater than 4–6 milliamps occurs, the “missing” current may be traveling through a person and the device trips instantaneously, disconnecting the potentially fatal circuit.

Although GFCIs were first required near swimming pools, the?National Electrical Code?requirements have greatly expanded over the years to include bathrooms, kitchen sinks, garages and more.

Current through air: arc flash

When current flows across an air gap, usually from inadvertent contact or component failure, a potentially deadly arc flash occurs. NFPA 70E, Standard for Electrical Safety in the Workplace, first used the term “arc flash” in the 1995 edition. IEEE 1584, IEEE Guide for Performing Arc-Flash Hazard Calculations, provides a model to predict the severity of an arc flash in terms of incident energy in calories per centimeter squared (cal/cm2).

Arc duration

One of the most significant variables defining the severity of an arc flash is the arc duration. If the duration doubles, the incident energy doubles. Cut it in half, and the incident energy is reduced by half. To reduce the severity of an arc flash, many innovative methods have been developed to control the duration.

Reducing the duration seems like a simple concept—set all adjustable devices to trip as quickly as possible. However, if every device tripped as fast as possible, a wider-spread outage could occur during a fault. To minimize the extent of the outage, devices located closer to the source will generally be selected or set to operate more slowly, allowing devices closer to the load to trip first. This concept is known as selective coordination. However, to better protect electrical workers, devices should operate as fast as possible. This poses quite a conflict between competing objectives: increasing safety with fast operation, or increase reliability with slow operation. A solution is to use devices with normal settings to minimize the outage as well as fast settings that can be temporarily enabled when energized work will be performed.

Another problem can occur when larger protective devices are used. The larger the device, the more current is required to trip instantaneously (typically a few cycles). If the prospective arcing short-circuit current is too low, it may trip in a time-delay region, taking much longer and resulting in a much greater (and more dangerous) incident energy. For example, if the prospective incident energy is 6 cal/cm2?based on an instantaneous time of three cycles, and if the same device trips in the time delay region, taking 10 times as long—30 cycles—the prospective incident energy would also increase by a factor of 10.

To address the issues with larger devices, 2020?NEC?Section 240.87, Arc Energy Reduction, states, “Where the highest continuous current trip setting for which the actual overcurrent device installed in a circuit breaker is rated or can be adjusted is 1,200A or higher, 240.87(A) and (B) shall apply.”

(A), Documentation, refers to documentation demonstrating that the method selected will operate below the available arcing current. (B), Method to Reduce Clearing Time, lists methods such as an arc-energy-reduction maintenance switch that can be used as a temporary setting so the instantaneous function operates at less than the available arcing current.

Direction

NFPA 70E defines arc-resistant equipment as “equipment designed to withstand the effects of an internal arcing fault and that directs the internally released energy away from the employee.” Arc-resistant equipment is designed in accordance with IEEE C37.20.7, Guide for Testing Switchgear Rated Up to 52 kV for Internal Arcing Faults. The overall concept is that if the doors on the equipment are properly closed and an arc flash occurs, the equipment contains the energy and redirects it away from the worker. Figure 3 shows arc-rated switchgear.

Several years ago, I accidentally witnessed 15-kilovolt arc-resistant switchgear being tested. The test was being conducted at the opposite end of a large facility where I was conducting separate arc flash tests. Just before their test, an alarm sounded indicating everyone should take cover. Kaboom! It sounded like the end of the world. I immediately ran to see what it was.

With smoke still rolling out of the end of the duct from the successful test, I quickly realized that the earth-shattering noise is a much better alternative than having all that energy directed toward the worker.

Distance

Increasing the distance between the worker and a prospective arc flash can greatly reduce the incident energy to the worker. Many devices, such as those shown in Figure 4, have been introduced to remotely operate electrical equipment. This equipment will not prevent an arc flash, but if it does happen, it can greatly increase the distance and helps place the worker outside of the hazard area known as the arc flash boundary.

Progress continues

These are just a few of the innovations to reduce the risk of shock, electrocution and arc flash. Is the additional cost of these technologies worth it? Try asking that question to a friend or family member of a victim.

This article was originally published in the May 2022 Edition of Electrical Contractor Magazine.