Where Does Grounded Electricity Actually Go?

Electricity does not take the path of least resistance; rather, it takes all paths available in accordance with their relative conductivity, meaning that current distributes proportionally across multiple paths.

“You may have heard that electricity takes the path of least resistance, but that's not really true. Electricity takes all the paths it can in accordance with their relative conductivity.”

Whether current flows from a grounded generator into the earth depends entirely on the mental model one holds about what the earth represents in an electrical circuit.

“Your answer depends completely on your mental model of what the earth represents in an electrical circuit. After all, the idea of a circuit is just an abstraction of some really complicated electromagnetic processes, and that's even more true on the grand scale of the power grid.”

Power line technicians are often encouraged to hop on one foot away from a ground fault to reduce the chance of step potential, because hopping minimizes the voltage difference between the two feet and thus reduces the current flowing through the body.

“In fact, power line technicans are often encouraged to hop on one foot away from a ground fault to reduce the chance of step potential. It sounds silly, but it might save their life.”

DC electrode systems create magnetic fields that can affect compass readings and magneto-sensitive fish like salmon and eels, and ocean electrodes can cause electrolysis, breaking down seawater into toxic chemicals like chloroform and bromoform.

“They create magnetic fields that can affect compass readings and magneto-sensitive fish like salmon and eels. In ocean electrodes, the current can cause electrolysis, breaking down seawater into toxic chemicals like chloroform and bromoform.”

It is not possible to identify specifically which power plant serves a particular city or building because power gets intermingled on the grid, flowing along transmission lines according to the difference in electrical potential rather than following fixed routes.

“A common question I get about the electrical grid is how to know specifically which power plant serves a city or a building. It's kind of like asking what tree or plant created the oxygen that you breath. Technically, it's more likely to be one close to you than very far away, but that's not quite how it works. Power gets intermingled on the grid - that's why it's called the grid in the first place - and it just flows along the lines in accordance with the difference in potential.”

Stray electrical currents flowing into pipelines and other buried structures through ground return paths can cause corrosion, and return currents from traction motors in electric trains can flow into ground and corrode adjacent pipelines while occasionally interfering with buried telecommunication lines.

“And, stray electrical currents in the ground can flow into pipelines and other buried structures, causing them to corrode. This is also a problem with some electric trains that use the rail as a return path. You may have heard that electricity takes the path of least resistance, but that's not really true. Electricity takes all the paths it can in accordance with their relative conductivity. So, even though a big steel rail is a lot more conductive than the earth, return current from traction motors can and does flow into ground, sometimes corroding adjacent pipelines, and occasionally interfering with buried telecommunication lines too.”

Current in grounded systems flows through the ground (the earth itself) as a return path, not into the ground as a sink; the ground is essentially just another wire, albeit a high-resistance one compared to copper or aluminum conductors.

“as soon as you add a phase-to-ground short circuit, the diagram looks much different... there's now a path for fault current to flow through the ground back to the source. And that's the answer to the question in the title of this video: electrical current (in nearly all cases) doesn't flow into the earth; it flows through the earth. The ground is really just another wire.”

In a three-phase AC power system, during a ground fault (such as a tree branch knocking down a power line), the weak electromagnetic coupling that normally keeps phase-to-ground voltages balanced is overpowered, causing all phases to experience a voltage shift with respect to ground, though phase-to-phase voltages remain unchanged.

“Watch what happens during a ground fault. This could be a tree branch knocking down a power line or a conductor being blown into contact with a steel tower or any other number of problems that lead to a short between one phase and ground. Now, all of the sudden, that weak coupling force keeping the phase-to-ground voltages balanced is overpowered, and all the phases experience a voltage shift with respect to the ground. But, the phase-to-phase voltages don't change.”

Engineers designing power plants, substations, and transmission lines must calculate the maximum safe touch potential and step potential limits for humans and design grounding systems to ensure these limits are never exceeded.

“The engineers who design power plants, substations, and transmission lines have to consider what touch potentials and step potentials can be safely withstood by a person and design grounding systems to make sure that they never exceed that level.”

The Pacific DC Intertie carries power from the Pacific Northwest into Los Angeles and is equipped with elaborate grounding systems: in Oregon, over 1000 electrodes are buried in a ring with a circumference of 2 miles (3.2 kilometers), and in California, huge electrodes are submerged in the Pacific Ocean a few miles offshore.

“For example, the Pacific DC Intertie that carries power from the pacific northwest into Los Angeles has elaborate grounding systems at both ends. In Oregon, over 1000 electrodes are buried in a ring with a circumference of 2 miles or 3.2 kilometers. In California, the grounding system consists of huge electrodes submerged in the Pacific Ocean a few miles off the shore.”

Close to a ground electrode, the concentric shells are spaced tightly together toward a single point or line, creating high resistance; this spacing is related to soil resistivity and represents a serious safety issue.

“But up close to the electrode, the shells are spaced tightly together toward a single point or line. That spacing is related to the resistance of the soil, and it can represent a pretty serious safety issue.”

Even bipolar (two-conductor) high voltage DC transmission lines often include grounding systems so they can use ground return during an outage or emergency if one pole is out of service.

“even the bipolar lines often include grounding systems so they they can use ground return during and outage or emergency if one pole is out of service.”

In small, low-voltage devices like battery-powered toys, the difference in electrical potential between components and the actual earth is not important, but this is not true for high-voltage systems connected to the grid because the phase-to-ground voltage differences become significant safety and cost concerns.

“On small, low voltage devices (like battery powered toys), the difference in potential between components on the circuit board and the actual earth isn't all that important, but that's not true for high voltage systems connected to the grid.”

In appliances with metal casings, current flowing from a live wire into the metal case creates a hazard, but adding a grounding conductor (parallel low-resistance return path) to the housing allows fault current to flow at high levels, triggering a breaker to shut off the circuit and preventing fire or electric shock.

“Under normal conditions, current flows from the live or hot wire through a heating element and into the neutral wire to return to the grid, completing the circuit. But, if something comes loose inside the toaster, the live or energized side of your electrical supply could come into contact with that metal case, making it energized too. This could start a fire, or in the worst case, shock someone who touches the case. So, many appliances are required to have another conductor attached to the housing, giving the current a parallel, low-resistance return path. That low resistance means lots of current will flow, triggering a breaker to shut off the circuit.”

Protective relays that monitor parts of the power grid detect problems by measuring fault current, and nearly all rely on fault current being much higher than normal electrical loads to distinguish between normal operation and short circuits.

“Nearly all the protective devices, called relays, that monitor parts of the power grid for problems rely on fault current to tell the difference between normal electrical loads and short circuits. The simplest way to do that is make sure the fault current is much higher than the normal loads.”

Most substations are equipped with a grid of buried conductors rather than a single grounding electrode to minimize resistance in the earth connection, reducing voltage gradients and thus step and touch potentials.

“For example, most substations are equipped not just with a single grounding electrode but a grid of buried conductors to minimize resistance in the earth connection.”

In an isolated generator connected only to a grounding electrode with no other circuit completion, current will not flow into the ground because there is no return path to the source.

“Imagine this scenario: You have a diesel-powered generator on a stand that is electrically isolated from the ground. Run a wire from the energized slot of an outlet to an electrode driven into the ground. Don't connect anything to the ground or neutral slots. Now imagine starting the generator. What happens? Does current flow from the energized wire into the ground or not?”

Lightning is a type of static electricity that flows out of the ground into the atmosphere (most frequently) or into the ground from the atmosphere, restoring an imbalance of charge; the mechanism is not fully understood.

“Unlike a conventional circuit where current is alway moving, lightning is a type of static electricity. It's not flowing… until it is. And unlike fault current that only uses the ground as a conduit, the current from a lightning strike really does just flow into the ground, or most frequently, out of the ground and into the atmosphere, restoring an imbalance of charge created by the movement of air or water… or something else. We really don't understand lightning that well.”

Adding salt water to saturated soil dramatically increases its electrical conductivity, allowing significant current to flow, because electrolytes in the salt water enable the soil to conduct electricity more effectively than pure water.

“If I add just a little bit of salt water to that standing water, immediately you see that the resistivity goes down and the lightbulb is able to light. And if I let that salt water soak into the soil, now the sand is able to conduct electricity too.”

Dry sand is a poor conductor of electricity, and soil and rock conductivity varies widely based on soil type, season, weather, temperature, and moisture content, making the resistance of the earth connection highly variable and dependent on local conditions.

“It turns out that dry sand is a pretty good insulator. In fact, soil and rock vary widely in how well they conduct electrical current. The resistivity changes with soil type, seasons, weather, temperature, and moisture content.”

Voltage is not a property of a single wire or location, but rather the difference in electrical potential between two points, which is why engineers arbitrarily assign a common reference point (ground) zero potential for measurement convenience.

“That's one thing that can be confusing about voltage: it doesn't actually refer to a single wire or trace or location, but the difference in electrical potentials between two points. For convention, we pick a common reference point, assume it has zero potential to make the math simple, and call it ground, even if there's no reference to the actual ground below our feet.”