Electrical shock and electrocution potential.
By Ed Butts, PE, CPI
I have written several columns over the years about recognizing various hazards and observing safety procedures in the water well industry. These have included personal stories of crushed and broken toes and fingers, slips and falls, bad backs from incorrect lifting, and electrical exposures.
Although many old-timers may recognize some of the following text from past columns, it is my hope that the new kids who haven’t read the columns can benefit from this one. After all, if any of my warnings and suggestions can possibly save a life, what better reward can there be?
So, as what I consider to be a fitting conclusion to this yearlong series on electrical topics, I wish to revisit the subject of electrical safety with an emphasis on the potential for electrical shock and electrocution plus new sections on arc flash and blast exposures and preventive measures.
First Rule: Know How Much and Where Contact Can Kill You
As we all know from regularly working with electricity, there are several components to AC or DC electrical power, particularly the big three: voltage, current, and resistance.
Voltage can be considered as the fundamental force that pushes electric current through a circuit. As an analogy to hydraulics, it is the same as the pressure. In low-voltage systems, alternating current (AC) power is used for most residential and industrial pumping purposes in the United States and is generally available in 120, 240, 480, and 575 volts.
Current refers to the amount of electricity (i.e., electrons) that flow through a circuit each second. Current is measured in amperes or milliamperes (1 mA = 1/1000 of an ampere) and is analogous to the flow rate in hydraulic systems. The amount of electric current that flows through the body determines the various effects from an electric shock.
Resistance, the last component of basic electrical circuit terms, is the value of pushback the circuit offers to impede the flow of current (amps) against the applied pressure (voltage). Resistance is typically measured in ohms and is analogous to the sum of friction losses in hydraulic systems.
Combining these factors yields the following word relationship: Depending on the resistance of a circuit, a certain amount of current will flow for any given voltage.
Or mathematically, as Ohm’s Law: Volts = Amps × Ohms, or Amps = Volts ÷ Ohms.
The volts and ohms in the potential for an electrical shock will play a role, but it is the current flow that determines the damaging physiological effects. Most current-related effects result from heating of tissues and stimulation of muscles and nerves. Stimulating nerves and muscles can result in problems ranging from a slight fall due to recoiling from pain to serious and permanent respiratory problems up to cardiac arrest.
Relatively small amounts of current are needed to potentially cause these physiological effects. The body resists the flow of current, as more than 99% of the body’s resistance to the flow of electrical current exists at the skin level.
As resistance is measured in ohms, a calloused and dry hand may possess more than 100,000 ohms of resistance because of a thick outer layer of dead cells in the outermost layer of the epidermis. A wet skin displays a resistance closer to just 1000 ohms.
The internal body or hand-to-foot resistance is usually about 300-500 ohms, related to the wet and relatively salty tissues that lie beneath the skin. The skin’s resistance can be effectively bypassed if there is a breakdown of skin from high voltage, a cut, a deep abrasion, or immersion in water.
The International Electrotechnical Commission (IEC) offers the values shown in Table 1 for the total body impedance of a hand-to-hand circuit for dry skin, large contact areas, based on 50 Hz, AC currents. The columns contain the distribution of the impedance (AC resistance) in the total population. For example, at 100 VAC, 50% of the population had an impedance of 1875 ohms (Ω) or less.
At a voltage level of 25 VAC and average (50% percentile) hand-to-hand resistance, an individual receiving an electrical shock may easily encounter 25 VAC ÷ 1875 ohms = 0.013 amps. This is 13 times greater than the perceptible level of 1 mA (0.001 amps) and just below the maximum current an average person can grasp and then release from (16 mA or 0.016 amps shown in Table 2).
This demonstrates how little voltage is needed to generate an electric current flow and resulting shock. The skin acts similar to an electrical device, such as a capacitor, in that it allows more current to flow if a voltage is changing rapidly. A rapidly changing voltage will be applied to the palm and fingers of one’s hand if it is holding a metal tool that suddenly touches a voltage source. This type of contact will pass a much greater current amplitude in the body than would otherwise occur. The level of AC current and the degree of potential damage are listed in Table 2:
Electric Current and the Human Body
Working around electrical power as we do, exposure to possible electrical shock and burns is apparent, or is it? Many individuals in our industry, especially those who regularly work around three-phase power, tend to become somewhat cavalier and downright bold over time when working around the seemingly less powerful 120-volt or 230-volt, single-phase power supplies.
This attitude creates one of the fastest ways I can imagine landing yourself or a co-worker in the hospital or morgue. For those of you old enough to remember what I am about to relate, think back. For those of you too young, listen up!
Pump classes put on by a popular motor manufacturer many years ago featured a character named “Pete Pumpman.” One of the messages he related to me as a rookie pumpman is how little current and voltage it takes to kill you—not just shock you, but actually kill you!
Consider this: Depending on the actual circumstances, a current drawing as low as 5 to 200 milliamps (0.005 to 0.2 amps) through the heart can cause fibrillation and possible death. For comparison, a single 100-watt light bulb draws around 0.8 amps, roughly four times the level of current that can kill you.
Everyone wants to always talk about voltage levels and the associated hazards—but forget voltage for a minute. The real underestimated electrical hazard is the current. And if a 100- watt light bulb can do that to you, just imagine what a dead-grounded ½ hp motor that draws 10 amps is capable of, and especially if you happen to be standing in water! The bottom line is this: Never, and I do mean never, underestimate the hazards associated with so-called “low voltage.”
As seen in Figures 1A and 1C, a current path through the heart, regardless of the actual path taken to ground, whether the foot or arm, can cause defibrillation of the heart, rapidly leading to serious injury or death. A current path that flows straight through the right arm and then out the right foot, as shown in Figure 1B, may result in a much less damaging injury since the current did not travel through the heart.
A technician must always be aware of the fact that electrical voltage must have a path for electrical current to flow from an electrical source through a human body and back to the ground before bodily damage or injury can occur. Although there is never any guarantee of this, disrupting, shielding, or preventing this route of current flow will usually avoid serious injury.
Beyond the route of current flow, additional factors such as skin condition (sweatiness or clammy), ambient temperature, duration of contact, whether the power source is AC or DC voltage, and humidity can all impact the flow of electrical current through a body.
Figure 1. Typical electrical routes through the human body.
Common Electrical Hazards
Many electrical hazard exposures are common and irrespective of the voltage. Thus, there are a few recommendations to follow regarding electrical hazards you should always heed.
- Only qualified personnel should be allowed to perform any electrical connections, repair, and wiring. If you are not totally qualified to perform electrical work, the rule is basically simple: Just don’t do it!
- Before beginning any electrical work, the source of electrical power must be fully deenergized and locked out to prevent reenergization, using approved lockout-tagout methods (Figure 2), except where the equipment must remain energized to effectively troubleshoot or test the system.
Figure 2. Example of lockout-tagout.
- Capacitors, such as those used in single-phase control boxes or for power factor correction, can pack a serious electrical wallop. Always discharge starting and running capacitors before touching the terminals or testing them, especially when using an ohmmeter unless you wanted to buy a new one anyway.
- Electrical feedbacks can occur in all kinds of situations, especially with 230-volt systems with one of two breakers tripped or off. Unfortunately, many household electrical systems use untied single-pole circuit breakers to provide power to a 230 VAC residential pump. Although this is an obvious code violation, it occurs with enough frequency that every pump technician must be aware of this potential.
This means checking every circuit between the two energized power legs and not to ground, as circuit feedback through relays, capacitors, solenoids, motors, or other devices can allow power to bleed back through the device with enough left over to cause electrocution.
5. Proper grounding and bonding of an electrical system is a critical component for ensuring a safe installation. Do not disconnect, violate, or modify any grounding or bonding conductor unless supervised by a qualified electrician.
Arc Flash and Blast Hazards
An arc flash is the result of a rapid release of electrical energy due to an arcing fault that occurs between a phase busbar and another phase busbar, neutral, or a ground. During an arc fault the air becomes the conductor, so anybody within the vicinity of the impacted air also becomes a possible victim.
Figure 3. Electrical workspace boundaries.
Arc faults and flashes are generally limited to systems where the bus voltage is in excess of 120 volts. Thus, lower voltage levels will normally not sustain an arc. An arc fault is similar in nature to the arc obtained during electric resistance arc welding. And as with arc welding, a fault has to be manually started by something creating the path of conduction or due to a component failure, such as a breakdown in insulation or an equipment short.
The cause of a short normally burns away during the initial flash, with the resulting arc fault sustained by the establishment of a highly conductive plasma. This plasma will conduct as much energy from the electrical source that is available and is only limited by the impedance of the arc and robustness of the equipment. This massive energy discharge burns the bus bars and vaporizes copper, causing an explosive volumetric increase in heated air.
An arc blast has been conservatively estimated to contain an expansion of up to 40,000 to 1. This fiery explosive blast usually devastates everything within its path, creating and spraying deadly shrapnel as it dissipates from the source, which depending on the proximity, can generate multiple injuries, acute health-related problems, and even death.
For example, the pressure created from the blast may exceed 2000 pounds per square foot, which has been known to knock workers off ladders, shatter windows, and collapse the lungs of nearby people.
These events also occur rapidly with speeds often exceeding 700 miles per hour, making it virtually impossible for a worker to step aside to get out of the way in time. Thus, the hot gases expelled from the blast also carry the products of the arc with them, including small droplets of molten metal, similar to the effects of the buckshot delivered from a fired shotgun.
These small droplets can readily travel through workers’ clothing and become impinged in or even lodged underneath the skin and into the body cavity, sometimes causing severe internal bleeding and damage.
Next, the sudden and intense blinding light emitted by the blast will undoubtedly result in weld type flash burns to the eyes of nearby observers at a minimum and up to temporary or permanent blindness in the worse cases.
Lastly, the sound these blasts create are known to exceed 160 decibels. This is greater than standing next to and hearing a jet airplane during takeoff, which can easily rupture eardrums and cause permanent hearing loss.
Figure 4: Typical arc flash label for electrical equipment.
These are all possible individual outcomes from an arc flash that considered as a whole can result in debilitating and permanent health issues.
The development of a short circuit and a resulting arc flash is dependent on several factors, including the primary and secondary voltages, the primary available short circuit current (symmetrical or asymmetrical), transformer size and impedance, type (copper or aluminum), size and length of supply conductors or busbar, motor amperage, and equipment bracing.
The arc fault current is usually much less than the available bolted fault current and below the rating of most circuit breakers. Therefore, unless these devices have been specifically selected to handle the calculated arc fault condition, they will generally not trip, and the full force of an arc flash will likely occur.
Determining the incident-energy is the primary result of an arc flash risk assessment. Most engineers now perform these calculations with an arc flash computer program rather than tedious hand calculations.
IEEE Standard 1584-2018 (“Guide for Performing Arc Flash Hazard Calculations”) provides the equations commonly used for arc flash studies. The results include the prospective incident energy expressed in calories per square centimeter (a calorie is a measure of heat) at a specific working distance for each piece of electrical equipment that is part of the study.
IEEE 1584 defines the working distance as “the distance between the potential arc source and the face and chest of the worker.” Incident energy varies exponentially with the inverse of the distance involved. Therefore, the greater the distance from the arc flash to the worker, the less incident energy developed and vice versa.
IEEE 1584 lists typical working distances of 18 inches for low-voltage equipment such as panels and motor control centers, 24 inches for low-voltage switchgear, and 36 inches for medium-voltage equipment.
However, the specific task must also be considered when defining this distance. For example, a 480V panelboard may require only 18 inches of clearance. The electrical equation for the energy emitted from an arc fault is equal to volts × current × time. The transition from an arc fault to arc flash or blast takes a finite time, increasing in intensity as the pressure wave develops. The challenge is to sense the arc fault current and disconnect the voltage in a timely but expedient manner before it develops into a more serious arc flash and blast condition.
Employees working within the flash protection boundary must wear nonconductive head and personal protection wherever there is a potential danger of injury from electric shock or burns from the flash or flying debris resulting from the electrical explosion.
The National Fire Protection Agency has its NFPA Standard 70E which is the “Standard for Electrical Safety in the Workplace.” It covers personnel responsibilities (both the employer and the worker have specific responsibilities for safety), training requirements, establishment of an electrical safety program, and an electrically safe working condition.
For arc flash hazard considerations, the focus is Article 130, “Working On or Near Live Parts.” The basic requirement is that live parts more than 50 volts to ground to which an employee might be exposed should be placed into an electrically safe work condition prior to working on or being exposed to them unless the employer can demonstrate the equipment has been disconnected or must be worked on live.
In this case, live work requires an “Energized Electrical Work Permit,” for which the requirements are stated in Article 130.1(A)(2). Some specific exemptions are allowed to the requirement for an electrical work permit such as testing or troubleshooting that are performed by qualified persons.
Typical approach boundaries to live parts are illustrated in Figure 3. These form a series of boundaries from an exposed, energized electrical conductor or circuit parts. The requirements for crossing each of these boundaries become increasingly restrictive as the worker moves closer to the exposed live parts. The limited, restricted, and prohibited approach boundaries are shock protection boundaries and are defined in NFPA 70E, Table 130.2(C).
Qualified workers can approach live parts 50 volts or higher up to the restricted approach boundary. But they can only cross this boundary if they are insulated or guarded and no uninsulated part of the body crosses the prohibited approach boundary if they are insulated from any other conductive object, or if the live part is insulated from the person and from any other conductive objects at a different potential.
Unqualified workers must remain outside the limited approach boundary unless they are escorted by a qualified person. Those unqualified cannot cross the restricted approach boundary alone.
Proper observance and adherence to keeping out of the individual boundaries and use of personal protective equipment (PPE) are the best ways to avoid personal injury from arc flash and general electrical hazards. OSHA has issued Consensus Standards which define exposure to arc flash hazards, responsibilities of the appropriate parties, and when protective equipment is required.
OSHA 29 CFR Section 1910.3(b)(1) addresses the historical relationship between the OSHA Act and NFPA 70 and 70E. Don’t be fooled into thinking this only applies to three-phase, 480 VAC circuits. Many technicians have been seriously injured or killed from an arc flash or blast occurring from a single-phase 208-volt electrical system.
For systems with potential concerns, a consulting electrical engineer should be retained to conduct an arc flash analysis and consider the location of the electrical equipment—as placing equipment within an electrical room normally restricts access to qualified personnel whereas equipment located within an ordinary room with general access may raise safety concerns with unqualified occupants.
Arc flash studies can be detailed and complex, requiring important considerations and decisions regarding the variables used in the study. Each variable must be carefully evaluated because they can each or all significantly impact the results. Once the arc flash analysis is complete, approved labels should be created and affixed to each respective piece of equipment to identify the value of incident energy, arc flash boundary, nominal system voltage, device name, and required level of PPE. Refer to Figure 4 for an example.
A Personal Experience
The potential of an arc flash incident should never be taken lightly, and I can attest to this firsthand. I experienced the painful impact of an electrical arc twice in my career.
The first was caused by a hairline crack in a welding visor and the second was from the following incident. During the mid-1970s, I was assigned to a service call for a non-functioning, old 480-volt, three-phase irrigation well pump. On arrival, I noted the primary utility line fuses were intact, so it was fairly obvious a problem with the disconnect or pump panel was the most likely culprit.
After disabling the 1950s vintage fused disconnect switch, I opened the door to check incoming power, and a loose fuse clip on the line side fell across two live power lines, resulting in a line-to-line dead short with an arc flash explosion occurring virtually in my face. The flash was so bright that I was instantly blinded and had to crawl back to my service truck to call for help.
Fortunately, after a trip to the local emergency room and irrigating my eyes for a few hours, my vision gradually returned that same day. The only other repercussion from this event was severe burning pain in both eyes for a week, which was partially soothed with salve but also finally dissipated.
The moral to this story is to never be cavalier, in a hurry, or take shortcuts. Always assume the potential of an arc flash is real and present on all electrical panels, even those that have seemingly been shut off, deenergized, and locked out. Stand to the side when opening panel doors and always wear eye protection when working with or while troubleshooting electrical systems. It took this event for me to learn my lesson; hopefully you learn from it too.
This concludes this column on electrical safety and our year-long series on electrical topics. We will embark next month on a series on the advantages and disadvantages of variable flow and head systems, including control valves and various types of pump or motor variable speed systems, including variable frequency drives (VFDs).
Until then work safe and smart.
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at email@example.com.