The Occupational Safety and Health Administration considers electrical safety to be a focused inspection item for all compliance officers. The OSHA Institute mandates that its electrical regulations be included in the construction (Subpart S) and general industry (Subpart K) training modules for the 10-hour and 30-hour Outreach Training Programs. MSHA also mandates electrical safety training in its eight-hour Outreach Training. According to the Bureau of Labor Statistics, electrocution is the fourth most common cause of death among construction workers, accounting for 17 percent of all fatalities. Subsections 1926.402 through 1926.408 in OSHA’s Construction Standard (Subpart K) apply to the safe work practices for the installation of temporary and permanent electrical equipment on the jobsite. Subsection (1926.416), entitled “Safety-Related Work Practices,” pertains to the hazards and appropriate mitigation methods related to existing permanent installations in place prior to any construction.
Roofers very often encounter dangerous levels of electricity in the form of the primary sub-feed, or the service entrance drop to most residences. In the case of industrial roofing, the overhead primary may involve an extremely dangerous three-phase, high-voltage exposure. Quite often, depending on design criteria, these conductors may not be insulated. Each conductor may be installed with an adequate lateral separation distance from the other phases and vertical distance from ground or structure below to provide adequate insulation. Air, when dry and at an adequate distance, will provide sufficient insulation from short circuit through a leg line or directly to ground through another conductor. Within the safe work proximity distances, however, the air does not provide sufficient insulation values to protect workers.
The primary electrical service drop to most residences can pose a life-threatening danger to roofers and other contractors.
The Snake
Whenever I train building trade workers in electrical safe work practices, I always tell them about the snake first. I inform them that at 7 o’clock that morning, while setting up for class, I noticed a Dendroaspis polylepis (black mamba) in the rear corner of the classroom. One bite from this 8-foot to 14-foot creature subjects the victim with enough venom to kill within minutes. Without immediate and concerted medical rescue efforts and sufficient antivenom, the mortality rate approaches 100 percent. Noting that the snake is not back there now, I confirm I spotted it only hours earlier. Normally not aggressive, black mambas are extremely territorial and nervous. If left alone, they are generally reclusive and tend to their own business. However, if approached and threatened, the snake quickly becomes the aggressor, often attacking much larger prey delivering rapid, multiple deadly strikes. A Mamba is a very fast snake, traveling steadily at 12 mph even in rough terrain. Outrunning this snake is unlikely. Its venom is strong neurotoxin. It is lethal to adult humans in as little as a 10 mg dose, but a mature mamba can delivered up to 400 mg in a single, crippling strike. While it’s striking distance is generally 4 to 6 feet, a large specimen can raise itself almost to shoulder height, significantly increasing its range and decreasing strike time. Death is due to asphyxia as the victim’s diaphragm becomes paralyzed and the victim slowly suffocates.I suggest that whatever each trainee is planning on doing today, he or she should keep an eye out for a large, dark, fast, aggressive and extremely deadly object. I also recommend that only those in the class who have been thoroughly trained and drilled by an experienced herpetologist in snake handling techniques and safety precautions should knowingly approach this species. Most importantly, I recommend that no one try to capture or handle this particular snake without first making sure you have a foolproof plan detailing exactly how to accomplish that task within this particular building. I suggest they develop a checklist for anyone who might come across this snake.
By the time my snake warning session comes to an end, eye contact guarantees that I have everyone’s undivided attention. I then produce a heavy object in a large canvas bag stenciled “Danger - Live Specimen.” I untie the top and offer the bag to anyone to remove its contents. I get no volunteers. With a heavy, leather gauntlet glove I reach into the bag and withdraw a 3-foot long, 2-inch diameter section of 50 kV service conductor and toss it quickly onto the floor. By then I have many in the class on their feet and stumbling backwards over their chairs. Only now, I believe, may we begin seriously discussing high-voltage electricity.
A thorough job safety analysis should involve a 360-degree view to detect all potential hazards and identify any electrical wiring.
Increasing Awareness
There is little that can be done to protect someone from high voltage without stimulating an increased awareness level. Every roofer must become, in essence, a high-voltage detector. The first step in this awareness is a thorough site inspection and job safety analysis (JSA). It may not be unusual for an apprentice roofer to scout out the roof for 20 minutes and never look overhead to notice a suspended electrical wire crossing over the deck, while the experienced journeyman knows enough to take in the 360-degree view for hazards.OSHA’s electrical standard states in 1926.416(a)(3): “Before work is begun the employer shall ascertain by inquiry or direct observation, or by instruments, whether any part of an energized electric power circuit, exposed or concealed, is so located that the performance of the work may bring any person, tool or machine into physical or electrical contact with the electric power circuit.”
At midday, all that can be typically seen is a thin black line against a bright blue sky. The diameter (gauge), elevation, insulation or voltage capacity of this wire is often impossible to determine by someone unskilled in electricity, positioned 20 feet or more below the line. Whether or not anyone has sufficient training to identify the potential voltage, every roofer on your crews should be trained to be aware of wires over or attached to the roof deck. While practicing this state of awareness is always recommended, it is the employer’s subsequent responsibility to do something about that snake once its presence is identified. The aforementioned OSHA standard goes on to stipulate: “The employer shall post and maintain proper warning signs where such a circuit exists. The employer shall advise employees of the location of such lines, the hazards involved, and the protective measures to be taken.”
While the decision to work within the safe work limits for high voltage rests primarily with the employer, only those designed as qualified persons (QP) may be permitted enter the hazard zone. Engineering controls should be implemented first to determine if the electrical hazard might be entirely eliminated during the roofers’ exposure. This would necessitate the implementation of a compliant lockout/tagout program whereby the sources of any/all potential energies are identified, their switchgear opened and padlocked and tagged by exposed personnel, and the circuit conductors removed and placed on the ground as a redundant protection method. Most importantly, the QP would re-test the circuit for the presence of voltage to verify a “zero energy state.” There are certain conditions where lockout/tagout may not be feasible, and then only trained and qualified personnel are permitted to enter within close proximity to the electrical field. Their training must promote a thorough understanding of how electricity flows through a circuit and the effects the flow of electrons has on the proximate environment. Slow and methodical safe work practices in proximity to high-voltage sources prove to be the only physical protection afforded to qualified workers who understand the hazards.
The Garden Hose
As an enthusiastic proponent of the K.I.S.S. rule (“Keep it simple, stupid”), I am always drawn to the analogy of electrical current in a conductor resembling water passing through a garden hose. The pump down in the well represents the hydroelectric turbine built into the base of a dam or the steam turbine in the boiler house. The water traveling through the hose represents the electrons moving through the conductor.The greater the gauge (diameter) of a conductive wire, the less resistance and the more electrons transferred (amperage) in any length of time at a given voltage. The amperage of the current is representative of the volume of electrical current (in amperes) produced in an allotted length of time, analogous to how many gallons per minute (gpm) of water passes any one point in the hose. The line’s voltage (in volts) represents the amount of electromotive force being applied to the copper or aluminum electrons to excite them enough to break their electromagnetic bonds and move “downstream” as a current. This is analogous to the hydraulic pressure (psi) that the water pump produces on the water moving through the hose at any one time.
The garden hose and the electrical conductor are both imperfect methods of conveyance. All crystalline metal wires produce a certain amount of impedance to the flow of electrons. This resistance is measured in ohms. The garden hose creates a certain coefficient of friction and turbulence from the sides of the tube, and this affects the efficiency of any water flow along its length, as do such things as elbows, valve bodies, dirt buildup and constrictions.
Whenever any type of mechanical, chemical, thermal, hydraulic, gravitational or electrical energy is transferred from one state to another there is a total net result incorporating some real percentage of loss due to the applied dynamics. This concept is summed up in Ohm’s Law, which states E = I/R, where E = voltage (volts), I = current (amps) and R = resistance (ohms). There is a direct relationship between voltage and current. That is, the more volts, the more current is produced. There is an inverse proportion of voltage and current to resistance; the more resistance in a circuit, the less voltage and amperage produced by that circuit.
In order to produce high voltages (over 600 volts), higher amperage and lower resistance are required. This often occurs by means of dividing the single-phase alternating current (AC) sine wave into three separate AC phases by lagging the cycles over time at approximately one-third the total voltage. In essence, the circuit now also travels through conductors of a larger sectional area and with less resistant metals, which have free electrons to offer. Thus they are able to carry high voltages from 600 volts to 150 kV (1kV = 1,000 volts). Any current with more than 100 kV is considered extremely high voltage.
Whenever we consider the energy required for free electrons to move “downstream” in a conductor, we describe an electromagnet, which produces a concentric field of invisible, yet measurable, magnetic energy around the conductor. This is called an induction field. The current traveling through the copper (or aluminum) wire will generate this variable field. Given enough reasons (impedance downstream), it may detour its electron route from the copper crystals into the magnetically charged induction field as a shortcut to ground. This detour will produce an arc blast as the energy heats and expands the air molecules creating light, heat, sound and a concussive pressure wave. Sometimes the human body becomes involved in such a shortcut, and we call this an electric shock. Should the victim’s neurobiological systems become disrupted beyond their ability to recover in a timely manner, the victim dies of electrocution. While most electrocution fatalities in this country occur at levels below 600 volts (low voltage), it is more often the amount of amperage that proves fatal. Often death is due to the internal and external tissue burns created when a large amount of current (amps) reaches a poor physical conductor, producing a significant rise in the body’s resistance. The micro-current controlling our heartbeat is interrupted, causing fibrillation or total heart failure. While some victims may survive brief high-voltage, high-amperage electric shocks, others may fatally succumb to much lower levels applied for a longer interval.
Alternating current (AC) flows back and forth through the conductor in alternating equal and opposite charges in each cycle. Ever since it’s inception by Westinghouse in the late 1800s, AC current in the United States has been cycling at 60 cycles per second (60 Hz). AC transformers step the current up or down, enabling electrical transmission over greater distances than direct current allows. High-voltage current (greater than 600 volts) is often sent from a substation, usually at very high voltages (50,000 to 100,000 volts). These overhead primary transmission lines are not physically insulated, but rather separated into three time-staggered phases. Each current phase is transmitted in its own conductor, separated by minimum distances through the air. Once the electrical transmission reaches a customer’s property, this distribution current is reduced in voltage by means of a “step-down” transformer. Depending on the amount of current required by the end user, these wires may be either insulated or uninsulated, single phase or three phase. The circuit is then grounded at its final distribution point, allowing for a completed circuit. It is not until the overhead lines carrying the electrical current pass over an elevated surface, such as a roof or other structure, that proximity problems may arise.
The problems that develop with electrical proximity fall into two main categories: direct contact and electromagnetic (EM) induction. In direct contact, a grounded worker contacts a live electrical conductor with a body part or a conductive tool or piece of equipment. The electrical current may take the path of least resistance through his body to ground rather than continue down a lengthy conductor to the circuit grounding rod. The injuries incurred are dependent upon the voltage, amperage and time of exposure. If the victim is in “full grip” on the conductor and the amperage is greater than 20 mA (milli-amps), then the person may not be able to release the grip and break contact. As the electrons are rapidly exchanged (amperage) in the copper molecules of the conductor according to the force applied (line voltage), a concentric, electro-magnetic (EM) field is generated around the wire at a radial distance dependant on the voltage. This EM field is induced only when the current is flowing. Open the switch and stop the flow of electrons and the EM effect is also interrupted.
OSHA stipulates in 1910.333(c )(I)(A): “When an unqualified person is working in an elevated position near overhead lines, the location shall be such that the person and the longest conductive object he or she may contact cannot come closer to any unguarded, energized overhead line that the following distances: (1) For voltages to ground 50 kV or below - 10 feet. (2) For voltages to ground over 50kV -10 feet plus 4 inches for every 10 kV over 50 kV.”
Apart from the type and thickness of insulation, the distance that the EM field may extend beyond the surface of any conductor is also affected by the atmosphere’s relative humidity (RH, or the percentage of water vapor per volume of air). While a low-voltage EM field may require a 10-foot safe working distance (SWD) at 70 percent RH, a SWD of 40 feet or more may be necessary after a day of rain (99 percent RH). A windy day may also contribute to swaying of the lines, decreasing the separation distances significantly.
It is important to understand the physics of electrical generation and the principles of conduction if we are to accurately identify electrical hazards on the roof. In my next article, I will discuss procedures to protect roofers from high-voltage electrical currents in their elevated environment. Detection, control and isolation are keys for any employer to implement “observe and act” protocols established in OSHA’s Subpart K, electrical safety standard.
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