For this issue concerning public spaces, it seemed appropriate to feature something that the press often calls “stray voltage.” Several recent incidents involving utility Con Edison made headlines in the New York press. In a Daily News article, Adam Lisbert wrote, “The utility said yesterday that its fleet of 15 voltage-sensing trucks now sweeps the city all year long. In 2007, the crews found 5,427 electrified objects—streetlights, sidewalk hatches, fences and scaffolds, all of which were isolated and repaired.” This program is, to some degree, a reaction to the unfortunate death of Jodie Lane while walking her dogs in the East Village in 2004.
Another Daily News article by Jimmy Vielkind describes how Con Edison is hiring livery cab drivers to protect the public by guarding stray-voltage hot spots until crews can fix the problem. Drivers sit in their cars for days with a card advising that “a stray voltage hazard was discovered here. The coned/taped off area contains an extremely dangerous electrified object or structure.”
Shielded from the headlines is a working group formed within the Power and Energy Society of the Institute of Electrical and Electronics Engineers (IEEE) to work on the same subject, though the words in the P1695 Working Group on Voltages at Publicly and Privately Accessible Locations meeting are “step potential,” “contact voltage” and “unintended touch.” Research on the Internet finds the use of additional terms on the subject, such as ground-current pollution, objectionable current flow, stray current, microshock and macroshock.
The subject isn’t new. Farmers have sued electric utilities for reduced milk production due to the effects of such leakage on their cows, and there have been people getting shocked when they touch metal ladders coming out of an in-ground pool. So it seemed straightforward to make the public aware by defining the subject, indicating the hazards and identifying possible solutions. Well, to paraphrase a colleague, “It was like trying to herd stray cats.”
Not only were definitions and causes many and conflicting (not surprising that the utility often blames the facility and vice versa), but there is debate about the level of current flow in a human or animal caused by a voltage between one object (usually metal) and the person or animal contacting it with part of their body and ground or earth. The impedance of the human or animal at that particular voltage level and frequency determines how much current will flow through their body. Depending on the age of the person, skin resistance, moisture on the skin, soil conductivity below them, current flowing through that soil, etc., the amount of current flowing through them will differ.
Similarly imprecise is how much current flow is hazardous. The data in the table is taken from various reference sources and is presented for informational purposes as to the range of possibilities. It is not intended to be taken as necessarily precise or accurate.
So, if by now you are as confused about the subject as I am, this article is a success. If you also are motivated to make sure GFCI outlets and/or breakers are installed in all areas where the “shock” potential exists, especially as defined in NFPA 70, National Electrical Code, then the article is a double success. As for the causes and solutions, well, we’ll save that one for another day when maybe there are fewer strays to herd to get to the facts.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.