Coupled voltages Ungrounded metal objects close to
electric field sources such as neon signs or conductors carrying
alternating currents may have measurable voltage levels caused by
capacitive coupling. Since voltages detected by high-impedance instruments disappear or become greatly reduced when a low impedance is substituted, the effect is sometimes called
phantom voltage (or
ghost voltage). The term is often used by electricians, and might be seen, for example, when measuring the voltage at a lighting fixture after removing the bulb. It is common to measure phantom voltages of 50–90 V in testing the wiring of ordinary 120 V circuits with a high-impedance instrument. The voltage produced may read almost to the full supply voltage, but the
capacitance or
mutual inductance between the wires of
building wiring systems is typically quite low and incapable of supplying significant amounts of
current. However, in
overhead transmission work on or near high-voltage lines, safety rules require connecting a conductor to
earth ground during maintenance. That is since induced voltages and currents on a conductor may cause electrocution or serious injury.
Capacitive leakage through insulation Alternating current is different from direct current in that the current can flow through what would ordinarily seem to be a physical barrier. In a
series circuit, a
capacitor blocks direct current but passes alternating current. In power transmission systems, one side of the circuit, known as the
neutral, is grounded to dissipate
static electricity and to reduce hazardous voltages caused by insulation failure and other electrical faults. Even a person standing on an insulated surface may get a shock only by touching the
hot wire because of the person's body being capacitively coupled to the ground upon which the person stands.
Induced voltages Classical
electromagnetic induction can occur when long conductors form an open
grounded loop under and parallel to transmission or distribution lines. In those cases, current is induced in the loop when a person makes contact with it and ground. Since this involves real current flow, it is potentially hazardous. This type of induced current occurs most often on long fences and distribution lines built under high-power
transmission lines.
Degraded insulation on power conductors Stray voltage may leak via damaged or degraded insulation. Failing insulation is essentially a high impedance fault which will allow current to flow through any available path to ground, a condition which can cause shocks or fires if left unmitigated. This leakage can occur when there is damage caused by physical, thermal, or chemical stresses to insulation on power lines, especially but not limited to underground or underwater cables. Examples of this damage are swollen or cracked insulation from overheating, abrasions caused by digging or ground seizing, and corrosion damage from salt or oil exposure. Electrical leakage can occur also from moisture, salt, dust, and dirt buildup on open air insulators in overhead power distribution. If the leakage in these cases is severe enough, it can lead to a utility pole fire.
Leakage from single-wire earth return The term "stray voltage" is used for the gradient (rate of change with respect to distance) of
electrical potential in the surface of the soil, associated with
single-wire earth return electricity distribution systems used in some rural locations. This gradient is low at points far away from the earth return connections, but increases near the ground rods where the metallic circuit enters the earth.
Neutral return currents through the ground In
three phase four-wire ("wye") electrical power systems, when the load on the phases is not exactly equal, there is some current in the neutral conductor. Because both the primary and secondary of the distribution transformer are grounded, and the primary ground is grounded at more than one point, the earth forms a parallel return path for the neutral current, allowing part of the neutral current to continuously flow through the earth. This arrangement is partially responsible for stray voltage. Stray voltage is a result of the design of a 4 wire distribution system and as such has existed as long as such systems have been used. Stray voltage became a problem for the dairy industry some time after electric milking machines were introduced, and large numbers of animals were simultaneously in contact with metal objects grounded to the electric distribution system and the earth. Numerous studies document the causes, physiological effects, and prevention, of stray voltage in the farm environment. Today, stray voltage on farms is regulated by state governments and controlled by the design of
equipotential planes in areas where livestock eat, drink or give milk. Commercially available neutral isolators also prevent elevated potentials on the utility system neutral from raising the voltage of farm neutral or ground wires.
Railway stray current Typically a rail transit systems will have at least one of the rails as a return conductor for the traction current. This arrangement is common, based on economic considerations, since it does not require the installation of an additional return conductor. This rail is in contact with the earth at many places throughout its length. Since current will follow every parallel path between source and load, some part of the traction current will also flow through the earth. This is normally referred to as leakage current or stray current. The amount of leaking current depends on the conductance of the return tracks compared to the soil; and on the quality of the insulation between the tracks and soil. Where the railway uses
direct current, this stray current can cause damage to other buried metallic objects by
electrolysis and accelerate corrosion of metal objects in contact with the soil. ==Stray Voltage Effects==