Where Does Grounded Electricity Actually Go?

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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