The National Fire Protection Association (NFPA) has released a 2015 edition of NFPA 70E: Standard for Electrical Safety in the Workplace. It covers and assists in complying with the Occupational Safety and Health Association's requirements, including changes to requirements since the last edition. For personal safety in forensic studies it is important to stay informed of these updates. NFPA 70E covers arc flash risk assessment, establishment of arc flash boundaries, requirements of Personal Protective Equipment (PPE), use of PPE within the arc flash boundary, and equipment labeling requirements regarding arc flash hazards. The following examples illustrate the importance of staying abreast of the latest standards.
All of these injuries were preventable with proper protocol and proper PPE. Most were caused by untrained or undertrained personnel.
Arc flash resulting from 480 VAC, three-phase accidents is a relatively common occurrence. Temperatures of up to 35,000° Fahrenheit (° F) are generated by an arc flash incident (temperature of the sun is said to be about 9,000-10,000° F). As if the temperature of the arc flash envelope is not enough, there is also a concussion from an arc blast that can blow a person several feet away from the point of incident causing additional severe injury. There is also the probability of molten particles of copper being emitted from the arc blast site (copper melts at about 1980° F) and contacting human skin. Copper vaporizes suddenly and expands by a factor of 67,000 in volume. Arc flash also presents an obvious fire hazard. Several videos are available on YouTube that clearly illustrate the effects of 480 VAC, three-phase arc flash incidents.
Electrical shock and electrocution prevention are other subjects addressed by NFPA 70E that have been useful in injury studies. One case involved a maintenance worker in the process of troubleshooting an electro-mechanical machine. The worker apparently sought guidance with someone else at a remote location using a cell phone. The worker was positioned between the machine and a fused disconnect switch on the wall behind the machine, touching both as he turned power back on for the machine. Because of a short circuit condition in the machine's control circuits and a poor ground at the disconnect box, the worker became a parallel path to ground and was electrocuted. It only takes a small amount of electrical current to cause an electrocution, as described in more detail below. Many times it occurs to people who should be aware of the existing conditions. Sometimes the qualifications of the maintenance personnel who are "Jacks-of-all-Trades are questionable."
It is electrical current that kills, not the voltage (although the voltage has a correlation). A small current of about 6 milli-amperes (mA) (0.006 amperes) is often referred to as the "let-go" current. This is the maximum amount of electrical current in which a human can "let go" of an energized object. Although it is somewhat unpleasant to sustain, it generally does not impair control of the muscles or cause permanent damage. Currents of between about 9 to 25 mA are generally painful and difficult to impossible to release from grasped energized objects by the hand. Currents from 60 to 100 mA are the most dangerous currents that can electrocute a person. Within this range, ventricular fibrillation takes place, the heart stops and inhibition of respiration is likely to occur. Smaller currents within this range can cause damage to the heart and result in heart attacks later in the day or night. Currents above these levels will cause tissue to burn, also resulting in severe injury or death.
In general, the body resistivity can be considered as about 500 ohms per limb and 500 ohms for the trunk. So a path from one hand through the body to the other hand, or a path from one hand through the body to a wet boot or shoe, would be considered about 1500 ohms total. Chemical compositions of the human body vary from person to person, and a person that is hot and sweaty may be less resistive. Using ohms law (E = I x R, where "E" is voltage, "I" is the current and "R" is the resistance), one can calculate the approximate limits of electrical current for a given situation. Therefore, 0.006A x 1500 Ω = 9 volts (V) (theoretically) would shock a person. Further, 0.06A x 1500Ω = 90 volts could kill a person across arm to arm or arm to foot with wet feet to soil if the current path is not broken. This explains the painful touch to a 120 VAC source. It is recognized that this common household voltage can be fatal.
Concrete is porous and absorbs moisture. It is not so different than standing on soil outside. That is why an unfinished basement or a garage is required to have ground fault circuit interrupter (GFCI) receptacles installed unless the circuit is dedicated for one appliance only and there is no possibility of anything else being connected to it. GFCI duplex outlets contain electronic components that can sense small imbalances in the current exiting the "hot" lead and returning on the neutral lead. A balance of the two readings indicates there is no leakage to ground. However, if there is an imbalance as small as 4-5 mA (0.004 to 0.005 amps) then the sensing circuit will trip the GFCI and the receptacle output is essentially de-energized.
Outlets on the outside of a building are also required to be GFCI protected, either by GFCI receptacles or a GFCI circuit breaker in the panelboard. Crawling through a crawlspace can be dangerous if exposed wiring is damaged or if appliances such as air handlers, sump pumps or lights are present and have faults in them. On farms where cattle barns have items such as drinking fountains and steel stanchions, a failed heater in a drinking fountain can energize the metal skin of the barn and steel stanchions and prove fatal to both humans as well as animals if the circuits are not properly GFCI protected and the barn is not properly grounded (including the metal siding). This is especially critical during the winter and spring months when the ground is wet and personnel's footwear are likely to be wet.
Since owners of small farms or businesses are used to doing their own work or hiring "jack of all trades" contractors, errors in wiring are common, and can contribute to shock and electrocution accidents. The thermostat of a livestock water tank heater is designed to break the "hot" side of the 120 VAC circuit, but should the polarity be reversed at some point in the circuit, the hot side is not broken by the thermostat and a failed heating element is energized at all times. If a forensic engineer is at a site and a cord-powered light or tool is used, it is imperative that he or she use a GFCI type outlet as the source. It is recommended that the forensic engineer have a GFCI outlet box to use in case a readily available GFCI receptacle is not found. The forensic engineer should treat all equipment as potentially energized and test and verify before touching.
NFPA 70E addresses the importance of an electrical safety program, having trained, qualified personnel versus unqualified personnel performing electrical work, relationships of contractors, use of test instruments, portable electric equipment, extension cords, GFCI protection, and other hazards associated with electrical energy in the workplace. A thorough knowledge of the new 2015 Edition of the NFPA 70E manual and its use is critical for the electrical forensic engineer to evaluate cause, responsibility and potential subrogation for clients.
DONAN, a family owned company founded in 1947, provides full-service Forensic Engineering and Fire Investigation Services. For over 60 years, Donan Engineering has been providing unbiased answers to their client's complicated questions. Their team of licensed and experienced forensic experts provides investigative services in the fields of engineering investigations, fire and explosion origin & cause, roofing investigations, and component testing. The diversity of their multidisciplinary staff allows them to provide not only a complete service, but a complete service whose conclusions can be successfully and completely supported.
©Copyright - All Rights Reserved
DO NOT REPRODUCE WITHOUT WRITTEN PERMISSION BY AUTHOR.