Equipotential Planes

Rural Power Systems

 Two Current Loops
 Neutral Interconnection
  Stray Voltage Section Outline
  1. Voltage Gradients in/on the Earth [Gradient Gd]
    1. What is a Gradient? [What is Gd?]
    2. Spreading Gradients in Two-Rod Study Case [Spreading Gd]
    3. Field Gradients in Two-Rod Study Case [Field Gd]
    4. Earth Surface Voltage Gradients Experienced As Stray Voltage [Gd Causes SV]
  2. Origins of Stray Voltage [SV Origins]
    1. Rural Power Systems [Power Systems]
    2. Supply-Side Gradients [Supply Side Gd]
      1. Supply-Side Field Gradients [Field Gd]
      2. Supply-Side Spreading Gradients [Spreading Gd]
      3. Combined Supply-Side Gradients [Combined Gd]
    3. Utilization-Side Gradients [User Side Gd]
      1. Ground Rod Gradients [Ground Rod Gd]
      2. Pipe Gradients [Pipe Gd]
      3. Combined Utilization-Side Gradients [Combined Gd]
    4. Supply-Side and Utilization-Side Gradients Together [Both Gradients]
    5. Intermingling [Intermingling]
    6. Differences Between Current Sources [Differences]
    7. Neutral-to-Earth Voltage [Neutral Voltage]
  3. Stray Voltage Exposure Prerequisites [Exposure Vital]
  4. SV Contacts [SV Contacts]
    1. Step Voltage [Step Voltage]
    2. Touch Voltage [Touch voltage]
    3. Avoidance [Avoidance]
  5. Stray Voltage vs. Time [Time Factor]
    1. Long-Term Variability (Steady State) [Long-Term]
    2. Transients [Transients]
  6. Definition of Stray Voltage[SV Definitions]
    1. Definitions By Authorities [by Authorities]
      1. Public Service Commission of Wisconsin (1989) [PSCW – 1989]
      2. U.S. Department of Agriculture (1991) [USDA – 1991]
      3. Science Advisors to Minnesota Public Utilities Commission (1996) [Minnesota – 1996]
    2. Key Points in the Definitions [Key Points]
      1. Small Voltage [Small Voltage]
      2. Simultaneous Contact [Contact]
      3. Current Flow [Current Flow]
    3. Uses of the Term [Extras]
      1. Humans vs. Cows [Humans]
      2. Non-Power Sources [Other Sources]

Source: http://www.lanera.comm/stray_voltage


Effects on the human body

The human body may be considered a good electrical conductor. In a variable field (for example sinusoidal at a 50 Hz frequency as that produced by the power grid), the body is traversed by a current at the same frequency as that of the ambient field.

Induced currents by MF and EF

The external variable electric field hardly penetrates the body: for an external electric field of a few kV/m, only a few mV/m develops in the body. It mainly causes a charge migration at the body surface. The result is thus a surface current and a residual current through the body.

The external variable magnetic induction field is only lightly disturbed by the body’s presence. Current loops appear in the body that attempt to cancel the external field. In normal conditions of exposure to the 50 Hz magnetic field, these induced currents are well below the natural currents in the body (called “endogenous currents”): for an external field of 0.15 mT, the induced current is about 5000 times smaller than the endogenous currents (*).

(*) The low frequency fields exposure standard setting criterion is the avoidance of induced current densities that are higher than those naturally present in the body. In 50 Hz, the maximum exposure limit is 10 mA/m² for workers and 2 mA/m² for the general public.

For further information on this subject, you may refer to the module “Health effects?” under the “Health” tab on our website.

Perception of 50 Hz fields

As we just saw, only very weak currents are induced in the body by an electric field. In general, these currents are not felt as they are not intense enough to excite nerve cells and muscles. The perception threshold varies from individual to individual.

Perception threshold of 50 Hz fields

When the electric field exceeds a 20 kV/m threshold, we perceive a light tingling of the skin and have goose bumps. The phenomenon is called piloerection. It is similar to what happens in the hair-raising experiment in static electricity.

Under certain conditions, we may also indirectly perceive the electric field:

1) Feeling a small electric shock when touching a mass isolated from the ground that is located below high voltage power lines for example: it is due to capacitive coupling.

capacitive coupling

The car below the high voltage power line is subjected to an electric field: it does induce a displacement of charges. The car acquires a certain potential, different than that of the approaching person.

When he touches the metallic surface, the two potentials will even out (*). The electric shock may be unpleasant, but it is not dangerous.

It is this type of phenomenon to which animals drinking at a trough under a high voltage line are subjected. The solution consists of correctly grounding the trough

(*) This phenomenon may look like an electrostatic discharge, but the amount of current and the duration of the discharge are vastly different.

2) Lighting of fluorescent tube

When approaching a fluorescent tube towards the conductors of a high voltage power line, the tube lights up, albeit weakly. Why? The electric field induces a voltage in the tube which excites the gas inside, leading eventually to the emission of light.

Note: The fluorescent tube operating principle is described in the module “Uses of electricity”.

3) Corona discharge noise

Corona discharges are phenomena associated with very strong electric fields. They are manifested as a luminous halo around high voltage overhead power lines under certain conditions (*). These discharges are the cause of sometimes unpleasant noise.

(*) The presence of small protuberances on the surface of conductors, for example a drop of water or snow flakes, or even an insect, cause large increases of the electric field. The corona effect varies drastically in function of the conditions of the external surface and of the atmosphere.

Exposure to a magnetic field similarly induces only very weak currents in the body. At the commonly encountered magnitudes, they are imperceptibles, just like those induced by electric fields.

Only an exposure to much stronger magnetic fields can lead to detectable effects. For example, when subjected to a magnetic field of 10 mT at 50 Hz (that is about 1000 times the maximum seen under a high voltage power line), flashing lights appear in the field of vision. They are called magnetophosphenes. These flashing lights are caused by induced currents at the retina level. These microcurrents disappear as soon as the exposure ends. There is no accumulation of this effect with repeated exposure as is the case with X rays for example.

Under certain conditions, we may also indirectly perceive the magnetic field; particularly notable are interferences from electrical appliances:

Normal operation of an electrical appliance may be disturbed by the electromagnetic field generated by another nearby electrical apparatus. These disturbances are called electromagnetic interference. To avoid difficulties caused by interference, electromagnetic compatibility rules governing electrical equipment must be observed.


It is important not to confuse biological effects and interferences of an electromagnetic field with some electronic device. Some materials are very sensitive to low frequency magnetic fields. For example, the cathode ray tube computer screen can be perturbed by a magnetic field as low as 1 µT. The interference is due to the refresh rate of the screen display which is close to 50 Hz.

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