I wrote this up some time ago, in response to a question about the purpose for ground rods and the manner in which they “stabilize the voltage to ground.” I wanted to write this for an audience with no formal training in electrical engineering concepts. Unfortunately, as a result I was not able to give a short answer. So bear with me, if you will.
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Voltage is the difference in “potential” between two points. “Potential” can only exist in the presence of an electric field. Suppose you had in your hand a charged object (i.e., metal ball to which a charge had been applied and that you hold with insulating gloves). Suppose you are surrounded by an electric field (the source of which need not be known, so let’s just assume it is there). Finally, suppose you intend to walk across the room, while holding the charged object. Depending on the orientation of the field (i.e., which way is Plus and which way is Minus), and depending on the nature of a packet of charge (i.e., again, whether it is plus or minus), the field will either help you by pushing the charge in your direction of travel, or it will fight you by pushing it in the other direction. Some amount of energy will be needed, in order to move this charged object through the field. The notion of “voltage between two points,” or “potential difference between two points,” has to do with the amount of energy it takes to move a charge from one point to the other.
“Potential” is a bit of an obscure concept. But you can look at it like this.
Suppose we live in separate houses on the same road. It is a very, very long road, and it is straight and level. There are no turnoffs available, no intersections of other roads. There is a town somewhere way, way beyond your house. I am not going to tell you the name of the town, nor how far that town is from your house. Suppose that if a car started at “that town” and drove to your house, it would take 70 gallons of gas for the trip. Suppose that if the same car driving the same way under the same conditions drove from “that town” to my house, it would take 90 gallons of gas to make the trip. Can you tell me how much gas it would take for that car to go from your house to my house? Did you guess “20 gallons”? Did you need to know the location of “that town,” in order to arrive at your answer?
Now suppose there is an electric field in the vicinity of our two houses and all along the road. Suppose someone wants to move a packet of charge (let’s say it is one coulomb of charge) from “that town way, way over there” (the usual engineering term for this is “from infinity”) to your house. Let’s ignore the gasoline needed to move the vehicle that distance, and only look at the extra energy needed to move a charge that distance in the presence of the electric field. It will take some amount of energy to make that trip. Let’s say it took 70 joules of energy. Suppose instead they moved the same amount of charge from “that town way, way over there” to my house, and it took 90 joules of energy to make the trip. Can you tell me how much energy it would take to move the charge from your house to my house? Did you guess “20 joules”? Did you need to know the location of “that town,” in order to arrive at your answer?
So then, what is the voltage between your house and mine? It is a difference in potential of 20 joules for every coulomb of charge you try to move from your house to mine. A “Volt” is defined as “one joule per coulomb.” Thus, the voltage between your house and mine would be twenty volts.
Now to the question of how a ground rod “stabilizes” a power system. Consider first an ungrounded system (i.e., no ground rods). The Equipment Grounding Conductors are bonded to the neutral conductors (can’t call them “grounded conductors”) at the main service point. But that point is not connected (via a Grounding Electrode Conductor) to planet Earth.
Consider a coffee pot on the kitchen counter. It operates at 120 volts. Consider the wires that enter the coffee pot. If there are 120 volts between the hot conductor and the neutral conductor, then what is the voltage between the hot conductor and planet Earth? While we’re at it, what is the voltage between the neutral conductor and planet Earth? These are like asking the distance from “that town” to your house, or the distance from “that town” to my house. The answer is that you don’t know. And as far as a light bulb, a TV set, or our coffee pot are concerned, it does not matter. You could have a voltage of 1,120 volts from the hot wire to planet Earth, and a voltage of 1000 volts from the neutral wire to planet Earth, and the difference is still 120 volts, so therefore the coffee pot will work properly.
So why is this a problem? Because the internal wiring of the coffee pot is designed to stop current from leaking from the energized parts to the outside world, provided that the voltage between the energized parts and the planet is not over 600 volts (the standard insulation rating for most of what we deal with). But in my example, the voltage is almost double the insulation’s rating. That would not be safe. You could get current leaking from the internal wires to the outside of the coffee pot.
There is another reason that this situation would not be safe. This time let’s presume the insulation system does its job, and there is no leakage current from the internal wires to the outside case. You normally think it to be safe to touch the metal outer case of the coffee pot. But is it safe? What is the voltage between the case and planet Earth (the planet by the way to which your feet are attached at the moment)? Well, there was 1120 volts from hot to ground, and 1000 volts from neutral to ground, and the neutral wire is attached (at the service) to the EGC, and the EGC is attached (internal to the coffee pot) to the external case of the coffee pot, so my guess is that there would be close to 1000 volts between the case (to which your hands are attached) and the planet (to which your feet are attached).
How do we stop that from happening? By connecting the neutral wire (at the main service) to planet Earth. That causes the voltage at the neutral and the voltage at the dirt to be almost the exact same value. That in turn causes the voltage between the hot conductor and planet Earth to be no more than about 120 volts. There may be some variation from moment to moment. But the voltage rating of the insulation systems of household electrical appliances will not be exceeded. In addition, the voltage between your hands and your feet, as you touch the external metal parts of kitchen appliances, will be close to zero at all times.
It is in this sense that the voltage has been stabilized.