Trying to educate people on the proper words to describe power quality phenomena has been an endless battle since the IEEE Std 1159 first came out in 1994. Referring to a voltage transient as either a spike or surge is still common. The same is true with regard to stray voltage and contact voltage. IEEE 1695-3025, Guide to Understanding, Diagnosing and Mitigating Stray and Contact Voltage, hasn’t prevented the misuse of stray voltage.
Per IEEE 1695, stray voltage results from the normal delivery or use of electricity that may be present between two conductive surfaces and that can be simultaneously contacted by people or animals.
Issues with changes in milk production on dairy farms have been attributed to stray voltage for years. Stray voltages are usually at low-voltage levels and not related to electrical faults. Even when all of the wiring and delivery of electricity is according to the National Electrical Code and National Electrical Safety Code, a perceptible voltage may be present. Harmonic currents in the neutral, especially when excessive triplen harmonics are present, can raise the neutral voltage from its usual “close to zero” potential.
Contact voltage results from electrical faults between two conductive surfaces that a person or animal contacts simultaneously. Contact voltage can exist at levels that may be hazardous. Reports of injuries and fatalities from people and animals touching light poles or metal grates in cities (especially male dogs when relieving themselves) are instances of contact voltages.
The following case study was presented by a construction worker who isn’t an electrician but assists ECs on jobs. When she first encountered the incident, she began inquiring into what happened to prevent a repeat.
A case study
A single-family residence built in 1956 has a four-bay detached garage with a metal chain link fence running alongside the garage up to the rear of the house, where there is a gate to allow access into the backyard and rear door. Mr. A, the homeowner, was working in the garage with electrical equipment, including a compressor. Ms. B, the woman who lived there, opened the gate from the driveway. When one hand was on the fence post and the other on the gate, she was electrically shocked, resulting in her body stiffening up and then being thrown to the ground. Fortunately, she was able to get up and enter the house before going to the hospital to be examined.
As is often the case, answers are derived before all the facts are understood, and misconceptions must be eliminated. A voltmeter reading across the two metal parts where Ms. B placed her hands confirmed significant voltage, which went away when the compressor in the garage was de-energized. Further tests by the electrician confirmed the 240V compressor had a ground fault.
It was incorrectly assumed that the meter on the side of the garage was provided with a separate service from the utility company. The utility service went to the house, and a BX cable in metal conduit traveling in close proximity to the fence provided the service to the 60A subpanel in the garage.
With no dedicated grounding conductor, the integrity of the conduit and armor covering were highly compromised, making an inadequate grounding path back to the bonding point in the main service panel.
Though the exact electrical path between the ground-faulted compressor and the metal fence is yet to be confirmed, it does explain what happened to Ms. B, and not to Mr. A when he touched the fence with one hand. When she opened the gate, the side of the fence that was no longer connected to the energized portion of the fence originating from the garage returned to a ground potential. By having one hand on the energized side and one on the ground potential side, the voltage was across Ms. B’s body, and enough current flowed through her for a substantial shock.
So what happened?
Why didn’t a breaker or GFCI trip? The latter question was obvious: the garage had no GFCIs. The 5-mA trip level of a GFCI would have prevented Ms. B’s shock. The fault current wasn’t high enough to trip the 20A breaker. Another code issue was the grounding electrode outside the garage, as well as the grounded conductor (neutral) and grounding conductor being bonded in the subpanel, though it was fed from a house service where the bonding and grounding should have happened.
The homeowners agreed that the garage electrical circuits and its feed from the house need to be brought up to code. This contact voltage incident could have had a more serious outcome if a child had been the victim of what was initially called stray voltage. Perhaps we should be more concerned with educating the public and contractors on how to best prevent harm from electricity, rather than what the proper label is.
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About The Author
BINGHAM, a contributing editor for power quality, can be reached at 908.499.5321.