Rural Power Systems
|A typical rural power system, in its last few miles to the farmstead, is a single-phase line, one of the branches of a three-phase power delivery system. The single-phase line is made up of a phase wire energized at a high electric potentialand insulated from the earth, and a neutral conductor, which is at earth potential because it is connected to the earth.|
|Near the farmstead, the phase wire is connected to one terminal and the neutral wire to the other terminal on the supply side of the transformer. The purpose of the transformer is to lower the voltage to safer, more practical levels. This ability to reduce voltage to usable levels is a major benefit of alternating current systems.|
|Coming out of the transformer is the standard three-wire 120/240-volt service found in most homes. The service wires are typically run to an outdoor utility meter installed at a key location on a farmstead to measure electric power usage. From there, the wiring typically continues to a disconnecting panel from which all circuits feeding the farmstead originate.|
|Two Current Loops|
|In order for any electrical system to operate, there must be a closed electric circuit loop. At a farm, there are two basic loop circuits involved, each with its own current: the high-voltage circuit (primary) on the supply side of the transformer and the low-voltage circuit (secondary) on the utilization side. The high-voltage circuit is physically isolated from the low-voltage circuit. Inside the utility transformer, the two circuits are magnetically coupled, allowing electric energy to be transferred by means of a magnetic field. The current on the primary circuit is separate from current(s) on the secondary circuit, although the two currents do mix on grounded conductors and in the earth.|
|An external connection is intentionally established between the primary neutral conductor of the high-voltage circuit feeding the transformer and the secondary neutral conductor of the low-voltage circuit coming out of the transformer. This allows protective devices such as fuses to de-energize the high-voltage circuit in the event of a fault inside the transformer (high-voltage wires coming into contact with low-voltage wires). If not for the connection between the primary and secondary neutrals, the low-voltage circuit could become energized at a high voltage. It is a widely used and time-proven construction practice required by electrical code. It protects people and animals from harm in case of system breakdown.|
|The neutral interconnection wire creates a network between the neutral and the earth that extends across the transformer as shown in the diagram below.|
|This network allows current to cross over from one neutral conductor onto the other. This intermingling makes tracing and learning about earth currents more complicated, as we shall see later [Intermingling].|
|Stray Voltage Section Outline|
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.
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.
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.
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.