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Conventional
Stray Voltage |
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Stray Voltage Links: Wisconsin Public Service Commission Wisconsin Agricultural Survey Statistics Michigan Agricultural Electrical Council Ontario Ministry of Agriculture USDA Bulletin on Utility Distribution Neutral Sizing National Food & Energy Council Dr.
Stringfellow Dr. Stringfellow Business Home
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Introduction
Conventional stray voltage is a small power-frequency voltage present between two points that can be contacted by a person or an animal. Since perception and physical responses are related to current flow through tissue, the latter depends not just on the voltage across the contact points but also the impedance of the circuit. This impedance includes that of the source, the external impedance between the person or animal and the contact points, and the internal impedance of the tissue. At power frequencies (50 or 60 Hz), all of these impedances are essentially resistive. Sensitivity to Electricity A common myth is that animals are much more sensitive to electricity than humans, a common statement being that dairy cows are a hundred times more sensitive than humans. This is just not true. Animals such as dairy cows are made of exactly the same tissue as humans and are therefore likely to have similar thresholds of perception and response. The reason why animals may perceive stray voltage when humans do not is their lower external contact resistance and lower internal body resistance. Animal contact resistance is lower because they have bare feet and are often in direct contact with wet surfaces, whereas humans usually are wearing boots and sometimes gloves in the same environment. Much research has been done on sensitivity of both humans and animals to power-frequency currents. The threshold depends on contact area and varies between individuals, but is as low as 0.05 mA for humans using a 0.01 square mm contact area up to around 1 mA for a 1 square cm contact area. In dairy cows, perception thresholds for muzzle to feet contact and from foot to foot are comparable to humans with similar contact area at around 1 mA to 2 mA. Contact and Body Resistance Contact resistance depends strongly on whether skin is wet or dry, humans varying from more than 10 kilohms dry to 1000 ohms wet. Dairy cows are in the range from 1,200 ohms to 300 ohms. Body resistance depends on the path. Humans total about 500 ohms from one hand to both feet, dairy cows about 300 ohms from muzzle to four feet and 500 ohms from front to hind feet. Contact and Step Voltages for Perception Given a total circuit resistance of typically 1,500 ohms for a human and 500 ohms for a cow, the smallest touch and step voltages that can produce perceptible currents are in the order of 1.5 Volts for a human and 0.5 volts for a dairy cow. Contact and Step Voltages for Severe Reaction Research on humans and animals strongly suggests that significant reaction to electric shock occurs at levels approximately ten times higher than perception, namely about 15 volts for humans and 5 volts for dairy cows in a wet environment. The 15 Volt level is a power-frequency limit set in several North American safety codes for human contact in wet environments.
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