We will, for brevity, assume that the reader possesses an elementary understanding of basic electrical theory. That is to say, that electromotive force, or difference of potential is conventially expressed in volts, the rate of flow of electrons is expressed in amps, and the resistance to this flow is expressed in ohms. Finally, that the mathematical expression of this relationship called Ohm's Law, states that the electromotive force in volts equals the product of the rate of flow in amps, and the resistance as measured in ohms.
As high voltage electrical workers, we commonly encounter utility lines that transmit thousands of volts. These "distribution lines" would be like the power lines within a city, or even in a residential neighborhood. However, when bulk power must be transmitted in large quantities over very long distances, a "transmission line" is used instead. These are generally the very tall poles or steel towers that may be observed spanning across open country, though in large metropolitan or industrial areas, they may be located more locally. In any case, these transmit electricity along the order of hundreds of thousands of volts and thousands of amps. For simplicity in civil planning, these lines may run parallel to one another for dozens or even hundreds of miles.
Maintenance may be performed on these lines in two ways; either energized or de-energized. Energized work is highly specialized, and is used for only small repairs. Major overhauls, such as replacing electrical conductors must be performed while the line is de-energized. However, it is not as simple as opening a circuit breaker.
The problem arises from the nature of alternating current. Because voltage is "induced" into any conductor that is exposed to a moving magnetic field, a line that has been removed from a source of power (like a generator) may still possess current and voltage in substantial quantities. Therefore, in order for the worker to safely make contact with this conductor, its power must somehow be "dissipated". This is the function of a procedure called "grounding", so named for the universal electrical connection to Earth. A source of electrical energy connected to "ground" or earth. At this point, the voltage is considered to be effectively zero, because everything is attached to the earth by gravity. The line is then considered "dead". Any transient current is dissipated safely into the earth. It is then safe to work on the line.
Except for the times that it isn't.
The problem is, when a line is receiving electrical energy from a source like induction, the ground wires may be carrying several amps of current constantly. Ohm's law states that voltage is equal to the product of amps and resistance in ohms. While variable, it is common for substrates to have hundreds of ohms of resistance per foot. This means that with, for example, one amp being dissipated into the earth, it is possible to have a voltage present equal to the product of amps (1) and resistance, like 100 ohms: 100 volts. This voltage may be experienced in the distance of one foot to another.
This hazard is known as ground rise potential. It basically says that, at the point where current enters the ground, (especially in high amounts) everything is not equally grounded. Rather, it is not at the same electrical potential. Instead, the area near the point of current injection must, by ohms law, possess voltage relative to another point across which there is resistance. After all, if there is current flow, and there is resistance to flow, there must already (or also) be electromotive force, (voltage) equal to the product of these two.
In the real world, this only presents a hazard when large amounts of power are present. Something like a downed power line that remains energized can produce energy on this scale. Stepping out of a vehicle that is in contact with an energized conductor is a classic example. The difference in voltage from one step to another is called "step potential".
This is also why lightning can affect a person without actually striking him. Incidentally, that is why standing next to a tree in an electrical storm is ill-advised. While the person and the tree are both grounded, and at theoretically the same potential, they may be separated by hundreds of ohms of soil. At thousands of amps of current, the product is a very high voltage.
This was the most important benefit that I received from the training. It is essential to be cognizant of circulating current and the resistance of a grounding substance, because therein lies the key to avoiding the hazards present by voltage that may be present, even (and especially) near ground wires.
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