Low Neutral to Earth Voltages Reduce Stray Voltage Problems
Note: The term “stray voltage” must “NOT”
be confused with the term “contact voltage”. The term “stray voltage” is associated with elevated neutral to earth voltages caused by the flow of return currents in the earth. Stray voltages are generally in the range of 10 volts or less and not considered a threat to human life. On the other hand, the term “contact voltage” represents a potentially hazardous situation where an energized conductor has made contact with a normally de-energized surface. Stray voltage sources can be difficult to identify and resolution may be time consuming. In contrast, contact voltage sources are easily identified and can be corrected immediately.
- Tingling or shocking sensation when touching metal objects
- Tingling or shocking sensation in swimming pools
- Tingling or shocking sensation when touching docks while in the water
- Farm animals behaving strangely when contacting water troughs, stanchions or other metal surfaces
- Dairy cattle symptoms include: reluctance to enter a milking parlor, uneasiness inside a milking parlor, uneven milk output, reduced milk production and health and breeding problems
- Normal operating distribution system
- Customer wiring or equipment problems
- High resistance utility system neutral connections
- High utility system neutral currents
- High resistance grounds
- High efficiency devices that create triplen harmonics
- Multiple circuit interaction
- COMMON SOLUTIONS
- Correct wiring and equipment problems
- Repair neutral connections
- Reduce neutral current
- Lower resistance to ground
- Bond grounds of different potential
- Neutral Isolation devices listed for the purpose
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 tingling or shocking sensations for people and animals. This only occurs when the person or animal becomes part of an electrical path between the grounded object and "remote" earth.
Outdoor electric distribution systems typically use neutral wires that are grounded and connected to the earth, in many locations. This is done for safety reasons. The many connections to earth allow some neutral current to flow through the earth instead of the neutral wire. Voltage drop occurs on the neutral and ground due to the current flowing through the neutral and ground impedances. This results in a voltage difference between the neutral and "remote" earth. Neutral to Earth Voltage, or NEV, appears at pole grounds and customers' main panels. Some neutral to earth voltage appears at grounded equipment because equipment grounding conductors are bonded to the neutral in the main panel.
A person or animal touching the earth and an electrically grounded object may experience a tingling or shocking sensation if the neutral to earth voltage is sufficient enough to be felt. One or two volts may be perceptible in certain situations. Water faucets, swimming pools, docks and dairy parlors are examples of locations typically associated with NEV issues.
Figure 1 shows a voltmeter measuring NEV in a swimming pool. Touching a hand rail, concrete deck or metal coping while in the water can result in a tingling or shocking sensation if perceptible neutral to earth voltages are present. Installing a pool in accordance with the 2005, or later, editions of the National Electrical Code will generally prevent both contact voltage issues and NEV issues in swimming pools.
Neutral to earth voltages around docks can be measured in much the same way as in swimming pools. Installing a dock in accordance with the National Electrical Code will protect swimmers in the vicinity of the dock from contact voltage. It is extremely important that all electrical circuits on a dock be protected by Ground Fault Current Interrupters, or GFCIs, because GFCIs help protect against contact voltage. GFCIs must be installed in accordance with the manufacturer’s instructions. They must also be maintained and tested regularly to ensure proper operation. Installing a dock in accordance with the NEC will not address stray voltage, or NEV, issues.
Stray voltage can make farm animals reluctant to drink from grounded water troughs. Dairy production may be affected if cattle feel a tingling or shocking sensation from metal stanchions at milking time. Figure 2 shows a voltmeter measuring NEV and the electrical path to an animal.
If a contact voltage issue or an NEV issue is suspected, a qualified electrician or the local utility should be contacted to investigate the situation and take appropriate action. Contact voltage issues should be resolved immediately. NEV issues should be further investigated and addressed appropriately by the local utility and the facility owner.
Neutral to earth voltages should be kept below perception levels when practical. Stray voltages above one volt should be reduced in dairy parlors. Eight volts or more of NEV should be reduced where practical. The need to reduce the voltage increases with higher NEV values.
Customer wiring problems and equipment grounding errors should be identified and corrected.
Wiring problems or short circuits on systems owned by nearby customers should also be identified and corrected.
Swimming pools should be installed in accordance with the 2005, or later, editions of the National Electrical Code. Bonding should be improved on existing pools when possible.
If possible, unplug all power to a dock when swimming in the vicinity of the dock. This is best accomplished by using cords that disconnect the hot wire, neutral wire, and ground wire when unplugged. New power installations around water should include an isolation transformer, where allowed.
High resistance neutral connections 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 way to detect bad connections.
Phase current balance is important. In general, balanced circuits have less neutral current and lower neutral current means lower NEV. Balance circuits and reduce neutral currents when beneficial. Be aware that balancing a circuit may not resolve an NEV issue if significant amounts of triplen harmonic currents are present.
If possible, convert 120 volt single phase loads to 240 volts to help reduce NEV caused by secondary neutral current.
Install an equipotential plane, ground ring or additional ground rods as the situation dictates. Equipotential planes can help by keeping conductive materials in a given area at, or near, the same potential. This is a must where farm animals are present. Ground rings can be used to intercept ground currents and divert them directly to the system neutral. Additional ground rods may reduce NEV by lowering the net resistance to neutral currents.
Stray voltage issues can be difficult to diagnose and solve. Additional information is available in EPRI Report TR-113566, dated September 1999 and titled “Identifying, Diagnosing and Resolving Residential Shocking Incidents” or the “Agricultural Wiring Handbook Eleventh Edition 1996”, published by the National Food and Energy Council, Columbia, Missouri.
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 and/or utility personnel for all wiring projects.