There are three principle hazards in working with electrical equipment. Any of these can be lethal and cause injury or property loss. They are electrical shock, arc flash, and electrical fires.
Electrical shock strikes without warning. It is immediate and instantaneous, but its effects may persist for years. Sometimes an individual exposed to electrical shock seems to recover right away, choosing to return to work, only to expire hours or days later due to hidden heart and vascular damage.
Most electrical shock happens upon direct contact with an energized conductive object. Secondary utility circuits, energized below the National Electrical Code (NEC) cutoff level of 150 Vac, do not arc through an appreciable distance in dry air, so direct contact is necessary to constitute a hazard.
Electric shocks may be line-to-line or line-to-ground. High-frequency current is a little less hazardous thanks to the skin effect, but dc or 50-60 Hz current penetrates deep into the body. Veins and arteries are direct pipelines to the heart, and blood is a heavily ionized liquid with robust charge carriers. Nerves are conductive wires that terminate at the brain.
Because electrical energy has a convulsive effect on muscle tissue, shock victims tend to grip and be unable to release metal wires, water pipe and conduit for as long as they remain energized. To prevent a secondary event, a rescuer should begin by shutting off the power or otherwise breaking the circuit, perhaps using a dry wooden stick. Then, call 911 to get help on the way before administering CPR, which will rarely revive an unresponsive victim. Its purpose is to keep the blood oxygenated until emergency help arrives.
It is highly recommended that electrical workers become certified in CPR. This process is much simpler than obtaining EMT certification, which requires extensive instruction and study.
Electrical fatalities have gone way down in recent years because of the introduction and wide-spread use of the ground-fault circuit interrupter (GFCI). This is a simple little device that can be inserted just like a circuit breaker into an electrical distribution panel or, alternately, it replaces the conventional wall receptacle. In still another form, it can be a device that is built into the power cord of certain tools or appliances such as steam cleaners and power washers.
A GFCI measures the amount of electrical current in the supply wire and compares it to that in the return conductor. If there is a substantial difference, above 6 mA, that means the electricity is going somewhere it shouldn’t. There is a fault to ground. Often, in such a situation, the metal enclosure of a hand tool such as an electric drill has become energized because of a chafing wire inside or a bad bearing allowing the rotor to contact the inside of the enclosure. If the current going to ground happens to be passing through a human, there can be an event ranging from mild shock to sudden electrocution. Sensing this missing electrical current, the GFCI instantly interrupts the power supply, and that is how lives are saved. The NEC specifies locations that are required to be GFCI-protected, and the list is growing with each new Code cycle.
Another electrical hazard, found particularly in an industrial workplace, is arc flash. This hazard does not depend on direct contact with a conductive object. The defining electrical parameter is available fault current, which is a consequence of Ohm’s law. Arc flash causes more fatalities than electric shock. As resistance falls closer to zero, current flow approaches infinity. Thus, a narrow conductive path — as established when a dropped wrench bridges high-voltages lugs landing large current-carrying conductors — can explode with sudden violence. The temperature can surpass that of the sun’s surface. Arc flash can cause severe injury to workers as far away as 20 ft. from the fault.
The severity of an arc flash injury is determined first and foremost by proximity to the electrical fault. Prior to working on electrical equipment that can’t be de-energized, workers must observe specific spatial boundaries, taking into consideration the available fault current as determined by the utility together with length and diameter of local conductors, current-limiting devices, and fusing.
The outer radius is known as the flash protection boundary. At this distance from energized equipment, an unprotected worker could sustain second-degree burns. Within the limited-approach boundary, the shock hazard rises because of proximity to an exposed live object. Within the restricted approach boundary, there is a still greater shock hazard. Within the prohibited approach boundary, the shock hazard is equivalent to touching a live part.
The most effective protection strategy is to de-energize the applicable circuit. Protective measures that mitigate the hazard and allow closer approach to a potential arc fault include protective equipment such as protective clothing and insulated tools.
Another hazard in the use of electricity is electrical fire. In the home, electrical fires cause nearly 500 fatalities annually. Just as GFCIs mitigate shock hazard, AFCIs (arc-fault circuit interrupters) prevent electrical fires.
An arc fault is an intermittent electrical connection that generates heat every time it interrupts and re-establishes the circuit. The greater the amount of current and the more “on” time, the more heat is generated. If heat is not safely dissipated, the temperature will rise. In the presence of combustible material, there is the potential for electrical fire.
An arc fault in electrical wiring may be a parallel, series or line-to-ground partial break in a conductor. Typically there is a small gap, but the two pieces do not pull far enough apart to completely interrupt the circuit, so there is an intermittent arc that is accompanied by light, heat and a faint buzzing sound. A parallel arc fault may trip a conventional circuit breaker but a series arc fault will not cause a breaker to cut out. Arc faults, especially the parallel variety, may burn clear after a while. But before that happens, nearby combustible material may ignite.
The AFCI outwardly resembles the GFCI – a breaker or receptacle with a test/reset button – but electronically they are totally different. While the GFCI compares amounts of supply and return current, the AFCI listens for 100 kHz oscillating current with fast rise and fall times and varying duty cycle. It responds to these conditions by opening the circuit. Often the cause of the fault is a tradesman’s errant screw or nail and this can cause down the road a fiery conflagration with tragic consequences.
When doing any type of electrical work that involves an increase in loading or usage, or extension of circuits, it makes sense to perform a worksite survey to ensure that there are no deficiencies or vulnerabilities that need to be addressed.
Aluminum wire is widely used and it is intrinsically safe provided design and installation requirements are rigorously observed. Copper wire is preferable in terms of conductivity and terminations, but realistically it is not going to be used in longer runs and greater diameters except where cost is not a factor.
Aluminum is less conductive than copper, which means that size-for-size, aluminum has less ampacity, must dissipate more heat, and can safely carry less current than copper. This difference is effectively addressed in the NEC, which in extensive ampacity tables requires larger sizes in aluminum for given current loads. So regarding temperature rise and potential insulation damage, the two metals are equivalent.
The other caution with aluminum is the terminations. Where aluminum wire is landed at an entrance panel, meter socket or at any lug, there is the tendency for the metal to creep so in time, the termination may experience corrosion, which degrades the connection and results in still more heat when there is significant current. The condition can avalanche to the extent that combustible material such as a wooden wall outside the enclosure may ignite.
Aluminum wire can, however, be safely terminated, provided at the time of installation, corrosion inhibitor is applied according to manufacturer’s instructions, which are part of the UL listing. After wire-brushing the mating surfaces, this paste-like substance is applied where wire and connector touch, and the lug is torqued the correct amount. Electrical corrosion inhibitor contains suspended zinc particles, which prevent formation of aluminum oxide. The carrier material, moreover, excludes atmospheric oxygen thereby preventing corrosion.
Complete coverage of best wiring practices would require a thick volume. Thorough familiarity with the NEC is essential for safe electrical design and installation. The above comments are intended to point out some of the issues involved.