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? 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. Grounding is one of the most confusing
and misunderstood aspects of the grid, so you can be pardoned for being a little perplexed.
For example, if I run a wire from the positive side of a battery into the ground,
nothing happens.
But, when an energized power line falls from a pole, there’s definitely current flowing into
the ground then. Cloud-to-ground lightning strikes move huge electrical currents into or out of the
earth, but my little thought experiment of a generator connected to a grounding electrode
won’t create any current at all. I’ll explain why in a minute. Even on a electrical diagram,
ground is just this magical symbol that hangs off the circuit willy nilly. But, connections between
an electrical circuit and the ground serve quite a few different and critical purposes. And I have
some demonstrations set up in the studio to help explain. I think you’re going to look at the power
grid in a whole new way after this, but just don’t try these experiments at home. I’m Grady
and this is Practical Engineering. In today’s episode, we’re talking about electrical grounding.
Why do we ground electrical circuits in the first place? Maybe the easiest way to answer
that question is to show you what happens when we don’t. For as much importance as it gets in the
electrical code, it might surprise you that it’s not always such a big deal, and in some cases,
can even be beneficial. After all, lots of small electrical circuits lack a connection
to the ground, even if part of the circuit is literally called, “ground.” In that case,
that term really just refers to a common reference point from which voltages are measured. 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. 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. Let me show you why:
This is a diagram of a typical power system on the grid. The coils of a generator are shown on
the left. When a magnetic field rotates past these coils, it generates electric current on
the conductors, and (very generally) this is how we get the three phase AC power that is
the backbone of most electric grids today. Look at nearly any transmission line, and you’ll see
three main conductors that (again, very generally) correspond to this diagram. But what you don’t see
here is a connection to ground. Let me put another diagram underneath where distance is equal to
voltage. You can see our three conductors all have the same phase-to-phase voltage,
and they have the same phase-to-ground voltage too. Everything is balanced. But,
in this example, that connection to the ground isn’t very strong, resulting just from the
electromagnetic fields of the alternating current (called capacitive coupling).
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. In fact, a ground fault on an ungrounded
power system usually doesn’t cause any immediate problems. The motors and transformers and other
loads on the system don’t really care about the phase-to-ground voltage because they’re
hooked up between phases. This is one benefit of an ungrounded power system: in many cases
it can keep working even during a ground fault. But, of course, there are some downsides too.
In the example I showed, the phase-to-ground voltages of the two unfaulted conductors
rise to almost twice what they would be in a balanced condition. Here’s why that matters:
Higher voltage requires more insulation which means more cost. Especially on large transmission
lines where insulation means literally holding the conductors great distances away from each
other and the ground, those costs can add up quick. It might seem like an esoteric problem
for an electrical engineer, but in practice, it just means that ungrounded power systems
can be a lot more expensive (a problem anyone can understand). But that’s just the start.
Look back at our diagram and you can see the faulted phase potential is equal to
ground potential. In other words, their difference is zero. There’s no voltage,
and when you have zero voltage, you also have zero current. No electricity is flowing from
the conductor into the ground. Or at least not very much is. You still have the capacitive
coupling between the unfaulted conductors that allows a little bit of current to flow,
but it’s not much. And that matters because nearly all the devices that
would protect a system from a problem (like a ground fault) need some current to flow.
If you know much about wiring in buildings, you might be familiar with the classic example of a
toaster with a metal case. It could be any appliance, but let’s use a toaster. 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.
And, it’s not just the breakers in your house that work this way. 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. In the case of
the damaged toaster, that fault current flowed through a conductor that is called “ground” (but
is actually just a parallel wire that connects to the neutral in your electrical panel). But,
in the case of substations and transmission lines, the fault current path is the actual ground.
Let’s look back at the diagram and convert it to a grounded system. If I add a strong bond
to ground at the generator, things don’t look much different in the unfaulted condition. But
as soon as you add a phase-to-ground short circuit, the diagram looks much different.
First, the other phases don’t experience a shift in their phase-to-ground potential.
But secondly, 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. Although not a great one. Let me show you an example.
I have a narrow acrylic box full of dry sand.
I put a copper rod into the sand on either side of the box and connected a circuit
with a lightbulb so that the current has to flow across the sand from one electrode to the other.
When I turn on the switch, nothing happens. 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. For example, let’s try to wet this sand and see if it makes a difference.
Still nothing. Even completely saturating the sand with tap water, only a tiny current flows.
You can barely see anything in the lightbulb, but the current meter shows a tenth of an amp now.
Soil resistivity also changes with the chemical constituents in the soil,
which is why I’m having trouble getting any current to flow through the sand. There just
aren’t enough electrolytes. Even with a layer of standing water on top of the sand doesn’t conduct
much current at all. 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.
This resistivity of soil to conduct current is pretty important. Earth isn’t a great wire,
but what it lacks in conductivity, it makes up for in size. You can kind of image
current flowing from a ground electrode into the surrounding soil as a series of concentric shells,
each representing a drop in voltage between the faulted conductor and the ground potential. Each
shell has more surface area for current to flow and so has lower resistance until eventually
there’s practically no resistance at all. 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.
Here’s a little demonstration I set up to show how this works.
This is a length of nichrome wire connected between mains voltage with a few power resistors
in between to limit the current. When I flip the switch, electrical current flows through the wire,
simulating a ground fault. This length of NiChrome wire is resistive to the flow of current just like
the soil would be in a ground fault condition. You can see it heat up when I flip the switch.
That means the electric potential along this wire is different at every point. I
can show that just by measuring the voltage with a meter at a few different locations.
Remember that voltage is the difference in potential between two points,
or in the case of Zap McBodySlam here, between two feet. When Zap steps on the
wire, his legs are are at two different electric potentials, and unfortunately,
human bodies are better conductors than the ground. That difference in electric potential
creates a voltage that drives current up into one leg and down out of the other. In this case,
I just have that voltage turning on a little light, but depending on how high that voltage is,
and how well Zap is insulated from it, this step potential can be a matter of life or
death. In fact, power line technicans are often encouraged to hop on one foot away from a ground
fault to reduce the chance of a step potential. It sounds silly, but it might save their life.
Similarly, power technicians often come into contact with the metal cases around
equipment regularly. So, if a ground fault happens on a piece of equipment,
and the resistance of the grounding system is too high, there can be a voltage between
the ground and the metal case, again creating the possibility of a voltage across a person’s body,
called touch potential. 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.
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. You might also notice
that many substations use crushed rock as the ground surface. That’s not just because linesmen
don’t like to mow the grass. It’s because the crushed rock, like the dry sand in my demo,
doesn’t conduct electricity well and minimizes the chance of standing water.
But, not all power systems use the ground just as a safety measure. There are systems where
the earth is actually the primary return path for current to flow. The ground is essentially
the neutral line. Electrical distribution systems called “Single Wire Earth Return”
or SWER are used in a few places around the world to deliver electrical power in rural
areas. Using the earth as a return path can save cost, since you only have to run a single wire,
but of course there are safety and technical challenges too.
Similarly, there are some high voltage transmission lines across the word that
use direct current (like a battery) instead of AC. We’ll save a detailed discussion of
these systems for another day, because there is a lot of fascinating engineering involved. But,
I did want to mention them here, because many of these lines are equipped with really elaborate
grounding systems. Although most High Voltage DC transmission lines use two conductors (positive
and negative), some only use one with the return current flowing through the earth or the sea. And,
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. 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.
Unlike AC return currents that generally follow a path that matches the transmission line,
DC currents can flow through the entire earth. In essence, the electrodes are completely decoupled.
That does mean they’re susceptible to some environmental issues though. 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. 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.
I’ve conveniently left out lightning from this discussion until now.
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. But an additional and vital reason we ground electrical systems is so that,
if lightning strikes, that current has a direct path to the ground. If it didn’t,
it might arc across gaps or build up charge in the system, creating a fire or damaging equipment.
It’s not just lighting, ground faults, and circuit return current that flows through the earth. Lots
of other natural mechanisms cause current to flow below our feet, including solar wind,
changes in earth’s magnetic field, and more. These are collectively known as a telluric currents,
and they intermingle below the surface with the currents that we send into the ground.
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. And the ground works in a similar way. You can’t necessarily
draw lines of current flow between sources and loads, lightning strikes, and telluric phenomena.
The truth of how current flows in the ground is a little more complicated than that;
it all kind of mixes together down there to some extent. But above the surface, it really isn’t so
complicated. Current doesn’t flow to the ground; it flows through the ground and back up. If there
is electricity moving into the ground from an energized conductor, go back to the source of
that conductor and see what’s happening. For the grid, it’s probably a transformer or electrical
generator, in either case, a simple coil of wire. And, the electrical current flowing
out of the coil has to be equal to the electrical current flowing into it, whether that current is
coming from one of the other phases, a neutral line, or an electrode buried in the ground.
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