STRAY VOLTAGE: KNOW YOUR TERMS
CONFUSION ABOUT THE TERM STRAY VOLTAGE IS A SERIOUS CONCERN. Historically, the term was associated with an elevated voltage that is difficult to mitigate but is not a threat to human life. These days, however, usage of the term is associated with much higher, dangerous voltages that can easily be corrected when discovered.
The issues associated with distribution system grounding, in particular stray voltage, have caused much confusion recently. Using the historic definition of the term, stray voltage has been a relatively dormant issue; it was associated with low-voltage magnitudes not considered dangerous. But recent events have caused confusion about the usage of this term, and the result is that some are now associating stray voltages with lethal voltages, those that could cause death.
GROUNDING MATTERS
There are many ways to ground the primary distribution system: the four-wire multi-grounded, the five-wire system, the four-wire uni-grounded, the three-wire uni-grounded, and the ungrounded wye and delta systems. There is no best system; they all have pros and cons.
The four-wire multi-grounded system used in the United States is a good choice. But, like most choices, one must be cognizant of the characteristics of this grounding system, one of which is that unbalanced currents flowing through the neutral and earth can cause stray voltages.
WHERE TERMS ORIGINATE
The IEEE Guide for the Application of Neutral Grounding in Electrical Utility Systems, Part IV — Distribution (IEEE C62.92.4-1991), is primarily concerned with the classification of distribution system grounding as it relates to temporary overvoltages and the selection criteria for surge arresters. In the appendix, this guide covers techniques for the interconnection of primary and secondary neutrals, which mitigate stray voltages. It is important to note that the term stray voltage is never specifically addressed. Historically, stray voltage was a concern on farms, where milking machines in the presence of stray voltages caused microshocks that resulted in decreased milk production and other behavioral abnormalities in cows.
Special isolating devices have been available for quite some time to isolate transformer primary and secondary neutrals to prevent stray voltages to the customer neutral from the utility system. While the IEEE guide recognizes the issue of stray voltage, it does not address such issues as how much of it should be allowed and how to mitigate it.
GROUND IMPACT
The statement that “good grounding solves all power-quality problems including stray voltage” belongs with the statement “lightning never strikes twice in the same place.” Tell that to workers at the Empire State Building. Without several relevant qualifiers, both of these statements are nonsense. In the case of grounding, sometimes good grounds are beneficial, sometimes they are a problem source and sometimes they make little difference — it all depends. Following is the impact of grounds on:
- Fault currents. Good grounding has little effect on the magnitudes of fault currents.
- EMF. Bad grounding reduces EMF.
- Line protection (using arresters). Good grounds are not required.
- Shield wire protection. Good grounding is required.
- Lightning arresters. Although it depends on the application, good grounds are generally not required.
- Temporary overvoltages. For a four-wire multi-grounded system, the impact of ground rod resistance is relatively minimal.
- Stray voltage. Good grounds can improve stray voltage but also can make it worse. Also, better grounding to resolve stray voltage issues may not be an effective solution.
CORRECT TERMINOLOGY
The term stray voltage is becoming all things to all people. The following are terms often interchanged with the term stray voltage, some of which are incorrect and causing a lot of the present confusion:
As generally defined by utility engineers, stray voltage refers to the persistent voltage imposed on the distribution primary neutral. Stray voltages are mostly due to return currents from unbalanced loads. This is a normal condition of a four-wire, multi-grounded system. In the context of the last 40 years, this voltage is associated with problems in dairy farms and, generally, the voltages do not exceed about 8 V. Stray voltages are not lethal.
Transient overvoltages (TOVs) are sometimes referred to as stray voltage, but they are not. TOVs are 60-Hz line-to-neutral overvoltages that occur on the unfaulted phases of a four-wire multi-grounded system during a fault (Fig. 1). As the nomenclature implies, TOVs are transient or temporary in nature. They are not persistent. A single line-to-ground fault creates a shift in the neutral voltage of approximately 35%. While this voltage is relatively high, it only lasts as long as it takes the protective equipment to clear the fault (normally in tens of cycles, usually less than a second). This is not stray voltage.
This term is normally used to address the condition in which the “hot” lead (120 V or more) contacts the casing of a device, such as a streetlight. This situation can be dangerous and possibly result in death. Contact voltage is not stray voltage, although it is sometimes misused in this context.
HARD TO IDENTIFY
Stray voltage is caused by voltage drop and ground currents that could have their origin either on the utility system or the customer premises itself. The problem can be very difficult to analyze since the return path of the unbalanced currents is especially complex. In many circumstances, system changes to mitigate the problem can cause the opposite effect. Throughout the years, the greatest interest in stray voltage has been in the area of dairy farming, because cows are sensitive to microshocks caused by stray voltage, which can affect milk production.
The path of unbalanced current flow on a distribution system is not obvious. One thing that greatly complicates an accurate model is that the loads are distributed, making the flow of current between the neutral and earth very complex.
Figures 2 and 3 illustrate the typical effect of unbalanced current flow on stray voltage. Figure 2 shows that the stray voltage level at substation is high, as it also is at the end of the feeder. There are two interesting points to make. First, the stray voltages near the substation are of opposite polarity to those at the end of the line (current reversal), and the voltages in the middle of the feeder are relatively low. Also, it is interesting to note that if the substation ground is good (1 V), the situation gets worse in some areas and better in others.
Figure 3 shows the effect of changing the system pole ground rod resistances from 5 Ω to 50 Ω. As can be seen, stray voltages are reduced, but not as much as one might think. In the areas with the highest stray voltage, the benefit of improving grounding is questionable.
RECENT EVENTS
Many in the electric utility industry are concerned that state regulatory bodies (public service commissions or public utility commissioners), which are not necessarily technical in nature, will define issues and set arbitrary standards.
For example, one state commission has taken the instance of a tragedy to misuse the term stray voltage. Furthermore, this commission has suggested testing standards that have not been reviewed by objective technical experts as being either necessary or effective. The tragedy, as we understand it, resulted from what could only be a contact voltage, and not at all a stray voltage. Our concern is that a regulation issued by one governing agency that misuses these terms will eventually affect other jurisdictions. Some states have set low limits on stray voltage that utilities in other states don't believe are possible to meet.
DETERIORATED CABLE NEUTRALS
Synergetic Design (Raleigh, North Carolina, U.S.), which operates in 18 states providing technical services to large investor-owned municipals and electric membership cooperatives, recently performed a survey related to the impact of exposed deteriorated neutrals on stray voltage levels found in primary cable. The results show that many utilities have switched to jacketed cable to avoid this condition in the future.
About 20% of the respondents were concerned with deteriorated neutrals causing shocks on such things as swimming pools, fences and water faucets. One utility indicated it drew arcs during the repair of a corroded neutral, even after the load was transferred. About 25% of the respondents indicated they had experienced shocks directly related to deteriorated neutrals. Figure 4 shows responses to the survey question about the level of stray voltages recorded in areas with deteriorated neutrals.
FEEDER INTERACTION
In many cases, the problem of locating a source of the stray voltage is extremely complicated due to additional feeders in the same area. A portion of the neutral current returns to the substation via the ground rods and the earth. These currents also may return on additional feeder neutrals, should such a return path have lower or comparable impedance.
Reducing the impedance of the ground rods has additional effects. The impedances of the ground rods at specific areas on a feeder are critical to the magnitude of the stray voltage. Lowering the ground rod impedance initially appears to be a solution for reducing the neutral-to-earth potential at a specific location. However, this also provides a lower impedance return path for any return current in the area. This presents the possibility of increasing the neutral current at the location of the reduced impedance ground rod. This reduction in the ground rod impedance in one problem area can increase the stray voltage in other areas on the feeder.
INDUSTRY INVOLVEMENT
The subject of stray voltage, once only mentioned in relation to dairy farms, has become an issue in the electric utility industry for other reasons, not the least of which is safety. The impact of confusing the term stray voltage with higher abnormal and potentially dangerous contact voltages will likely cause considerable cost with virtually no benefit.
Additionally, the distraction of this confusion and unnecessary testing will likely siphon funds away from other projects that have more safety value for the customer. The general reduction of industry participation in the standards-writing function of IEEE has created a situation in which state commissions and lawyers — without the participation of industry experts and standards groups — may end up dictating stray voltage standards that are entirely incorrect. They could also mandate safe levels of stray voltage that would virtually be impossible to attain. More attention to this area is needed by our industry.
Provided By:
Jim Burke is a fellow of the IEEE and an executive consultant with Synergetic Design in Raleigh, North Carolina, U.S. He was chair of the IEEE Working Group on Distribution Neutral Grounding, which developed the present guide in 1991.
jimburke@synergeticdesign.com
Keary Dosier is an engineer with Synergetic Design. He has been working in the area of stray voltage for more than a year. He completed his degree in electrical engineering at North Carolina State University in December 2005.
krdosier@bellsouth.net
