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Paper No. 983007
An ASAE Meeting PresentationELECTRIC FENCE SYSTEM INFLUENCE ON ANIMAL CONTACT VOLTAGE
Authors:
Robert J. Fick, Ph.D., P.E.
Agricultural Engineering Department
Michigan State University
East Lansing, MI,
USATruman C. Surbrook, Ph.D., P.E.
Agricultural Engineering Department
Michigan State University
East Lansing, MI, USAMark G., Holycross, Graduate Assistant
Agricultural Engineering Department
Michigan State University
East Lansing, MI, USAJonathon R. Althouse, MS
Agricultural Engineering Department
Michigan State University
East Lansing, MI, USAWritten for Presentation at the 1998 International Meeting Sponsored by ASAE Orlando, FL, July 11 to 16, 1998
Summary: Grass contact with an electric fence can provide a pathway for enough voltage between the fencer ground terminal and remote earth to deliver an uncomfortable shock. The possibility of a fence charger reaching an animal contact is reduced by improving farm grounding. It is possible to get perceivable neutral-to-earth voltage due to a high resistance neutral to a building with a fence charger. With a ground rod located one meter from the fence system ground, enough voltage appeared at the second ground to be perceived by a human. Connecting the ground to other grounds reduced the voltage. A Fluke 87 was able to detect fencer pulses when the pulse magnitude was large enough to be perceived, but was not representative of the peak voltage output.
Keywords: Stray Voltage, Electric Fence, Transient Voltage, Grounding
The authors are solely responsible for the content of this technical presentation. The technical presentation does not necessarily reflect the official position of ASAE, and its printing and distribution does not constitute an endorsement of views which may be expressed. Technical presentations are not subject to the formal peer review process by ASAE editorial committees; therefore, they are not to be presented as refereed publications. Quotation from this work should state that it is from a presentation made by Fick, R.J. et al, at the 1996 Annual ASAE meeting. EXAMPLE: Fick,R.J., et al, " ELECTRIC FENCE SYSTEM INFLUENCE ON ANIMAL CONTACT VOLTAGE ." Presented at the 1998 International Meeting, Paper No. 983007. ASAE, 2950 Niles Rd., St. Joseph, MI 49085-9659, USA. For information about securing permission to reprint or reproduce a technical presentation, please address inquires to ASAE. ASAE, 2950 Niles Rd., St. Joseph, MI 49085-9659, USA. Voice: 616-429-0300 FAX: 616-429-3852 E-mail:hq@asae.org
Introduction: Low impedance electric fence energizers are capable of making contact with vegetation and still maintaining the capability of delivering a repelling shock to livestock. The internal impedance of the energizer is low thus allowing a large current flow to occur for a controlled time period. The out put energy of the energizer is a function of the output voltage and the time interval of the pulse. Manufacturers directions for these fence energizers emphasize the importance of establishing a good low resistance grounding system for the energizer that is separated from the electrical system ground,or any metal equipment with which the livestock may make contact. The energizer is nothing more than a source of a high voltage short duration pulse of voltage which will result in a short duration current pulse when a circuit is completed. The wire mounted on insulators is one terminal, and the earth is the other terminal. Current will flow when the circuit is completed by livestock, vegetation,or equipment making contact between the wire and the earth. A low resistance grounding electrodeis needed to complete the circuit back to the energizer. If vegetation or an animal makes contact with the fence, the circuit is completed and current flows to earth and returns to the energizer by way of the grounding electrode.
Ohm's law describes the electrical condition where current flowing over a resistance results in a voltage. If current is flowing in the electric fence circuit, there will be a voltage developed between the fence system ground rod and the adjacent earth during the time interval when the current if flowing. If the current is a short duration pulse, the voltage at the ground rod will be a short duration pulse. If this fence system ground rod makes contact with the electrical system grounds,a neutral-to-earth voltage pulse will be experienced at various locations on the farm. Figure 1 shows a properly installed electric fence system that is making contact with vegetation, and a current pulse results every time the wire is energized. Figure 2 shows the voltage pulse from the fence system ground rod to an adjacent point on the earth. The duration of the pulse is a function of the energizer design. The magnitude of the voltage at the ground rod depends upon the level of current flow in the earth and the resistance of the grounding electrode. The higher the grounding electrode resistance is, the higher the voltage between the grounding electrode and the earth will be . If the energizer grounding electrode is connected to the electrical system grounds, then the entire farm grounding system will act as the ground return path for the fence energizer. This will result in a voltage pulse being present at locations throughout the farm.
Figure 1 A pulse of current will flow through the earth and return to the energizer when vegetation, livestock, or equipment makes contact with the electric fence.
Figure 2 A voltage pulse will occur between the fence energizer grounding electrode and an adjacent point on the earth every time an earth current pulse occurs.
Voltages from electric fencers are frequently reported showing up at waterers and other animal contacts by neutral-to-earth voltage investigators, but publications quantifying these voltages have not been found. Sheldon (1993) looked at fencer outputs for different controllers and fence configurations. He measured voltage at fence contacts along the length of the fence for different fence configurations.
Gettman (1983) addressed the testing of the standards and performance characteristics of electric fences by Underwriter Laboratories in its Standard for Safety for Electric Fence Controllers,UL 69 (1993). If a fence controller is found to meet the requirements specified in UL 69, and if the manufacturer agrees to subscribe to UL's Follow-Up Service, UL can extend Listing coverage to that particular controller and authorize the manufacturer to use the appropriate UL Registered Mark on those controllers which meet UL's requirements. The output of the fencer must not exceed a curve of current plotted against pulse duration. Both of the fence controllers we tested were Unregistered. Many of the higher powered low impedance chargers available are not UL Registered.
Objectives: Low impedance electric fence systems have been claimed to be a source of electrical current that affected animal behavior and possibly productivity. This research examines the operating characteristics of a low impedance electric fence system to determine any possible way during operation it can create a voltage between animal contact points. Current flow was measured through a simulated animal at the contact points using a resistor of a value comparable to that of livestock. Specific situations examined were:
- An electric fence system grounded to the farm electrical system grounding system with the fence making contact with grass.
- The electrical system ground or metal livestock equipment located in close proximity to the electric fence system grounding electrode with the fence making contact with grass.
- A high resistance neutral supplying a 120 volt electric fence system properly installed with and without grass contact with the fence.
A commonly available digital multimeter that has been used to measure transient voltages was also tested and compared with an oscilloscope to determine if it can be a reliable means of predicting whether a voltage may be present between animal contact points resulting from electric fence operation. Human subjects also tested voltages between animal contact points to determine if voltages by these three methods could be perceived.
Experimental Procedure: Only low impedance fence energizers that were listed by Underwriters' Laboratories were tested as a part of this research. The first energizer had an output rating of 6 Joules and was a Farmworks® Model 4400-B, and the second energizer did not have an output energy rating and was a Parker McCrory Fieldmaster® Model FM-1. The first step in the testing was to determine the amount of current that would flow when the fence wire made contact with sandy soil with a dry surface, and the current flow that would occur as the fence wire made contact with grass. Current flow was measured with a Tektronix TekScope® Model THS720 Std with a Tektronix ac current probe Model P6021 capable of detecting short duration current pulses. The voltage was measured across a 550 ohm carbon resistor connected between the fence energizer grounding terminal and are mote ground rod which had a resistance of 25.1 ohms. Grounding electrode resistances were measured using the three-point fall of potential method with a Biddle Megger® DET 5/2.
Human perception was determined by using a minimum of three individuals. A glass beaker was filled with salt water and an electrode inserted into the salt water was connected to the ground terminal of the fence energizer. A copper cylinder 1.6 cm in diameter and 5 cm long was wetted with salt water and grasped in the palm of the hand. The cylinder was connected to the same ground rod as the 550 ohm resistor. While grasping the cylinder in the palm of the hand, the index finger was inserted into the beaker of salt later.
A Fluke Model 87 digital multimeter has been used in the field to measure the short duration transient voltages. In the 1 ms maximum mode, the meter records the maximum voltage value that occurs over a one millisecond interval and averages that value with all previous maximum values recorded. As the voltage pulse decreases in duration from 1 ms, the meter records a value that is less than the peak voltage. This meter was used to measure the voltage across the 550 ohm resistor and compared with the peak voltage measured with the oscilloscope.
The electric fence wire was mounted so that a 15 meter length was making contact with green grass. The wire was mounted 0.75 meters above the ground. The first experiment evaluated two different energizers with four levels of fence system grounding resistance. This was to simulate the condition where the fence system was grounded to the farm electrical system. The four levels of fence system ground resistance were 40.0 ohms, 25.4 ohms, 10.3 ohms, and 2.9 ohms. In addition to capturing the wave form of the current through the 550 ohm resistor and the voltage across the resistor, the oscilloscope determined the peak value of the voltage and the peak value of the current. Perception was determined by human subjects. A schematic diagram of the experiment is shown in Figure 3.
Figure 3 A schematic diagram of the electric fence circuit in experiment one where the fence was faulted to earth through grass and the voltage was measured from the earth to the ground terminal of the fence energizer.
The second experiment was concerned with the flow of current in the 120 volt supply to the energizer and whether that current was of sufficient value to cause a voltage drop on the neutral supplying the building. If there was a high resistance in the neutral supplying the building, a neutral-to-earth voltage pulse would occur due to the current flow to the fence energizer. Two fence energizers were tested at three levels of neutral resistance. Neutral resistances tested were 2.0 ohms,1.02 ohms, and 0.12 ohms. The supply end of the neutral resistance was connected to earth through an 11.2 ohm grounding electrode and at the load end through a 25.1 ohm grounding electrode. The fence energizer was properly installed and grounded to a 10.3 ohm grounding electrode. The experiment was conducted with the fence faulted to earth through grass and the fence not faulted to earth. A schematic diagram of the experiment two is shown in Figure 4.
Figure 4 A schematic diagram of the electric fence circuit in experiment two where the neutral supplying the fence energizer simulating the neutral of a feeder to the building has three different levels of conductor resistance.
Experiment three examined the influence of a gradient near the fence energizer grounding electrode on an adjacent ground rod. Two grounding electrodes located one meter apart were tested with one ground rod connected to the fence energizer with voltage measured to the other ground rod. In the first part of the experiment that ground rod was tested by itself, and in the second part it was connected to an 11.2 ohm grounding electrode located several hundred feet away. The first part of the experiment was similar to locating metal livestock equipment close to a fence energizer grounding electrode. The electric fence was faulted to earth through grass contact to create a voltage gradient near the fence energizer grounding electrode. The schematic diagram of this experiment is shown in Figure 5.
Figure 5 A schematic diagram of the electric fence circuit in experiment three where a second ground rod was located one meter from the fence energizer ground rod. The circuit was modified later so the ground rod was connected to an 11.2 ohm ground.
Results and Discussion: Experiment one examined the voltage from the fence energizer grounding electrode to earth with different grounding resistances as shown in Figure 3. A summary of the data is shown in
Table1. The voltages in the table are measured across a 550 ohm resistor connected from the fence energizer ground to remote earth.
Energizer Energizer Fluke 87 Tektronix Perception
model ground 1ms peak peak peak human
ohms volts volts amps
4400 40.0 64.4 176 1.48 Pain
4400 25.4 28.4 116 0.92 Uncomfortable
4400 10.3 25.3 46 0.41 Noticeable
4400 2.9 2.4 16 0.10 Very slight
FM-1 40.0 21.2 172 1.76 Slight
FM-1 25.4 2.4 122 1.08 None
FM-1 10.3 0.1 40 0.38 None FM-1 2.9 0.1 22 0.18 None
Figure 6 Voltage from fence energizer ground to earth across a 550 ohm resistor and current through the resistor for the model 4400 fence energizer with fence energizer grounding resistance of 25.4ohms.
Figure 7 Voltage from fence energizer ground to earth across a 550 ohm resistor and current through the resistor for the model FM-1 fence energizer with fence energizer grounding resistance of 25.4ohms.
Figure 8 Peak voltage of the wave form for the model 4400 fence energizer with different levels of fence energizer grounding resistance.
In experiment two the 120 volt line feeding the fence energizer was studied to determine the influence of neutral resistance on neutral-to-earth voltage at the building where the fence energizer is installed. The question was whether the current flow to the fence energizer is of sufficient magnitude to create a neutral-to-earth voltage pulse that can be perceived by livestock in the building. Peak values of neutral current for the model 4400 fence energizer are given in Table 2 along with the voltages measured from the neutral in the building to the earth. The peak neutral currents were in excess of 20 amperes when the fence energizer put a voltage on the fence wire. It did not make a difference whether the fence was faulted to the earth. Figure 9 shows the flow of neutral current for the model FM-1 energizer and it can be seen that the line current peaks at the moment voltage is applied to the fence. Figure 9 shows the voltage drop across the resistance in the neutral supplying the fence energizer and the peak current drawn by the fence energizer. It can be seen by the data in Table 2 that for the FM-1 energizer the voltage did not reach a level that would cause human perception. Figure 10 shows the neutral current and neutral voltage drop for the model 4400 fence energizer. In Table 2, it can be seen that a neutral-to-earth voltage pulse could be perceived for the model 4400 energizer.
Table 2 Experiment two with resistance in the neutral conductor of the supply to the fence energizer and the resulting voltage from the building neutral to earth.
Energizer Fault Neutral Fluke 87 Tektronix Perception
model resistance 1ms peak peak peak neutral human
ohms volts volts amps amps
4400 Yes 2.0 5.5 11.2 ------ 24.0 Slight
4400 No 2.0 5.8 12.0 0.112 24.0 Slight
4400 Yes 1.02 2.6 8.2 0.068 23.6 Very slight
4400 No 1.02 2.8 8.2 0.080 25.2 Very slight
4400 Yes 0.12 2.5 3.8 0.048 28.0 None
4400 No 0.12 2.5 4.6 0.036 26.8 None
FM-1 Yes 2.0 1.0 0.7 0.020 0.18 None
FM-1 No 2.0 1.0 ---- ------ 0.15 None
Figure 9 The neutral current and neutral voltage drop are shown for the model FM-1 fence energizer with 2 ohms of resistance in the neutral supplying the energizer. The voltage and current maximums occur at the moment voltage is applied to the wire.
Figure 10 The neutral current and neutral voltage drop are shown for the model 4400 fence energizer with 2 ohms of resistance in the neutral supplying the energizer. The voltage and current maximums occur at the moment voltage is applied to the wire.
Experiment three investigated the influence of the voltage gradient near the fence energizer grounding electrode on a ground rod located at a distance of only one meter. This test was only conducted for the model 4400 faulted to earth through grass. The data are shown in Table 3. For the first test the ground rod was not connected to any external grounds. For the second test the ground rod was connected to another grounding electrode locates several hundred feet away. For the first test the voltage measured from the isolated ground rod to a reference ground was sufficiently high to cause human perception. As soon as the ground rod was connected to another ground rod the voltage dropped dramatically to a level well below human perception. If the ground rod had been located closer than one meter, the voltage most likely would have been higher. This test does show that it is possible to achieve a voltage at the perception level by locating metal objects that can be contacted very close to the fence energizer grounding electrode.
Table 3 Experiment three with the electrical system ground rod located one meter from the fence energizer ground rod with the fence faulting to earth through vegetation.
Energizer Energizer Fluke 87 Tektronix Perception
model ground 1ms peak peak peak human
volts volts volts amps
ground rod isolated
4400 18.8 4+ 10.9 0.092 Slight
ground rod connected to another 11.2 ohm ground
4400 16.9 0.9 9.4 0.084 None
Examination of the data in the Tables shows that the voltage measurements made by the Fluke 87 digital multimeter on the one millisecond maximum setting did not accurately represent the peak voltage of the wave form. The peak voltage for the model FM-1 and 4400 were similar for the same conditions, but examination of Figure 2 shows that the pulse width for the model FM-1 was much shorter than for the model 4400. As a result the Fluke 87 multimeter yielded a lower voltage reading for the model FM-1 than for the 4400 for approximately the same peak voltage. Comparing the perception of the different experiments with the voltage recorded by the Fluke 87 tells a different story. Except at very low voltages, the Fluke 87 voltage set on the 1 ms maximum setting was a reasonable indicator of the level of perception at least for these two fence energizers. The subjective perception level for at least three human subjects compared with the Fluke 87 voltage reading is summarized in Figure 11.
Figure 11 The human perception compared with the Fluke 87 voltage readings set to record the 1millisecond maximum.
Conclusions
- Given enough weed contact with the fence, voltages between the fencer ground terminal and remote earth will be high enough to deliver an uncomfortable shock.
- The pulse or pulses that occurred during line charging can cause a neutral-to -earth voltage that is high enough to be perceived.
- If a voltage is present, a Fluke 87 Multimeter can generally be used to detect that voltage, but because pulse length affects the reading of the meter, it is not as reliable at determining if the voltage will be of a magnitude that will be perceived.
Acknowledgments: Support for this project was provided by the Michigan State University Agricultural Experimentation Station and the Michigan Agricultural Electric Council.
References: Gettman, K.E. "Standards, Performance Characteristics and the Testing of Electric FenceControllers". Presented at the 1983 Winter International Meeting, Paper No. 834554. ASAE, 2950 Niles Rd., St. Joseph, MI 49085-9659, USA.
Sheldon, L. "Field Tests of Electric Fence Controllers". Presented at the 1993 Winter International Meeting, Paper No. 933523. ASAE, 2950 Niles Rd., St. Joseph, MI 49085-9659, USA.
Underwriters Laboratory. Standard for Safety for Electric Fence Controllers, UL 69. 1993. Underwriters Laboratory Inc., 333 Pfingsten Road, Northbrook, IL 60062-2096. AgE M
