Why small contact voltage exposures are not lethal to humans
It is often stated in the electrical industry, “It’s not the voltage that kills you; it’s the current.” However, this expression is only partially correct.
In reality, a standard time/current curve exists for determining whether a shock will prove fatal to a human or not. This time/current curve — or slight variations of it — is the basis for the 50V safety level used by a number of standards-making organizations, including OSHA, NFPA, ANSI, IEEE, UL, and other agencies. The time/current curve’s validity is backed up by actual mortality information. Excluding operators of DC electric welding equipment, no case of less than or equal to 50V electrocution to a human is known to exist.
Knowledge — and practical application of that information — should be used to assist state regulators, electric utilities, and testing firms to better manage emergency response actions after identifying contact voltages and to effectively coordinate routine repairs. This information can also be used to argue against well-meaning, but misinformed, special interest groups that are actively trying to require the immediate repair of any publicly accessible object found to have 1VAC or greater on it.
Voltage and current both must be present at the correct levels (or greater) before a shock will prove fatal. However, electrical safety is still typically taught using simplistic current charts showing only amperage as the source of injury or electrocution. One might question the validity of these types of tables, because there is no apparent consensus standard as to the meaning and effects of different levels of current on a human. OSHA uses at least two different sets of numbers in its literature. The first is found in OSHA Publication 3075, “Controlling Electrical Hazards.” The second is located on its Construction eTool web page at: http://www.osha.gov/SLTC/etools/construction/electrical_incidents/eleccurrent.html.
Application of these current tables may have some limited use in general public education, but they have almost no practical application in real-world scenarios, because they differ too much to be applied consistently. That is, there are too many variations of these charts in existence for them to be considered a factual document — or even an accurate guide, for that matter.
Work by Charles F. Dalziel, Richard H. Kaufmann, Edward C. Cantwell, and others have shown that it is the volt-amps (VA) energy, or power measured in watt-seconds (W-sec), that ultimately determines if an electrical shock will be lethal — not strictly a number on a generic current chart.
Dalziel made many contributions to the field of electrical safety, including the invention of the ground fault circuit interrupter (GFCI) in 1961, but it is not clear if he was aware of the time/current constant (k) = 0.027. However, in his paper, “Effects of Electricity on Man” Table I, the maximum safe power a person could be exposed to for short-duration (less than or equal to 3 sec) shocks is 13.5 W-sec. A W-sec is the product of watts (W) multiplied by time (t). There is never a situation where a 5V, 60-Hz shock will prove fatal to a human when applied externally to the body, because it will never exceed the 13.5 W-sec value. If cardiac disruption has not occurred in about 3 sec, it never will (paraphrased from Dalziel’s research). This low power level is the reason why low-voltage (<50V) shocks have never been fatal — there is not enough electrical energy available to disrupt bodily functions. Dalziel’s observation of 13.5 W-sec is confirmed by Kaufmann’s time/current formula (I2t=0.027).
Exposure to 25V is also clearly safe with no risk to human life. It is only when we reach around 50V that we start having issues with shocks that are capable of delivering more than 13.5 W-sec in less than 3 sec. At 50V, the power equals 5W, and the calculated exposure time is 2.7 sec. (5 × 2.7 = 13.5). This assumes a “worse-case” body resistance of 500 ohms. Most people in reasonably good physical and mental shape can still react quickly enough to release themselves in 2.7 sec or less from a 50V shock. When the voltage level is 50V and above, however, it delivers too much energy in less than 3 sec, which leads to electrical fatalities.
In recent years, a number of states and several major cities have adopted “contact voltage” testing rules to routinely test all publicly accessible conductive surfaces that may become energized due to an electrical fault, such as street lamps, traffic signals, handhole enclosures, manhole covers, and other similar objects. Most contact voltage-testing programs make an exception for inspecting conductive surfaces that are found along highways, toll roads, and the interstate. The reason behind this line of thinking is that motor vehicle traffic may pose a greater safety risk for the technician performing the electrical safety testing than the public making contact with the potentially energized surface. Yet, motorists frequently use light poles at night, often parking directly underneath them, to provide illumination while they change a flat tire or troubleshoot a mechanical problem with their vehicle.
Direct current (DC) voltage systems are also not tested under most programs. Subway and elevated train systems are obviously not considered publicly accessible surfaces under normal circumstances, but traction devices (trolleys), street-cars, and electric buses that often operate in the 600VDC to 700VDC range are at pedestrian level. DC voltage leaks from transit systems — sometimes called stray current — have been implicated in a number of pipeline failures in our largest cities. Leaking DC can, and has, “burned” holes in underground water and gas pipelines, causing leaks and ruptures of those vessels. Direct current can also shock or electrocute people, but at recognized higher voltage and current levels than AC.
While we can prove both by mathematics and from real-life medical records that human exposure to 50VAC and less is not lethal, these lesser voltages should not be ignored when found during contact voltage surveys. All confirmed contact voltages — regardless of value — need to be properly documented and reported.
My concern is to avoid a repeat of what has transpired in New York State, where every indication of contact voltage of 4.5V or greater requires an immediate physical guarding until emergency repair or maintenance crews can respond to “make it safe.” In New York City alone, an average of 550 surfaces are found each month with a voltage level of 4.5V or greater, but only a handful of these is above 50V. Nevertheless, all 550 are treated equally, as if each were an emergency situation that is immediately dangerous to life or health (IDLH). This overreaction has cost New Yorkers millions of dollars annually, yet it does not make the public any safer.
Other states like Maryland and Rhode Island have, or are considering, 1VAC findings as an emergency response situation. Electrical measurements in the 1V range are often influenced by errors introduced by the voltmeter operator, so these two states will undoubtedly have even more “emergency” situations than New York to deal with. The city of Seattle, on the other hand, has taken a more practical approach, making 30VAC as the voltage action level — a number that allows them to add on a “safety buffer” to recognized standards and save the rate payers from excessive repair costs.
Voigtsberger manages the mobile contact voltage testing program at Premier Utility Services LLC based in Hauppauge, N.Y. He has more than 30 years of experience in the electrical safety inspection/ testing industry and can be reached at firstname.lastname@example.org.