Can someone explain grounding to me?

Factual Questions
21

Here is my extremely non-technical understanding of the situation.

A normal circuit is a loop, right? Hence the name. You have a powered side “hot” and a return side “cold”. Normally power goes from the hot, through the wiring of your appliance, to the cold. However, sometimes there will be a problem with your appliance - poor insulation, a shorted/exposed wire, whatever. This problem will lead there to be a path between the hot terminal and the outside of your applicance. If you then touch this appliance, the power will flow from the hot, through you, to the ground that you’re standing on. Yes, this will hurt.

A ground is a third wire which is achored to the outside and/or metal frame of your appliance, and runs directly to a grounding point (already exhaustively discussed). If there is a fault in your appliance, the power will flow down this wire to the ground. Frequently, this will trip your circuit breaker and tip you off to a problem. If it doesn’t trip the circuit breaker, it will still protect you if you touch the faulty device, because it provides a better (more conductive) path for the power to reach ground than your body.

Does that help?

mischievous

22

Lib
Here’s an example of your “last person” question. If you and six of your friends(?) all hold hands and the last person grabs a copper water pipe (water pipes run throught the earth and are usually a good ground source) and the first guy jams the butterknife into the electric toaster - well, all of you get to do the Jerry Lewis dance and each of you that survive will swear that you got the most evil shock.

And here’s an analogy that may be helpful. Think of electricity (electrons) as if they were little cars and wires are their highways and roads. These highways channel the electrons through very specific places (your appliances) and the shear volume of their flow causes heat, magnetism, light, whatever. If we happen to allow those cars (electrons) loose into the dessert (earth/ground) they will scatter so much that any heat, magnetism, light, whatever is negligible.

If they are turned loose into the desert before we use them it’s impossible to use them in our appliances any more. If we turn them loose after they go through the appliances then no biggie.

The only time there is any concern over grounding is if, as Trisk said above, the conductivity of the ground is poor, or there is a possibility that large amounts of current will flow through the ground. Even in fairly conductive soil your friendly power company will bury a mesh of copper cable to increase the dissipation of current around its power plants and substations.

23

loose into the dessert?

My bad, please don’t let electrons loose onto the apple pie.
maybe the desert would work better.

24

I’d expect that to turn the base into a large lightning rod. Did they end up having trouble with lightning strikes or induced currents during solar storms?

25

One problem is that “grounding” is such a broad field that such a simple sounding thing as “explain grounding” leads to a whole college course and a library full of writing.

There is grounding for safety; grounding for noise reduction; grounding as a reference point for measurements; grounding as … and on and on.

26

Enough so that a seperate system of linghtning rods was also present. Thunderstorms were not all that common, though.

Tris

27

Can you narrow down what you’re not getting? Is it understanding the flow of electricity, or how the earth itself can act as a reference, or…?

28

Eustace Soares, P.E. wrote a book entitled "Designing Electrical Distribution Systems for Safety" back in 1966, and this text is considered a classic. Many of his recommendations have been incorporated in Article 250 of the NEC, that part of the Code dealing with grounding.

Every year, the International Association of Electrical Inspectors offers field courses around the US and Canada, one of them being the full day Soares Grounding Seminar. To say that I found it highly educational would be understatement.

I believe the textbook can be purchased by contacting the IAEI. A Google search under the gentleman’s name returned a handful of hits from the Bussman Company, manufacturers of overcurrent protective devices.

As far as a system of driven grounding electrodes becoming an attractor for lightning, the purpose of a ground ring is to ensure dissipation of stray currents for equipment protection. Inasmuch as earth is already the ‘ground’ reference, attraction of lightning strike is only increased when a point of ground is made taller, e.g. a tall tree, church steeple, or similar raised structure.

Regarding the third wire on an appliance, make that the fourth. 240 volt appliances had three wire connections to conserve copper for the second world war. The frame was tied to neutral, however the neutral still carries current. A few Code cycles back, the fourth conductor was added, such that electric ranges, clothes dryers, and the like are now connected with two hots, one neutral, and a chassis ground.

29

LSL’s link was pretty good. And all the explanations here have covered the problem, I’m sure, but for some reason, I just always seem to find at leasr one thing that jerks my head out of the loop. For example, I liked the idea of circuit and circuitous being related — for some reason, that relation had never dawned on me. So the electrical circuit is a circle. But a circle to where? Okay, we have the Power Plant, some power towers, some rural power lines, a single line to my house, a wire in the blender, and then a wire into the ground. So, then what? All the electrons (the cars in the desert, I guess) stumble and flitter all around until they make their way back to the Power Plant? Mischievous spoke of a power (hot) and return (cold) side. If the electricity is going into the ground, what’s the point of having a return side? (I’m assuming that’s a second wire.) And then, there’s the AC/DC stuff. I understand the idea basically, I think. But why on earth would you want the current to be constantly changing directions? Doesn’t that greatly diminish the velocity at which the current can travel?

30

Velocity is irrelevant. Having the current switch directions rapidly has several advantages, one of which is that you can use simple devices like transformers to easily step up the voltage very high for long-distance transmission, then step it down again for use. This is important because if electricity were transmitted at the 120 V we use it at in our homes, the wires would have to be extremely thick and heavy to carry all the current. But, per Ohm’s Law, increasing the voltage lowers the current, so the transmission line wires can be much thinner. DC requires more complex (and therefore more expensive) devices to accomplish this same feat.

31

Velocity is irrelevant? Do you mean because the Power Plant is so close to my house that half the speed of light is just as good as the full speed?

32

Electricity doesn’t necessarily travel through a wire at c anyway. It is limited by the speed fo light in the medium surrounding the wire. If that happens to be air, then the speed of propagation will approach c, however if it’s something like, say, teflon insulation the velocity will be much less, say between 60% and 70% of c. However the velocity of current has little, if anything, to do with its ability to do work. That depends on factors like current and voltage, as well as the impedances of the source and load(s), among other factors.

33

[…possible light going off in head…] I’ll mull that over. Thanks.

34

Think of electricity as a flow of energy.

Try not to think of it as a flow of electrons etc, which might imply speed.

What happens is that the generator pumps energy around a system, it does not matter how fast that energy travels as such.

Imagine now that the amount of power delivered depends upon just how much you open the gate for that energy to flow.

The amount of energy delivered depends upon the amount you open the gate by.

Water is often used as an analogy in such situations.

Water does not need to move fast to deliver lots of energy, by opening the gate wide enough, lots of fluid energy will be delivered no matter what the ‘speed’

Imagine you are at some point lower than the water source, the lower you are in comparison, then the greater the potential flow to you.
You then use that flow to do some useful work for you, such as drive a paddle wheel.

Once the fluid has passed by you and dropped off the energy by spinning the wheel it reaches a lowest point.

This system is special though, because all the fluid does is carry energy, and energy can only be transferred by the fluid in the system.

Somehow you have to get the fluid that has transferred its useful energy, back to the pump.

The result is that you have to have a return line.

In electrical terms, the differance in height between the pump and the user is called potential which is given in Volts, and the greater this is than the greater the amount of power that can be delivered for a given volume of fluid.

We can call the volume of fluid the current, in electrical terms I.
Clearly the power is a combination of the potential differance and the volume of fluid that has moved.

Thus Power =V*I

Electrons, electricity, or whatever, are just a method of delivering energy.
Now to get to the awkward part.

Using yet another analogy.

Imagine you have a tape measure, which has markings in inches upon it, but no numbers at all.

We know that to travel along this measure, 1inch travel causes a change of 1 volt.

At the moment we don’t know how great is the total length, we do not know the start point, or even which direction to take to make the voltage go up and down.

It seems sensible for now to start at one end of the tape and call it 0 Volts, and each inch of travel is 1Volt.

With some kind of measuring device, we can compare the voltage at our start point with the voltage at any other point.

Lets say that the measure is 10 inches, and when we travel from one end to the other we find that the differance between the two ends turns out to be 10 volts.

It turns out that the start point is actually 10 Volts higher than the finish point, thus the finish is -10Volts in comparison to the start.

We don’t use that exact terminology, instead it would be correct to say that the finish is -10Volts with respect to the start point.( w.r.t )

Whenever you make any electrical voltage measurement you need two points to compare.

Now you can see that we didn’t actually have to start at the very beginning of the tape if we wanted to be clever, we could start slap bang in the middle.

If we called the middle of the tape 0Volts we would travel in one direction and get a differance in one direction or +5 Volts w.r.t the 0Volt point.

If we went the opposite direction then we could achieve -5 Voltsw.r.t 0 Volts.

The 0 Volt point is selected for our convenience, we could place it anywhere, just so long as we always assume that the total net voltage never exceeds or under achivees the maximum possible voltage between the furthest most two ends.

One way of achieving this, to make sure our selected point is at 0Volts, is to connect it to earth. We then measure all the voltages in this system w.r.t this earth point.

If we were to introduce a second earth at some differant point something odd happens, it turns out that since earth is the same value no matter where it is connected in the system, it acts as a way of going around the circuit and bypasses it in the region between the two earths.This is than called a short circuit, due to that bypass.

By using an earth in a circuit we effectively tie one part of it down in electrical terms, which means that the rest of the circuit can only ever reach a maximum value relative to this fixed point.

This short circuit idea is very important in electrical safety terms.

If you have an electric toaster, the normal working path for the current to take would be down one wire, throught the heater element and back through the other wire.

If some fault develops, so that the circuit somenow toiuches the metal casing, then power could flow through that casing, into the unfortunate person who happened to touch it, and into ground and back to the mains supply which has one end connected to earth.

This could easily kill.

What is done is that the casing has another wire connected to it, this is the earth, and if you follow this wire all the way back it would join to the electrical system earth.

If a part of the live circuit touches the metal casing, this voltage is sent direct to earth, and it dramatically increases the flow of current.What has happened is that this power is being shunted to earth and is bypassing the normal circuit, we have a controlled short circuit, because we have not one earth, but two.

This will then cause a specially prepared element of wire in the supply line to get, hot, melt, and thus disconnect the toaster - the fuse.

This is one application of the use of an earth, but there are many others.

In terms of electrical principles earths all work in the same way, but depending upon the specific use we might choose to exploit one particular aspect above others.

35

One of the analogies that I used when teaching the concept of AC vs. DC circuits was that of a belt-drive system.

Picture a crank-driven driveshaft connected to another shaft via a belt. (An example of a drive belt is the belt you have to replace on your car’s engine.) The goal is to turn the second shaft, which is friction-braked. Turning the crank thus produces frictional heat at the far end.

As soon as you start turn the driveshaft with the crank, you start producing heat at the far end. You don’t have to wait until the section of belt at your end actually reaches the far end before you start producing heat at the far end. This is analogous to a DC circuit transferring energy to a light bulb much faster than the drift speed of electrons (which is only a few centimeters per second).

Alternatively, you could just turn the crank back and forth. This will cause the shaft at the far end to also turn back and forth, and still produce frictional heat. Any given section of drive belt pretty much stays in place, but the work (energy) you put into the driveshaft is still rapidly transferred to the shaft at the far end. This is analogous to an AC circuit, in which the polarity is constantly reversing, but energy is still transferred in one direction from the generator to the load.

Hope this helps!

36

Ah but the “single line” to your house is actually two wires (I know, it’s 3 wires but let’s take this one step at a time). There is 120 v. ac between the two wires. Current flows out of the source on one wire, through your blender, washer motor, light bulb or what have you and back to the source on the other wire. The direction of flow reverses at the rate of the line frequency. The current that comes into your house does not go through the earth ground. The connection to the earth ground is for the purpose of keeping the whole electrical grid from developing high voltages with respect to the ground which keeps people from being killed when they plug in a toaster. Every so often along a power transmission line, there is an earth ground. On the high voltage lines, the ones on the big steel towers, I think there is an earth ground at every tower. In addition there are earth grounds at all of those substations that you see here and there. The ones that are surrounded by a chain link fence and have a whole bunch of big metal cans inside with long, rippled ceramic insulators sticking out of them. And in cities there is a ground at every building that is connected to the grid.

Just down from my house in the easement is a power pole with a transformer on it. Power is distributed around town at 2200 v. (maybe it’s 4400 v. but that’s a detail) and the transformer steps it down from the high voltage to the house voltage. On that pole from one terminal of the transformer is a thick, bare copper wire that runs to a grounding rod beside the pole.

Now back to that wire into the house. I said it is actually three wires in the typical house. There are two 120 v. circuits that come to the house. They are out of phase. Let’s label the wires black, white and red. The white wire is the “ground” or “neutral.” And right here we run into the first terminology hitch. That use of the term “ground” isn’t the same use as in the foregoing but bear with me. This “ground” is the voltage level to which all other voltages in the circuit are referred. In other words, the voltage on that white line is 0 v. by definition. That’s why it is also sometime called the “neutral” line, or just “neutral.”

Ok. The black wire goes positive from the same voltage as the neutral to some voltage magnitude, with its value at each instant of time being proportional to a sine curve, and at the same time the red wire goes negative also along a sine curve to the same magnitude in the negative direction. Then the black and red wire voltages return to the voltage of the white wire with the magnitudes following the sine curve. And then the black wire goes negative with respect to the white while the red goes positive and then both return to 0 v. and they repeat that cycle over and over. Now if the voltage is measured between either the red or the black to the white wire you will get 120 v. Between the red and the black you will measure 240 v. so you can run your clothes dryer and electric stove, and maybe your spa. This system allows the use of high power devices that would take excessive current at 120 v. to operate at 240 v. while no voltage in the building exceeds 120 v. above the neutral level. And the white, neutral wire is tied to the earth ground at the electrical panel (the box with all the circuit breakers), and at that point only. All of the current used in your house goes through these wires and none of it through the earth.

37

Go to your room! Because I said so!

[sub]Oh, nevermind[/sub]

38

Out of phase? Is this right? I understood that a 240v service (for residential use) was achieved with a single phase transformer. The secondary windings are designed to have 240v between them and the neutral is center tapped, thus 120v between either end of the winding and the neutral tap. Across the entire winding is 240v, but it is in phase. It has to be. Three phase power is typically not run to houses.

39

If they are in phase then there is no voltage between them at all. As one peaks at +120, the other peaks at +120. What potential exists between that?

40

It depends on what you want to call 0 V. In the case of typical residential service, the center tap is connected to Earth ground and is called 0 V. In this case, each of the two outer legs are 180[sup]o[/sup] our of phase whith each other, with one at +120 V and the other at -120 V. If you were to tie one of the outer legs to ground instead, that would become your 0 V reference and the center tap and the other outer leg would be in phase with each other, with the CT at +120 V and the other outer leg at +240 V. This is never done in practice, that I’m aware of.