The Equipotential Plane in New Livestock Facilities

Transcription

1 The Equipotential Plane in New Livestock Facilities Keith G. Tinsey, P.E., Truman C. Surbrook P.E. and Jonathan R. Althouse Agricultural Engineering Department Michigan State University Introduction An equipotential plane is a method of using a conductive network installed in the floor of livestock facilities in order to minimize neutral-to-earth voltage at animal contact points. Neutral-to-earth voltage can be termed stray voltage if it occurs at a significant level between two points that an animal can contact simultaneously. Stray voltage is of great concern to livestock producers and of particular concern to dairy producers. At significant levels, stray voltage will cause decreased milk production and affect animal health due to changes in behavior such as eating or drinking. Installing an equipotential plane when constructing new or remodeling existing facilities can help minimize the possibility of stray voltage influencing livestock. Stray Voltage In order to understand how an equipotential plane can minimize neutral-to-earth voltage at animal contact points, one must first understand stray voltage. Stray voltage does not directly affect animals, but indirectly through avoidance behavior. For example, when cows change their behavior it can affect their performance. Cows may avoid drinking, eating, letting down their milk or entering stalls. If cows are nervous in the milking parlor, they may not milk out properly, which can affect production and health. If cows receive a shock when drinking or eating, they may consume less and this would affect milk production. Or, if cows receive a shock when they enter an area, they may avoid that area making management more difficult. The terms neutral-to-earth voltage and stray voltage are used because they are more accurately measured in the field as compared to measuring current. It is current flowing through an animal that affects an animal s behavior. Controlled laboratory studies have shown that cows can detect relatively low levels of current, as low as one milliamp (ma). However, it takes at least 4 ma to cause behavioral changes. (Effects of Electrical Voltage/Current on Farm Animals, How to Detect and Remedy Problems, USDA, ARS Agricultural Handbook Number 696). To understand how stray voltage is caused, it is important to recognize that electricity always has a source. For a farm it is the transformer mounted on a utility pole or a cement pad on the ground. Any current that gets out of its conductor is looking for a path back to the transformer. Conductors supplying a building can be a source of neutral-to-earth voltage. Every time electricity flows along a conductor, some of its energy is lost in the form of a voltage drop. That means the connection to earth at one building is at a different voltage than the grounding connection at another building. This causes a very small current to flow into the earth at each building. The result is neutralto-earth voltage as illustrated in Figure 1. If wires between buildings are too small for the load to be supplied, the voltage drop may be great enough to elevate neutral-to-earth voltage to levels high enough to be felt by livestock.

2 Figure 1. High resistance in a neutral conductor acts as a restriction that forces an increased current flow in the earth that results in neutral-to-earth voltage. Stray voltage occurs when livestock come in contact with metal equipment that has a different electrical potential than the surface the animal is standing on. As the animal becomes part of the electrical circuit, the difference in electrical potential will cause current to flow. Current may then travel through the animal to earth in order to return to the electrical source. neutral-to-earth voltage causing stray voltage is typically less than 10 Volts AC 60 Hz steady state (VAC). Voltages above 10 VAC are usually caused by electrical wiring problems such as nearby ground faults and should be considered safety hazards that require immediate attention. Sources of neutral-to-earth voltage include corroded or poor neutral wire connections, damaged neutral wires and normal ground current from the electrical distribution system. The electrical distribution system includes both on farm wiring as well as off farm electrical utility distribution. Voltage drop on grounded utility lines can sometimes result in stray voltage levels high enough to be felt by livestock. Ground faults at a neighbor s property such as at a water pump can cause stray voltage. A farm can become part of the neighbor s ground fault circuit through the utility s grounded conductor. Some utilities separate the grounded conductor and the farm neutral conductor at the transformer to prevent the farm from becoming part of one of these off farm neutral-to-earth voltage circuits. An example of stray voltage occurring is when livestock touch a metal object, such as an electrically heated metal livestock waterer, the animal s body is placed across the neutral-to-earth voltage and current flows through the animal and into the earth as shown in figure 2. If an equipotential plane were in place, the waterer would be

3 connected to the equipotential plane. Neutral-to-earth voltage would be minimized because the waterer and the equipotential plane would be at the same electrical potential. A voltage difference would not exist between the concrete floor and the waterer. This is illustrated in the bottom view of figure 2. Therefore, current would not flow through the animal because the waterer and the concrete floor are at the same voltage potential. However, there would be a voltage difference between the concrete floor and the adjacent earth, thus creating a step potential when livestock walk onto or off of the concrete floor. Figure 2. Without an equipotential plane, livestock can be exposed to stray voltage. An equipotential plane eliminates the voltage difference across the body of livestock, but a voltage is created between the concrete floor and adjacent earth.

4 Equipotential Plane An equipotential plane is a method to prevent livestock from being affected even if stray voltage occurs on a farm. An equipotential plane utilizes a conductive network embedded in the floor of livestock facilities connected to the metal stalls and equipment that livestock touch. If the stalls and floor are connected together, a voltage cannot develop between them. More specifically, an equipotential plane is a surface where wire mesh, reinforcing bars, or other conductive elements are embedded in concrete, bonded to all extensive metal structures and fixed non-electrical metal equipment in livestock areas that may become energized, and bonded to the electrical grounding system of the facility. Detailed instructions for the installation of an equipotential plane are provided in ASAE Engineering Practice EP473.2 Jan 2001, Equipotential Plane in Animal Containment Areas. Equipotential planes have been discussed in conjunction with livestock facilities as early as 1976 by Craine. Research has been conducted to investigated the installation and effectiveness of equipotential planes (Kammel 1987, Althouse 1990). Equipotential planes have been recommended in concrete floors of livestock confinement buildings for many years and are required to be installed by the National Electrical Code. The National Electrical Code Article , Equipotential Planes and Bonding of Equipotential Planes, requires the equipotential plane to be bonded to the electrical panel grounding bus with a copper wire not smaller than size 8 AWG as illustrated in Figure 3. Equipotential planes are required in all concrete floor confinement areas that contain metal equipment that is accessible to livestock and is likely to become energized. Figure 3. Protect the 8 AWG copper conductor with a PVC sleeve and bond the equipotential plane to the building service panel ground bus.

5 Outdoor confinement areas shall have equipotential planes installed around likewise equipment. The outdoor equipotential plane shall encompass the area around the equipment where the livestock stands while accessing the equipment. Equipment examples are automated feeders and electrically heated waterers. The National Electrical Code , states that dirt confinement areas that contain metallic equipment that is accessible to livestock and is likely to become energized do not require an equipotential plane, however, all circuits providing electric power to such equipment shall have Ground Fault Circuit Interrupter (GFCI) protection. Many agricultural producers and farm builders are not aware of this equipotential plane requirement since most agricultural installations are not required to have the electrical wiring inspected. The installation of the metal reinforcing mesh for the concrete floor is generally not a task for the electrician. The builder or concrete contractor needs to be made aware of the equipotential plane requirement. Installation The purpose is to prevent livestock from being exposed to stray voltage that may develop between metal equipment and the floor. The easiest equipotential plane installation technique is to install sheets of concrete reinforcement steel mesh before the floor concrete is poured. To be most effective, weld the sheets together at one or more places. The sheets of steel mesh can also be connected together with a clamp or wire wrap. Then weld the sheets to stall dividers and other metal equipment. It may be more convenient to braze a copper wire to the steel mesh. The wire must be size 8 AWG or larger and extended up out of the concrete for the electrician to connect to equipment and the electrical panel grounding bus. As the copper wire comes up out of the concrete, sleeve it with a short stub of PVC conduit, if possible, to prevent the wire from breaking in the future, (See Figure 3). Slatted floors found in swine facilities and some dairy facilities require a different approach to equipotential plane installation. It is not practical to bond the reinforcing steel of precast concrete slats to the equipotential plane. Bonding the slats is not necessary provided the reinforcing steel in the supporting walls for the slats is bonded to metal that can be contacted by livestock. The reinforcing steel in the supporting walls is also required to be bonded to the grounding bus of the building service panel with a copper conductor not smaller than size 8 AWG. This is illustrated in Figure 4. The equipotential plane consists of reinforcing steel embedded and interconnected within the pit walls and floor. The animals are still protected because the slats cannot develop an electric potential that is different from the equipotential system within the pit walls and floor. All metallic animal contact surfaces must also be bonded to the equipotential plane in a slatted floor facility, (Agricultural Wiring Handbook, 13 th Edition).

6 Figure 4. Equipotential plane employed in a slatted floor building system. Where slats rest on pit walls and the pit walls and floor are connected as an equipotential plane. Retrofitting ASAE EP473 discusses two methods of retrofitting an equipotential plane in an existing facility. The first method of retrofitting is to overlay new concrete 2 to 3 inches thick over existing concrete with the equipotential plane embedded as described above. The surface of the existing floor must be properly prepared in order to achieve adequate bonding between the existing floor and new floor. The second method is to saw grooves in the existing concrete and grout in size 8 AWG copper conductors. The groove should be approximately 0.25 inch wide by 1.0 to 1.5 inch deep. Installing a bare copper wire in a dairy stanchion barn is shown in Figure 5. The conductors should be spaced 12 to 18 inches and be placed in the areas where the animal s feet will be located. Conductors should also be installed in the concrete feed manger. A quick-setting expanding grout is recommended. It is recommended to consult ASAE Engineering Practice EP473.2 JAN 2001, Equipotential Plane in Animal Containment Areas for further installation details.

7 Figure 5. Installation of bare copper wire into a saw cut groove in the concrete floor of a dairy stanchion barn prior to grouting. Outdoor Confinement Areas One area often overlooked is at livestock waters installed in open areas. Instead of installing a small concrete pedestal for the waterer, one should consider pouring a concrete slab large enough so livestock can stand on the concrete while drinking. Install the reinforcing steel mesh in the concrete with a copper wire emerging below the waterer for easy bonding, (Fick, 1999) The equipotential plane surrounding the waterer should be bonded to the frame of the waterer. It is essential that an equipment grounding wire be run with the circuit wires that connects the frame of the waterer to the grounding bus of the electrical panel supplying the waterer. As illustrated in Figure 6, provide at least one voltage gradient path for livestock to walk onto the concrete. A recommended publication is also available for installing electrically heated waterers, ASAE Engineering Practice EP342.2 DEC 1999, Safety for Electrically Heated Waterers.

8 Voltage Gradient Ramp If there is a significant stray voltage situation occurring on the farm, an animal within the equipotential plane should not be affected. As described, voltage differences should not exist when an animal is within an equipotential plane. However, in the event that the stray voltage level at a farm is at a level that animals can feel and animals must enter or exit the equipotential plane on a regular basis, a voltage gradient ramp will be needed at places where livestock enter or exit the equipotential plane. This need is due to a voltage potential that will exist between an animal s front and hind legs as they step onto or off of the equipotential plane. This is commonly referred to as a step potential. Note in the top view of Figure 2 that there is a voltage between the metal water bowl and the floor, but no voltage between the floor and remote earth. In a real barn the voltages will not be quite so dramatic, but Figure 2 is only intended to illustrate a point. Now note in the bottom view of Figure 2 that when the equipotential plane is installed, the voltage difference between the water bowl and the floor goes to zero, but now there is a voltage between the floor and remote earth. Livestock can experience a voltage between front and back feet when stepping onto or off of the equipotential plane. A voltage transition or ramp can be installed to prevent the livestock from experiencing a perceptible current when walking onto the equipotential plane. One way to make a voltage ramp is to drive 8 foot long ground rods into the earth at a 45 degree angle at each livestock entrance as shown in Figure 6. Four ground rods spaced 1 foot apart are usually adequate unless the entrance is extra wide. A copper wire not smaller than size 8 AWG is to be used to bond the ground rods together and to the equipotential plane. Connection to the ground rods can be made with grounding clamps rated for direct burial. Figure 6. Pour a concrete slab large enough for livestock to stand on while drinking at waterers with an equipotential plane in the concrete and a voltage gradient ramp.

9 Robotic Milking It is important to have an adequate equipotential plane system in conjunction with robotic milking installations. House, 2001, states that Measurements taken from cow contact points on both types of robotic milking systems in Ontario indicate the metal equipment is case grounded and provides a potential cow contact for stray voltage. Since the metal floor is an integral part of the milking box, the cow is on an equipotential plane while in the stall and therefore protected from stray voltage during milking. Cows are exposed to a step potential when entering and leaving the box. This step potential can be minimized or eliminated by connecting the metal of the robotic milking equipment to the equipotential plane or by installing a voltage gradient ramp leading up to the robotic milking system. Proper Equipment Grounding Electrical equipment should be maintained in good repair. The bare or green safety grounding wire must be properly connected for all electrical equipment. If anything goes wrong in the wiring or equipment, that grounding wire will carry fault current safely back to the grounding terminal of the electrical panel. If this equipment ground wire is damaged or missing, the fault current will seek an alternate path back to the electrical panel. Alternate pathways may include metallic objects, the earth or any humans and animals connected to them. For added protection, install Ground Fault Circuit Interrupter (GFCI) protection type receptacles in damp and wet areas where portable tools will be used. Summary Farm wiring as well as the utility electrical system can be a cause of stray voltage. A properly installed equipotential plane is one method of mitigating the effects of neutral-to-earth voltage if neutral-to-earth voltage exists, however it does not address the sources of neutral-to-earth voltage. Sources should be eliminated if possible. Equipotential planes installed in milking barns, feeding areas, and around waterers can help prevent livestock from being affected by stray voltage if an electrical problem develops at a farm. Recommended publications for properly installing equipment and equipotential planes include: ASAE Engineering Practice EP342.2 DEC 1999, Safety for Electrically Heated Waterers. ASAE Engineering Practice EP473.2 Jan 2001, Equipotential Plane in Animal Containment Areas Installation Guidelines: Equipotential Planes for Stray Voltage Reduction National Electrical Code

10 References Althouse, Jonathon A Master of Science Thesis. Affecting Neutral-to-Earth Voltage Levels Through Modifications of the Farm Electrical Grounding System. Agricultural Engineering Department, Michigan state University, East Lansing, MI, ASAE. 2001a. EP 473. Equipotential Plane in Animal Containment Areas. ASAE Standards, 45th edition. Amer. Soc. Agric. Engrs., St. Joseph, MI, ASAE. 1999b. EP 342. Safety for Electrically Heated Livestock Waterers. ASAE Standards, 45th edition. Amer. Soc. Agric. Engrs., St. Joseph, MI, Craine, L.B Establishment of an Equipotential Plane in the Milking Area to Alter the Grounded Neutral Voltage to Earth. First Progress Report to Northwest Dairyman s Association. Fick, Robert J. and Surbrook, Truman C. May Farm Wiring to Prevent Stray Voltage. Hoard s Dairyman Magazine, Fort Atkinson, WI Kammel, David W. and Jones, Bret Analysis of Equipotential Plane Installations. ASAE Paper No Amer. Soc. Agric. Engrs., St. Joseph, MI, House, Harold K., Rodenburg, Jack and Kelton, David F Experience With Automated Milking Systems in Ontario. ASAE Paper No Amer. Soc. Agric. Engrs., St. Joseph, MI, National Electrical Code 2002, National Fire Protection Association, Inc., One Batterymarch Park, Quincy, MA National Food and Energy Council, Inc Agricultural Wiring Handbook 13 th Edition. 601 Business Loop, 70 West, Suite 216D, Columbia, MO USDA Effects of Electrical Voltage/Current on Farm Animals, How to Detect and Remedy Problems, ARS Agricultural Handbook Number 696. United States Department of Agriculture. Wisconsin Farm Electric Council Equipotential Planes for Stray Voltage Reduction, Installation Guidelines.