Tech Tip 09
Low Neutral Voltages Reduce Stray Voltage Problems
- Farm Animals Acting Like They Are Shocked From Stanchions or Water Fountains.
- Shocks While Touching Metal Objects.
- Shock Sensations in Swimming Pools.
- High Resistance Neutral Connections.
- Customer Wiring or Equipment Problems.
- High Neutral Currents.
- High Resistance Grounds.
- Repair Neutral Connections.
- Correct Wiring and Equipment Problems.
- Reduce Neutral Current.
- Lower Resistance to Ground.
Electrically grounded equipment normally has a small voltage with respect to earth even with proper wiring and grounding. Sometimes the voltage is high enough to cause shock sensations for animals and people. This only occurs when the animal or person becomes part of an electrical path between the grounded object and "remote" earth.
Outdoor electric distribution systems typically use neutral wires that are grounded, connected to the earth, in many locations. The many connections to earth allow some neutral current to flow through the earth instead of the neutral wire. This causes a voltage difference between the neutral and "remote" earth a few feet away. Neutral to Earth Voltage, or NEV, appears at pole grounds and customers' main panels. Some voltage appears at grounded equipment because equipment grounding wires are bonded to the neutral in the main panel. Figure 1 shows a voltmeter measuring NEV and the electrical path to an animal.
An animal touching the earth and an electrically grounded object will have some of the voltage across their body when they touch it. The voltage across the animal is an example of Stray Voltage. Just one volt may cause a slight shock sensation for animals and people in certain situations.
Stray Voltage can make farm animals reluctant to drink from grounded water fountains. Dairy production may be affected if cattle feel shocks from metal stanchions at milking time. People may feel a shock if they touch a metal object while also touching wet concrete, soil, or other metal objects. Swimming pools and water lines are two examples.
Figure 1: Neutral and Ground Path
It is desirable to keep NEV below perception levels when practical. Stray Voltages above one volt should be reduced especially when it causes problems. Two to four volts of NEV should be reduced where practical. Urgency to reduce the voltage increases rapidly with higher voltages.
High resistance neutral connections can cause high NEV and should be replaced. A clamp-on amp meter will show if current is not flowing through poor connections. A digital volt meter directly across connections or adjacent pole grounds is another good way to detect bad connections.
Phase current balance is important. Balanced circuits have less neutral current. Lower neutral current means lower NEV. This works on three phase and on single phase circuits. Converting 120 volt single phase loads to 240 volts help reduce NEV caused by secondary neutral current.
Additional ground rods reduce NEV by lowering the net resistance to neutral currents. Customer wiring problems and equipment grounding errors are also important factors. Even wiring problems or short circuits on nearby customers can cause problems.
NEV and stray voltage problems can be difficult to identify, diagnose and solve. Additional information, USDA Handbook 696 for example, is available. Consult with Technical Services for help preventing, identifying and solving stray voltage problems.
The information and diagrams presented herein are for general educational purposes only, and should not be relied upon as instructions for customer self-wiring. Customers should at all times seek the assistance of qualified electricians or utility personnel for all wiring projects.