Site Content

In an electrical circuit, electricity moves from a source to a load, such as lights or a motor, and back to the source, completing the path. The source for the farm is the transformer. The source for the utility is the substation. We have heard that electricity seeks the earth. In a sense, it does, but really it is trying to complete the path to the transformer. There is a relationship between the voltage, the force to make the electricity flow; the current, the amount of electricity flowing in the circuit; and the resistance, the things in the circuit that try to hold back the current. With a constant resistance, a higher voltage causes a higher amount of current to flow. With a constant voltage, a lower resistance will result in a higher current. Many of us have heard the phrase that electricity follows the path of least resistance. Actually, electricity follows all available paths. More will flow through the path of least resistance, but some will flow through all paths.

A normal farm electrical system is made up of several basic circuits. Four wires are included in wiring. Three wires normally carry electricity. Two of these are hot wires and the third is the neutral. The fourth wire is an equipment grounding wire that goes to all electrical equipment. It is also attached to anything else that might become energized. The equipment grounding wire exists to protect people and animals from electrocution. If a hot wire shorts out to a piece of equipment, a low resistant path through the equipment grounding wire causes a circuit breaker to trip. In a three wire system, the grounding wire is attached to a ground rod, as well as the neutral, at each building electric service panel - in a 4-wire system they are separate.  A 240-volt load uses only the two hot wires. A 120-volt load uses a hot wire and the neutral. Sometimes two 120-volt circuits are wired using the same neutral. Lighting circuits are an example of circuits that are wired this way. The neutral carries the difference between the currents of the two circuits. If one circuit is using 8 amps and the other is using 6 amps, the neutral will carry 2 amps. If they are using the same current, the neutral current will be zero.

The utility system, to a single phase farm, is made up of two wires. One wire is the hot wire and the other is a neutral. The neutral is connected to the earth through ground rods along the along the distribution line and at the transformers. The voltage between the hot and neutral wires is normally 7,200 volts.

A transformer is used to step down the voltage from 7,200 volts to the 120/240 volt system used on the farm. The utility neutral is connected to the farm neutral at the transformer. This is used as system protection in the case of lightning on either side of the transformer. The ground rods of both systems are used to dissipate the lightning’s energy.

A basic electrical circuit (discussed earlier) is a complete path from the source to a load and back to the source. There are normally three wires going to each building, two hot and a neutral. The current travels from the transformer to each building on the two hot wires and back on the neutral. Since the neutral is connected to the equipment grounding wire at the building electrical panel, the neutral current can also go to the transformer through the earth at the ground rod or through anything else that is connected to the equipment grounding wire and the earth. An animal becomes part of a path when touching a device that is grounded, such as a water fountain, feed rail or stall.

Equipotential planes were designed to help eliminate stray voltage. They are now required by the National Electric Code to be installed in all animal confinement installations. A wire mesh, or re-rod connected together, is installed in the concrete and connected to every piece of metal installed in the concrete that the animals may touch. This plane is also attached to the building electrical grounding system. The theory is that the equipotential plane will eliminate any voltage in the animal environment; therefore, eliminating the possibility of stray voltage. Water fountains installed on an equipotential plane should be grounded to the plane at that location, as well as having the equipment grounding wire that goes back to the panel.  

Four wire systems to farm buildings with single phase service is another attempt to cure or prevent stray voltage problems. This would be a fifth wire on a three phase service. The National Electric Code now requires this to all new farm buildings. A grounding wire is installed between the farm disconnecting means and the building electrical panel. The neutral and the grounding wires are not connected at any point beyond the farm disconnect. The reasoning here is that the neutral current is prevented from going through the earth and increasing the voltage between grounded equipment and the earth. 

 The other part of basic stray voltage is the utility. As mentioned earlier, current travels from a source to a load and back to the source. The utility source is the substation. The electricity travels from the substation out to the various farms, houses, and businesses then goes back to the substation. As with the farm, the earth is a natural and normal path for the electricity to flow. The utility system is grounded at the substation, along the utility poles, and is connected to the farm neutral at the transformer. The farm grounding system, then, becomes another path for the utility neutral current going to the substation. The utility system comes from the substation with three-phase power and branches off with single phase lines to most of the farms. Utilities attempt to keep the three-phase power balanced as much as possible, which keeps the neutral current on the three-phase lines at a minimum. The single phase lines can be many miles long depending on the system. There is only a hot wire and a neutral on the single phase lines. The utility neutral resistance increases with the length of the wire; increasing the neutral to earth voltage. As expected, the neutral to earth voltage is highest at the farms near the end of the single phase lines, where stray voltage is most likely to occur. I have also found stray voltage problems on the farms close to the substation, since the currents in the earth will go up through that farm’s grounding system into the utility neutral and on to the substation.

The farm should have a ground rod at each building, making it a good source to the earth. Equipotential planes installed in the livestock buildings create an even better farm grounding system. Some utilities have installed extra ground rods on their system in an attempt to reduce the amount of stray voltage. This does not always work, since the problem is the length of the neutral and the good grounding of the farm

A neutral isolator can be installed at the transformer to separate the utility neutral and the farm neutral. In an event where there is a large voltage difference between the utility neutral and the farm neutral, such as a lightning strike, the isolator will close, let the power through, and then open back up again. Isolators will close around 24-volts or 30-volts depending on the isolator. A neutral isolator will cause an increase in neutral to earth voltage on the utility side of the transformer and a decrease on the farm side. A nearby farm may then have a stray voltage problem due to the increased utility voltage. If the other farm is located over a mile away, I would not expect to see them affected, but I would monitor them.

One of our initial measurements, on a single phase farm, is to measure the neutral to earth voltage and turn on the vacuum pump. The vacuum pump is a large 240-volt load, which does not use the farm neutral. If the neutral to earth voltage increases, there is an increase in neutral current going into the earth. Since the load is 240-volts, the increase comes from the utility neutral. Any additional 240-volt loads on the farm will increase the utility neutral current. This includes the cooling compressors, electric water heater in the barn and house, silo motors, feed mixers, grain bin fans, and the oven in the house. This will normally be highest in the fall when the bin fans are operating, feeding is done during milking, and either heating or air conditioning in the house.

Had a farm that suspected he had stray voltage. The farmer said he would have his electrician check things out. I called him several weeks later. He said the neutral to earth voltage in the barn went up when they turned on various motors. They then shut the power to the barn off. The neutral to earth voltage in the barn went up as they turned on the oven and other loads in the house. They weren’t sure how to explain what was happening. I said I could come, but would want the utility there when I got there. He agreed. We measured the neutral to earth voltage in the barn and turned on the vacuum pump and cooling compressors. The voltage increased with the loads. The utility then temporarily disconnected the bonds between the utility and farm neutrals. We then observed the neutral to earth voltage while turning on the same loads. The voltage did not change. The utility decided they did have a problem and agreed to install a neutral isolator.

Had a farmer talk to me at a meeting and said their heifers would not let their milk down. This caused the farm to cull them. This had been occurring for the last several years. He decided to have it checked by us. He had two services at his barn. The service near his milking parlor already had a neutral isolator. The other service was in the back part of the barn where he had a phase convertor to create three-phase power for his silos and feed mixing equipment. This service did not have a neutral isolator. His neutral to earth voltage did go up some with the vacuum pump, but not significantly. He had some cows that were producing around 140 pounds of milk. I wasn’t sure if there really was a stray voltage problem. Talking to the wife during the investigation, she said the heifers came in for milking about the same time the feed mixing equipment and silo motors were used. That large increase in power did raise the voltage in the parlor to a high level. The heifers were being subjected to a lot of stray voltage when the other cows were not. The utility installed a neutral isolator at the second service. The problem with milk let down on the the heifers was solved.

One thing I have mentioned to college dairy students is to “listen to the cows”. As you know from your experiences, paying to attention to the animals helps you in your diagnosis of health problems.

We try to keep an open mind when I go out to farms. We know and understand livestock so we pay attention to the animals and listen to the you to help figure out what is happening. We then take measurements to determine if the electrical system is telling us the same thing.