What is NEV? | How can NEV affect livestock? | What conditions cause NEV to occur?
Defective electric fence | How can stray voltage be identified on a farm?
How can NEV conditions be corrected? | What can be done to prevent NEV?
What is Neutral-to-Earth Voltage (NEV)?
The term NEV is used to describe a measurable level of voltage which may occur between a metal object and the adjacent floor or earth. Figure 1 shows a voltmeter being used to measure NEV between an animal's water bowl and the floor. Simply stated, voltage is the pressure that pushes electrical current through wiring systems, lights, and electrical equipment. When a person or an animal feels a tingle or a shock, the person or animal is actually feeling electrical current flowing through the body or a portion of the body. When the voltage between a metal object and the adjacent floor or earth is very low, the amount of electrical current flowing through an animal's or person's body is so low that it cannot be felt. But sometimes, a condition within the wiring on a farm, the wiring at a neighbor's property, or the power lines supplying a farm may cause the voltage between equipment and the adjacent earth or floor to be felt by an animal or person. This NEV can range from a slight tingling or burning sensation at a cut to an uncomfortable jolt for animals. If a person feels an uncomfortable jolt, the condition is probably not NEV but a serious problem with the wiring system or equipment. Such a condition requires immediate attention to find the cause. An investigation needs to be conducted to measure voltage levels on the farm and take corrective action if necessary.
Neutral-to-earth and stray voltage may occur between metal objects and the adjacent floor or earth.
The electrical wiring on a farm has a conductor which is grounded to the earth. This conductor is called the neutral conductor and generally is the white wire within the wiring and equipment. The purpose of grounding this wire is to make the electrical system as safe as possible and is required by the electrical code. Also, the primary power lines supplying a farm usually have a conductor which is grounded to the earth to provide maximum safety and reliability. This is shown in Figure 2. Storms and accidents that damage the primary power lines could result in a dangerous voltage which could cause personal injury if the electrical system is not grounded. A grounded electrical system will prevent dangerous voltages from occurring on a customer's wiring system and grounded metal equipment such as water pipes and stalls in a barn. A local accident will result in only the affected electrical system being de-energized, which minimizes the interruption of power to other customers. For example, lightning damage to a transformer will result in only that transformer being disconnected when the transformer fuse blows.
Electrical systems that are grounded to the earth for safety and reliability have a small amount of current flowing through the earth when electrical power is being used. The earth and the grounded primary neutral conductor act as two parallel conductors. The ground rods at each transformer pole and at some poles along the line act as connections between the neutral wire on the poles and the earth. This is illustrated in Figure 2. Normal power line current flows back to the source (substation) on the neutral wire, with some flowing through the earth by way of the ground rods. This is normal, and it can result in a small level of NEV between metal objects and the adjacent floor or earth. A damaged neutral wire or a corroded connector, however, can result in an increased current flow on one or more of these ground rods, resulting in an increase in current flow to the earth and an increase in the NEV level at that ground rod.
Electrical systems are grounded to the earth
to provide maximum safety and reliability.
|Primary and Secondary Current|
Permission to reprint all information in this bulletin granted by James M. Schrandt and
Truman C. Surbrook, who originally prepared the bulletin. © 1993 Consumers Energy
Note: Some modifications have been made to the bulletin for use on this web site.