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Stray Voltage Update 97

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Stray Voltage Update 97 Conference Paper Presented at the 1997 Rural Electric Power Conference Minneapolis, Minnesota 20 – 22 April, 1997 Sponsored by the Institute of Electrical and Electronics Engineers, Inc. Stray Voltage Update 97 Douglas J. Reinemann, Ph.D. University of Wisconsin – Madison Department of Biological Systems Engineering Milking Research and Instruction Lab LaVerne E. Stetson USDA-ARS, Lincoln Nebraska Daniel M. Dasho, and Mark A. Cook Public Service Commission of Wisconsin Rural Electric Power Services Stray Voltage Analysis Team STRAY VOLTAGE UPDATE 97 D.J. Reinemann, L. E. Stetson, D. Dasho, M. A. Cook ABSTRACT and secondary grounded neutral system. The amount of current flowing and the The USDA handbook " Effects of Electrical impedance of the grounded neutral network Voltage/Current on Farm Animals" was determine the level of voltage. published in 1991. This handbook serves as an excellent summary of stray voltage In addition to these steady voltages, short research and practice up to the date of its duration, or transient voltages, also exist on publication. This paper will provide an the grounded neutral network. These update of stray voltage issues which have transient voltages may have a phase duration emerged and the results of recent research ranging from 8.3 mS, (corresponding to 60 and field surveys on stray voltage and Hz power frequency) to less that 1 µs. The related electrical phenomena which has main source of these transient voltages is occurred since the publication of the USDA switching of electrical equipment. Recent Handbook. research has been performed to more accurately determine animal response to WHAT IS STRAY VOLTAGE? these electrical phenomena. The definition of stray voltage in the USDA Motor Starting Transients handbook is: "Stray voltage is a small voltage (less than A 60 Hz transient voltage will occur when 10 volts) measured between two points an electric motor is started. These are that can be simultaneously contacted by an among the most common types of transient animal. Because animals respond to the voltage encountered at animal contact current produced by a voltage and not to locations (Dasho et al, 94). Elevated current the voltage directly, the source of the flow on the primary neutral occurs when voltage must be able to produce current 240 V motors are started. The current flows flow greater than the threshold current into the impedance of the distribution needed to elicit a response from an animal system neutral and produces a voltage when an animal, or an equivalent electrical transient. The voltage magnitude is load, contact both points." dependent on the starting current of the motor, transformer ratio, and the impedance This definition is still considered an accurate of the grounded neutral system. If the description of the stray voltage phenomena. primary and secondary neutrals are bonded, It was meant to apply to steady 60-Hertz a portion of this voltage will appear on the (Hz) ac voltages and also applies to steady farm secondary neutral system. 50 Hz ac voltages. The sources of steady 60 Hz ac stray voltages are well documented In the same way, elevated current flow and solutions well understood. occurs mainly on the secondary neutral when 120 V motors are started. The Steady 60 Hz voltages are a normal transient voltage produced by 120 V motors consequence of the use of electrical power will depend upon the transient current flow resulting in current flow on both the primary on the secondary neutral and the impedance 1 of the grounded neutral system. A portion of problematic they must be measured this voltage will appear on the primary correctly and compared with animal neutral if the primary and secondary neutrals sensitivity at the appropriate phase duration. are bonded. The definitions used in this paper are These motor starting transients consist of defined using the waveforms in Figure 1. from 1 to 20 cycles of 60 Hz (8.3 ms phase Wave A is single-cycle, sinusoidal in shape duration) voltage and current. The and monophasic, going from 0 to some peak maximum voltage and current usually positive value then back to zero. The phase occurs during the first or second cycle. duration of wave A is 3.1 time units and the peak voltage is 5 V. If motor starting transients are a concern, the following steps can be taken to reduce their level. Use 240 volt motors whenever Figure 1. Example Waveforms possible. Inspect motors for wiring in Time accordance with the National Electric Code 0123 including proper conductor size, and type of 6 connections. Make sure that motors are appropriately sized to the load and not 4 started or stopped under abnormally heavy 2 load. Power factors should be kept above 0 (80%). Voltage -2 The motor starting current can be measured and compared to the rated starting current -4 listed on the nameplate to determine if the Wave A Wave B motor is being started under heavy load or if the motor is defective. Contact the power Wave B is also single cycle, sinusoidal in supplier to determine the maximum motor shape but is biphasic, going from zero to size that can be started on the distribution some positive peak value then passing system. Soft start motors or other current through zero to a negative peak value and limiting devices will reduce starting returning to zero voltage. The Phase currents. duration of wave B is 1.6 time units and the peak voltage is 3 V. Short Duration Transients When dealing with short duration impulses, Transient voltages with short duration the peak voltage and phase duration are (phase duration less than 8.3 ms or convenient numbers to measure with frequencies higher than 60 Hz) have two electronic recording equipment such as an main sources: improperly installed electric oscilloscope. The RMS (Root Means fencers, cow trainers and crowd gates, and Square) value of these transient waveforms switching of electrical equipment. The may be difficult to measure or calculate. If sensitivity of animals to transient voltages is biphasic waves are not symmetric about the highly dependent on the phase duration of zero axes, the ratio of the RMS to peak these voltages. In order to determine value will be different than for the whether these short-duration events may be 2 symmetric sinusoids usually encountered in Switching of electrical devices may produce steady 60- Hz stray voltages. transient voltages with phase duration less than 10 µs. These short duration voltages For waveforms with phase duration greater contain very little energy and decay very than about 1 ms, animal sensitivity is quickly. They appear only very close their determined mainly by the peak current. For point of origin. Field data indicates that the waveforms with phase duration less than main sources of short duration transient 100 µs sensitivity is determined mainly by voltage and current in cow contact areas the charge of the impulse. originate on the farm being investigated (Dasho, 1994). Electric fencers and similar devices produce a very high voltage (5000 - 20,000 V) Low impedance neutral systems including impulse with very short phase duration (10 - conductors, and grounding and equipotential 300 µs). These impulses are generally a planes in animal confinement locations are single cycle and may be mono phasic (only effective means to reduce steady 60 Hz cow a positive spike) or biphasic (spiking both contact voltage as well as transient voltages. positive and negative). This type of impulse is designed to produce aversion but not ANIMAL RESPONSE cause harm to the animal. A response curve showing both expected These devices have been used for many behavioral and milk production responses to years to contain animals to certain areas. current and voltage is presented in USDA The vast body of field experience with these Handbook 696. This figure is the consensus devices suggests that intermittent exposure opinion of the authors who conducted and to voltage transients is not problematic. reviewed data from numerous experiments. It does not attempt to illustrate the variations Field experience has shown that improper found among experiments or within installation of electric fencers, trainers and experiments. crowd gates are a common source of short duration transients in cow contact locations This figure is meant to apply to continuous (Dasho, 1994). Common installation exposure to steady 60 Hz, voltage and problems include inadequate separation of current. The current level given for the ground for these devices and other perception is from 1 to 3 mA, RMS with no grounding, improper insulation on live or production loss anticipated in this range. grounding conductors and unintentional Moderate behavioral responses are expected shorting to metallic objects. in the range from 3 to 6 mA. Any loss in production is not due to physiological If the impulses from these devices appear change in animals. frequently in animal contact locations at sufficient levels, animals may avoid those locations. Switching and other Transient Voltages 3 Response to 60 Hz Voltage and Current In these experiments cows were exposed to the transient current (1 cycle, 60 Hz) Research conducted at the University of whenever they attempted to drink. In Wisconsin (UW) has resulted in practice transient events are not typically characterization of the sensitivity of a large always present. number of dairy cows to 60 Hz transient voltages (Reinemann et al, 1996). The Predictability of response was improved distribution of responses for 120 cows over previous studies by taking into account exposed to a single-cycle, 60 Hz current is individual animal sensitivity e.g. each cow shown in Figure 2. This is a response level was exposed relative to her individual at which cows first showed signs that they behavioral response level. This result were aware of the presence of the stimulus.
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